[
    {
        "text": "Does HL Tau Disk Polarization in ALMA Band 3 Come from Radiatively\n  Aligned Grains?: Disk polarization in (sub)millimeter dust continuum is a rapidly growing\nfield in the ALMA era. It opens up the exciting possibility of detecting and\ncharacterizing magnetic fields and grain growth in disks around young stellar\nobjects. However, to use polarization for probing the disk properties, its\nproduction mechanism must be ascertained first. To date, the conventional\nmechanism involving magnetically aligned grains fails to explain the\npolarization patterns detected in most disks. This is especially true for the\ninclined disk of HL Tau in ALMA Band 3 (wavelength $\\sim 3$ mm), which has an\nelliptical polarization pattern. The elliptical pattern was taken as evidence\nfor polarized emission by dust grains aligned with their long axes\nperpendicular the direction of the radiative flux. We show that the radiatively\naligned grains produce a circular, rather than elliptical, polarization pattern\neven in inclined disks such as HL Tau. An elliptical polarization pattern can\nbe produced if the grains are aligned aerodynamically by the difference in\nrotation speed between the dust and gas through the Gold mechanism. However, a\nstrong azimuthal variation in polarized intensity is expected for both the\nradiative and aerodynamic alignment, but not observed in the HL Tau disk in\nALMA Band 3. We conclude that neither of these two mechanisms alone can explain\nthe data and the origin of the 3 mm polarization remains a mystery. We\nspeculate that this mystery may be resolved by a combination of both direct\nemission and scattering by aerodynamically aligned grains.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Wide-angle protostellar outflows driven by narrow jets in stratified\n  cores: Most simulations of outflow feedback on star formation are based on the\nassumption that outflows are driven by a wide angle \"X-wind,\" rather than a\nnarrow jet. However, the arguments initially raised against pure jet-driven\nflows were based on steady ejection in a uniform medium, a notion that is no\nlonger supported based on recent observations. We aim to determine whether a\npulsed narrow jet launched in a density-stratified, self-gravitating core could\nreproduce typical molecular outflow properties, without the help of a\nwide-angle wind component. We performed axisymmetric hydrodynamic simulations\nusing the MPI-AMRVAC code with optically thin radiative cooling on timescales\nup to 10000 yrs. Then we computed and compared the predicted properties with\nobservational data. First, the jet-driven shell expands faster and wider\nthrough a core with steeply decreasing density than through an uniform core.\nSecond, when blown into the same singular flattened core, a jet-driven shell\nhas a similar width as a wide-angle wind-driven shell in the first few hundred\nyears, but a decelerating expansion on long timescales. The flow adopts a\nconical shape and a base opening angle reaching up to $90\\unicode{xb0}$. Third,\nafter $\\sim$ 10000 yrs, a pulsed jet-driven shell shows fitting features and a\nqualitative resemblance with recent observations of protostellar outflows with\nthe Atacama Large Millimeter Array (ALMA), such as HH46-47 and CARMA-7. In\nparticular, similarities are seen in the shell widths, opening angles,\nposition-velocity diagrams, and mass-velocity distribution, with some showing a\ncloser resemblance than in simulations based on a wide-angle \"X-wind\" model.\nTherefore, a realistic ambient density stratification in addition to\nmillenia-long integration times are equally essential to reliably predict the\nproperties of outflows driven by a pulsed jet and to confront them with the\nobservations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Models for velocity decrease in HH34: The conservation of the energy flux in turbulent jets that propagate in the\ninterstellar medium (ISM) allows us to deduce the law of motion when an inverse\npower law decrease of density is considered. The back-reaction that is caused\nby the radiative losses for the trajectory is evaluated. The velocity\ndependence of the jet with time/space is applied to the jet of HH34, for which\nthe astronomical data of velocity versus time/space are available. The\nintroduction of precession and constant velocity for the central star allows us\nto build a curved trajectory for the superjet connected with HH34. The bow\nshock that is visible in the superjet is explained in the framework of the\ntheory of the image in the case of an optically thin layer.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Cool dwarfs stars from the Torino Observatory Parallax Program: We investigate and parameterise high proper motion red stars in the Torino\nObservatory Parallax Program. Observations of 27 objects were made over the\nperiod 1994 - 2001 on the 1.05m Torino telescope. The trigonometric parallaxes\nand proper motions were determined using standard techniques. We determine\nparallaxes and proper motions, and by comparison to models we infer masses,\nages, and {metallicities}. Of the 27 objects, 22 are within 25pc and 4 appear\nto be subdwarfs. There are published parallaxes for 18 objects, and all but 4\nagree to within 2$\\sigma$. The discrepancies are discussed.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Realistic model atmosphere and revised abundances of the coolest Ap star\n  HD101065: Among the known Ap stars, HD101065 is probably one of the most interesting\nobjects, demonstrating very rich spectra of rare-earth elements (REE). Strongly\npeculiar photometric parameters of this star that can not be fully reproduced\nby any modern theoretical calculations, even those accounting for realistic\nchemistry of its atmosphere. In this study we investigate a role of missing REE\nline opacity and construct a self-consistent atmospheric model based on\naccurate abundance and chemical stratification analysis. We employed the\nLLmodels stellar model atmosphere code together with DDAFit and SynthMag\nsoftware packages to derive homogeneous and stratified abundances for 52\nchemical elements and to construct a self-consistent model of HD101065\natmosphere. The opacity in REE lines is accounted for in details, by using\nup-to-date extensive theoretical calculations. We show that REE elements play a\nkey role in the radiative energy balance in the atmosphere of HD101065, leading\nto the strong suppression of the Balmer jump and energy redistribution very\ndifferent from that of normal stars. Introducing new line lists of REEs allowed\nus to reproduce, for the first time, spectral energy distribution of HD101065\nand achieve a better agreement between the unusually small observed Str\\\"omgren\nc1 index and the model predictions. Using combined photometric and\nspectroscopic approaches and based on the iterative procedure of abundance and\nstratification analysis we find effective temperature of HD101065 to be\nTeff=6400K.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Contributions of structural variations to the asymptotic mixed-mode\n  coupling Factor in red giant stars: The advent of ultra-precise photometry space missions enable the possibility\nof investigating stellar interior with mixed modes. The structural variations\ninduced by the discontinuity of the chemical composition left behind during the\nfirst dredge--up is an important feature in the stellar mid-layers located\nbetween the hydrogen-burning shell and the base of the convective zone of red\ngiants, as the mixed-mode properties can be significantly affected by these\nvariations. In this paper, the contributing factors to variations of the\nmixed-mode coupling factor, $q$, are discussed with stellar models. In general,\nthe structural variations give rise to a subtle displacement in the Lamb\nfrequency and a sharp change in the buoyancy frequency, which lead to\nvariations in the value of $q$ computed using the asymptotic formalisms that\nassuming a smooth background free of structural variations. The impact of these\ntwo factors can be felt in detectable mixed modes in low-luminosity red giants.\nFurthermore, the different nature of variations of the two characteristic\nfrequencies with radius near the base of the convective zone, produces a sudden\nincrease in $q$ in evolved red giants. This is followed by a quick drop in $q$\nas the star evolves further along the red giant branch.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Frequency analysis of Delta Scuti stars towards the Galactic bulge: We have performed a frequency analysis of 10,092 Delta Scuti-type stars\ndetected in the fourth phase of the Optical Gravitational Lensing Experiment\n(OGLE) towards the Galactic bulge, which is the most numerous homogeneous\nsample of Delta Scuti stars observed so far. The main goal was to search for\nstars pulsating in at least two radial modes simultaneously. We have found 3083\ncandidates for such stars, which is the largest set obtained to date. Among\nthem, 2655 stars pulsate in two radial modes, 414 stars pulsate in three radial\nmodes, and 14 stars pulsate in four radial modes at the same time. We report\nthe identification of 221 Delta Scuti stars pulsating in the fundamental mode,\nfirst overtone, and third overtone simultaneously. We show the most populated\nPetersen and Bailey diagrams and discuss statistical properties of the\nidentified frequencies based on this numerous sample. Additionally, we present\ntheoretical predictions of period ratios for Delta Scuti stars pulsating in\novertones from the fourth to the seventh.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The M Supergiant High Mass X-Ray Binary 4U 1954+31: The X-ray binary 4U 1954+31 has been classified as a Low Mass X-ray Binary\n(LMXB) containing a M giant and a neutron star (NS). It has also been included\nin the rare class of X-ray symbiotic binaries (SyXB). The Gaia parallax,\ninfrared colors, spectral type, abundances, and orbital properties of the M\nstar demonstrate that the cool star in this system is not a low mass giant but\na high mass M supergiant. Thus, 4U 1954+31 is a High Mass X-ray Binary (HMXB)\ncontaining a late-type supergiant. It is the only known binary system of this\ntype. The mass of the M I is 9$^{+6}_{-2}$ M$_\\odot$ giving an age of this\nsystem in the range 12 - 50 Myr with the NS no more than 43 Myr old. The spin\nperiod of the NS is one of the longest known, 5 hours. The existence of M I\nplus NS binary systems is in accord with stellar evolution theory, with this\nsystem a more evolved member of the HMXB population.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Strength and Variability of the Helium 10830 \u00c5 Triplet in Young\n  Stars, with Implications for Exosphere Detection: Young exoplanets trace planetary evolution, particularly the atmospheric mass\nloss that is most dynamic in youth. However, the high activity level of young\nstars can mask or mimic the spectroscopic signals of atmospheric mass loss.\nThis includes the activity-sensitive He 10830 \\AA\\ triplet, which is an\nincreasingly important exospheric probe. To characterize the He-10830 triplet\nat young ages, we present time-series NIR spectra for young transiting planet\nhosts taken with the Habitable-zone Planet Finder. The He-10830 absorption\nstrength is similar across our sample, except at the fastest and slowest\nrotation, indicating that young chromospheres are dense and populate metastable\nhelium via collisions. Photoionization and recombination by coronal radiation\nonly dominates metastable helium population at the active and inactive\nextremes. Volatile stellar activity, such as flares and changing surface\nfeatures, drives variability in the He-10830 triplet. Variability is largest at\nthe youngest ages before decreasing to $\\lesssim5-10$ m\\AA\\ (or 3%) at ages\nabove 300 Myr, with 6 of 8 stars in this age range agreeing with no intrinsic\nvariability. He-10830 triplet variability is smallest and age-independent at\nthe shortest timescales. Intrinsic stellar variability should not preclude\ndetection of young exospheres, except at the youngest ages. We recommend\nout-of-transit comparison observations taken directly surrounding transit and\nobservation of multiple transits to minimize activity's effect. Regardless,\ncaution is necessary when interpreting transit observations in the context of\nstellar activity, as many scenarios can lead to enhanced stellar variability\neven on timescales of an hour.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Constraining modified gravity from tidal phenomena in binary stars: In beyond-Horndeski theories of gravity, the Vainshtein screening mechanism\nmight only be partially effective inside stellar objects. This results in a\nmodification of the pressure balance equation inside stars, often characterized\nby a single parameter ($\\Upsilon$) in isotropic systems. We show how to\nconstrain such theories of modified gravity, using tidal effects. We study such\neffects in cataclysmic variable star binaries and numerically obtain limits on\nthe critical masses of the donor stars, below which they are tidally disrupted,\nby modeling them in beyond-Horndeski theories. This is contrasted with values\nof the donor masses, obtained using existing observational data, by a Monte\nCarlo error progression method. A best fit scenario of the two yields a\nparametric constraint in the theories that we consider, within the\napproximations used. Here, we obtain the allowed range $ 0 \\le \\Upsilon \\le\n0.47 $.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Eleven Exoplanet Host Star Angular Diameters from the CHARA Array: We directly measured the angular diameters for 11 exoplanet host stars using\nGeorgia State University's CHARA Array interferometer and calculated their\nlinear radii and effective temperatures. The sample tends towards evolving or\nevolved stars and includes one dwarf, four subgiants, and six giants. We then\nestimated masses and ages for the stars using our effective temperatures\ncombined with metallicity measurements from the literature.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Tests of Sunspot Number Sequences: 2. Using Geomagnetic and Auroral Data: We compare four sunspot-number data sequences against geomagnetic and\nterrestrial auroral observations. The comparisons are made for the original\nSIDC composite of Wolf-Zurich-International sunspot number [$R_{ISNv1}$], the\ngroup sunspot number [$R_{G}$] by Hoyt and Schatten (Solar Phys., 1998), the\nnew \"backbone\" group sunspot number [$R_{BB}$] by Svalgaard and Schatten (Solar\nPhys., 2016), and the \"corrected\" sunspot number [$R_{C}$] by Lockwood at al.\n(J.G.R., 2014). Each sunspot number is fitted with terrestrial observations, or\nparameters derived from terrestrial observations to be linearly proportional to\nsunspot number, over a 30-year calibration interval of 1982-2012. The fits are\nthen used to compute test sequences, which extend further back in time and\nwhich are compared to $R_{ISNv1}$, $R_{G}$, $R_{BB}$, and $R_{C}$. To study the\nlong-term trends, comparisons are made using averages over whole solar cycles\n(minimum-to-minimum). The test variations are generated in four ways: i) using\nthe IDV(1d) and IDV geomagnetic indices (for 1845-2013) fitted over the\ncalibration interval using the various sunspot numbers and the phase of the\nsolar cycle; ii) from the open solar flux (OSF) generated for 1845 - 2013 from\nfour pairings of geomagnetic indices by Lockwood et al. (Ann. Geophys., 2014)\nand analysed using the OSF continuity model of Solanki at al. (Nature, 2000)\nwhich employs a constant fractional OSF loss rate; iii) the same OSF data\nanalysed using the OSF continuity model of Owens and Lockwood (J.G.R., 2012) in\nwhich the fractional loss rate varies with the tilt of the heliospheric current\nsheet and hence with the phase of the solar cycle; iv) the occurrence frequency\nof low-latitude aurora for 1780-1980 from the survey of Legrand and Simon (Ann.\nGeophys., 1987). For all cases, $R_{BB}$ exceeds the test terrestrial series by\nan amount that increases as one goes back in time.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Population Synthesis of Black Hole Binaries with Normal-Star Companions:\n  I. Detached Systems: Optical observations of normal-stars in binary systems with massive unseen\nobjects have been proposed to search for candidate black holes (BHs) and\nprovide a direct measurement of their dynamical masses. In this paper, we have\nperformed binary population synthesis calculations to simulate the potential\npopulation of detached binaries containing BHs and normal-star companions in\nthe Galaxy. We focus on the influence of the BH progenitors. In the traditional\nmodel, BHs in binaries evolve from stars more massive than $ \\sim25M_{\\odot} $.\nHowever, it is difficult for this model to produce BH low-mass X-ray binaries.\nRecent investigations on massive star evolution suggest that the BH progenitors\nmay have masses as low as $ \\sim15M_{\\odot} $. Based on this model, we provide\nthe expected distributions of various parameters for detached BH binaries with\nnormal-star companions, including the component masses, the orbital parameters\nof the binary systems, the radial velocity semi-amplitudes, and the astrometric\nsignatures of the optical companions. %the detached BH systems with low-mass\ncompanions can be easily produced in our simulations. Our calculations show\nthat there are more than thousands of such detached binaries in the Galaxy, and\nhundreds of them are potentially observable systems with luminous companions\nbrighter than 20 mag. In addition, detached BH binaries are dominated by those\nwith main-sequence companions and only a few percent of them are expected to\nhave giant companions.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Gaia-ESO Survey. Mg-Al anti-correlation in iDR4 globular clusters: We use Gaia-ESO Survey iDR4 data to explore the Mg-Al anti-correlation in\nglobular clusters, that were observed as calibrators, as a demonstration of the\nquality of Gaia-ESO Survey data and analysis. The results compare well with the\navailable literature, within 0.1 dex or less, after a small (compared to the\ninternal spreads) offset between the UVES and the GIRAFFE data of 0.10-0.15 dex\nwas taken into account. In particular, we present for the first time data for\nNGC 5927, one of the most metal-rich globular clusters studied in the\nliterature so far with [Fe/H]=-0.49 dex, that was included to connect with the\nopen cluster regime in the Gaia-ESO Survey internal calibration. The extent and\nshape of the Mg-Al anti-correlation provide strong constraints on the multiple\npopulation phenomenon in globular clusters. In particular, we studied the\ndependency of the Mg-Al anti-correlation extension with metallicity,\npresent-day mass, and age of the clusters, using GES data in combination with a\nlarge set of homogenized literature measurements. We find a dependency with\nboth metallicity and mass, that is evident when fitting for the two parameters\nsimultaneously, but no significant dependency with age. We confirm that the\nMg-Al anti-correlation is not seen in all clusters, but disappears for the less\nmassive or most metal-rich ones. We also use our dataset to see whether a\nnormal anti-correlation would explain the low [Mg/$\\alpha$] observed in some\nextragalactic globular clusters, but find that none of the clusters in our\nsample can reproduce it, and more extreme chemical compositions (like the one\nof NGC 2419) would be required. We conclude that GES iDR4 data already meet the\nrequirements set by the main survey goals, and can be used to study in detail\nglobular clusters even if the analysis procedures were not specifically\ndesigned for them.",
        "category": "astro-ph_SR"
    },
    {
        "text": "SST/CRISP Observations of Convective Flows in a Sunspot Penumbra: Context. Recent discoveries of intensity correlated downflows in the interior\nof a sunspot penumbra provide direct evidence for overturning convection,\nadding to earlier strong indications of convection from filament dynamics\nobserved far from solar disk center, and supporting recent simulations of\nsunspots.\n  Aims. Using spectropolarimetric observations obtained at a spatial resolution\napproaching 0'.'1 with the Swedish 1-m Solar Telescope (SST) and its\nspectropolarimeter CRISP, we investigate whether the convective downflows\nrecently discovered in the C i line at 538.03 nm can also be detected in the\nwings of the Fe i line at 630.15 nm\n  Methods. We make azimuthal fits of the measured LOS velocities in the core\nand wings of the 538 nm and 630 nm lines to disentangle the vertical and\nhorizontal flows. To investigate how these depend on the continuum intensity,\nthe azimuthal fits are made separately for each intensity bin. By using\nspatially high-pass filtered measurements of the LOS component of the magnetic\nfield, the flow properties are determined separately for magnetic spines\n(relatively strong and vertical field) and inter-spines (weaker and more\nhorizontal field).\n  Results. The dark convective downflows discovered recently in the 538.03 nm\nline are evident also in the 630.15 nm line, and have similar strength. This\nconvective signature is the same in spines and inter-spines. However, the\nstrong radial (Evershed) outflows are found only in the inter-spines.\n  Conclusions. At the spatial resolution of the present SST/CRISP data, the\nsmall-scale intensity pattern seen in continuum images is strongly related to a\nconvective up/down flow pattern that exists everywhere in the penumbra. Earlier\nfailures to detect the dark convective downflows in the interior penumbra can\nbe explained by inadequate spatial resolution in the observed data.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Presupernova neutrinos: realistic emissivities from stellar evolution: We present a new calculation of neutrino emissivities and energy spectra from\na massive star going through the advanced stages of nuclear burning\n(presupernova) in the months before becoming a supernova. The contributions\nfrom beta decay and electron capture, pair annihilation, plasmon decay, and the\nphotoneutrino process are modeled in detail, using updated tabulated nuclear\nrates. We also use realistic conditions of temperature, density, electron\nfraction and nuclear isotopic composition of the star from the state of the art\nstellar evolution code MESA. Results are presented for a set of progenitor\nstars with mass between 15 $M_\\odot$ and 30 $M_\\odot$. It is found that beta\nprocesses contribute substantially to the neutrino emissivity above realistic\ndetection thresholds of few MeV, at selected positions and times in the\nevolution of the star.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A solar flare disturbing a light wall above a sunspot light bridge: With the high-resolution data from the Interface Region Imaging Spectrograph,\nwe detect a light wall above a sunspot light bridge in the NOAA active region\n(AR) 12403. In the 1330 A slit-jaw images, the light wall is brighter than the\nambient areas while the wall top and base are much brighter than the wall body,\nand it keeps oscillating above the light bridge. A C8.0 flare caused by a\nfilament activation occurred in this AR with the peak at 02:52 UT on 2015\nAugust 28, and the flare's one ribbon overlapped the light bridge which was the\nobservational base of the light wall. Consequently, the oscillation of the\nlight wall was evidently disturbed. The mean projective oscillation amplitude\nof the light wall increased from 0.5 Mm to 1.6 Mm before the flare, and\ndecreased to 0.6 Mm after the flare. We suggest that the light wall shares a\ngroup of magnetic field lines with the flare loops, which undergo a magnetic\nreconnection process, and they constitute a coupled system. When the magnetic\nfield lines are pushed upwards at the pre-flare stage, the light wall turns to\nthe vertical direction, resulting in the increase of the light wall's\nprojective oscillation amplitude. After the magnetic reconnection takes place,\na group of new field lines with smaller scales are formed underneath the\nreconnection site and the light wall inclines. Thus, the projective amplitude\ndecreases remarkably at the post-flare stage.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the nature of the WO3 star DR1 in IC 1613: We present the results of a quantitative spectroscopic analysis of the\noxygen-sequence Wolf- Rayet star DR1 in the low-metallicity galaxy IC 1613. Our\nmodels suggest that the strong oxygen emission lines are the result of the high\ntemperature of this WO3 star and do not necessarily reflect a more advanced\nevolutionary stage than WC stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spectro-interferometry of the Be star delta Sco: Near-Infrared Continuum\n  and Gas Emission Region Sizes in 2007: We present near-infrared H and K-band spectro-interferometric observations of\nthe gaseous disk around the primary Be star in the delta Sco binary system,\nobtained in 2007 (between periastron passages in 2000 and 2011). Observations\nusing the CHARA/MIRC instrument at H-band resolve an elongated disk with a\nGaussian FWHM 1.18 x 0.91 mas. Using the Keck Interferometer, the source of the\nK-band continuum emission is only marginally spatially resolved, and\nconsequently we estimate a relatively uncertain K-band continuum disk FWHM of\n0.7 +/- 0.3 mas. Line emission on the other hand, He1 (2.0583 micron) and Br\ngamma (2.1657 micron), is clearly detected, with about 10% lower visibilities\nthan those of the continuum. When taking into account the continuum/line flux\nratio this translates into much larger sizes for the line emission regions: 2.2\n+/- 0.4 mas and 1.9 +/- 0.3 mas for He1 and Br gamma respectively. Our KI data\nalso reveal a relatively flat spectral differential phase response, ruling out\nsignificant off-center emission. We expect these new measurements will help\nconstrain dynamical models being actively developed in order to explain the\ndisk formation process in the delta Sco system and Be stars in general.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Nonequilibrium ionization and ambipolar diffusion in solar magnetic flux\n  emergence processes: Magnetic flux emergence has been shown to be a key mechanism for unleashing a\nwide variety of solar phenomena. However, there are still open questions\nconcerning the rise of the magnetized plasma through the atmosphere, mainly in\nthe chromosphere, where the plasma departs from local thermodynamic equilibrium\n(LTE) and is partially ionized. We aim to investigate the impact of the\nnonequilibrium (NEQ) ionization and recombination and molecule formation of\nhydrogen, as well as ambipolar diffusion, on the dynamics and thermodynamics of\nthe flux emergence process. Using the Bifrost code, we performed 2.5D numerical\nexperiments of magnetic flux emergence from the convection zone up to the\ncorona. The experiments include the NEQ ionization and recombination of atomic\nhydrogen, the NEQ formation and dissociation of H2 molecules, and the ambipolar\ndiffusion term of the Generalized Ohm's Law. Our experiments show that the LTE\nassumption substantially underestimates the ionization fraction in most of the\nemerged region, leading to an artificial increase in the ambipolar diffusion\nand, therefore, in the heating and temperatures as compared to those found when\ntaking the NEQ effects on the hydrogen ion population into account. We see that\nLTE also overestimates the number density of H2 molecules within the emerged\nregion, thus mistakenly magnifying the exothermic contribution of the H2\nmolecule formation to the thermal energy during the flux emergence process. We\nfind that the ambipolar diffusion does not significantly affect the amount of\ntotal unsigned emerged magnetic flux, but it is important in the shocks that\ncross the emerged region, heating the plasma on characteristic times ranging\nfrom 0.1 to 100 s. We also briefly discuss the importance of including elements\nheavier than hydrogen in the equation of state so as not to overestimate the\nrole of ambipolar diffusion in the atmosphere.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Corona-Australis DANCe. I. Revisiting the census of stars with Gaia-DR2\n  data: Corona-Australis is one of the nearest regions to the Sun with recent and\nongoing star formation, but the current picture of its stellar (and substellar)\ncontent is not complete yet. We take advantage of the second data release of\nthe Gaia space mission to revisit the stellar census and search for additional\nmembers of the young stellar association in Corona-Australis. We applied a\nprobabilistic method to infer membership probabilities based on a\nmultidimensional astrometric and photometric data set over a field of 128\ndeg$^{2}$ around the dark clouds of the region. We identify 313\nhigh-probability candidate members to the Corona-Australis association, 262 of\nwhich had never been reported as members before. Our sample of members covers\nthe magnitude range between $G\\gtrsim5$ mag and $G\\lesssim20$ mag, and it\nreveals the existence of two kinematically and spatially distinct subgroups.\nThere is a distributed `off-cloud' population of stars located in the north of\nthe dark clouds that is twice as numerous as the historically known `on-cloud'\npopulation that is concentrated around the densest cores. By comparing the\nlocation of the stars in the HR-diagram with evolutionary models, we show that\nthese two populations are younger than 10 Myr. Based on their infrared excess\nemission, we identify 28 Class II and 215 Class III stars among the sources\nwith available infrared photometry, and we conclude that the frequency of Class\nII stars (i.e. `disc-bearing' stars) in the on-cloud region is twice as large\nas compared to the off-cloud population. The distance derived for the\nCorona-Australis region based on this updated census is $d=149.4^{+0.4}_{-0.4}$\npc, which exceeds previous estimates by about 20 pc.In this paper we provide\nthe most complete census of stars in Corona-Australis available to date that\ncan be confirmed with Gaia data.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Cross and magnetic helicity in the outer heliosphere from Voyager 2\n  observations: Plasma velocity and magnetic field measurements from the Voyager 2 mission\nare used to study solar wind turbulence in the slow solar wind at two different\nheliocentric distances, 5 and 29 astronomical units, sufficiently far apart to\nprovide information on the radial evolution of this turbulence. The magnetic\nhelicity and the cross-helicity, which express the correlation between the\nplasma velocity and the magnetic field, are used to characterize the\nturbulence. Wave number spectra are computed by means of the Taylor hypothesis\napplied to time resolved single point Voyager 2 measurements. The overall\npicture we get is complex and difficult to interpret. A substantial decrease of\nthe cross-helicity at smaller scales (over 1-3 hours of observation) with\nincreasing heliocentric distance is observed. At 5 AU the only peak in the\nprobability density of the normalized residual energy is negative, near -0.5.\nAt 29 AU the probability density becomes doubly peaked, with a negative peak at\n-0.5 and a smaller peak at a positive values of about 0.7. A decrease of the\ncross-helicity for increasing heliocentric distance is observed, together with\na reduction of the unbalance toward the magnetic energy of the energy of the\nfluctuations. For the smaller scales, we found that at 29 AU the normalized\npolarization is small and positive on average (about 0.1), it is instead zero\nat 5 AU. For the larger scales, the polarization is low and positive at 5 AU\n(average around 0.1) while it is negative (around - 0.15) at 29 AU.",
        "category": "astro-ph_SR"
    },
    {
        "text": "An `Analytic Dynamical Magnetosphere' formalism for X-ray and optical\n  emission from slowly rotating magnetic massive stars: Slowly rotating magnetic massive stars develop \"dynamical magnetospheres\"\n(DM's), characterized by trapping of stellar wind outflow in closed magnetic\nloops, shock heating from collision of the upflow from opposite loop\nfootpoints, and subsequent gravitational infall of radiatively cooled material.\nIn 2D and 3D magnetohydrodynamic (MHD) simulations the interplay among these\nthree components is spatially complex and temporally variable, making it\ndifficult to derive observational signatures and discern their overall scaling\ntrends.Within a simplified, steady-state analysis based on overall conservation\nprinciples, we present here an \"analytic dynamical magnetosphere\" (ADM) model\nthat provides explicit formulae for density, temperature and flow speed in each\nof these three components -- wind outflow, hot post-shock gas, and cooled\ninflow -- as a function of colatitude and radius within the closed (presumed\ndipole) field lines of the magnetosphere. We compare these scalings with\ntime-averaged results from MHD simulations, and provide initial examples of\napplication of this ADM model for deriving two key observational diagnostics,\nnamely hydrogen H-alpha emission line profiles from the cooled infall, and\nX-ray emission from the hot post-shock gas. We conclude with a discussion of\nkey issues and advantages in applying this ADM formalism toward derivation of a\nbroader set of observational diagnostics and scaling trends for massive stars\nwith such dynamical magnetospheres.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Possible Period for the K-band Brightening Episodes of GX 17+2: The low mass X-ray binary and Z source GX 17+2 undergoes infrared K-band\nbrightening episodes of at least 3.5 magnitudes. The source of these episodes\nis not known. Prior published K-band magnitudes and new K-band measurements\nacquired between 2006 and 2008 suggest that the episodes last at least 4 hours\nand have a period of 3.01254 $\\pm$ 0.00002 days. Future bright episodes can be\npredicted using the ephemeris JD_{max} (n) = 2454550.79829 + (3.01254 $\\pm$\n0.00002)(n) days. A growing body of evidence suggests that the GX 17+2 could\nhave a synchrotron jet, which could cause this activity.",
        "category": "astro-ph_SR"
    },
    {
        "text": "An Incipient Debris Disk in the Chamaeleon I Cloud: The point at which a protoplanetary disk becomes a debris disk is difficult\nto identify. To better understand this, here we study the $\\sim$40~AU\nseparation binary T~54 in the Chamaeleon I cloud. We derive a K5 spectral type\nfor T~54~A (which dominates the emission of the system) and an age of\n$\\sim$2~Myr. However, the dust disk properties of T~54 are consistent with\nthose of debris disks seen around older and earlier-type stars. At the same\ntime, T~54 has evidence of gas remaining in the disk as indicated by [Ne II],\n[Ne III], and [O I] line detections. We model the spectral energy distribution\nof T~54 and estimate that $\\sim$3$\\times$10$^{-3}$ Earth-masses of small dust\ngrains ($<$0.25~$\\mu$m) are present in an optically thin circumbinary disk\nalong with at least $\\sim$3$\\times$10$^{-7}$ Earth-masses of larger\n($>$10~$\\mu$m) grains within a circumprimary disk. Assuming a solar-like\nmixture, we use Ne line luminosities to place a minimum limit on the gas mass\nof the disk ($\\sim$3$\\times$10$^{-4}$ Earth-masses) and derive a gas-to-dust\nmass ratio of $\\sim$0.1. We do not detect substantial accretion, but we do see\nH$\\alpha$ in emission in one epoch, suggestive that there may be intermittent\ndumping of small amounts of matter onto the star. Considering the low dust\nmass, the presence of gas, and young age of T~54, we conclude that this system\nis on the bridge between the protoplanetary and debris disk stages.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Massive star-formation toward G28.87+0.07 (IRAS 18411-0338) investigated\n  by means of maser kinematics and radio to infrared, continuum observations: We used the Very Long Baseline Array (VLBA) and the European VLBI Network\n(EVN) to perform phase-referenced VLBI observations of the three most powerful\nmaser transitions associated with the high-mass star-forming region\nG28.87+0.07: the 22.2 GHz H$_{2}$O, 6.7 GHz CH$_{3}$OH, and 1.665 GHz OH lines.\nWe also performed VLA observations of the radio continuum emission at 1.3 and\n3.6 cm and Subaru observations of the continuum emission at 24.5 $\\mu$m. Two\ncentimeter continuum sources are detected and one of them (named \"HMC\") is\ncompact and placed at the center of the observed distribution of H$_{2}$O,\nCH$_{3}$OH and OH masers. The bipolar distribution of line-of-sight (l.o.s)\nvelocities and the pattern of the proper motions suggest that the water masers\nare driven by a (proto)stellar jet interacting with the dense circumstellar\ngas. The same jet could both excite the centimeter continuum source named \"HMC\"\n(interpreted as free-free emission from shocked gas) and power the molecular\noutflow observed at larger scales -- although one cannot exclude that the\nfree-free continuum is rather originating from a hypercompact \\ion{H}{2}\nregion. At 24.5 $\\mu$m, we identify two objects separated along the north-south\ndirection, whose absolute positions agree with those of the two VLA continuum\nsources. We establish that $\\sim$90% of the luminosity of the region\n($\\sim$\\times10^{5} L_\\sun$) is coming from the radio source \"HMC\", which\nconfirms the existence of an embedded massive young stellar object (MYSO)\nexciting the masers and possibly still undergoing heavy accretion from the\nsurrounding envelope.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Deriving photospheric parameters and elemental abundances for a sample\n  of stars showing the FIP effect: One puzzling question in solar physics is the difference between elemental\nabundances in the photosphere and the corona. Elements with low first\nionization potential (FIP) can be overabundant in the corona compared to the\nphotosphere under certain circumstances. The same phenomenon has been observed\non a handful of stars, while a few of them show the inverse effect. But not all\nthe stars in the original sample had precise photospheric abundances derived\nfrom optical spectra, so for some the solar values were adopted. In this work\nwe make homogeneous abundance measurements from optical spectroscopy.\n  We collected spectra of 16 stars showing the FIP effect with the 1-m RCC\ntelescope of Konkoly Observatory, with resolution of $\\lambda / \\Delta \\lambda\n\\sim 21\\,000$. We determine the fundamental astrophysical parameters\n($T_\\mathrm{eff}$, $\\log g$, $[M/H]$, $\\xi_\\mathrm{mic}$, $v \\sin i$) and\nindividual elemental abundances with the SME spectral synthesis code using\nMARCS2012 model atmosphere and spectral line parameters from the Vienna Atomic\nLine Database (VALD).",
        "category": "astro-ph_SR"
    },
    {
        "text": "Pre-flare activity and magnetic reconnection during the evolutionary\n  stages of energy release in a solar eruptive flare: In this paper, we present a multi-wavelength analysis of an eruptive\nwhite-light M3.2 flare which occurred in active region NOAA 10486 on November\n1, 2003. Excellent set of high resolution observations made by RHESSI and TRACE\nprovide clear evidence of significant pre-flare activities for ~9 minutes in\nthe form of an initiation phase observed at EUV/UV wavelengths followed by the\nX-ray precursor phase. During the initiation phase, we observed localized\nbrightenings in the highly sheared core region close to the filament and\ninteractions among short EUV loops overlying the filament which led to the\nopening of magnetic field lines. The X-ray precursor phase is manifested in\nRHESSI measurements below ~30 keV and coincided with the beginning of flux\nemergence at the flaring location along with early signatures of the eruption.\nFrom the RHESSI observations, we conclude that both plasma heating and electron\nacceleration occurred during the precursor phase. The main flare is consistent\nwith the standard flare model. However, after the impulsive phase, intense HXR\nlooptop source was observed without significant footpoint emission. More\nintriguingly, for a brief period the looptop source exhibited strong HXR\nemission with energies up to 100 keV and significant non-thermal\ncharacteristics. The present study indicates a causal relation between the\nactivities in the preflare and main flare. We also conclude that pre-flare\nactivities, occurred in the form of subtle magnetic reorganization along with\nlocalized magnetic reconnection, played a crucial role in destabilizing the\nactive region filament leading to solar eruptive flare and associated\nlarge-scale phenomena.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Magnetohydrodynamic wave mode identification in circular and elliptical\n  sunspot umbrae: evidence for high order modes: In this paper we provide clear direct evidence of multiple concurrent higher\norder magnetohydrodynamic (MHD) modes in circular and elliptical sunspots by\napplying both Proper Orthogonal Decomposition (POD) and Dynamic Mode\nDecomposition (DMD) techniques on solar observational data. These techniques\nare well documented and validated in the areas of fluid mechanics, hydraulics,\nand granular flows, yet are relatively new to the field of solar physics. While\nPOD identifies modes based on orthogonality in space and it provides a clear\nranking of modes in terms of their contribution to the variance of the signal,\nDMD resolves modes that are orthogonal in time. The clear presence of the\nfundamental slow sausage and kink body modes, as well as higher order slow\nsausage and kink body modes have been identified using POD and DMD analysis of\nthe chromospheric H$\\alpha$ line at 6562.808~{\\AA} for both the circular and\nelliptical sunspots. Additionally, to the various slow body modes, evidence for\nthe presence of the fast surface kink mode was found in the circular sunspot.\nAll the MHD modes patterns were cross-correlated with their theoretically\npredicted counterparts and we demonstrated that ellipticity cannot be neglected\nwhen interpreting MHD wave modes. The higher-order MHD wave modes are even more\nsensitive to irregularities in umbral cross-sectional shapes, hence this must\nbe taken into account for more accurate modelling of the modes in sunspots and\npores.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The frequency of large variations in the near-infrared fluxes of T Tauri\n  stars: Variability is a characteristic feature of young stellar objects (YSOs) and\ncould contribute to the large scatter observed in HR diagrams for star forming\nregions. For typical YSOs, however, the long-term effects of variability are\npoorly constrained. Here I use archived near-infrared photometry from 2MASS,\nUKIDSS, and DENIS to investigate the long-term variability of high-confidence\nmembers of the four star forming regions Rho-Oph, ONC, IC348, and NGC1333. The\ntotal sample comprises more than 600 objects, from which ~320 are considered to\nhave a disk. The dataset covers timescales up to 8 yr. About half of the YSOs\nare variable on a 2sigma level, with median amplitudes of 5-20%. The fraction\nof highly variable objects with amplitudes >0.5 mag in at least two\nnear-infrared bands is very low - 2% for the entire sample and 3% for objects\nwith disks. These sources with strong variability are mostly objects with disks\nand are prime targets for follow-up studies. The variability amplitudes are\nlargest in NGC1333, presumably because it is the youngest sample of YSOs. The\nfrequency of highly variable objects also increases with the time window of the\nobservations (from weeks to years). These results have three implications: 1)\nWhen deriving luminosities for YSOs from near-infrared magnitudes, the typical\nerror introduced by variability is in the range of 5-20% percent and depends on\ndisk fraction and possibly age. 2) Variability is a negligible contribution to\nthe scatter in HR diagrams of star forming regions (except for a small number\nof extreme objects), if luminosities are derived from near-infrared magnitudes.\n3) Accretion outbursts with an increase in mass accretion rate by several order\nof magnitudes, as required in scenarios for episodic accretion, occur with a\nduty cycle of >2000-2500 yr in the Class II phase. (abridged)",
        "category": "astro-ph_SR"
    },
    {
        "text": "A flash of polarized optical light points to an aspherical \"cow\": The astronomical transient AT2018cow is the closest example of the new class\nof luminous, fast blue optical transients (FBOTs). Liverpool Telescope RINGO3\nobservations of AT2018cow are reported here, which constitute the earliest\npolarimetric observations of an FBOT. At 5.7 days post-explosion, the optical\nemission of AT2018cow exhibited a chromatic polarization spike that reached ~7%\nat red wavelengths. This is the highest intrinsic polarization recorded for a\nnon-relativistic explosive transient, and is observed in multiple bands and at\nmultiple epochs over the first night of observations, before rapidly declining.\nThe apparent wavelength dependence of the polarization may arise through\ndepolarization or dilution of the polarized flux, due to conditions in\nAT~2018cow at early times. A second ``bump\" in the polarization is observed at\nblue wavelengths at ~12 days. Such a high polarization requires an extremely\naspherical geometry that is only apparent for a brief period (<1 day), such as\nshock breakout through an optically thick disk. For a disk-like configuration,\nthe ratio of the thickness to radial extent must be ~10%.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Correlation Analysis of Mode Frequencies with Activity Proxies at\n  Different Phases of the Solar Cycle: We analyze intermediate degree p- and f-mode eigenfrequencies measured by\nGONG and MDI/SOHO for a complete solar cycle to study their correlation with\nsolar activity. We demonstrate that the frequencies do vary linearly with the\nactivity, however the degree of correlation differs from phase to phase of the\ncycle. During the rising and the declining phases, the mode frequencies are\nstrongly correlated with the activity proxies whereas during the low- and\nhigh-activity periods, the frequencies have significantly lower correlation\nwith all the activity proxies considered here.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Theoretical seismology in 3D : nonlinear simulations of internal gravity\n  waves in solar-like stars: Internal gravity waves (hereafter IGWs) are studied for their impact on the\nangular momentum transport in stellar radiation zones and the information they\nprovide about the structure and dynamics of deep stellar interiors. We here\npresent the first 3D nonlinear numerical simulations of IGWs excitation and\npropagation in a solar-like star. The aim is to study the behavior of waves in\na realistic 3D nonlinear time dependent model of the Sun and to characterize\ntheir properties. We compare our results with theoretical and 1D predictions.\nIt allows us to point out the complementarity between theory and simulation and\nto highlight the convenience but also the limits of the asymptotic and linear\ntheories. We show that a rich spectrum of IGWs is excited by the convection,\nrepresenting about 0.4% of the total solar luminosity. We study the spatial and\ntemporal properties of this spectrum, the effect of thermal damping and\nnonlinear interactions between waves. We give quantitative results about the\nmodes frequencies, evolution with time and rotational splitting and we discuss\nthe amplitude of IGWs considering different regimes of parameters. This work\npoints out the importance of high performance simulation for its\ncomplementarity with observation and theory. It opens a large field of\ninvestigation concerning IGWs propagating nonlinearly in 3D spherical\nstructures. The extension of this work to other types of stars, with different\nmasses, structures and rotation rates will lead to a deeper and more accurate\ncomprehension of IGWs in stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "High-resolution spectroscopic view of planet formation sites: Theories of planet formation predict the birth of giant planets in the inner,\ndense, and gas-rich regions of the circumstellar disks around young stars.\nThese are the regions from which strong CO emission is expected. Observations\nhave so far been unable to confirm the presence of planets caught in formation.\nWe have developed a novel method to detect a giant planet still embedded in a\ncircumstellar disk by the distortions of the CO molecular line profiles\nemerging from the protoplanetary disk's surface. The method is based on the\nfact that a giant planet significantly perturbs the gas velocity flow in\naddition to distorting the disk surface density. We have calculated the\nemerging molecular line profiles by combining hydrodynamical models with\nsemianalytic radiative transfer calculations. Our results have shown that a\ngiant Jupiter-like planet can be detected using contemporary or future\nhigh-resolution near-IR spectrographs such as VLT/CRIRES or ELT/METIS. We have\nalso studied the effects of binarity on disk perturbations. The most\ninteresting results have been found for eccentric circumprimary disks in\nmid-separation binaries, for which the disk eccentricity - detectable from the\nasymmetric line profiles - arises from the gravitational effects of the\ncompanion star. Our detailed simulations shed new light on how to constrain the\ndisk kinematical state as well as its eccentricity profile. Recent findings by\nindependent groups have shown that core-accretion is severely affected by disk\neccentricity, hence detection of an eccentric protoplanetary disk in a young\nbinary system would further constrain planet formation theories.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Characterising face-on accretion onto and the subsequent contraction of\n  protoplanetary discs: Observations indicate that stars generally lose their protoplanetary discs on\na timescale of about 5 Myr. Which mechanisms are responsible for the disc\ndissipation is still debated. Here we investigate the movement through an\nambient medium as a possible cause of disc dispersal. The ram pressure exerted\nby the flow can truncate the disc and the accretion of material with no\nazimuthal angular momentum leads to further disc contraction. We derive a\ntheoretical model from accretion disc theory that describes the evolution of\nthe disc radius, mass, and surface density profile as a function of the density\nand velocity of the ambient medium. We test our model by performing\nhydrodynamical simulations of a protoplanetary disc embedded in a flow with\ndifferent velocities and densities. We find that our model gives an adequate\ndescription of the evolution of the disc radius and accretion rate onto the\ndisc. The total disc mass in the simulations follows the theoretically expected\ntrend, except at the lowest density where our simulated discs lose mass owing\nto continuous stripping. This stripping may be a numerical rather than a\nphysical effect. Some quantitative differences exist between the model\npredictions and the simulations. These are at least partly caused by numerical\nviscous effects in the disc and depend on the resolution of the simulation. Our\nmodel can be used as a conservative estimate for the process of face-on\naccretion onto protoplanetary discs, as long as viscous processes in the disc\ncan be neglected. The model predicts that in dense gaseous environments, discs\ncan shrink substantially in size and can, in theory, sweep up an amount of gas\nof the order of their initial mass. This process could be relevant for planet\nformation in dense environments.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Period and amplitude variations in post-common-envelope eclipsing\n  binaries observed with SuperWASP: Period or amplitude variations in eclipsing binaries may reveal the presence\nof additional massive bodies in the system, such as circumbinary planets. Here,\nwe have studied twelve previously-known eclipsing post-common-envelope binaries\nfor evidence of such light curve variations, on the basis of multi-year\nobservations in the SuperWASP archive. The results for HW Vir provided strong\nevidence for period changes consistent with those measured by previous studies,\nand help support a two-planet model for the system. ASAS J102322-3737.0\nexhibited plausible evidence for a period increase not previously suggested;\nwhile NY Vir, QS Vir and NSVS 14256825 afforded less significant support for\nperiod change, providing some confirmation to earlier claims. In other cases,\nperiod change was not convincingly observed; for AA Dor and NSVS 07826147,\nprevious findings of constant period were confirmed. This study allows us to\npresent hundreds of new primary eclipse timings for these systems, and further\ndemonstrates the value of wide-field high-cadence surveys like SuperWASP for\nthe investigation of variable stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "CoBiToM Project -- II: Evolution of contact binary systems close to the\n  orbital period cut-off: Ultra-short orbital period contact binaries (Porb < 0.26 d) host some of the\nsmallest and least massive stars. These systems are faint and rare, and it is\nbelieved that they have reached a contact configuration after several Gyrs of\nevolution via angular momentum loss, mass transfer and mass loss through\nstellar wind processes. This study is conducted in the frame of Contact\nBinaries Towards Merging (CoBiToM) Project and presents the results from light\ncurve and orbital analysis of 30 ultra-short orbital period contact binaries,\nwith the aim to investigate the possibility of them being red nova progenitors,\neventually producing merger events. Approximately half of the systems exhibit\norbital period modulations, as a result of mass transfer or mass loss\nprocesses. Although they are in contact, their fill-out factor is low (less\nthan 30 per cent), while their mass ratio is larger than the one in longer\nperiod contact binaries. The present study investigates the orbital stability\nof these systems and examines their physical and orbital parameters in\ncomparison to those of the entire sample of known and well-studied contact\nbinaries, based on combined spectroscopic and photometric analysis. It is found\nthat ultra-short orbital period contact binaries have very stable orbits, while\nvery often additional components are gravitationally bound in wide orbits\naround the central binary system. We confirmed that the evolution of such\nsystems is very slow, which explains why the components of ultra-short orbital\nperiod systems are still Main Sequence stars after several Gyrs of evolution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Nearby Young, Active, Late-type Dwarfs in Gaia's First Data Release: The Galex Nearby Young Star Survey (GALNYSS) has yielded a sample of\n$\\sim$2000 UV-selected objects that are candidate nearby ($D\n\\stackrel{<}{\\sim}$150 pc), young (age $\\sim$10--100 Myr), late-type stars.\nHere, we evaluate the distances and ages of the subsample of (19) GALNYSS stars\nwith Gaia Data Release 1 (DR1) parallax distances $D \\le 120$ pc. The overall\nyouth of these 19 mid-K to early-M stars is readily apparent from their\npositions relative to the loci of main sequence stars and giants in Gaia-based\ncolor-magnitude and color-color diagrams constructed for all Galex- and\nWISE-detected stars with parallax measurements included in DR1. The isochronal\nages of all 19 stars lie in the range $\\sim$10--100 Myr. Comparison with\nLi-based age estimates indicates a handful of these stars may be young\nmain-sequence binaries rather than pre-main sequence stars. Nine of the 19\nobjects have not previously been considered as nearby, young stars, and all but\none of these are found at declinations north of $+$30$^\\circ$. The Gaia DR1\nresults presented here indicate that the GALNYSS sample includes several\nhundred nearby, young stars, a substantial fraction of which have not been\npreviously recognized as having ages $\\stackrel{<}{\\sim}$100 Myr.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Corrected Weight Functions for Stellar Oscillation Eigenfrequencies: Kawaler et al. (1985) present a variational expression for the\neigenfrequencies associated with stellar oscillations. We highlight and correct\na typographical error in the weight functions appearing in these expressions,\nand validate the correction numerically.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Stellar atmospheres, atmospheric extension and fundamental parameters:\n  weighing stars using the stellar mass index: One of the great challenges in understanding stars is measuring their masses.\nThe best methods for measuring stellar masses include binary interaction,\nasteroseismology and stellar evolution models, but these methods are not ideal\nfor red giant and supergiant stars. In this work, we propose a novel method for\ninferring stellar masses of evolved red giant and supergiant stars using\ninterferometric and spectrophotometric observations combined with spherical\nmodel stellar atmospheres to measure what we call the stellar mass index,\ndefined as the ratio between the stellar radius and mass. The method is based\non the correlation between different measurements of angular diameter, used as\na proxy for atmospheric extension, and fundamental stellar parameters. For a\ngiven star, spectrophotometry measures the Rosseland angular diameter while\ninterferometric observations generally probe a larger limb-darkened angular\ndiameter. The ratio of these two angular diameters is proportional to the\nrelative extension of the stellar atmosphere, which is strongly correlated to\nthe star's effective temperature, radius and mass. We show that these\ncorrelations are strong and can lead to precise measurements of stellar masses.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Isochronal age scale of young moving groups in the solar neighbourhood: We present a self-consistent, absolute isochronal age scale for young (< 200\nMyr), nearby (< 100 pc) moving groups, which is consistent with recent lithium\ndepletion boundary ages for both the Beta Pic and Tucana-Horologium moving\ngroups. This age scale was derived using a set of semi-empirical\npre-main-sequence model isochrones that incorporate an empirical colour-Teff\nrelation and bolometric corrections based on the observed colours of Pleiades\nmembers, with theoretical corrections for the dependence on log g. Absolute\nages for young, nearby groups are vital as these regions play a crucial role in\nour understanding of the early evolution of low- and intermediate-mass stars,\nas well as providing ideal targets for direct imaging and other measurements of\ndusty debris discs, substellar objects and, of course, extrasolar planets.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Understanding the origin of the magnetic field morphology in the\n  wide-binary protostellar system BHR 71: We present 1.3 mm ALMA observations of polarized dust emission toward the\nwide-binary protostellar system BHR 71 IRS1 and IRS2. IRS1 features what\nappears to be a natal, hourglass-shaped magnetic field. In contrast, IRS2\nexhibits a magnetic field that has been affected by its bipolar outflow. Toward\nIRS2, the polarization is confined mainly to the outflow cavity walls. Along\nthe northern edge of the redshifted outflow cavity of IRS2, the polarized\nemission is sandwiched between the outflow and a filament of cold, dense gas\ntraced by N$_2$D$^+$, toward which no dust polarization is detected. This\nsuggests that the origin of the enhanced polarization in IRS2 is the\nirradiation of the outflow cavity walls, which enables the alignment of dust\ngrains with respect to the magnetic field -- but only to a depth of ~300 au,\nbeyond which the dust is cold and unpolarized. However, in order to align\ngrains deep enough in the cavity walls, and to produce the high polarization\nfraction seen in IRS2, the aligning photons are likely to be in the mid- to\nfar-infrared range, which suggests a degree of grain growth beyond what is\ntypically expected in very young, Class 0 sources. Finally, toward IRS1 we see\na narrow, linear feature with a high (10-20%) polarization fraction and a well\nordered magnetic field that is not associated with the bipolar outflow cavity.\nWe speculate that this feature may be a magnetized accretion streamer; however,\nthis has yet to be confirmed by kinematic observations of dense-gas tracers.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A 9-Month Hubble Space Telescope Near-UV Survey of M87. I. Light and\n  Color Curves of 94 Novae, and a Re-determination of the Nova Rate: M87 has been monitored with a cadence of 5 days over a 9 month-long span\nthrough the near-ultraviolet (NUV:F275W) and optical (F606W) filters of the\nWide Field Camera 3 (WFC3) of the $\\textit{Hubble Space Telescope}$. This\nunprecedented dataset yields the NUV and optical light and color curves of 94\nM87 novae, characterizing the outburst and decline properties of the largest\nextragalactic nova dataset in the literature (after M31 and M81). We test and\nconfirm nova modelers' prediction that recurrent novae cannot erupt more\nfrequently that once every 45 days; show that there are zero rapidly recurring\nnovae in the central $\\sim$ 1/3 of M87 with recurrence times $ < $ 130 days;\ndemonstrate that novae closely follow the K-band light of M87 to within a few\narcsec of the galaxy nucleus; show that nova NUV light curves are as\nheterogeneous as their optical counterparts, and usually peak 5 to 30 days\nafter visible light maximum; determine our observations' annual detection\ncompleteness to be 71 - 77\\%; and measure the rate Rnova of nova eruptions in\nM87 as $352_{-37}^{+37}$/yr. The corresponding luminosity-specific classical\nnova rate for this galaxy is $7.91_{-1.20}^{+1.20}/yr/10^{10}L_\\odot,_{K}$.\nThese rates confirm that ground-based observations of extragalactic novae miss\nmost faint, fast novae and those near the centers of galaxies. An annual M87\nnova rate of 300 or more seems inescapable. A luminosity-specific nova rate of\n$\\sim$ $7 - 10/yr/10^{10}L_\\odot,_{K}$ in ${\\it all}$ types of galaxies is\nindicated by the data available in 2023.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Atomic diffusion in solar-like stars with MESA. Comparison with the\n  Montreal/Montpellier and CESTAM stellar evolution codes: The stellar evolution code Modules for Experiments in Stellar Astrophysics\n(MESA) is public and is widely used by the community. It includes the\npossibility of taking several non-standard processes such as atomic diffusion\ninto account. Even if the effect of gravitational settling is considered a\nstandard ingredient in stellar modelling today, this is not the case for\nradiative accelerations. The specific treatment of atomic diffusion along with\nthe radiative accelerations has never been compared with other stellar\nevolution codes. Benchmarking these codes is important because improved\naccuracy is required in order to analyse data from present and future space\nmissions, such as the \\textit{Kepler}, Transiting Exoplanet Survey Satellite\n(TESS), and PLAnetary Transits and Oscillations of stars (PLATO) missions.\n  The aim of this paper is to compare MESA models including atomic diffusion\n(with radiative accelerations) with models computed with the\nMontreal/Montpellier stellar evolution code and with the Code d'Evolution\nStellaire Adaptatif et Modulaire (CESTAM). Additionally, we assess the impact\nof some MESA options related to atomic diffusion.\n  We calculated atomic diffusion, including radiative accelerations, following\nthe abundance profiles of 14 elements with MESA models. This was then compared\nwith 1.1 and 1.4~$M_{\\odot}$ models computed with the Montreal/Montpellier and\nCESTAM codes. Various tests of MESA options for atomic diffusion were also\ncarried out by varying only one of them at a time.\n  We find that the abundance profiles of the considered elements in the MESA\nmodels compare rather well with the models computed with the two other codes\nwhen atomic diffusion options are carefully set. We also show that some options\nin MESA are crucial for a proper treatment of atomic diffusion.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spectroscopic and Photometric Study of the Asymptotic Giant Branch Star\n  T Cephei: Spectroscopy and photometry of the Asymptotic Giant Branch star T Cephei were\nrecorded concurrently on 36 nights during its 387 day pulsation cycle in 2022.\nPhotometry was used to calibrate all spectra in absolute flux. We report on the\nvariation of B and V magnitudes, B-V colour index, spectral type, effective\ntemperature and Balmer emission line flux during one complete pulsation cycle.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Short term period variable stars observed at OAUNI: We present the first scientific results of the program on short term period\nvariable stars observed using the OAUNI facility at the peruvian Andes. These\nresults include good quality light curves of delta Scuti stars, rapidly\noscillating stars along with eclipsing and cataclysmic binaries. The\nphotometric precision reached by the available instrumental and equipment, and\nused in the several scientific subprograms, has satisfied the initial\nexpectations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Wide-angle protostellar outflows driven by narrow jets in stratified\n  cores: Most simulations of outflow feedback on star formation are based on the\nassumption that outflows are driven by a wide angle \"X-wind,\" rather than a\nnarrow jet. However, the arguments initially raised against pure jet-driven\nflows were based on steady ejection in a uniform medium, a notion that is no\nlonger supported based on recent observations. We aim to determine whether a\npulsed narrow jet launched in a density-stratified, self-gravitating core could\nreproduce typical molecular outflow properties, without the help of a\nwide-angle wind component. We performed axisymmetric hydrodynamic simulations\nusing the MPI-AMRVAC code with optically thin radiative cooling on timescales\nup to 10000 yrs. Then we computed and compared the predicted properties with\nobservational data. First, the jet-driven shell expands faster and wider\nthrough a core with steeply decreasing density than through an uniform core.\nSecond, when blown into the same singular flattened core, a jet-driven shell\nhas a similar width as a wide-angle wind-driven shell in the first few hundred\nyears, but a decelerating expansion on long timescales. The flow adopts a\nconical shape and a base opening angle reaching up to $90\\unicode{xb0}$. Third,\nafter $\\sim$ 10000 yrs, a pulsed jet-driven shell shows fitting features and a\nqualitative resemblance with recent observations of protostellar outflows with\nthe Atacama Large Millimeter Array (ALMA), such as HH46-47 and CARMA-7. In\nparticular, similarities are seen in the shell widths, opening angles,\nposition-velocity diagrams, and mass-velocity distribution, with some showing a\ncloser resemblance than in simulations based on a wide-angle \"X-wind\" model.\nTherefore, a realistic ambient density stratification in addition to\nmillenia-long integration times are equally essential to reliably predict the\nproperties of outflows driven by a pulsed jet and to confront them with the\nobservations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Long-term variability of T Tauri stars using WASP: We present a reference study of the long-term optical variability of young\nstars using data from the WASP project. Our primary sample is a group of\nwell-studied classical T Tauri stars (CTTS), mostly in Taurus-Auriga. WASP\nlightcurves cover timescales up to 7 years and typically contain 10000-30000\ndatapoints. We quantify the variability as function of timescale using the\ntime-dependent standard deviation 'pooled sigma'. We find that the overwhelming\nmajority of CTTS has low-level variability with sigma<0.3mag dominated by\ntimescales of a few weeks, consistent with rotational modulation. Thus, for\nmost young stars monitoring over a month is sufficient to constrain the total\namount of variability over timescales up to a decade. The fraction of stars\nwith strong optical variability (sigma>0.3mag) is 21% in our sample and 21% in\nan unbiased control sample. An even smaller fraction (13% in our sample, 6% in\nthe control) show evidence for an increase in variability amplitude as a\nfunction of timescale from weeks to months or years. The presence of long-term\nvariability correlates with the spectral slope at 3-5mu, which is an indicator\nof inner disk geometry, and with the U-B band slope, which is an accretion\ndiagnostics. This shows that the long-term variations in CTTS are predominantly\ndriven by processes in the inner disk and in the accretion zone. Four of the\nstars with long-term variations show periods of 20-60d, significantly longer\nthan the rotation periods and stable over months to years. One possible\nexplanation are cyclic changes in the interaction between the disk and the\nstellar magnetic field.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Magnetic OB[A] Stars with TESS: probing their Evolutionary and\n  Rotational properties -- The MOBSTER Collaboration: In this contribution, we present the MOBSTER Collaboration, a large community\neffort to leverage high-precision photometry from the Transiting Exoplanet\nSurvey Satellite (\\textit{TESS}) in order to characterize the variability of\nmagnetic massive and intermediate-mass stars. These data can be used to probe\nthe varying column density of magnetospheric plasma along the line of sight for\nOB stars, thus improving our understanding of the interaction between surface\nmagnetic fields and massive star winds. They can also be used to map out the\nbrightness inhomogeneities present on the surfaces of Ap/Bp stars, informing\npresent models of atomic diffusion in their atmospheres. Finally, we review our\ncurrent and ongoing studies, which lead to new insights on this topic.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Understanding the origin of the [OI] low-velocity component from T Tauri\n  stars: The formation time, masses, and location of planets are strongly impacted by\nthe physical mechanisms that disperse protoplanetary disks and the timescale\nover which protoplanetary material is cleared out. Accretion of matter onto the\ncentral star, protostellar winds/jets, magnetic disk winds, and\nphotoevaporative winds operate concurrently. Hence, disentangling their\nrelative contribution to disk dispersal requires identifying diagnostics that\ntrace different star-disk environments. Here, we analyze the low velocity\ncomponent (LVC) of the Oxygen optical forbidden lines, which is found to be\nblueshifted by a few km/s with respect to the stellar velocity. We find that\nthe [OI] LVC profiles are different from those of [NeII] at 12.81\\mu m and CO\nat 4.7\\mu m lines pointing to different origins for these gas lines. We report\na correlation between the luminosity of the [OI] LVC and the accretion\nluminosity Lacc. We do not find any correlation with the X-ray luminosity,\nwhile we find that the higher is the stellar FUV luminosity, the higher is the\nluminosity of the [OI] LVC. In addition, we show that the [OI]6300\\AA/5577\\AA\\\nratio is low (ranging between 1 and 8). These findings favor an origin of the\n[OI] LVC in a region where OH is photodissociated by stellar FUV photons and\nargue against thermal emission from an X-ray-heated layer. Detailed modeling of\ntwo spectra with the highest S/N and resolution shows that there are two\ncomponents within the LVC: a broad, centrally peaked component that can be\nattributed to gas arising in a warm disk surface in Keplerian rotation (with\nFWHM between ~40 and ~60 km/s), and a narrow component (with FWHM ~10 km/s and\nsmall blueshifts of ~2 km/s) that may arise in a cool (<1,000 K) molecular\nwind.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Shearing box simulations in the Rayleigh unstable regime: We study the stability properties of Rayleigh unstable flows both in the\npurely hydrodynamic and magnetohydrodynamic (MHD) regimes for two different\nvalues of the shear $q=2.1, 4.2$ ($q = - d\\ln\\Omega / d\\ln r$) and compare it\nwith the Keplerian case $q=1.5$. We find that the $q>2$ regime is unstable both\nin the hydrodynamic and in the MHD limit (with an initially weak magnetic\nfield). In this regime, the velocity fluctuations dominate the magnetic\nfluctuations. In contrast, in the $q<2$ (magnetorotational instability (MRI))\nregime the magnetic fluctuations dominate. This highlights two different paths\nto MHD turbulence implied by the two regimes, suggesting that in the $q>2$\nregime the instability produces primarily velocity fluctuations that cause\nmagnetic fluctuations, with the causality reversed for the $q<2$ MRI unstable\nregime. We also find that the magnetic field correlation is increasingly\nlocalized as the shear is increased in the Rayleigh unstable regime. In\ncalculating the time evolution of spatial averages of different terms in the\nMHD equations, we find that the $q>2$ regime is dominated by terms which are\nnonlinear in the fluctuations, whereas for $q<2$, the linear terms play a more\nsignificant role.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Solar supergranular fractal dimension dependence on the Solar cycle\n  phase: We study the complexity of the supergranular network through fractal\ndimension by using Ca II K digitized data archive obtained from Kodaikanal\nsolar observatory. The data consists of 326 visually selected supergranular\ncells spread across the 23rd solar cycle. Only cells that were well-defined\nwere chosen for the analysis and we discuss the potential selection effect\nthereof, mainly that it favors cells of a smaller size (< 20 Mm). Within this\nsample, we analyzed the fractal dimension of supergranules across the Solar\ncycle and find that it is anticorrelated with the activity level.",
        "category": "astro-ph_SR"
    },
    {
        "text": "FUSE, STIS, and Keck spectroscopic analysis of the UV-bright star vZ\n  1128 in M3 (NGC 5272): We present a spectral analysis of the UV-bright star vZ 1128 in M3 based on\nobservations with the Far Ultraviolet Spectroscopic Explorer (FUSE), the Space\nTelescope Imaging Spectrograph (STIS), and the Keck HIRES echelle spectrograph.\nBy fitting the H I, He I, and He II lines in the Keck spectrum with non-LTE\nH-He models, we obtain Teff = 36,600 K, log g = 3.95, and log N(He)/N(H) =\n-0.84. The star's FUSE and STIS spectra show photospheric absorption from C, N,\nO, Al, Si, P, S, Fe, and Ni. No stellar features from elements beyond the iron\npeak are observed. Both components of the N V 1240 doublet exhibit P~Cygni\nprofiles, indicating a weak stellar wind, but no other wind features are seen.\nThe star's photospheric abundances appear to have changed little since it left\nthe red giant branch (RGB). Its C, N, O, Al, Si, Fe, and Ni abundances are\nconsistent with published values for the red-giant stars in M3, and the\nrelative abundances of C, N, and O follow the trends seen on the cluster RGB.\nIn particular, its low C abundance suggests that the star left the asymptotic\ngiant branch before the onset of third dredge-up.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Discovery of electron cyclotron MASER emission from the magnetic Bp star\n  HD 133880 with the Giant Metrewave Radio Telescope: We report the discovery of coherent radio emission from the young,\nrapidly-rotating magnetic Bp star HD 133880 at a frequency of 610 MHz with the\nGiant Metrewave Radio Telescope (GMRT). This is only the second magnetic star\nin which coherent radio emission has been detected. In our observations of HD\n133880 covering the full rotational cycle of the star (except for a phase\nwindow 0.17-0.24), we witness an abrupt order-of-magnitude flux enhancement\nalong with $\\approx100$ percent right circular polarization. We attribute this\nphenomenon to coherent Electron Cyclotron MASER Emission. We attribute the lack\nof left circularly polarised emission to the asymmetric topology of the star's\nmagnetic field. The phase of enhancement, $0.73$, differs from the previously\nreported phase of enhancement, $0.16$, (at 610 MHz Chandra et al. 2015) by\none-half cycle. However, no flux enhancement is found at phase $0.16$ in our\ndata, which could be due to an unstable or drifting emission region, or a\nconsequence of the reported changes of the star's rotational period. Either of\nthese factors could have shifted the enhancement to the above-mentioned phase\nwindow not sampled by our observations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Constraining the neutrino magnetic dipole moment from white dwarf\n  pulsations: Pulsating white dwarf stars can be used as astrophysical laboratories to\nconstrain the properties of weakly interacting particles. Comparing the cooling\nrates of these stars with the expected values from theoretical models allows us\nto search for additional sources of cooling due to the emission of axions,\nneutralinos, or neutrinos with magnetic dipole moment. In this work, we derive\nan upper bound to the neutrino magnetic dipole moment using an estimate of the\nrate of period change of the pulsating DB white dwarf star PG 1351+489. By\ncomparing the theoretical rate of change of period expected for this star with\nthe rate of change of period with time of PG 1351+489, we assess the possible\nexistence of additional cooling by neutrinos with magnetic dipole moment. Our\nmodels suggest the existence of some additional cooling in this pulsating DB\nwhite dwarf, consistent with a non-zero magnetic dipole moment. Our upper limit\nfor the neutrino magnetic dipole moment is somewhat less restrictive than, but\nstill compatible with, other limits inferred from the white dwarf luminosity\nfunction or from the color-magnitude diagram of the Globular cluster M5.\nFurther improvements of the measurement of the rate of period change of the\ndominant pulsation mode of PG 1351+489 will be necessary to confirm our bound.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Generalized multi-polytropic Rankine-Hugoniot relations and the entropy\n  condition: The study aims at a derivation of generalized \\RH relations, especially that\nfor the entropy, for the case of different upstream/downstream polytropic\nindices and their implications. We discuss the solar/stellar wind interaction\nwith the interstellar medium for different polytropic indices and concentrate\non the case when the polytropic index changes across hydrodynamical shocks. We\nuse first a numerical mono-fluid approach with constant polytropic index in the\nentire integration region to show the influence of the polytropic index on the\nthickness of the helio-/astrosheath and on the compression ratio. Second, the\nRankine-Hugonoit relations for a polytropic index changing across a shock are\nderived analytically, particularly including a new form of the entropy\ncondition. In application to the/an helio-/astrosphere, we find that the size\nof the helio-/astrosheath as function of the polytropic index decreases in a\nmono-fluid model for indices less than $\\gamma=5/3$ and increases for higher\nones and vice versa for the compression ratio. Furthermore, we demonstrate that\nchanging polytropic indices across a shock are physically allowed only for\nsufficiently high Mach numbers and that in the hypersonic limit the compression\nratio depends only on the downstream polytropic index, while the ratios of the\ntemperature and pressure as well as the entropy difference depend on both, the\nupstream and downstream polytropic indices.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Search for systemic mass loss in Algols with bow shocks: Aims. Various studies indicate that interacting binary stars of Algol type\nevolve non-conservatively. However, direct detections of systemic mass loss in\nAlgols have been scarce so far. We study the systemic mass loss in Algols by\nlooking for the presence of infrared excesses originating from the thermal\nemission of dust grains, which is linked to the presence of a stellar wind.\n  Methods. In contrast to previous studies, we make use of the fact that\nstellar and interstellar material is piled up at the edge of the astrosphere\nwhere the stellar wind interacts with the interstellar medium. We analyse WISE\nW3 $12\\,\\mu$m and WISE W4 $22\\,\\mu$m data of Algol-type binary Be and B[e]\nstars and the properties of their bow shocks. From the stand-off distance of\nthe bow shock we are able to determine the mass-loss rate of the binary system.\n  Results. Although the velocities of the stars with respect to the\ninterstellar medium are quite low, we find bow shocks present in two systems,\nnamely $\\pi$ Aqr, and $\\varphi$ Per; a third system, CX Dra, shows a more\nirregular circumstellar environment morphology which might somehow be related\nto systemic mass loss. The properties of the two bow shocks point to mass-loss\nrates and wind velocities typical of single B stars, which do not support an\nenhanced systemic mass loss.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Factory and The Beehive I. Rotation Periods For Low-Mass Stars in\n  Praesepe: Stellar rotation periods measured from single-age populations are critical\nfor investigating how stellar angular momentum content evolves over time, how\nthat evolution depends on mass, and how rotation influences the stellar dynamo\nand the magnetically heated chromosphere and corona. We report rotation periods\nfor 40 late-K to mid-M stars members of the nearby, rich, intermediate-age\n(~600 Myr) open cluster Praesepe. These rotation periods were derived from ~200\nobservations taken by the Palomar Transient Factory of four cluster fields from\n2010 February to May. Our measurements indicate that Praesepe's mass-period\nrelation transitions from a well-defined singular relation to a more scattered\ndistribution of both fast and slow rotators at ~0.6 Msun. The location of this\ntransition is broadly consistent with expectations based on observations of\nyounger clusters and the assumption that stellar-spin down is the dominant\nmechanism influencing angular momentum evolution at 600 Myr. However, a\ncomparison to data recently published for the Hyades, assumed to be coeval to\nPraesepe, indicates that the divergence from a singular mass-period relation\noccurs at different characteristic masses, strengthening the finding that\nPraesepe is the younger of the two clusters. We also use previously published\nrelations describing the evolution of rotation periods as a function of color\nand mass to evolve the sample of Praesepe periods in time. Comparing the\nresulting predictions to periods measured in M35 and NGC 2516 (~150 Myr) and\nfor kinematically selected young and old field star populations suggests that\nstellar spin-down may progress more slowly than described by these relations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "OGLE-2016-BLG-1003: First Resolved Caustic-crossing Binary-source Event\n  Discovered by Second-generation Microlensing Surveys: We report the analysis of the first resolved caustic-crossing binary-source\nmicrolensing event OGLE-2016-BLG-1003. The event is densely covered by the\nround-the-clock observations of three surveys. The light curve is characterized\nby two nested caustic-crossing features, which is unusual for typical\ncaustic-crossing perturbations. From the modeling of the light curve, we find\nthat the anomaly is produced by a binary source passing over a caustic formed\nby a binary lens. The result proves the importance of high-cadence and\ncontinuous observations, and the capability of second-generation microlensing\nexperiments to identify such complex perturbations that are previously unknown.\nHowever, the result also raises the issues of the limitations of current\nanalysis techniques for understanding lens systems beyond two masses and of\ndetermining the appropriate multiband observing strategy of survey experiments.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Study of the Orbital Periods of Deeply Eclipsing SW Sextantis Stars: Results are presented of a five-year project to study the orbital periods of\neighteen deeply eclipsing novalike cataclysmic variables, collectively known as\nSW Sextantis stars, by combining new measurements of eclipse times with\npublished measurements stretching back in some cases over fifty years. While\nthe behaviour of many of these binary systems is consistent with a constant\norbital period, it is evident that in several cases this is not true. Although\nthe time span of these observations is relatively short, evidence is emerging\nthat the orbital periods of some of these stars show cyclical variation with\nperiods in the range 10-40 years. The two stars with the longest orbital\nperiods, V363 Aur and BT Mon, also show secular period reduction with rates of\n-6.6 \\times 10^{-8} days/year and -3.3 \\times 10^{-8} days/year. New\nephemerides are provided for all eighteen stars to facilitate observation of\nfuture eclipses.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Conditions for Photospherically Driven Alfvenic Oscillations to Heat the\n  Solar Chromosphere by Pedersen Current Dissipation: A magnetohydrodynamic model that includes a complete electrical conductivity\ntensor is used to estimate conditions for photospherically driven, linear,\nnon-plane Alfvenic oscillations extending from the photosphere to the lower\ncorona to drive a chromospheric heating rate due to Pedersen current\ndissipation that is comparable to the net chromospheric net radiative loss of\n$\\sim 10^7$ ergs-cm$^{-2}$-sec$^{-1}$. The heating rates due to electron\ncurrent dissipation in the photosphere and corona are also computed. The wave\namplitudes are computed self-consistently as functions of an inhomogeneous\nbackground (BG) atmosphere. The effects of the conductivity tensor are resolved\nnumerically using a resolution of 3.33 m. The oscillations drive a\nchromospheric heating flux $F_{Ch} \\sim 10^7 - 10^8$ ergs-cm$^{-2}$-sec$^{-1}$\nat frequencies $\\nu \\sim 10^2 - 10^3$ mHz for BG magnetic field strengths $B\n\\gtrsim 700$ G and magnetic field perturbation amplitudes $\\sim 0.01 - 0.1$\n$B$. The total resistive heating flux increases with $\\nu$. Most heating occurs\nin the photosphere. Thermalization of Poynting flux in the photosphere due to\nelectron current dissipation regulates the Poynting flux into the chromosphere,\nlimiting $F_{Ch}$. $F_{Ch}$ initially increases with $\\nu$, reaches a maximum,\nand then decreases with increasing $\\nu$ due to increasing electron current\ndissipation in the photosphere. The resolution needed to resolve the\noscillations increases from $\\sim 10$ m in the photosphere to $\\sim 10$ km in\nthe upper chromosphere, and is proportional to $\\nu^{-1/2}$. Estimates suggest\nthat these oscillations are normal modes of photospheric flux tubes with\ndiameters $\\sim 10-20$ km, excited by magnetic reconnection in current sheets\nwith thicknesses $\\sim 0.1$ km.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Precision southern hemisphere VLBI pulsar astrometry II: Measurement of\n  seven parallaxes: Accurate measurement of pulsar distances via astrometry using very long\nbaseline interferometry enables the improvement of Galactic electron density\ndistribution models, improving distance estimates for the vast majority of\npulsars for which parallax measurements are unavailable. However, pulsars at\nsouthern declinations have been under-represented in previous interferometric\nastrometry campaigns. In order to redress this imbalance, we have conducted a\ntwo-year astrometric campaign targeting eight southern pulsars with the\nAustralian Long Baseline Array. The program summarized in this paper has\nresulted in the measurement of seven new pulsar parallaxes, with success on\nobjects down to a mean flux density of 0.8 mJy at 1600 MHz. Our results\nhighlight the substantial uncertainties that remain when utilizing free\nelectron density models for individual pulsar distances. Until this study, PSR\nJ0630-2834 was believed to convert 16% of its spin-down energy into x-rays, but\nour measured parallax distance of 332 (+52 -40) pc has revised this value to\n<1%. In contrast, PSR J0108-1431 was found to be almost a factor of two more\ndistant than previously thought, making its conversion of spin-down energy to\nx-rays the most efficient known (>1%). The 8.5 second radio pulsar J2144-3933\nwas found to be closer than previously predicted, making its apparent 1400 MHz\nradio luminosity the lowest of any known pulsar (20 microJy kpc^2). We have\nexamined the growing population of neutron stars with accurate parallaxes to\ndetermine the effect of distance errors on the underlying neutron star velocity\ndistribution, and find that typical distance errors may be biasing the\nestimated mean pulsar velocity upwards by 5%, and are likely to exaggerate the\ndistribution's high-velocity tail.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Estimating Stellar Atmospheric Parameters by Automated Methods Using\n  SSLs: Libraries of stellar spectra, such as ELODIE (Prugniel & Soubiran 2001),\nCFLIB (Valdes et al. 2004), or MILES (S\\'anchez-Bl\\'azquez et al. 2006), are\nused for a variety of applications, and especially in modelling stellar\npopulations (e. g. Le Borgne et al. (2004)). In that context, apart from the\ncompleteness and quality of these spectral databases (Singh et al. 2006), the\naccurate calibration of stellar atmospheric parameters, temperature (Teff),\nsurface gravity (log g), and metallicity ([Fe/H]), is known to be critical\n(Prugniel et al. 2007; Percival & Salaris 2009). We discuss the technique of\ndetermining stellar atmospheric parameters accurately by `full spectrum\nfitting'.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Validation of the Exoplanet Kepler-21b using PAVO/CHARA Long-Baseline\n  Interferometry: We present long-baseline interferometry of the Kepler exoplanet host star\nHD179070 (Kepler-21) using the PAVO beam combiner at the CHARA Array. The\nvisibility data are consistent with a single star and exclude stellar\ncompanions at separations ~1-1000 mas (~ 0.1-113 AU) and contrasts < 3.5\nmagnitudes. This result supports the validation of the 1.6 R_{earth} exoplanet\nKepler-21b by Howell et al. (2012) and complements the constraints set by\nadaptive optics imaging, speckle interferometry, and radial velocity\nobservations to rule out false-positives due to stellar companions. We conclude\nthat long-baseline interferometry has strong potential to validate transiting\nextrasolar planets, particularly for future projects aimed at brighter stars\nand for host stars where radial velocity follow-up is not available.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Accretion process, magnetic fields, and apsidal motion in the pre-main\n  sequence binary DQ Tau: Classical T Tauri stars (CTTSs) are young stellar objects that accrete\nmaterials from their accretion disc influenced by their strong magnetic field.\nThe magnetic pressure truncates the disc at a few stellar radii and forces the\nmaterial to leave the disc plane and fall onto the stellar surface by following\nthe magnetic field lines. However, this global scheme may be disturbed by the\npresence of a companion interacting gravitationally with the accreting\ncomponent. This work is aiming to study the accretion and the magnetic field of\nthe tight eccentric binary DQ Tau, composed of two equal-mass ($\\sim$ 0.6 \\msun\n) CTTSs interacting at different orbital phases. We investigated the\nvariability of the system using a high-resolution spectroscopic and\nspectropolarimetric monitoring performed with ESPaDOnS at the CFHT. We provide\nthe first ever magnetic field analysis of this system, the Zeeman-Doppler\nimaging revealed a stronger magnetic field for the secondary than the primary\n(1.2 kG and 0.5 kG, respectively), but the small-scale fields analysed through\nZeeman intensification yielded similar strengths (about 2.5 kG). The magnetic\nfield topology and strengths are compatible with the accretion processes on\nCTTSs. Both components of this system are accreting, with a change of the main\naccretor during the orbital motion. In addition, the system displays a strong\nenhancement of the mass accretion rate at periastron and apastron. We also\ndiscovered, for the first time in this system, the apsidal motion of the\norbital ellipse.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Modelling the occurrence of grand minima in sun-like stars using a\n  dynamo model: In this work, we have studied the variability and frequency of occurrence of\nthe grand minima using kinematic dynamo models of one solar mass star with\ndifferent rotation rates and depths of convection zones. We specify the\nlarge-scale flows (differential rotations and meridional circulations) from\ncorresponding hydrodynamic models. We include stochastic fluctuations in the\nBabcock-Leighton source for the poloidal field to produce variable stellar\ncycles. We observe that the rapidly rotating stars produce highly irregular\ncycles with strong magnetic fields and rarely produce Maunder-like grand\nminima, whereas the slowly rotating stars (Sun and longer rotation period)\nproduce smooth cycles of weaker strength and occasional grand minima. In\ngeneral, the number of the grand minima increases with the decrease in rotation\nrate. These results can be explained by the fact that with the increase of\nrotation period, the supercriticality of the dynamo decreases, and the dynamo\nis more prone to produce extended grand minima in this regime.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Magnetic field and prominences of the young, solar-like, ultra-rapid\n  rotator V530 Per: We investigate signatures of magnetic fields and activity at the surface and\nin the prominence system of the ultra-rapid rotator V530 Per, a G-type\nsolar-like member of the young open cluster $\\alpha$~Persei. This object has a\nrotation period shorter than all stars with available magnetic maps. With a\ntime-series of spectropolarimetric observations gathered with ESPaDOnS over 2\nnights on the CFHT, we reconstruct the surface brightness and large-scale\nmagnetic field of V530 Per using the Zeeman-Doppler imaging method, assuming an\noblate stellar surface. We also estimate the short term evolution of the\nbrightness distribution through latitudinal differential rotation. Using the\nsame data set, we finally map the spatial distribution of prominences through\ntomography of the H\\alpha emission. The brightness map is dominated by a large,\ndark spot near the pole, accompanied by a complex distribution of bright and\ndark features at lower latitudes. The magnetic field map is reconstructed as\nwell, most of the large-scale magnetic field energy is stored in the toroidal\nfield component. The main radial field structure is a positive region of about\n500 G, at the location of the dark polar spot. The brightness map of V530 Per\nis sheared by solar-like differential rotation, with a roughly solar value for\nthe difference in rotation rate between the pole and equator. \\halpha~is\nobserved in emission, and is mostly modulated by the stellar rotation period.\nThe prominence system is organized in a ring at the approximate location of the\nco-rotation radius, with significant evolution between the two observing\nnights. V530 Per is the first example of a solar-type star to have its surface\nmagnetic field and prominences mapped together, which will bring important\nobservational constraints to better understand the role of slingshot\nprominences in the angular momentum evolution of the most active stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "An Observational Perspective of Transitional Disks: Transitional disks are objects whose inner disk regions have undergone\nsubstantial clearing. The Spitzer Space Telescope produced detailed spectral\nenergy distributions (SEDs) of transitional disks that allowed us to infer\ntheir radial dust disk structure in some detail, revealing the diversity of\nthis class of disks. The growing sample of transitional disks also opened up\nthe possibility of demographic studies, which provided unique insights. There\nnow exist (sub)millimeter and infrared images that confirm the presence of\nlarge clearings of dust in transitional disks. In addition, protoplanet\ncandidates have been detected within some of these clearings. Transitional\ndisks are thought to be a strong link to planet formation around young stars\nand are a key area to study if further progress is to be made on understanding\nthe initial stages of planet formation. Here we provide a review and synthesis\nof transitional disk observations to date with the aim of providing timely\ndirection to the field, which is about to undergo its next burst of growth as\nALMA reaches its full potential. We discuss what we have learned about\ntransitional disks from SEDs, color-color diagrams, and imaging in the (sub)mm\nand infrared. We then distill the observations into constraints for the main\ndisk clearing mechanisms proposed to date (i.e., photoevaporation, grain\ngrowth, and companions) and explore how the expected observational signatures\nfrom these mechanisms, particularly planet-induced disk clearing, compare to\nactual observations. Lastly, we discuss future avenues of inquiry to be pursued\nwith ALMA, JWST, and next generation of ground-based telescopes.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Extension and validation of the pendulum model for longitudinal solar\n  prominence oscillations: Longitudinal oscillations in prominences are common phenomena on the Sun.\nThese oscillations can be used to infer the geometry and intensity of the\nfilament magnetic field. Previous theoretical studies of longitudinal\noscillations made two simplifying assumptions: uniform gravity and\nsemi-circular dips on the supporting flux tubes. However, the gravity is not\nuniform and realistic dips are not semi-circular. To understand the effects of\nincluding the nonuniform solar gravity on longitudinal oscillations, and\nexplore the validity of the pendulum model with different flux-tube geometries.\nWe first derive the equation describing the motion of the plasma along the flux\ntube including the effects of nonuniform gravity, yielding corrections to the\noriginal pendulum model. We also compute the full numerical solutions for the\nnormal modes, and compare them with the new pendulum approximation. We have\nfound that the nonuniform gravity introduces a significant modification in the\npendulum model. We have also found a cut-off period, i.e. the longitudinal\noscillations cannot have a period longer than 167 minutes. In addition,\nconsidering different tube geometries, the period depends almost exclusively on\nthe radius of curvature at the bottom of the dip. We conclude that nonuniform\ngravity significantly modifies the pendulum model. These corrections are\nimportant for prominence seismology, because the inferred values of the radius\nof curvature and minimum magnetic-field strength differ substantially from\nthose of the old model. However, we find that the corrected pendulum model is\nquite robust and is still valid for non-circular dips.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Destabilization of a Solar Prominence/Filament Field System by a Series\n  of Eight Homologous Eruptive Flares: Homologous flares are flares that occur repetitively in the same active\nregion, with similar structure and morphology. A series of at least eight\nhomologous flares occurred in active region NOAA 11237 over 16 - 17 June 2011.\nA nearby prominence/filament was rooted in the active region, and situated near\nthe bottom of a coronal cavity. The active region was on the southeast solar\nlimb as seen from SDO/AIA, and on the disk as viewed from STEREO/EUVI-B. The\ndual perspective allows us to study in detail behavior of the\nprominence/filament material entrained in the magnetic field of the\nrepeatedly-erupting system. Each of the eruptions was mainly confined, but\nexpelled hot material into the prominence/filament cavity system (PFCS). The\nfield carrying and containing the ejected hot material interacted with the PFCS\nand caused it to inflate, resulting in a step-wise rise of the PFCS\napproximately in step with the homologous eruptions. The eighth eruption\ntriggered the PFCS to move outward slowly, accompanied by a weak coronal\ndimming. As this slow PFCS eruption was underway, a final ejective flare\noccurred in the core of the active region, resulting in strong dimming in the\nEUVI-B images and expulsion of a coronal mass ejection (CME). A plausible\nscenario is that the repeated homologous flares could have gradually\ndestabilized the PFCS, and its subsequent eruption removed field above the\nacitive region and in turn led to the ejective flare, strong dimming, and CME.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Generalized three body problem and the instability of the core-halo\n  objects in binary systems: Goal of the presented research is to construct simplified model of the\ncore-halo structures in binary systems. Examples are provided by Thorne-Zytkov\nobjects, hot Jupiters, protoplanets with large moons, red supergiants in\nbinaries and globular clusters with central black hole. Instability criteria\ndue to resonance between internal and orbital frequencies in such a systems has\nbeen derived. To achieve assumed goals, generalized planar circular restricted\nthree body problem is investigated with one of the point masses, $M$, replaced\nwith spherical body of finite size. Mechanical system under consideration\nincludes two large masses $m$ and $M$ and the test body with small mass $\\mu$.\nOnly gravitational interactions are considered. Equations of motion are\npresented, and linear instability criteria are derived using quantifier\nelimination.\n  Motion of the test mass $\\mu$ is shown to be unstable due to resonance\nbetween orbital and internal frequencies if $\\frac{M}{d^3} < \\frac{4}{3} \\pi\n\\rho < \\frac{ M + 3 m \\left( 1+\\mu/M \\right)^{-1}}{d^3}$, where $\\rho$ is the\ncentral density of mass $M$, and $d$ distance between masses $m$ and $M$\n(circular orbit diameter).\n  The above result is important for core-collapse supernova theory, with mass\n$\\mu$ identified with helium core of the exploding massive star. The\ninstability cause off-center supernova \"ignition\" relative to the\ncenter-of-mass of the hydrogen envelope. The instability is also inevitable\nduring protoplanet growth, with hypothetical ejection of the rocky core from\ngas giants and formation of the \"puffy planets\" due to resonance with orbital\nfrequency. Hypothetical central intermediate black holes of the globular\nclusters are also in unstable position with respect to perturbations caused by\nthe Galaxy.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Clusters AgeS Experiment (CASE): Variable Stars in the Globular\n  Cluster M4: Based on over 3000 BV images of M4 collected in years 1995-2009 we obtain\nlight curves of 22 variables, 10 of which are newly detected objects. We\nidentify four detached eclipsing binaries and eight contact binaries. Accurate\nperiods are found for all but two variables. Nineteen variables are\nproper-motion members of the cluster, and the remaining three are field stars.\nFive variables are optical counterparts of X-ray sources. For one of the\nvariables unassociated with X-ray sources we report a flare lasting for about\n90 min and reaching an amplitude of 0.11 mag in V. One of the new contact\nbinaries has a record-low mass ratio q=0.06. Another four such systems show\nseason-to season luminosity variations probably related to magnetic activity\ncycles, whose lengths are surprisingly similar to that of the solar cycle\ndespite a huge difference in rotational periods. The location of contact\nbinaries on the color-magnitude diagram of M4 strongly suggests that at least\nin globular clusters the principal factor enabling EW systems to form from\nclose but detached binaries is stellar evolution. We identify 46 blue and\nyellow stragglers in M4 and discuss their properties. We also derive a map of\nthe differential extinction in the central part of M4, and determine the\nreddening of a selected reference region, E(B-V)= 0.392 mag.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Sunspot rotation. II. Effects of varying the field strength and twist of\n  an emerging flux tube: Context. Observations of flux emergence indicate that rotational velocities\nmay develop within sunspots. However, the dependence of this rotation on\nsub-photospheric field strength and twist remains largely unknown.\n  Aims. We investigate the effects of varying the initial field strength and\ntwist of an emerging sub-photospheric magnetic flux tube on the rotation of the\nsunspots at the photosphere.\n  Methods. We consider a simple model of a stratified domain with a\nsub-photospheric interior layer and three overlying atmospheric layers. A\ntwisted arched flux tube is inserted in the interior and is allowed to rise\ninto the atmosphere. To achieve this, the MHD equations are solved using the\nLagrangian-remap code, Lare3d. We perform a parameter study by independently\nvarying the sub-photospheric magnetic field strength and twist.\n  Results. Altering the initial field strength and twist significantly affects\nthe tube's evolution and the rotational motions that develop at the\nphotosphere. The rotation angle, vorticity, and current show a direct\ndependence on the initial field strength. We find that an increase in field\nstrength increases the angle through which the fieldlines rotate, the length of\nfieldlines extending into the atmosphere, and the magnetic energy transported\nto the atmosphere. This also affects the amount of residual twist in the\ninterior. The length of the fieldlines is crucial as we predict the twist per\nunit length equilibrates to a lower value on longer fieldlines. No such direct\ndependence is found when we modify the twist owing to the complex effect this\nhas on the tension force acting on the tube. However, there is still a clear\nordering in quantities such as the rotation angle, helicity, and free energy\nwith higher initial twist cases being related to sunspots that rotate more\nrapidly, transporting more helicity and magnetic energy to the atmosphere.",
        "category": "astro-ph_SR"
    },
    {
        "text": "FIP Bias Evolution in a Decaying Active Region: Solar coronal plasma composition is typically characterized by first\nionization potential (FIP) bias. Using spectra obtained by Hinode's EUV Imaging\nSpectrometer (EIS) instrument, we present a series of large-scale, spatially\nresolved composition maps of active region (AR) 11389. The composition maps\nshow how FIP bias evolves within the decaying AR from 2012 January 4-6.\nGlobally, FIP bias decreases throughout the AR. We analyzed areas of\nsignificant plasma composition changes within the decaying AR and found that\nsmall-scale evolution in the photospheric magnetic field is closely linked to\nthe FIP bias evolution observed in the corona. During the AR's decay phase,\nsmall bipoles emerging within supergranular cells reconnect with the\npre-existing AR field, creating a pathway along which photospheric and coronal\nplasmas can mix. The mixing time scales are shorter than those of plasma\nenrichment processes. Eruptive activity also results in shifting the FIP bias\ncloser to photospheric in the affected areas. Finally, the FIP bias still\nremains dominantly coronal only in a part of the AR's high-flux density core.\nWe conclude that in the decay phase of an AR's lifetime, the FIP bias is\nbecoming increasingly modulated by episodes of small-scale flux emergence, i.e.\ndecreasing the AR's overall FIP bias. Our results show that magnetic field\nevolution plays an important role in compositional changes during AR\ndevelopment, revealing a more complex relationship than expected from previous\nwell-known Skylab results showing that FIP bias increases almost linearly with\nage in young ARs (Widing $\\&$ Feldman, 2001, ApJ, 555, 426).",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Branchings of the Main s-process: Their Sensitivity to alpha-induced\n  Reactions on 13C and 22Ne and to the Uncertainties of the Nuclear Network: This paper provides a detailed analysis of the main component of the slow\nneutron capture process (the s-process), which accounts for the solar\nabundances of half of the nuclei with 90 <~ A <~ 208. We examine the impact of\nthe uncertainties of the two neutron sources operating in low-mass asymptotic\ngiant branch (AGB) stars: the 13C(alpha, n)16O reaction, which releases\nneutrons radiatively during interpulse periods (kT ~ 8 keV), and the\n22Ne(alpha, n)25Mg reaction, partially activated during the convective thermal\npulses (TPs). We focus our attention on the branching points that mainly\ninfluence the abundance of s-only isotopes. In our AGB models, the 13C is fully\nconsumed radiatively during interpulse. In this case, we find that the present\nuncertainty associated to the 13C(alpha, n)16O reaction has marginal effects on\ns-only nuclei. On the other hand, a reduction of this rate may increase the\namount of residual (or unburned) 13C at the end of the interpulse: in this\ncondition, the residual 13C is burned at higher temperature in the convective\nzone powered by the following TP. The neutron burst produced by the 22Ne(alpha,\nn)25Mg reaction has major effects on the branches along the s path. The\ncontributions of s-only isotopes with 90 <~ A <= 204 are reproduced within\nsolar and nuclear uncertainties, even if the 22Ne(alpha, n)25Mg rate is varied\nby a factor of two. Improved beta-decay and neutron capture rates of a few key\nradioactive nuclides would help to attain a comprehensive understanding of the\nsolar main component.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Toward understanding the early stages of an impulsively accelerated\n  coronal mass ejection: The expanding magnetic flux in coronal mass ejections (CMEs) often forms a\ncavity. A spherical model is simultaneously fit to STEREO EUVI and COR1 data of\nan impulsively accelerated CME on 25 March 2008, which displays a well-defined\nextreme ultraviolet (EUV) and white-light cavity of nearly circular shape\nalready at low heights ~ 0.2 Rs. The center height h(t) and radial expansion\nr(t) of the cavity are obtained in the whole height range of the main\nacceleration. We interpret them as the axis height and as a quantity\nproportional to the minor radius of a flux rope, respectively. The\nthree-dimensional expansion of the CME exhibits two phases in the course of its\nmain upward acceleration. From the first h and r data points, taken shortly\nafter the onset of the main acceleration, the erupting flux shows an\noverexpansion compared to its rise, as expressed by the decrease of the aspect\nratio from k=h/r ~ 3 to k ~ (1.5-2.0). This phase is approximately coincident\nwith the impulsive rise of the acceleration and is followed by a phase of very\ngradual change of the aspect ratio (a nearly self-similar expansion) toward k ~\n1.5 at h ~ 10 Rs. The initial overexpansion of the CME cavity can be caused by\nflux conservation around a rising flux rope of decreasing axial current and by\nthe addition of flux to a growing, or even newly forming,flux rope by magnetic\nreconnection. Further analysis will be required to decide which of these\ncontributions is dominant. The data also suggest that the horizontal component\nof the impulsive cavity expansion (parallel to the solar surface) triggers the\nassociated EUV wave, which subsequently detaches from the CME volume.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Radio Measurements of the stellar proper motions in the core of the\n  Orion Nebula Cluster: Using multi-epoch VLA observations, covering a time baseline of 29.1 years,\nwe have measured the proper motions of 88 young stars with compact radio\nemission in the core of the Orion Nebula Cluster (ONC) and the neighboring\nBN/KL region. Our work increases the number of young stars with measured proper\nmotion at radio frequencies by a factor of 2.5 and enables us to perform a\nbetter statistical analysis of the kinematics of the region than was previously\npossible. Most stars (79 out of 88) have proper motions consistent with a\nGaussian distribution centered on\n$\\overline{\\mu_{\\alpha}\\cos{\\delta}}=1.07\\pm0.09\\quad{\\rm mas\\,yr^{-1}}$, and\n$\\overline{\\mu_{\\delta}}=-0.84\\pm0.16\\quad{\\rm mas\\,yr^{-1}}$, with velocity\ndispersions of $\\sigma_{\\alpha}=1.08\\pm0.07\\quad{\\rm mas\\,\\,yr^{-1}},$\n$\\sigma_{\\delta}=1.27\\pm0.15\\quad{\\rm mas\\,\\,yr^{-1}}$. We looked for organized\nmovements of these stars but found no clear indication of radial\nexpansion/contraction or rotation. The remaining nine stars in our sample show\npeculiar proper motions that differ from the mean proper motions of the ONC by\nmore than 3-$\\sigma$. One of these stars, V 1326 Ori, could have been expelled\nfrom the Orion Trapezium 7,000 years ago. Two could be related to the\nmulti-stellar disintegration in the BN/KL region, in addition to the previously\nknown sources BN, I and n. The others either have high uncertainties (so their\nanomalous proper motions are not firmly established) or could be foreground\nobjects.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Horizontal flow fields observed in Hinode G-band images. I. Methods: Context: The interaction of plasma motions and magnetic fields is an\nimportant mechanism, which drives solar activity in all its facets. For\nexample, photospheric flows are responsible for the advection of magnetic flux,\nthe redistribution of flux during the decay of sunspots, and the built-up of\nmagnetic shear in flaring active regions. Aims: Systematic studies based on\nG-band data from the Japanese Hinode mission provide the means to gather\nstatistical properties of horizontal flow fields. This facilitates comparative\nstudies of solar features, e.g., G-band bright points, magnetic knots, pores,\nand sunspots at various stages of evolution and in distinct magnetic\nenvironments, thus, enhancing our understanding of the dynamic Sun. Methods: We\nadapted Local Correlation Tracking (LCT) to measure horizontal flow fields\nbased on G-band images obtained with the Solar Optical Telescope on board\nHinode. In total about 200 time-series with a duration between 1-16 h and a\ncadence between 15-90 s were analyzed. Selecting both a high-cadence (dt = 15\ns) and a long-duration (dT = 16 h) time-series enabled us to optimize and\nvalidate the LCT input parameters, hence, ensuring a robust, reliable, uniform,\nand accurate processing of a huge data volume. Results: The LCT algorithm\nproduces best results for G-band images having a cadence of 60-90 s. If the\ncadence is lower, the velocity of slowly moving features will not be reliably\ndetected. If the cadence is higher, the scene on the Sun will have evolved too\nmuch to bear any resemblance with the earlier situation. Consequently, in both\ninstances horizontal proper motions are underestimated. The most reliable and\nyet detailed flow maps are produced using a Gaussian kernel with a size of 2560\nkm x 2560 km and a full-width-at-half-maximum (FWHM) of 1200 km (corresponding\nto the size of a typical granule) as sampling window.",
        "category": "astro-ph_SR"
    },
    {
        "text": "From Protoplanetary Disks to Extrasolar Planets: Understanding the Life\n  Cycle of Circumstellar Gas with Ultraviolet Spectroscopy: Few scientific discoveries have captured the public imagination like the\nexplosion of exoplanetary science during the past two decades. This work has\nfundamentally changed our picture of Earth's place in the Universe and led NASA\nto make significant investments towards understanding the demographics of\nexoplanetary systems and the conditions that lead to their formation. The story\nof the formation and evolution of exoplanetary systems is essentially the story\nof the circumstellar gas and dust that are initially present in the\nprotostellar environment; in order to understand the variety of planetary\nsystems observed, we need to understand the life cycle of circumstellar gas\nfrom its initial conditions in protoplanetary disks to its endpoint as planets\nand their atmospheres. In this white paper response to NASA's Request for\nInformation \"Science Objectives and Requirements for the Next NASA UV/Visible\nAstrophysics Mission Concepts (NNH12ZDA008L)\", we describe scientific programs\nthat would use the unique capabilities of a future NASA ultraviolet\n(UV)/visible space observatory to make order-of-magnitude advances in our\nunderstanding of the life cycle of circumstellar gas.",
        "category": "astro-ph_SR"
    },
    {
        "text": "WR 63: A multiple system (O+O)+WR ?: The spectrum of the Wolf-Rayet (WR) star WR 63 contains spectral lines of two\ndifferent O stars that show regular radial velocity (RV) variations with\namplitudes of ~160 and ~225 km/s on a ~4.03 d period. The light-curve shows two\nnarrow eclipses that are 0.2 mag deep on the same period as the RV changes. On\nthe other hand, our data show no significant RV variations for the WR spectral\nlines. Those findings are compatible with WR 63 being a triple system composed\nof two non-interacting late O stars orbiting a WR star on a period larger than\n1000 days. The amplitude of the WR spectral line-profile variability reaches\n7-8% of the line intensity and seems related to a 0.04 mag periodic photometric\nvariation. Large wind density structures are a possible origin of this\nvariability, but our data are not sufficient to verify this. Our analysis shows\nthat, should the three stars be bound, they would be coeval with an age of\nabout 5.9+/-1.4 Myrs. The distance to the O stars is estimated to be 3.4+/-0.5\nkpc. Their dynamical masses are 14.3+/-0.1 and 10.3+/-0.1 M_sol. Using\nrotating, single star evolutionary tracks, we estimate their initial masses to\nbe 18+/-2 and 16+/-2 M_sol for the primary and the secondary, respectively.\nRegular spectral monitoring is required in the future to detect RV variations\nof the WR star that would prove that it is gravitationally bound to the close\nO+OB system and to determine its mass.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A deep catalogue of classical Be stars in the direction of the Perseus\n  Arm: spectral types and interstellar reddenings: We present a catalogue of 247 photometrically and spectroscopically confirmed\nfainter classical Be stars (13 < r < 16) in the direction of the Perseus Arm of\nthe Milky Way (-1 < b < +4, 120 < l < 140). The catalogue consists of 181\nIPHAS-selected new classical Be stars, in addition to 66 objects that were\nstudied by Raddi et al. (2013) more closely, and 3 stars identified as\nclassical Be stars in earlier work. This study more than doubles the number\nknown in the region. Photometry spanning 0.6 to 5 micron, spectral types, and\ninterstellar reddenings are given for each object. The spectral types were\ndetermined from low-resolution spectra (lambda / Delta-lambda ~ 800-2000), to a\nprecision of 1-3 subtypes. The interstellar reddenings are derived from the (r\n- i) colour, using a method that corrects for circumstellar disc emission. The\ncolour excesses obtained range from E(B-V) = 0.3 up to 1.6 - a distribution\nthat modestly extends the range reported in the literature for Perseus-Arm open\nclusters. For around half the sample, the reddenings obtained are compatible\nwith measures of the total sightline Galactic extinction. Many of these are\nlikely to lie well beyond the Perseus Arm.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Microlensing Binaries Discovered through High-Magnification Channel: Microlensing can provide a useful tool to probe binary distributions down to\nlow-mass limits of binary companions. In this paper, we analyze the light\ncurves of 8 binary lensing events detected through the channel of\nhigh-magnification events during the seasons from 2007 to 2010. The\nperturbations, which are confined near the peak of the light curves, can be\neasily distinguished from the central perturbations caused by planets. However,\nthe degeneracy between close and wide binary solutions cannot be resolved with\na $3\\sigma$ confidence level for 3 events, implying that the degeneracy would\nbe an important obstacle in studying binary distributions. The dependence of\nthe degeneracy on the lensing parameters is consistent with a theoretic\nprediction that the degeneracy becomes severe as the binary separation and the\nmass ratio deviate from the values of resonant caustics. The measured mass\nratio of the event OGLE-2008-BLG-510/MOA-2008-BLG-369 is $q\\sim 0.1$, making\nthe companion of the lens a strong brown-dwarf candidate.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Hubble Space Telescope reveals multiple Sub-Giant Branch in eight\n  Globular Clusters: In the last few years many globular clusters (GCs) have revealed complex\ncolor-magnitude diagrams, with the presence of multiple main sequences (MSs),\nbroaden or multiple sub-giant branches (SGBs) and MS turn offs, and broad or\nsplit red giant branches (RGBs). After a careful correction for differential\nreddening, high accuracy photometry with the Hubble Space Telescope presented\nin this paper reveals a broadened or even split SGB in five additional Milky\nWay GCs: NGC 362, NGC 5286, NGC 6656, NGC 6715, and NGC 7089. In addition, we\nconfirm (with new and archival HST data) the presence of a split SGB in 47Tuc,\nNGC 1851, and NGC 6388. The fraction of faint SGB stars with respect to the\nentire SGB population varies from one cluster to another and ranges from\n$\\sim$0.03 for NGC 362 to ~0.50 for NGC 6715. The average magnitude difference\nbetween the bright SGB and the faint SGB is almost the same at different\nwavelengths. This peculiarity is consistent with the presence of two groups of\nstars with either an age difference of about 1-2 Gyrs, or a significant\ndifference in their overall C+N+O content.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evidence of Electron Acceleration around the Reconnection X-point in a\n  Solar Flare: Particle acceleration is one of the most significant features that are\nubiquitous among space and cosmic plasmas. It is most prominent during flares\nin the case of the Sun, with which huge amount of electromagnetic radiation and\nhigh-energy particles are expelled into the interplanetary space through\nacceleration of plasma particles in the corona. Though it has been well\nunderstood that energies of flares are supplied by the mechanism called\nmagnetic reconnection based on the observations in X-rays and EUV with space\ntelescopes, where and how in the flaring magnetic field plasmas are accelerated\nhas remained unknown due to the low plasma density in the flaring corona. We\nhere report the first observational identification of the energetic non-thermal\nelectrons around the point of the ongoing magnetic reconnection (X-point); with\nthe location of the X-point identified by soft X-ray imagery and the localized\npresence of non-thermal electrons identified from imaging-spectroscopic data at\ntwo microwave frequencies. Considering the existence of the reconnection\noutflows that carries both plasma particles and magnetic fields out from the\nX-point, our identified non-thermal microwave emissions around the X-point\nindicate that the electrons are accelerated around the reconnection X-point.\nAdditionally, the plasma around the X-point was also thermally heated up to 10\nMK. The estimated reconnection rate of this event is ~0.017.",
        "category": "astro-ph_SR"
    },
    {
        "text": "High resolution soft X-ray spectroscopy and the quest for the hot (5-10\n  MK) plasma in solar active regions: We discuss the diagnostics available to study the 5-10 MK plasma in the solar\ncorona, which is key to understanding the heating in the cores of solar active\nregions. We present several simulated spectra, and show that excellent\ndiagnostics are available in the soft X-rays, around 100 Angstroms, as six\nionisation stages of Fe can simultaneously be observed, and electron densities\nderived, within a narrow spectral region. As this spectral range is almost\nunexplored, we present an analysis of available and simulated spectra, to\ncompare the hot emission with the cooler component. We adopt recently designed\nmultilayers to present estimates of count rates in the hot lines, with a\nbaseline spectrometer design. Excellent count rates are found, opening up the\nexciting opportunity to obtain high-resolution spectroscopy of hot plasma.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Cepheid Abundances: Multiphase Results and Spatial Gradients: Parameters and abundances have been derived for 435 Cepheids based on an\nanalysis of 1127 spectra. Results from five or more phases are available for 52\nof the program stars. The latter set of stars span periods between 1.5 and 68\ndays. The parameters and abundances show excellent consistency across phase.\nFor iron, the average range in the determined abundance is 0.11 from these 52\nstars. For 163 stars with more than one phase available the average range is\n0.07. The variation in effective temperature tracks well with phase, as does\nthe total broadening velocity. The gravity and microturbulent velocity follow\nphase, but with less variation and regularity.\n  Abundance gradients have been derived using GAIA DR2 parallax data (GAIA\nCollaboration et al. 2016, 2018), as well as Bayesian distance estimates based\nupon GAIA DR2 from Bailer-Jones et al. (2018). The abundance gradient derived\nfor iron is d[Fe/H]/dR = -0.05 dex/kpc, similar to gradients derived in\nprevious studies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Measurements of Non-Thermal Line Widths in Solar Active Regions: Spectral line widths are often observed to be larger than can be accounted\nfor by thermal and instrumental broadening alone. This excess broadening is a\nkey observational constraint for both nanoflare and wave dissipation models of\ncoronal heating. Here we present a survey of non-thermal velocities measured in\nthe high temperature loops (1--5MK) often found in the cores of solar active\nregions. This survey of $\\textit{Hinode}$ Extreme Ultraviolet Imaging\nSpectrometer (EIS) observations covers 15 non-flaring active regions that span\na wide range of solar conditions. We find relatively small non-thermal\nvelocities, with a mean value of 17km s$^{-1}$, and no significant trend with\ntemperature or active region magnetic flux. These measurements appear to be\ninconsistent with those expected from reconnection jets in the corona,\nchromospheric evaporation induced by coronal nanoflares, and Alfv\\'en wave\nturbulence models. Furthermore, because the observed non-thermal widths are\ngenerally small their measurements are difficult and susceptible to systematic\neffects.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Space, time and velocity association of successive coronal mass\n  ejections: Our aim is to investigate the possible physical association between\nconsecutive coronal mass ejections (CMEs). Through a statistical study of the\nmain characteristics of 27761 CMEs observed by SOHO/LASCO during the past 20\nyears. We found the waiting time (WT) or time elapsed between two consecutive\nCMEs is $< 5$ hrs for 59\\% and $< 25$ hrs for 97\\% of the events, and the CME\nWTs follow a Pareto Type IV statistical distribution. The difference of the\nposition-angle of a considerable population of consecutive CME pairs is less\nthan $30^\\circ$, indicating the possibility that their source locations are in\nthe same region. The difference between the speed of trailing and leading\nconsecutive CMEs follows a generalized Student t-distribution. The fact that\nthe WT and the speed difference have heavy-tailed distributions along with a\ndetrended fluctuation analysis shows that the CME process has a long-range\ndependence. As a consequence of the long-range dependence, we found a small but\nsignificative difference between the speed of consecutive CMEs, with the speed\nof the trailing CME being higher than the speed of the leading CME. The\ndifference is largest for WTs < 2 hrs and tends to be zero for WTs > 10 hrs,\nand it is more evident during the ascending and descending phases of the solar\ncycle. We suggest that this difference may be caused by a drag force acting\nover CMEs closely related in space and time.",
        "category": "astro-ph_SR"
    },
    {
        "text": "First Keck Nulling Observations of a Young Stellar Object: Probing the\n  Circumstellar Environment of the Herbig Ae star MWC 325: We present the first N-band nulling plus K- and L-band V2 observations of a\nyoung stellar object, MWC325, taken with the 85 m baseline Keck Interferometer.\nThe Keck nuller was designed for the study of faint dust signatures associated\nwith debris disks, but it also has a unique capability for studying the\ntemperature and density distribution of denser disks found around young stellar\nobjects. Interferometric observations of MWC 325 at K, L and N encompass a\nfactor of five in spectral range and thus, especially when spectrally dispersed\nwithin each band, enable characterization of the structure of the inner disk\nregions where planets form. Fitting our observations with geometric models such\nas a uniform disk or a Gaussian disk show that the apparent size increases\nmonotonically with wavelength in the 2-12 um wavelength region, confirming the\nwidely held assumption based on radiative transfer models, now with spatially\nresolved measurements over broad wavelength range, that disks are extended with\na temperature gradient. The effective size is a factor of about 1.3 and 2\nlarger in the L-band and N-band, respectively, compared to that in the K-band.\nThe existing interferometric measurements and the spectral energy distribution\ncan be reproduced by a flat disk or a weakly-shadowed nearly flat-disk model,\nwith only slight flaring in the outer regions of the disk, consisting of\nrepresentative \"sub-micron\" (0.1 um) and \"micron\" (2 um) grains of a 50:50\nratio of silicate and graphite. This is marked contrast with the disks\npreviously found in other Herbig Ae/Be stars suggesting a wide variety in the\ndisk properties among Herbig Ae/Be stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Perpendicular Diffusion of Energetic Particles in Collisionless Plasmas: A fundamental problem in plasma and astrophysics is the interaction between\nenergetic particles and magnetized plasmas. In the current paper we focus on\nparticle diffusion across the guide magnetic field. It is shown that the\nperpendicular diffusion coefficient depends only on the parallel diffusion\ncoefficient and the Kubo number. Therefore, one can find four asymptotic limits\ndepending on the values of these two parameters. These regimes are the\nquasilinear limit, the Kadomtsev & Pogutse limit, the scaling of Rechester &\nRosenbluth, and the scaling found by Zybin & Istomin. In the current article we\nfocus on the Rechester & Rosenbluth scenario because this was not discovered\nbefore in the context of collisionless plasmas. Examples and applications are\ndiscussed as well. We show that an energy independent ratio of perpendicular\nand parallel diffusion coefficients can be found and that this ratio can be\nvery small but also close to unity. This is exactly what one observes in the\nsolar wind.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Solar Flare Prediction and Feature Selection using Light Gradient\n  Boosting Machine Algorithm: Solar flares are among the most severe space weather phenomena, and they have\nthe capacity to generate radiation storms and radio disruptions on Earth. The\naccurate prediction of solar flare events remains a significant challenge,\nrequiring continuous monitoring and identification of specific features that\ncan aid in forecasting this phenomenon, particularly for different classes of\nsolar flares. In this study, we aim to forecast C and M class solar flares\nutilising a machine-learning algorithm, namely the Light Gradient Boosting\nMachine. We have utilised a dataset spanning 9 years, obtained from the\nSpace-weather Helioseismic and Magnetic Imager Active Region Patches (SHARP),\nwith a temporal resolution of 1 hour. A total of 37 flare features were\nconsidered in our analysis, comprising of 25 active region parameters and 12\nflare history features. To address the issue of class imbalance in solar flare\ndata, we employed the Synthetic Minority Oversampling Technique (SMOTE). We\nused two labeling approaches in our study: a fixed 24-hour window label and a\nvarying window that considers the changing nature of solar activity. Then, the\ndeveloped machine learning algorithm was trained and tested using forecast\nverification metrics, with an emphasis on evaluating the true skill statistic\n(TSS). Furthermore, we implemented a feature selection algorithm to determine\nthe most significant features from the pool of 37 features that could\ndistinguish between flaring and non-flaring active regions. We found that\nutilising a limited set of useful features resulted in improved prediction\nperformance. For the 24-hour prediction window, we achieved a TSS of 0.63\n(0.69) and accuracy of 0.90 (0.97) for $\\geq$C ($\\geq$M) class solar flares.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Penumbral micro-jets at high spatial and temporal resolution: Sunspot observations in chromospheric spectral lines have revealed the\nexistence of short-lived linear bright transients, commonly referred to as\npenumbral micro-jets (PMJs). Details on the origin and physical nature of PMJs\nare to large extend still unkown. We aim to characterize the dynamical nature\nof PMJs to provide guidance for future modelling efforts. We analyze high\nspatial (0.1 arcsec) and temporal resolution (1 s) Ca II H filtergram (0.1 nm\nbandwidth) observations of a sunspot obtained on two consecutive days with the\nSwedish 1-m Solar Telescope. We find that PMJs appear to be the rapid\nbrightening of an already existing (faint) fibril. The rapid brightening is the\nfast increase (typically less than 10 s) in intensity over significant length\n(several 100s of km) of the existing fibril. For most PMJs, we cannot identify\na clear root or source from where the brightening appears to originate. After\nthe fast onset, about half of the PMJs have a top that is moving with an\napparent velocity between 5 and 14 km/s, most of them upwards. For the other\nPMJs, there is no significant motion of the top. For about a third of the PMJs\nwe observe a splitting into two parallel and co-evolving linear features during\nthe later phases of the lifetime of the PMJ. We conclude that mass flows can\nplay only limited role in the onset phase of PMJs and that it is more likely\nthat we see the effect of a fast heating front.",
        "category": "astro-ph_SR"
    },
    {
        "text": "High-cadence spectroscopy of M-dwarfs. I. Analysis of systematic effects\n  in HARPS-N line profile measurements on the bright binary GJ 725A+B: Understanding the sources of instrumental systematic noise is a must to\nimprove the design of future spectrographs. In this study, we alternated\nobservations of the well-suited pair of M-stars GJ 725A+B to delve into the\nsub-night HARPS-N response. Besides the possible presence of a low-mass planet\norbiting GJ 725B, our observations reveal changes in the spectral energy\ndistribution (SED) correlated with measurements of the width of the\ninstrumental line profile and, to a lower degree, with the Doppler\nmeasurements. To study the origin of these effects, we searched for\ncorrelations among several quantities defined and measured on the spectra and\non the acquisition images.\n  We find that the changes in apparent SED are very likely related to flux\nlosses at the fibre input. Further tests indicate that such flux losses do not\nseriously affect the shape of the instrumental point spread function of\nHARPS-N, but identify an inefficient fitting of the continuum as the most\nlikely source of the systematic variability observed in the FWHM. This index,\naccounting for the HARPS-N cross-correlation profiles width, is often used to\ndecorrelate Doppler time-series. We show that the Doppler measurement obtained\nby a parametric least-squares fitting of the spectrum accounting for continuum\nvariability is insensitive to changes in the slope of the SED, suggesting that\nforward modeling techniques to measure moments of the line profile are the\noptimal way to achieve higher accuracy. Remaining residual variability at ~1\nm/s suggests that for M-stars Doppler surveys the current noise floor still has\nan instrumental origin.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Statistical Analysis of the Horizontal Divergent Flow in Emerging Solar\n  Active Regions: Solar active regions (ARs) are thought to be formed by magnetic fields from\nthe convection zone. Our flux emergence simulations revealed that a strong\nhorizontal divergent flow (HDF) of unmagnetized plasma appears at the\nphotosphere before the flux begins to emerge. In our earlier study, we analyzed\nHMI data for a single AR and confirmed presence of this precursor plasma flow\nin the actual Sun. In this paper, as an extension of our earlier study, we\nconducted a statistical analysis of the HDFs to further investigate their\ncharacteristics and better determine the properties. From SDO/HMI data, we\npicked up 23 flux emergence events over a period of 14 months, the total flux\nof which ranges from 10^{20} to 10^{22} Mx. Out of 23 selected events, 6 clear\nHDFs were detected by the method we developed in our earlier study, and 7 HDFs\ndetected by visual inspection were added to this statistic analysis. We found\nthat the duration of the HDF is on average 61 minutes and the maximum HDF speed\nis on average 3.1 km s^{-1}. We also estimated the rising speed of the\nsubsurface magnetic flux to be 0.6-1.4 km s^{-1}. These values are highly\nconsistent with our previous one-event analysis as well as our simulation\nresults. The observation results lead us to the conclusion that the HDF is\nrather a common feature in the earliest phase of AR emergence. Moreover, our\nHDF analysis has capability of determining the subsurface properties of\nemerging fields that cannot be directly measured.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The inner environment of protoplanetary disks with near infrared\n  spectro-interferometry: In this paper, I review how optical spectro-interferometry has become a\nparticularly well suited technique to study the close environment of young\nstars, by spatially resolving both their IR continuum and line emission\nregions. I summarize in which ways optical interferometers have brought major\ninsights about our understanding of the inner part of circumstellar disks, a\nregion in which the first stages of planet formation are thought to occur. In\nparticular, I emphasize how new methods are now enabling to probe the hot gas\nemission, in addition to the circumstellar dust.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The rotation rate of solar active and ephemeral regions -- II. Temporal\n  variations of the rotation rates: Systematic studies of the rotation rate of sunspot groups using white-light\nimages yield controversial results on the variations of the rotation rate:\nsunspot groups were found to either accelerate or decelerate systematically.\nThis disagreement might be related to shortcomings of the method used to probe\nthe rotation rate of sunspot groups. In contrast to previous works, in this\nstudy we use magnetic field maps to analyse the variations of the rotation rate\nof active regions. We found that an active region may exhibit either\nacceleration or deceleration during the emergence while the rotation rate\nremains almost unchanged during decay. Hence, we suppose that there is no\nsystematic geometrical inclination to the radial direction of the apex of the\nsubsurface magnetic flux loop forming an active region. A thorough comparison\nof the rotation rate of unipolar and bi/multipolar active regions revealed no\nsignificant changes in the rotation rate of decaying active regions. In\ncontrast to previous works, we presume the rotation rate to keep constant\n(within the expected uncertainties) during the evolution of an active region\nafter emergence.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A new ephemeris and an orbital solution of \u03b5 Aurigae: We collected rich series of RV measurements covering last 110 years and\nphotometric observations from the past 6 primary eclipses, complemented them by\nour new observations and derived a new precise ephemeris and an orbital\nsolution of epsilon Aur.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Flux-Rope Twist in Eruptive Flares and CMEs: due to Zipper and\n  Main-Phase Reconnection: The nature of three-dimensional reconnection when a twisted flux tube erupts\nduring an eruptive flare or coronal mass ejection is considered. The\nreconnection has two phases: first of all, 3D \"zipper reconnection\" propagates\nalong the initial coronal arcade, parallel to the polarity inversion line\n(PIL), then subsequent quasi-2D \"main phase reconnection\" in the low corona\naround a flux rope during its eruption produces coronal loops and chromospheric\nribbons that propagate away from the PIL in a direction normal to it.\n  One scenario starts with a sheared arcade: the zipper reconnection creates a\ntwisted flux rope of roughly one turn ($2\\pi$ radians of twist), and then main\nphase reconnection builds up the bulk of the erupting flux rope with a\nrelatively uniform twist of a few turns. A second scenario starts with a\npre-existing flux rope under the arcade. Here the zipper phase can create a\ncore with many turns that depend on the ratio of the magnetic fluxes in the\nnewly formed flare ribbons and the new flux rope. Main phase reconnection then\nadds a layer of roughly uniform twist to the twisted central core. Both phases\nand scenarios are modeled in a simple way that assumes the initial magnetic\nflux is fragmented along the PIL. The model uses conservation of magnetic\nhelicity and flux, together with equipartition of magnetic helicity, to deduce\nthe twist of the erupting flux rope in terms the geometry of the initial\nconfiguration.\n  Interplanetary observations show some flux ropes have a fairly uniform twist,\nwhich could be produced when the zipper phase and any pre-existing flux rope\npossess small or moderate twist (up to one or two turns). Other interplanetary\nflux ropes have highly twisted cores (up to five turns), which could be\nproduced when there is a pre-existing flux rope and an active zipper phase that\ncreates substantial extra twist.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The nature of Els\u00e4sser variables in compressible MHD: The Els\\\"{a}sser variables are often used in studies of plasma turbulence, in\nhelping differentiate between MHD waves propagating parallel or anti-parallel\nto the main magnetic field. While for pure Alfv\\'en waves in a homogeneous\nplasma the method is strictly valid, we show that compressible, magnetoacoustic\nwaves are in general described by both Els\\\"{a}sser variables. Furthermore, in\na compressible and inhomogeneous plasma, the pure MHD waves (Alfv\\'en, fast and\nslow) are no longer normal modes, but waves become linearly coupled or display\nmixed properties of Alfv\\'en and magnetoacoustic nature. These waves are\nnecessarily described by both Els\\\"{a}sser variables and therefore the\nEls\\\"{a}sser formalism cannot be used to strictly separate parallel and\nanti-parallel propagating waves. Nevertheless, even in an inhomogeneous plasma,\nfor a highly Alfv\\'enic wave the Els\\\"{a}sser variable corresponding to the\npropagation direction appears still dominating. We suggest that for Alfv\\'enic\nwaves, the relative amplitude of Els\\\"{a}sser variables depends on the local\ndegree of inhomogeneity and other plasma and wave properties. This finding has\nimplications for turbulence studies in inhomogeneous and compressible plasmas,\nsuch as the solar corona and solar wind.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Summary of IAU GA SpS5 - I. Obscured and distant clusters: This first part of Special Session 5 explored the current status of\ninfrared-based observations of obscured and distant stellar clusters in the\nMilky Way galaxy. Recent infrared surveys, either serendipitously or using\ntargeted searches, have uncovered a rich population of young and massive\nclusters. However, cluster characterization is more challenging as it must be\nobtained often entirely in the infrared due to high line-of-sight extinction.\nDespite this, much is to be gained through the identification and careful\nanalysis of these clusters, as they allow for the early evolution of massive\nstars to be better constrained. Further, they act as beacons delineating the\nMilky Way's structure and as nearby, resolved analogues to the distant\nunresolved massive clusters studied in distant galaxies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evolutionary models of the optical component of the LMC X-1/Star 32\n  binary system: Calculations carried out to model the evolution of Star 32 under different\nassumptions about the stellar wind mass-loss rate provide robust limits on the\npresent mass of the star. The obtained range is 31 to 35.5 Msun, which is in\nvery good agreement with the orbital solution of Orosz et al., namely 28.3 to\n35.3 Msun. The initial mass of Star 32 had to be in the range 35 to 40 Msun and\nthe present age of the system is 3.7 to 4.0 Myr.",
        "category": "astro-ph_SR"
    },
    {
        "text": "An overall view of temperature oscillations in the solar chromosphere\n  with ALMA: By direct measurements of the gas temperature, the Atacama Large\nMillimeter/sub-millimeter Array (ALMA) has yielded a new diagnostic tool to\nstudy the solar chromosphere. Here we present an overview of the\nbrightness-temperature fluctuations from several high-quality and\nhigh-temporal-resolution (i.e., 1 and 2 sec cadence) time series of images\nobtained during the first two years of solar observations with ALMA, in Band 3\nand Band 6, centred at around 3 mm (100 GHz) and 1.25 mm (239 GHz),\nrespectively. The various datasets represent solar regions with different\nlevels of magnetic flux. We perform Fast Fourier and Lomb-Scargle transforms to\nmeasure both the spatial structuring of dominant frequencies and the average\nglobal frequency distributions of the oscillations (i.e., averaged over the\nentire field of view). We find that the observed frequencies significantly vary\nfrom one dataset to another, which is discussed in terms of the solar regions\ncaptured by the observations (i.e., linked to their underlying magnetic\ntopology). While the presence of enhanced power within the frequency range 3-5\nmHz is found for the most magnetically quiescent datasets, lower frequencies\ndominate when there is significant influence from strong underlying magnetic\nfield concentrations (present inside and/or in the immediate vicinity of the\nobserved field of view). We discuss here a number of reasons which could\npossibly contribute to the power suppression at around 5.5 mHz in the ALMA\nobservations. However, it remains unclear how other chromospheric diagnostics\n(with an exception of Halpha line-core intensity) are unaffected by similar\neffects, i.e., they show very pronounced 3-min oscillations dominating the\ndynamics of the chromosphere, whereas only a very small fraction of all the\npixels in the ten ALMA data sets analysed here show peak power near 5.5 mHz.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Millimeter imaging of MWC 758: probing the disk structure and kinematics: We investigate the structure and kinematics of the circumstellar disk around\nthe Herbig Ae star MWC 758 using high resolution observations of the 12CO (3-2)\nand dust continuum emission at the wavelengths of 0.87 and 3.3 mm. We find that\nthe dust emission peaks at an orbital radius of about 100 AU, while the CO\nintensity has a central peak coincident with the position of the star. The CO\nemission is in agreement with a disk in keplerian rotation around a 2.0 Msun\nstar, confirming that MWC758 is indeed an intermediate mass star. By comparing\nthe observation with theoretical disk models, we derive that the disk surface\ndensity Sigma(r) steeply increases from 40 to 100 AU, and decreases\nexponentially outward. Within 40 AU, the disk has to be optically thin in the\ncontinuum emission at millimeter wavelengths to explain the observed dust\nmorphology, though our observations lack the angular resolution and sensitivity\nrequired to constrain the surface density on these spatial scales. The surface\ndensity distribution in MWC 758 disk is similar to that of ``transition''\ndisks, though no disk clearing has been previously inferred from the analysis\nof the spectral energy distribution (SED). Moreover, the asymmetries observed\nin the dust and CO emission suggest that the disk may be gravitationally\nperturbed by a low mass companion orbiting within a radius of 30 AU. Our\nresults emphasize that SEDs alone do not provide a complete picture of disk\nstructure and that high resolution millimeter-wave images are essential to\nreveal the structure of the cool disk mid plane.",
        "category": "astro-ph_SR"
    },
    {
        "text": "New R Coronae Borealis and DY Persei Candidates in the SMC: We report 3 new R Coronae Borealis and 63 new DY Persei candidates in the\nSmall Magellanic Cloud. Our analysis, based on data published by the OGLE team,\nconsisted in a search for the characteristic drops in brightness that define\nthese classes. All candidates had been previously classified as semi-regular or\nMira variables. We briefly remark upon the possible existence of a \"borderline\"\nDY Per-like star and a \"transitional\" DY Per/RCB star. Follow-up observations\nare needed to conclusively establish the nature of our candidates.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Measurement of the radial velocity of the Sun as a star by means of a\n  reflecting solar system body. The effect of the body rotation: Minor bodies of the solar system can be used to measure the spectrum of the\nSun as a star by observing sunlight reflected by their surfaces. To perform an\naccurate measurement of the radial velocity of the Sun as a star by this\nmethod, it is necessary to take into account the Doppler shifts introduced by\nthe motion of the reflecting body. Here we discuss the effect of its rotation.\nIt gives a vanishing contribution only when the inclinations of the body\nrotation axis to the directions of the Sun and of the Earth observer are the\nsame. When this is not the case, the perturbation of the radial velocity does\nnot vanish and can reach up to about 2.4 m/s for an asteroid such as 2 Pallas\nthat has an inclination of the spin axis to the plane of the ecliptic of about\n30 degrees. We introduce a geometric model to compute the perturbation in the\ncase of a uniformly reflecting body of spherical or triaxial ellipsoidal shape\nand provide general results to easily estimate the magnitude of the effect.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Supergranules as Probes of the Sun's Meridional Circulation: Recent analysis revealed that supergranules (convection cells seen at the\nSun's surface) are advected by the zonal flows at depths equal to the widths of\nthe cells themselves. Here we probe the structure of the meridional circulation\nby cross-correlating maps of the Doppler velocity signal using a series of\nsuccessively longer time lags between maps. We find that the poleward\nmeridional flow decreases in amplitude with time lag and reverses direction to\nbecome an equatorward return flow at time lags > 24 hours. These\ncross-correlation results are dominated by larger and deeper cells at longer\ntime lags. (The smaller cells have shorter lifetimes and do not contribute to\nthe correlated signal at longer time lags.) We determine the characteristic\ncell size associated with each time lag by comparing the equatorial zonal flows\nmeasured at different time lags with the zonal flows associated with different\ncell sizes from a Fourier analysis. This association gives a characteristic\ncell size of ~50 Mm at a 24 hour time lag. This indicates that the poleward\nmeridional flow returns equatorward at depths > 50 Mm -- just below the base of\nthe surface shear layer. A substantial and highly significant equatorward flow\n(4.6 +/- 0.4 m/s) is found at a time lag of 28 hours corresponding to a depth\nof ~70 Mm. This represents one of the first positive detections of the Sun's\nmeridional return flow and illustrates the power of using supergranules to\nprobe the Sun's internal dynamics.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Chemical composition of stars with massive planets: Stellar parameters of 25 planet-hosting stars and abundances of Li, C, O, Na,\nMg, Al, S, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Ni, Zn, Y, Zr, Ba, Ce, Pr, Nd, Sm and\nEu, were studied based on homogeneous high resolution spectra and uniform\ntechniques. The iron abundance [Fe/H] and key elements (Li, C, O, Mg, Si)\nindicative of the planet formation, as well as the dependencies of [El/Fe] on\n$T_{cond}$, were analyzed. The iron abundances determined in our sample stars\nwith detected massive planets range within -0.3<[Fe/H]<0.4. The behaviour of\n[C/Fe], [O/Fe], [Mg/Fe] and [Si/Fe] relative to [Fe/H] is consistent with the\nGalactic Chemical Evolution trends. The mean values of C/O and [C/O] are <C/O>=\n0.48 +/-0.07 and <[C/O]>=-0.07 +/-0.07, which are slightly lower than solar\nones. The Mg/Si ratios range from 0.83 to 0.95 for four stars in our sample and\nfrom 1.0 to 1.86 for the remaining 21 stars. Various slopes of [El/Fe] vs.\nTcond were found. The dependencies of the planetary mass on metallicity, the\nlithium abundance, the C/O and Mg/Si ratios, and also on the [El/Fe]-Tcond\nslopes were considered.",
        "category": "astro-ph_SR"
    },
    {
        "text": "SuperWASP observations of pulsating Am stars: We have studied over 1600 Am stars at a photometric precision of 1 mmag with\nSuperWASP photometric data. Contrary to previous belief, we find that around\n200 Am stars are pulsating delta Sct and gamma Dor stars, with low amplitudes\nthat have been missed in previous, less extensive studies. While the amplitudes\nare generally low, the presence of pulsation in Am stars places a strong\nconstraint on atmospheric convection, and may require the pulsation to be\nlaminar. While some pulsating Am stars have been previously found to be delta\nSct stars, the vast majority of Am stars known to pulsate are presented in this\npaper. They will form the basis of future statistical studies of pulsation in\nthe presence of atomic diffusion.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Towards a consistent model of the hot quadruple system HD 93206 = QZ\n  Carin\u00e6: II. N-body model: HD 93206 is early-type massive stellar system, composed of components\nresolved by direct imaging (Ab, Ad, B, C, D) as well as a compact sub-system\n(Aa1, Aa2, Ac1, Ac2). Its geometry was already determined on the basis of\nextensive photometric, spectroscopic and interferometric observations. However,\nthe fundamental absolute parameters are still not known precisely enough. We\nuse an advanced N-body model to account for all mutual gravitational\nperturbations among the four close components, and all observational data\ntypes, including: astrometry, radial velocities, eclipse timing variations,\nsquared visibilities, closure phases, triple products, normalized spectra, and\nspectral-energy distribution (SED). The respective model has 38 free\nparameters, namely three sets of orbital elements, component masses, and their\nbasic radiative properties ($T$, $\\log g$, $v_{\\rm rot}$). We revised the\nfundamental parameters of QZ Car as follows. For a model with the nominal\nextinction coefficient $R_V \\equiv A_V/E(B-V) = 3.1$, the best-fit masses are\n$m_1 = 26.1\\,M_{\\rm S}$, $m_2 = 32.3\\,M_{\\rm S}$, $m_3 = 70.3\\,M_{\\rm S}$, $m_4\n= 8.8\\,M_{\\rm S}$, with uncertainties of the order of $2\\,M_{\\rm S}$, and the\nsystem distance $d = (2800\\pm 100)\\,{\\rm pc}$. In an alternative model, where\nwe increased the weights of RV and TTV observations and relaxed the SED\nconstraints, because extinction can be anomalous with $R_V \\sim 3.4$, the\ndistance is smaller, $d = (2450\\pm 100)\\,{\\rm pc}$. This would correspond to\nthat of Collinder 228 cluster. Independently, this is confirmed by dereddening\nof the SED, which is only then consistent with the early-type classification\n(O9.7Ib for Aa1, O8III for Ac1). Future modelling should also account for an\naccretion disk around Ac2 component.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Collisional- and photo-excitations of Ca IV including strong 3.2 $\u03bc$m\n  emission line: We report a detailed study of features of electron-impact excitation (EIE) of\nCa IV for the first time using the relativistic Breit-Pauli R-Matrix method\nwith a large close coupling wavefunction expansion of 54 fine structure levels\nbelonging to n=2,3,4 complexes. Our study predicts presence of a strong 3.2\n$\\mu$m emission line in IR. The EIE collision strength ($\\Omega$) shows\nextensive resonances with enhanced background resulting in an effective\ncollision strength ($\\gamma$) of 2.2 at about 10,000 K that increases to 9.66\naround 300,000 K. The present results include collision strength of all 1431\nexcitations among the 54 levels and effective collision strength for a limited\nnumber of transitions of possible interest. We have found extensive resonances\nin the low energy region, convergence of the resonances, and of the partial\nwaves with the 54 levels wavefunction. At higher energy, the collision strength\ndecreases beyond the resonance region for forbidden transitions, is almost\nconstant or decreases slowly for dipole-allowed transitions with low oscillator\nstrengths, and rises with Bethe-Coulomb behavior of ln(E)to almost a plateau\nfor transitions with high f-values.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Bright Spectroscopic Binaries: II. A study of five systems with orbital\n  periods of P < 500 days: We present a detailed analysis of five bright spectroscopic binary systems\n(HD 18665, HD 27131, HD 171852, HD 215550, HD 217427) that have orbital periods\nof P < 500 days. We determined the complete set of orbital parameters using the\ntoolkit RadVel by analyzing the observed radial velocity curves. To study the\nproperties of the five systems, we also analyzed the intermediate resolution\nspectra (R = 20,000) observed with the TIGRE telescope and determined the\nstellar parameters of the primary stars using the toolkit iSpec. With Gaia\nEarly Data Release 3 parallaxes, a correction for interstellar extinction using\nthe 3D dust map, and bolometric corrections, we placed the stars in the\nHertzsprung-Russell diagram and compared the positions with stellar evolution\ntracks calculated with the Eggleton code to determine the masses and ages of\nthe primary stars. They have all evolved to the giant phase. Finally, we were\nable to determine the masses of the secondary stars and to estimate the orbital\ninclinations i of the binary systems.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Temperature and Distance Dependence of Tidal Circularization in Close\n  Binaries: A Catalog of Eclipsing Binaries in the Southern Hemisphere Observed\n  by the TESS Satellite: Tidal forces are important for understanding how close binary stars and\ncompact exoplanetary systems form and evolve. However, tides are difficult to\nmodel and significant uncertainties exist about the strength of tides. Here, we\ninvestigate tidal circularization in close binaries using a large sample of\nwell-characterised eclipsing systems. We searched TESS photometry from the\nsouthern hemisphere for eclipsing binaries. We derive best-fit orbital and\nstellar parameters by jointly modelling light curves and spectral energy\ndistributions. To determine the eccentricity distribution of eclipsing binaries\nover a wide range of stellar temperatures ($3\\,000-50\\,000\\,$K) and orbital\nseparations $a/R_1$ ($2-300$), we combine our newly obtained TESS sample with\neclipsing binaries observed from the ground and by the Kepler mission. We find\na clear dependency of stellar temperature and orbital separation in the\neccentricities of close binaries. We compare our observations with predictions\nof the equilibrium and dynamical tides. We find that while cool binaries agree\nwith the predictions of the equilibrium tide, a large fraction of binaries with\ntemperatures between $6\\,250\\,$K and $10\\,000\\,$K and orbital separations\nbetween $a/R_1 \\sim 4$ and $10$ are found on circular orbits contrary to the\npredictions of the dynamical tide. This suggests that some binaries with\nradiative envelopes may be tidally circularised significantly more efficiently\nthan usually assumed. Our findings on orbital circularization have important\nimplications also in the context of hot Jupiters where tides have been invoked\nto explain the observed difference in the spin-orbit alignment between hot and\ncool host stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Oxygen abundances in G- and F-type stars from HARPS: We present a detailed and uniform study of oxygen abundance from two\ndifferent oxygen lines at 6158$\\AA$ and 6300$\\AA$ in a large sample of\nsolar-type stars. The results are used to check the behaviour of these spectral\nlines as oxygen abundance indicators and to study the evolution of oxygen in\nthick and thin disk populations of the Galaxy. Equivalent width measurements\nwere carried out for the [OI]~6158$\\AA$ and OI~6300$\\AA$ lines. LTE abundances\nwere obtained from these two lines in 610 and 535 stars, respectively. We were\nable to measure oxygen abundance from both indicators in 447 stars, enabling\nus, for the first time, to compare them in a uniform way. Careful error\nanalysis has been performed. We found that oxygen abundances derived from the\n6158$\\AA$ and 6300$\\AA$ lines agree to within 0.1dex in 58\\% of the stars in\nour sample, and this result improves for higher signal-to-noise values. We\nconfirm an oxygen enhancement in stars of the thick disk, as has also been seen\nfor other $\\alpha$-elements. The new oxygen abundances confirm previous\nfindings for a progressive linear rise in the oxygen-to-iron ratio with a slope\nequal to 0.78 from solar metallicity to [Fe/H]$\\sim$-1. However, the slope we\nmeasured is steeper than the one found in previous studies based on the oxygen\ntriplet. Below [Fe/H]=$-$0.6 our stars show [O/Fe] ratios as high as $\\sim$0.8,\nwhich can be interpreted as evidence for oxygen overproduction in the Galactic\nthick disk. These high oxygen abundances do not pose a problem to\nchemodynamical models since there is a range of parameters that can accommodate\nour results.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Connecting the Wilson depression to the magnetic field of sunspots: Context: In sunspots, the geometric height of continuum optical depth unity\nis depressed compared to the quiet Sun. This so-called Wilson depression is\ncaused by the Lorentz force of the strong magnetic field inside the spots.\nHowever, it is not understood in detail yet, how the Wilson depression is\nrelated to the strength and geometry of the magnetic field or to other\nproperties of the sunspot.\n  Aims: We aim to study the dependence of the Wilson depression on the\nproperties of the magnetic field of the sunspots and how exactly the magnetic\nfield contributes to balancing the Wilson depression with respect to the gas\npressure of the surroundings of the spots.\n  Methods: Our study is based on 24 spectropolarimetric scans of 12 individual\nsunspots performed with Hinode. We derived the Wilson depression for each spot\nusing both, a recently developed method that is based on minimizing the\ndivergence of the magnetic field, and an approach developed earlier that\nenforces an equilibrium between the gas pressure and the magnetic pressure\ninside the spot and the gas pressure in the quiet Sun, thus neglecting the\ninfluence of the curvature force. We then performed a statistical analysis by\ncomparing the Wilson depression resulting from the two techniques with each\nother and by relating them to various parameters of the sunspots, such as their\nsize or the strength of the magnetic field.\n  Results: We find that the Wilson depression becomes larger for spots with a\nstronger magnetic field, but not as much as one would expect from the increased\nmagnetic pressure. This suggests that the curvature integral provides an\nimportant contribution to the Wilson depression, particularly for spots with a\nweak magnetic field. Our results indicate that the geometry of the magnetic\nfield in the penumbra is different between spots with different strengths of\nthe average umbral magnetic field.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Low-temperature chemistry between water and hydroxyl radicals: H/D\n  isotopic effects: Sets of systematic laboratory experiments are presented -- combining Ultra\nHigh Vacuum cryogenic and plasma-line deposition techniques -- that allow us to\ncompare H/D isotopic effects in the reaction of H2O (D2O) ice with the hydroxyl\nradical OD (OH). The latter is known to play a key role as intermediate species\nin the solid-state formation of water on icy grains in space. The main finding\nof our work is that the reaction H2O + OD --> OH + HDO occurs and that this may\naffect the HDO/H2O abundances in space. The opposite reaction D2O + OH --> OD +\nHDO is much less effective, and also given the lower D2O abundances in space\nnot expected to be of astronomical relevance. The experimental results are\nextended to the other four possible reactions between hydroxyl and water\nisotopes and are subsequently used as input for Kinetic Monte Carlo\nsimulations. This way we interpret our findings in an astronomical context,\nqualitatively testing the influence of the reaction rates.",
        "category": "astro-ph_SR"
    },
    {
        "text": "LMC Blue Supergiant Stars and the Calibration of the Flux-weighted\n  Gravity--Luminosity Relationship: High quality spectra of 90 blue supergiant stars in the Large Magellanic\nCloud are analyzed with respect to effective temperature, gravity, metallicity,\nreddening, extinction and extinction law. An average metallicity, based on Fe\nand Mg abundances, relative to the Sun of [Z] = -0.35 +/- 0.09 dex is obtained.\nThe reddening distribution peaks at E(B-V) = 0.08 mag, but significantly larger\nvalues are also encountered. A wide distribution of the ratio of extinction to\nreddening is found ranging from Rv = 2 to 6. The results are used to\ninvestigate the blue supergiant relationship between flux-weighted gravity, and\nabsolute bolometric magnitude. The existence of a tight relationship, the FGLR,\nis confirmed. However, in contrast to previous work the observations reveal\nthat the FGLR is divided into two parts with a different slope. For\nflux-weighted gravities larger than 1.30 dex the slope is similar as found in\nprevious work, but the relationship becomes significantly steeper for smaller\nvalues of the flux-weighted gravity. A new calibration of the FGLR for\nextragalactic distance determinations is provided.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The CARMENES search for exoplanets around M dwarfs: Convective shift and\n  starspot constraints from chromatic radial velocities: Context. Variability caused by stellar activity represents a challenge to the\ndiscovery and characterization of terrestrial exoplanets and complicates the\ninterpretation of atmospheric planetary signals.\n  Aims. We aim to use a detailed modeling tool to reproduce the effect of\nactive regions on radial velocity measurements, which aids the identification\nof the key parameters that have an impact on the induced variability.\n  Methods. We analyzed the effect of stellar activity on radial velocities as a\nfunction of wavelength by simulating the impact of the properties of spots,\nshifts induced by convective motions, and rotation. We focused our modeling\neffort on the active star YZ CMi (GJ 285), which was photometrically and\nspectroscopically monitored with CARMENES and the Telescopi Joan Or\\'o.\n  Results. We demonstrate that radial velocity curves at different wavelengths\nyield determinations of key properties of active regions, including spot\nfilling factor, temperature contrast, and location, thus solving the degeneracy\nbetween them. Most notably, our model is also sensitive to convective motions.\nResults indicate a reduced convective shift for M dwarfs when compared to\nsolar-type stars (in agreement with theoretical extrapolations) and points to a\nsmall global convective redshift instead of blueshift.\n  Conclusions. Using a novel approach based on simultaneous chromatic radial\nvelocities and light curves, we can set strong constraints on stellar activity,\nincluding an elusive parameter such as the net convective motion effect.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spectroscopic Observations of Selected Planetary Nebulae from the HASH\n  Database: We conducted research on the classification and physical properties of 10\nobjects from the HASH (Hong Kong/Australian Astronomical Observatory/Strasbourg\nObservatory H-alpha Planetary Nebula (PN)) database with small angular sizes (<\n8\\arcsec) in the northern hemisphere. The sample consisted of 6 Likely PNe, 2\nnew candidates, one emission-line star, and one object of unknown nature. Among\nthem, we observed 4 objects for the first time using the medium-resolution\nTFOSC spectrograph located on the RTT150 cm of the T\\\"UB\\.ITAK National\nObservatory (TUG). To investigate the classification of the observed objects,\nwe utilized the emission line ratios of [O III]/H$_{\\gamma}$, [O\nIII]/H$_{\\beta}$, [N II]/H$_{\\alpha}$ and [S II]/H$_{\\alpha}$ and diagnostic\ndiagrams such as the Sabbadin-Minello-Bianchini (SMB) and\nBaldwin-Phillips-Terlevich (BPT). When considering a broader range of\ndiagnostic criteria compared to those provided in the literature, our analyses\nresulted in the reclassification of 4 objects from Likely PNe to True PNe and\nthe retention of the previous classification for the remaining 6 objects. In\naddition, we obtained various physical conditions such as electron\ntemperatures, electron densities, logarithmic extinction coefficients, and\nexcitation classes for the 10 objects under study. Our analysis revealed that\nthe ionic abundances of the majority of these objects were in agreement with\nGalactic PNe. Our spectral observations have led to the updating of 10 PNe in\nthe HASH database.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Solar extreme events: Solar flares and CMEs have a broad range of magnitudes. This review discusses\nthe possibility of \"extreme events,\" defined as those with magnitudes greater\nthan have been seen in the existing historical record. For most quantitative\nmeasures, this direct information does not extend more than a century and a\nhalf into the recent past. The magnitude distributions (occurrence frequencies)\nof solar events (flares/CMEs) typically decrease with the parameter measured or\ninferred (peak flux, mass, energy etc. Flare radiation fluxes tend to follow a\npower law slightly flatter than $S^{-2}$, where S represents a peak flux; solar\nparticle events (SPEs) follow a still flatter power law up to a limiting\nmagnitude, and then appear to roll over to a steeper distribution, which may\ntake an exponential form or follow a broken power law. This inference comes\nfrom the terrestrial $^{14}$C record and from the depth dependence of various\nradioisotope proxies in the lunar regolith and in meteorites. Recently major\nnew observational results have impacted our use of the relatively limited\nhistorical record in new ways: the detection of actual events in the $^{14}$C\ntree-ring records, and the systematic observations of flares and \"superflares\"\nby the Kepler spacecraft. I discuss how these new findings may affect our\nunderstanding of the distribution function expected for extreme solar events.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Field Blue Stragglers and Related Mass Transfer Issues: This chapter contains my impressions and perspectives about the current state\nof knowledge about field blue stragglers (FBS) stars, drawn from an extensive\nliterature that I searched. I conclude my review of issues that attend FBS and\nmass transfer, by a brief enumeration of a few mildly disquieting observational\nfacts.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The data center for the Spectrometer and Telescope for Imaging X-rays\n  (STIX) onboard Solar Orbiter: The Spectrometer and Telescope for Imaging X-rays (STIX) on board Solar\nOrbiter observes solar X-ray emission in the range of 4 &ndash; 150 keV and\nproduces spectra and images of solar flares over a wide range of flare\nmagnitudes. During nominal operation, STIX continuously generates data. A\nconstant data flow requires fully automated data-processing pipelines to\nprocess and analyze the data, and a data platform to manage, visualize, and\ndistribute the data products to the scientific community. The STIX Data Center\nhas been built to fulfill these needs. In this paper, we outline its main\ncomponents to help the community better understand the tools and data it\nprovides. The STIX Data Center is operated at the University of Applied\nSciences and Arts Northwestern Switzerland (FHNW) and consists of automated\nprocessing pipelines and a data platform. The pipelines process STIX telemetry\ndata, perform common analysis tasks, and generate data products at different\nprocessing levels. They have been designed to operate fully automatically with\nminimal human intervention. The data platform provides web-based user\ninterfaces and application programmable interfaces for searching and\ndownloading STIX data products. The STIX Data Center has been operating\nsuccessfully for more than two years. The platform facilitates instrument\noperations and provides vital support to STIX data users.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Eclipsing Binary Stars: the Royal Road to Stellar Astrophysics: Russell (1948) famously described eclipses as the \"royal road\" to stellar\nastrophysics. From photometric and spectroscopic observations it is possible to\nmeasure the masses and radii (to 1% or better!), and thus surface gravities and\nmean densities, of stars in eclipsing binary systems using nothing more than\ngeometry. Adding an effective temperature subsequently yields luminosity and\nthen distance (or vice versa) to high precision. This wealth of directly\nmeasurable quantities makes eclipsing binaries the primary source of empirical\ninformation on the properties of stars, and therefore a cornerstone of stellar\nastrophysics. In this review paper I summarise the current standing of\neclipsing binary research, present an overview of useful analysis techniques,\nand conclude with a glance to the future.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Identifying Footpoints of Pre-eruptive and Coronal Mass Ejection Flux\n  Ropes with Sunspot Scars: The properties of pre-eruptive structures and coronal mass ejections (CMEs)\nare characterized by those of their footpoints, the latter of which thus\nattract great interest. However, how to identify the footpoints of pre-eruptive\nstructures and how to identify the footpoints with ground-based instruments,\nstill remain elusive. In this work, we study an arc-shaped structure intruding\nin the sunspot umbra. It is located close to the (pre-)eruptive flux rope\nfootpoint and is thus expected to help identify the footpoint. We analyse this\narc-shaped structure, which we name as \"sunspot scar\", in a CME event on 2012\nJuly 12 and in two CME events in observationally-inspired MHD simulations\nperformed by OHM and MPI-AMRVAC. The sunspot scar has a more inclined magnetic\nfield with a weaker vertical component and a stronger horizontal component\nrelative to that in the surrounding umbra and manifests as a light bridge in\nthe white light passband. The hot field lines anchored in the sunspot scar are\nspatially at the transition between the flux rope and the background coronal\nloops, and temporally in the process of the slipping reconnection which builds\nup the flux rope. The sunspot scar and its related light bridge mark the edge\nof the CME flux rope footpoint, and especially, the edge of the pre-eruptive\nflux rope footpoint in the framework of \"pre-eruptive structures being flux\nropes\". Therefore, they provide a new perspective for the identification of\npre-eruptive and CME flux rope footpoints, and also new methods for studying\nthe properties and evolution of pre-eruptive structures and CMEs with\nphotospheric observations only.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Elemental Abundance Analysis of Single and Binary Late-B Stars Using\n  Sub-meter Class Telescopes: HR 342, HR 769, HR 1284, and HR 8705: We test the capabilities of 0.4 m telescopes equipped with an \\'{e}chelle\nspectrograph to derive fundamental parameters and elemental abundances of four\nlate-B type stars: HR 342, HR 769, HR 1284, and HR 8705. The medium resolution\n(R~14000) spectra covering the wavelength range of 4380-7350 {\\AA} of the four\nstars have been obtained using the 40-cm-telescope in Ankara University Kreiken\nObservatory (AUKR). Using spectrum synthesis, we were able to derive the\nabundances of eleven chemical elements. We find that these stars do not show\nremarkable departures from the solar abundances, except for HR 8705 and the\nprimary component of HR 1284, which exhibit slight underabundances of a few\nelements, i.e., O, Mg, Al, Si, and Fe. We also find that HR 1284 is probably a\nnew spectroscopic binary star. In order to model the spectrum of this object,\none of us (TK) has developed a new graphic interface which allows us to\nsynthesize the composite spectrum of binary stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Age-Rotation-Activity Relation: From Myrs to Gyrs: Over the past 40 years, observational surveys have established the existence\nof a tight relationship between a star's age, rotation period, and magnetic\nactivity. This age-rotation-activity relation documents the interplay between a\nstar's magnetic dynamo and angular momentum evolution, and provides a valuable\nage estimator for isolated field stars. While the age-rotation-activity\nrelation has been studied extensively in clusters younger than 500 Myr,\nempirically measured rotation periods are scarce for older ages. Using the\nPalomar Transient Factory (PTF), we have begun a survey of stellar rotation to\nmap out the late-stage evolution of the age-rotation-activity relation: the\nColumbia/Cornell/Caltech PTF (CCCP) survey of open clusters. The first CCCP\ntarget is the nearby ~600 Myr Hyades-analog Praesepe, where PTF has produced\nlight curves spanning more than 3 months and containing >150 measurements for\n~650 cluster members. Analyzing these light curves, we have measured rotation\nperiods for 40 K & M cluster members, filling the gap between the periods\npreviously reported for solar-type Hyads (Radick et al. 1987, Prosser et al.\n1995) and for a handful of low-mass Praesepe members (Scholz et al. 2007). Our\nmeasurements indicate that Praesepe's period-color relation undergoes at\ntransition at a characteristic spectral type of ~M1 --- from a well-defined\nsingular relation at higher mass, to a more scattered distribution of both fast\nand slow-rotators at lower masses. The location of this transition is broadly\nconsistent with expectations based on observations of younger clusters and the\nassumption that stellar-spin down is the dominant mechanism influencing angular\nmomentum evolution at ~600 Myr. In addition to presenting the results of our\nphotometric monitoring of Praesepe, we summarize the status and future of the\nCCCP survey.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spectropolarimetric multi line analysis of stellar magnetic fields: In this paper we study the feasibility of inferring the magnetic field from\npolarized multi-line spectra using two methods: The pseudo line approach and\nThe PCA-ZDI approach. We use multi-line techniques, meaning that all the lines\nof a stellar spectrum contribute to obtain a polarization signature. The use of\nmultiple lines dramatically increases the signal to noise ratio of these\npolarizations signatures. Using one technique, the pseudo-line approach, we\nconstruct the pseudo-line as the mean profile of all the individual lines. The\nother technique, the PCA-ZDI approach proposed recently by Semel et al. (2006)\nfor the detection of polarized signals, combines Principle Components Analysis\n(PCA) and the Zeeman Do ppler Imaging technique (ZDI). This new method has a\nmain advantage: the polarized signature is extracted using cross correlations\nbetween the stellar spectra nd functions containing the polarization properties\nof each line. These functions are the principal components of a database of\nsynthetic spectra. The synthesis of the spectra of the database are obtained\nusing the radiative transfer equations in LTE. The profiles built with the\nPCA-ZDI technique are denominated Multi-Zeeman-Signatures. The construction of\nthe pseudo line as well as the Multi-Zeeman-Signatures is a powerful tool in\nthe study of stellar and solar magnetic fields. The information of the physical\nparameters that governs the line formation is contained in the final polarized\nprofiles. In particular, using inversion codes, we have shown that the magnetic\nfield vector can be properly inferred with both approaches despite the magnetic\nfield regime.",
        "category": "astro-ph_SR"
    },
    {
        "text": "HST hot-Jupiter transmission spectral survey: Haze in the atmosphere of\n  WASP-6b: We report Hubble Space Telescope (HST) optical to near-infrared transmission\nspectroscopy of the hot Jupiter WASP-6b, measured with the Space Telescope\nImaging Spectrograph (STIS) and Spitzer's InfraRed Array Camera (IRAC). The\nresulting spectrum covers the range $0.29-4.5\\,\\mu$m. We find evidence for\nmodest stellar activity of WASP-6b and take it into account in the transmission\nspectrum. The overall main characteristic of the spectrum is an increasing\nradius as a function of decreasing wavelength corresponding to a change of\n$\\Delta (R_p/R_{\\ast})=0.0071$ from 0.33 to $4.5\\,\\mu$m. The spectrum suggests\nan effective extinction cross-section with a power law of index consistent with\nRayleigh scattering, with temperatures of $973\\pm144$ K at the planetary\nterminator. We compare the transmission spectrum with hot-Jupiter atmospheric\nmodels including condensate-free and aerosol-dominated models incorporating Mie\ntheory. While none of the clear-atmosphere models is found to be in good\nagreement with the data, we find that the complete spectrum can be described by\nmodels that include significant opacity from aerosols including Fe-poor\nMg$_2$SiO$_4$, MgSiO$_3$, KCl and Na$_2$S dust condensates. WASP-6b is the\nsecond planet after HD189733b which has equilibrium temperatures near\n$\\sim1200$ K and shows prominent atmospheric scattering in the optical.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Binary Central Stars of Planetary Nebulae Identified With Kepler/K2: We present the identification of 34 likely binary central stars (CSs) of\nplanetary nebulae (PNe) from {\\it Kepler/K2} data, seven of which show\neclipses. Of these, 29 are new discoveries. Two additional CSs with more\ncomplicated variability are also presented. We examined the light curves of all\n`possible', `likely' and `true' PNe in every {\\it Kepler/K2} campaign (0\nthrough 19) to identify CS variability that may indicate a binary CS. For\nCampaigns 0, 2, 7, 15, and 16 we find 6 likely or confirmed variables among 21\nPNe. Our primary effort, though, was focused on Campaign 11 which targeted a\nGalactic bulge field containing approximately 183 PNe, in which we identified\n30 candidate variable CSs. The periods of these variables range from 2.3~h to\n30~d, and based on our analysis, most are likely to be close binary star\nsystems. We present periods and preliminary classifications (eclipsing, double\ndegenerate, or irradiated systems) for the likely binaries based on light curve\nshape. From our total sample of 204 target PNe, with a correction for\nincompleteness due to magnitude limits, we calculate a binary fraction of PN\ncentral stars to be 20.7 percent for all the observed PNe, or 23.5 percent if\nwe limit our sample only to `true' PNe. However these fractions are almost\ncertainly lower limits due to the large angular size of the \\emph{Kepler}\npixels, which leads to reduced sensitivity in detecting variability, primarily\nas a result of dilution and noise from the nebula and neighbouring stars. We\ndiscuss the binary population of CSs based on these results as part of the\ntotal known sample of close binary CSs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "PHL 417: a zirconium-rich pulsating hot subdwarf (V366 Aquarid)\n  discovered in K2 data: The Kepler spacecraft observed the hot subdwarf star PHL 417 during its\nextended K2 mission, and the high-precision photometric lightcurve reveals the\npresence of 17 pulsation modes with periods between 38 and 105 minutes. From\nfollow-up ground-based spectroscopy we find that the object has a relatively\nhigh temperature of 35 600 K, a surface gravity of $\\log g / {\\rm\ncm\\,s^{-2}}\\,=\\,5.75$ and a super-solar helium abundance. Remarkably, it also\nshows strong zirconium lines corresponding to an apparent +3.9 dex\noverabundance compared with the Sun. These properties clearly identify this\nobject as the third member of the rare group of pulsating heavy-metal stars,\nthe V366 Aquarii pulsators. These stars are intriguing in that the pulsations\nare inconsistent with the standard models for pulsations in hot subdwarfs,\nwhich predicts that they should display short-period pulsations rather than the\nobserved longer periods. We perform a stability analysis of the pulsation modes\nbased on data from two campaigns with K2. The highest amplitude mode is found\nto be stable with a period drift, $\\dot{P}$, of less than $1.1\\cdot10^{-9}$\ns/s. This result rules out pulsations driven during the rapid stages of helium\nflash ignition.",
        "category": "astro-ph_SR"
    },
    {
        "text": "New hydrodynamic solutions for line-driven winds of hot massive stars\n  using Lambert $W$-function: Hot massive stars present strong stellar winds that are driven by absorption,\nscattering and re\\-emission of photons by the ions of the atmosphere\n(\\textit{line-driven winds}). A better comprehension of this phenomenon, and a\nmore accurate calculation of hydrodynamics and radiative acceleration is\nrequired to reduce the number of free parameters in spectral fitting, to\ndetermine accurate wind parameters such as mass-loss rates and velocity\nprofiles.\n  We use the non-LTE model-atmosphere code CMFGEN to numerically solve the\nradiative transfer equation in the stellar atmosphere and to calculate the\nradiative acceleration $g_\\text{rad}(r)$. Under the assumption that the\nradiative acceleration depends only on the radial coordinate, we solve\nanalytically the equation of motion by means of the Lambert $W$-function. An\niterative procedure between the solution of the radiative transfer and the\nequation of motion is executed in order to obtain a final self-consistent\nvelocity field that is no longer based on any $\\beta$-law.\n  We apply the Lambert-procedure to three O supergiant stars ($\\zeta$-Puppis,\nHD~165763 and $\\alpha$-Cam) and discuss the Lambert-solutions for the velocity\nprofiles. It is found that, even without recalculation of the mass-loss rate,\nthe Lambert-procedure allows the calculation of consistent velocity profiles\nthat reduce the number of free parameters when a spectral fitting using CMFGEN\nis performed. Synthetic spectra calculated from our Lambert-solutions show\nsignificant differences compared to the initial $\\beta$-law CMFGEN models. The\nresults indicate the importance of consistent velocity profile calculation in\nthe CMFGEN code and its usage in a fitting procedure and interpretation of\nobserved spectra.",
        "category": "astro-ph_SR"
    },
    {
        "text": "V5856 Sagittarii/2016: Broad Multi-Epoch Spectral Coverage of a\n  Sustained High Luminosity Nova: Nova V5856 Sagittarii is unique for having remained more than nine magnitudes\nabove its pre-outburst brightness for more than six years. Extensive visible\nand IR spectra from the time of outburst to the present epoch reveal separate\nemitting regions with distinct spectral characteristics. Permitted emission\nlines have both broad and narrow components, whereas the forbidden line\nprofiles are almost entirely broad. The permitted line components frequently\ndisplay P Cygni profiles indicating high optical depth, whereas the broad\ncomponents do not show detectable absorption. The densities and velocities\ndeduced from the spectra, including differences in the O I 7773 and 8446 lines,\nare not consistent with an on-going wind. Instead, the prolonged high\nluminosity and spectral characteristics are indicative of a post-outburst\ncommon envelope that enshrouds the binary, and is likely the primary source of\nthe visible and IR emission.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the variation of the scaling exponent of the flare fluence with\n  temperature: Solar flares result in an increase of the solar irradiance at all\nwavelengths. While the distribution of the flare fluence observed in coronal\nemission has been widely studied and found to scale as f(E) ~ E^{-\\alpha}, with\n\\alpha slightly below 2, the distribution of the flare fluence in chromospheric\nlines is poorly known. We used the solar irradiance measurements observed by\nthe SDO/EVE instrument at a 10s-cadence to investigate if there is a dependency\nof the scaling exponent on the formation region of the lines (or temperature).\nWe analyzed all flares above the C1 level since the start of the EVE\nobservation (May 2010) to determine the flare fluence distribution in 16 lines\ncovering a large range of temperature, several of which were not studied\nbefore. Our results show a small downward trend with the temperature of the\nscaling exponent of the PDF, going from above 2 at lower temperature (a few\n10^4 K) to about1.8 for hot coronal emission (several 10^6 K). However, because\ncolder lines also have smaller contrast, we could not exclude that this\nbehavior is caused by including more noise for smaller flare for these lines.\nWe discuss the method and its limits and tentatively associate this possible\ntrend to the different mechanisms responsible for the heating of the\nchromosphere and corona during flares.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Radiation Magnetohydrodynamics Simulation of Proto-Stellar Collapse:\n  Two-Component Molecular Outflow: We perform a three-dimensional nested-grid radiation magneto-hydrodynamics\n(RMHD) simulation with self-gravity to study the early phase of the low-mass\nstar formation process from a rotating molecular cloud core to a first\nadiabatic core just before the second collapse begins. Radiation transfer is\nhandled with the flux-limited diffusion approximation, operator-splitting and\nimplicit time-integrator. In the RMHD simulation, the outer region of the first\ncore attains a higher entropy and the size of first core is larger than that in\nthe magnetohydrodynamics simulations with the barotropic approximation. Bipolar\nmolecular outflow consisting of two components is driven by magnetic Lorentz\nforce via different mechanisms, and shock heating by the outflow is observed.\nUsing the RMHD simulation we can predict and interpret the observed properties\nof star-forming clouds, first cores and outflows with millimeter/submillimeter\nradio interferometers, especially the Atacama Large Millimeter/submillimeter\nArray (ALMA).",
        "category": "astro-ph_SR"
    },
    {
        "text": "Eleven Exoplanet Host Star Angular Diameters from the CHARA Array: We directly measured the angular diameters for 11 exoplanet host stars using\nGeorgia State University's CHARA Array interferometer and calculated their\nlinear radii and effective temperatures. The sample tends towards evolving or\nevolved stars and includes one dwarf, four subgiants, and six giants. We then\nestimated masses and ages for the stars using our effective temperatures\ncombined with metallicity measurements from the literature.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Heating and Pulsations of V386 Serpentis after its 2019 Dwarf Nova\n  Outburst: Following the pulsation spectrum of a white dwarf through the heating and\ncooling involved in a dwarf nova outburst cycle provides a unique view of the\nchanges to convective driving that take place on timescales of months versus\nmillenia for non-accreting white dwarfs. In 2019 January the dwarf nova V386\nSer (one of a small number containing an accreting, pulsating white dwarf),\nunderwent a large amplitude outburst. Hubble Space Telescope ultraviolet\nspectra were obtained 7 and 13 months after outburst along with optical\nground-based photometry during this interval and high-speed photometry at 5.5\nand 17 months after outburst. The resulting spectral and pulsational analysis\nshows a cooling of the white dwarf from 21,020 K to 18,750 K (with a gravity\nlog(g) = 8.1) between the two UV observations, along with the presence of\nstrong pulsations evident in both UV and optical at a much shorter period after\noutburst than at quiescence. The pulsation periods consistently lengthened\nduring the year following outburst, in agreement with pulsation theory.\nHowever, it remains to be seen if the behavior at longer times past outburst\nwill mimic the unusual non-monotonic cooling and long periods evident in the\nsimilar system GW Lib.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Chromospheric cannonballs on the Sun: In the highly dynamic chromosphere, there exist many kinds of small-scale\nactivities, such as spicules, surges, and Ellerman bombs. Here, we report the\ndiscovery of a new phenomenon in the chromosphere observed with the New Vacuum\nSolar Telescope at the Fuxian Solar Observatory. In the high tempo-spatial\nresolution H$\\alpha$ images, some dark or bright structures are found to fly\nalong the curved trajectory, looking like cannonballs. Their average size,\nmass, and velocity are about 1.5 $\\times$ 10$^{9}$ km$^{3}$, 1.5 $\\times$\n10$^{8}$ kg, and 56 km s$^{-1}$, respectively. In the simultaneous\n(extreme-)ultraviolet images obtained by the Solar Dynamics Observatory, these\ncannonballs appear as brighter features compared to the surrounding area,\nimplying that there exists some kind of heating during this process. The\nphotospheric magnetograms show the magnetic flux emergence and interaction with\nthe pre-existing fields. These observations reveal that the cannonballs are\nchromospheric material blobs launched due to the magnetic reconnection between\nemerging magnetic flux and the pre-existing loops.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Prandtl number dependence of compressible convection: Flow statistics\n  and convective energy transport: (abridged) Context: The ratio of kinematic viscosity to thermal diffusivity,\nthe Prandtl number, is much smaller than unity in stellar convection zones.\nAims: To study the statistics of convective flows and energy transport as\nfunctions of the Prandtl number. Methods: Three-dimensional numerical\nsimulations convection in Cartesian geometry are used. The convection zone (CZ)\nis embedded between two stably stratified layers. Statistics and transport\nproperties of up- and downflows are studied separately. Results: The rms\nvelocity increases with decreasing Prandtl number. At the same time the filling\nfactor of downflows decreases and leads to stronger downflows at lower Prandtl\nnumbers, and to a strong dependence of overshooting on the Prandtl number.\nVelocity power spectra do not show marked changes as a function of Prandtl\nnumber. At the highest Reynolds numbers the velocity power spectra are\ncompatible with the Bolgiano-Obukhov $k^{-11/5}$ scaling. The horizontally\naveraged convected energy flux ($\\overline{F}_{\\rm conv}$) is independent of\nthe Prandtl number within the CZ. However, the upflows (downflows) are the\ndominant contribution to the convected flux at low (high) Prandtl number. These\nresults are similar to those from Rayleigh-Ben\\'ard convection in the low\nPrandtl number regime where convection is vigorously turbulent but inefficient\nat transporting energy. Conclusions: The current results indicate a strong\ndependence of convective overshooting and energy flux on the Prandtl number.\nNumerical simulations of astrophysical convection often use Prandtl number of\nunity. The current results suggest that this can lead to misleading results and\nthat the astrophysically relevant low Prandtl number regime is qualitatively\ndifferent from the parameters regimes explored in typical simulations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The shape of sunspots and solar activity cycles: The paper presents the results of the analysis of the geometric\ncharacteristics of sunspots for the period of 19-24 cycles of activity. The\nshape of sunspots was studied on the basis of the method of normalization of\nimages of sunspots to study the average profile of the spot. The deviation of\nthe shape of sunspots from the axisymmetric configuration is investigated. It\nwas found that the spots, as a rule, have an ellipsoid shape, and the major\naxis of the ellipse has a predominant inclination to the equator, opposite in\nthe Northern and Southern hemispheres. The angle of inclination of the sunspot\naxis corresponds to the angle of inclination of the bipoles in the activity\ncycles. The relationship between the shape of sunspots in the current cycle and\nthe amplitude of the next cycle of activity is found. The greater the\nelongation along the longitude of the current cycle of spots, the higher the\nnext cycle of activity will be.",
        "category": "astro-ph_SR"
    },
    {
        "text": "ALMA Observations of the Water Fountain Pre-Planetary Nebula IRAS\n  16342-3814: High-velocity bipolar jets and an Expanding Torus: We have mapped 12CO J=3-2 and other molecular lines from the \"water-fountain\"\nbipolar pre-planetary nebula (PPN) IRAS 16342-3814 with ~0.\"35 resolution using\nALMA. We find (i) two very high-speed knotty, jet-like molecular outflows, (ii)\na central high-density (> few x 10^6 cm^{-3}), expanding torus of diameter 1300\nAU, and (iii) the circumstellar envelope of the progenitor AGB, generated by a\nsudden, very large increase in the mass-loss rate to >3.5 x 10^{-4} Msun/yr in\nthe past ~455 yr. Strong continuum emission at 0.89 mm from a central source\n(690 mJy), if due to thermally-emitting dust, implies a substantial mass (0.017\nMsun) of very large (~mm-sized) grains. The measured expansion ages of the\nabove structural components imply that the torus (age~160 yr) and the younger\nhigh-velocity outflow (age~110 yr) were formed soon after the sharp increase in\nthe AGB mass-loss rate. Assuming a binary model for the jets in IRAS 16342, the\nhigh momentum rate for the dominant jet-outflow in IRAS 16342 implies a high\nminimum accretion rate, ruling out standard Bondi-Hoyle-Lyttleton wind\naccretion and wind Roche lobe overflow (RLOF) models with white-dwarf or\nmain-sequence companions. Most likely, enhanced RLOF from the primary or\naccretion modes operating within common envelope evolution are needed.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The orbital periods of subdwarf B binaries produced by the first stable\n  Roche overflow channel: Long-orbital-period subdwarf B (sdB) stars with main-sequence companions are\nbelieved to be the product of stable Roche Lobe overflow (RLOF), a scenario\nchallenged by recent observations. Here we represent the results of a\nsystematic study of the orbital-period distribution of sdB binaries in this\nchannel using detailed binary evolution calculations. We show that the observed\norbital-period distribution of long-period sdB binaries can be well explained\nby this scenario. Furthermore, we find that, if the progenitors of the sdB\nstars have initial masses below the helium flash mass, the sdB binaries\nproduced from stable RLOF follow a unique mass -- orbital period relation for a\ngiven metallicity $Z$; increasing the orbital period from $\\sim 400$ to $\\sim\n1100$\\,d corresponds to increasing the mass of the sdB star from $\\sim 0.40$ to\n$\\sim 0.49\\,M_\\odot$ for $Z=0.02$. We suggest that the longest sdB binaries\n(with orbital period $> 1100$\\,d) could be the result of atmospheric RLOF. The\nmass -- orbital period relation can be tested observationally if the mass of\nthe sdB star can be determined precisely, e.g.\\ from asteroseismology. Using\nthis relation, we revise the orbital period distribution of sdB binaries\nproduced by the first stable RLOF channel for the best fitting model of Han et\nal (2003), and show that the orbital period has a peak around 830\\,d.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the maximum black hole mass at solar metallicity: In high metallicity environments the mass that black holes (BHs) can reach\njust after core-collapse widely depends on how much mass their progenitor stars\nlose via winds. On one hand new theoretical and observational insights suggest\nthat early-stage winds should be weaker than what many canonical models\nprescribe. On the other hand the proximity to the Eddington limit should affect\nthe formation of optically thick envelopes already during the earliest stages\nof stars with initial masses $M_{\\rm ZAMS}\\gtrsim 100$ M$_\\odot$, hence\nresulting in higher mass-loss rates during the main sequence. We use the\nevolutionary codes MESA and Genec to calculate a suite of tracks for massive\nstars at solar metallicity Z$_\\odot=0.014$ which incorporate these changes in\nour wind mass loss prescription. In our calculations we employ moderate\nrotation, high overshooting and magnetic angular momentum transport. We find a\nmaximum BH mass $M_{\\rm BH, max}=28.3$ M$_\\odot$ at Z$_\\odot$. The most massive\nBHs are predicted to form from stars with $M_{\\rm ZAMS}\\gtrsim 250$ M$_\\odot$,\nwith the BH mass directly proportional to its progenitor's $M_{\\rm ZAMS}$. We\nalso find in our models that at Z$_\\odot$ almost any BH progenitor naturally\nevolves into a Wolf-Rayet star due to the combined effect of internal mixing\nand wind mass loss. These results are considerably different from most recent\nstudies regarding the final mass of stars before their collapse into BHs. While\nwe acknowledge the inherent uncertainties in stellar evolution modelling, our\nstudy underscores the importance of employing the most up-to-date physics in BH\nmass predictions.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Quasilinear Approach of the Whistler Heat-Flux Instability in the Solar\n  Wind: The hot beaming (or strahl) electrons responsible for the main electron\nheat-flux in the solar wind are believed to be self-regulated by the\nelectromagnetic beaming instabilities, also known as the heat-flux\ninstabilities. Here we report the first quasi-linear theoretical approach of\nthe whistler unstable branch able to characterize the long-term saturation of\nthe instability as well as the relaxation of the electron velocity\ndistributions. The instability saturation is not solely determined by the drift\nvelocities, which undergo only a minor relaxation, but mainly from a concurrent\ninteraction of electrons with whistlers that induces (opposite) temperature\nanisotropies of the core and beam populations and reduces the effective\nanisotropy. These results might be able to (i) explain the low intensity of the\nwhistler heat-flux fluctuations in the solar wind (although other explanations\nremain possible and need further investigation), and (ii) confirm a reduced\neffectiveness of these fluctuations in the relaxation and isotropization of the\nelectron strahl and in the regulation of the electron heat-flux.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The EBLM Project I-Physical and orbital parameters, including spin-orbit\n  angles, of two low-mass eclipsing binaries on opposite sides of the Brown\n  Dwarf limit: This paper introduces a series of papers aiming to study the dozens of low\nmass eclipsing binaries (EBLM), with F, G, K primaries, that have been\ndiscovered in the course of the WASP survey. Our objects are mostly single-line\nbinaries whose eclipses have been detected by WASP and were initially followed\nup as potential planetary transit candidates. These have bright primaries,\nwhich facilitates spectroscopic observations during transit and allows the\nstudy of the spin-orbit distribution of F, G, K+M eclipsing binaries through\nthe Rossiter-McLaughlin effect. Here we report on the spin-orbit angle of\nWASP-30b, a transiting brown dwarf, and improve its orbital parameters. We also\npresent the mass, radius, spin-orbit angle and orbital parameters of a new\neclipsing binary, J1219-39b (1SWAPJ121921.03-395125.6, TYC 7760-484-1), which,\nwith a mass of 95 +/- 2 Mjup, is close to the limit between brown dwarfs and\nstars. We find that both objects orbit in planes that appear aligned with their\nprimaries' equatorial planes. Neither primaries are synchronous. J1219-39b has\na modestly eccentric orbit and is in agreement with the theoretical\nmass--radius relationship, whereas WASP-30b lies above it.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Very Long Baseline Interferometry imaging of the advancing ejecta in the\n  first gamma-ray nova V407 Cyg: In 2010/3, the Large Area Telescope on board Fermi revealed a transient\ngamma-ray source, positionally coincident with the optical nova in the\nsymbiotic binary, V407Cyg. This event marked the first discovery of gamma-ray\nemission from a nova. We aimed to obtain resolved radio imaging of the material\ninvolved in the nova event; to determine the ejecta geometry and advance\nvelocity directly in the image plane; to constrain the physical conditions of\nthe system. We observed the source with the EVN and the VLBA over 16 epochs,\nbetween 20 days and 6 months after the optical discovery. The source is\ninitially very dim but it later shows a substantial increase in brightness and\na resolved shell-like structure 40 to 90 days after the optical event. The\nshell has a projected elliptical shape and is asymmetric in brightness and\nspectral index, being brighter and characterised by a rising spectrum at the\nS-E edge. We determine a projected velocity of ~3500 km/s in the initial phase,\nand ~2100 km/s between day 20 and 91. We also found an emitting feature about\n350 mas (940 AU) to the N-W, advancing at a projected velocity of ~700 km/s\nalong the polar axis of the binary. The total flux density in the VLBI images\nis significantly lower than that previously reported at similar epochs and over\nmuch wider angular scales with the VLA. Optical spectra demonstrated that in\n2010 we were viewing V407Cyg along the equatorial plane and from behind the\nMira. Our radio observations image the bipolar flow of the ejecta perpendicular\nto the orbital plane, where deceleration is much lower than through the\nequatorial plane probed by the truncated profile of optical emission lines. The\nseparated polar knot at 350 mas and the bipolar flow strictly resemble the\nsimilar arrangement seen in Hen 2-104. The observed ~700 km/s expansion\nconstrains the launch-date of the polar knot around 2004. [Abridged]",
        "category": "astro-ph_SR"
    },
    {
        "text": "Dissipation of magnetic fields in star-forming clouds with different\n  metallicities: We study dissipation process of magnetic fields in the metallicity range $0-1\nZ_{\\odot}$ for contracting prestellar cloud cores. By solving non-equilibrium\nchemistry for important charged species including charged grains, we evaluate\nthe drift velocity of the magnetic-field lines with respect to the gas. We find\nthat the magnetic flux dissipates in the density range $10^{12}{\\rm cm^{-3}}\n\\lesssim n_{\\rm H} \\lesssim 10^{17}{\\rm cm^{-3}}$ for the solar-metallicity\ncase at the scale of the core, which is assumed to be the Jeans scale. The\ndissipation density range becomes narrower for lower metallicity. The magnetic\nfield is always frozen to the gas below metallicity $\\lesssim\n10^{-7}-10^{-6}Z_\\odot$, depending on the ionization rate by cosmic rays and/or\nradioactivity. With the same metallicity, the dissipation density range becomes\nwider for lower ionization rate. The presence of such a dissipative regime is\nexpected to cause various dynamical phenomena in protostellar evolution such as\nthe suppression of jet/outflow launching and fragmentation of the circumstellar\ndisks depending on the metallicity.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The effect of the Solar wind on low-frequency observations of pulsars: We operate the six German stations of the LOw Frequency ARray as standalone\ntelescopes to observe more than 100 pulsars every week. To date, we have\ncollected almost four years of high-quality data at an unprecedented weekly\ncadence. This allows us to perform a wide variety of analyses aimed at\ncharacterising the magnetoionic plasma crossed by pulsar radiation. In\nparticular, our studies are focused on electron density variations in the\ninterstellar and interplanetary media, the Galactic and interplanetary magnetic\nfield, scintillation, and extreme scattering events. Here we report the first\nresults from our Solar wind monitoring campaign.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The chemical signatures of planetary engulfment events in binary systems: Planetary engulfment events involve the chemical assimilation of a planet\ninto a star's external layer. This can cause a change in the chemical pattern\nof the stellar atmosphere in a way that mirrors the composition of the rocky\nobject engulfed, with the refractory elements being more abundant than the\nvolatiles. Due to these stellar chemical changes, planetary engulfment events\ncan render the process of chemical tagging potentially inaccurate. A\nline-by-line differential analysis of twin stars in wide binary systems allows\nus to test the chemical homogeneity of these associations with typical\nindividual stellar Fe I uncertainties of 0.01 dex and eventually unveil\nchemical anomalies that could be attributed to planetary engulfment events. Out\nof the 14 systems analysed here, we report the discovery of the most chemically\ninhomogeneous system to date (HIP34407/HIP34426). The median difference in\nabundances of refractory elements within the pair is 0.19 dex and the trend\nbetween the differential abundances and condensation temperature suggests that\nthe anomaly is likely due to a planetary engulfment event. Within our sample,\nfive other chemically anomalous systems are found.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Multiwavelength observation of a large-scale flux rope eruption above\n  kinked mini-filament: We analyse multiwavelength observations of a western limb flare (C3.9)\noccurred in AR NOAA 111465 on 30 April 2012. The high resolution images\nrecorded by SDO/AIA 304, 1600 \\AA\\ and Hinode/SOT H$\\alpha$ show the activation\nof a mini-filament (rising speed$\\sim$40 km s$^{-1}$) associated with kink\ninstability and the onset of a C-class flare near the southern leg of the\nfilament. The first magnetic reconnection occurred at one of the footpoints of\nthe filament causing the breaking of its southern leg. The filament shows\nunwinding motion of the northern leg and apex in the counterclockwise direction\nand failed to erupt. A flux-rope (visible only in hot channels, i.e., AIA 131\nand 94 \\AA\\ channels and Hinode/SXT) structure was appeared along the neutral\nline during the second magnetic reconnection taking place above the kinked\nfilament. Formation of the RHESSI hard X-ray source (12-25 keV) above the\nkinked filament and simultaneous appearance of the hot 131 \\AA\\ loops\nassociated with photospheric brightenings (AIA 1700 \\AA) suggest the particle\nacceleration along these loops from the top of the filament. In addition, EUV\ndisturbances/waves observed above the filament in 171 \\AA\\ also show a close\nassociation with magnetic reconnection. The flux rope rises slowly ($\\sim$100\nkm s$^{-1}$) producing a rather big twisted structure possibly by reconnection\nwith the surrounding sheared magnetic fields within $\\sim$15-20 minutes, and\nshowed an impulsive acceleration reaching a height of about 80--100 Mm. AIA 171\nand SWAP 174 \\AA\\ images reveal a cool compression front (or CME frontal loop)\nsurrounding the hot flux rope structure.",
        "category": "astro-ph_SR"
    },
    {
        "text": "13C17O suggests gravitational instability in the HL Tau disc: We present the first detection of the 13C17O J=3-2 transition toward the HL\nTau protoplanetary disc. We find significantly more gas mass (at least a factor\nof ten higher) than has been previously reported using C18O emission. This\nbrings the observed total disc mass to 0.2 M, which we consider to be a\nconservative lower limit. Our analysis of the Toomre Q profile suggests that\nthis brings the disc into the regime of gravitational instability. The radial\nregion of instability (50-110 au) coincides with the location of a proposed\nplanet-carved gap in the dust disc and a spiral in the gas. We, therefore,\npropose that if the origin of the gap is confirmed to be due to a forming giant\nplanet, then it is likely to have formed via the gravitational fragmentation of\nthe protoplanetary disc.",
        "category": "astro-ph_SR"
    },
    {
        "text": "High-precision abundances of Sc, Mn, Cu, and Ba in solar twins. Trends\n  of element ratios with stellar age: A previous study of correlations between element abundance ratios, [X/Fe],\nand ages of solar twin stars is extended to include Sc, Mn, Cu, and Ba. HARPS\nspectra with S/N > 600 are used to derive very precise (+/- 0.01 dex)\ndifferential abundances, and stellar ages with internal errors less than 1 Gyr\nare obtained by interpolation in the logg - Teff diagram between isochrones\ncalculated with the Aarhus Stellar Evolution Code. For stars younger than 6\nGyr, [X/Fe] is tightly correlated with stellar age for all elements. For ages\nbetween 6 and 9 Gyr, the [X/Fe] - age correlations break down and the stars\nsplit up into two groups having respectively high and low [X/Fe] for the odd-Z\nelements. It is concluded that while stars in the solar neighborhood younger\nthan about 6 Gyr were formed from interstellar gas with a smooth chemical\nevolution, older stars have originated from regions enriched by supernovae with\ndifferent neutron excesses. Furthermore, the correlations between abundance\nratios and stellar age suggest that: i) Sc is made in Type II supernovae along\nwith the alpha-capture elements, ii) the Type II to Ia SNe yield ratio is about\nthe same for Mn and Fe, iii) Cu is mainly made by the weak s-process in massive\nstars, iv) the Ba/Y yield ratio for AGB stars increases with decreasing stellar\nmass, v) [Y/Mg] and [Y/Al] can be used as \"chemical clocks\" when determining\nages of solar metallicity stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Trends and Characteristics of High-Frequency Type II Bursts Detected by\n  CALLISTO Spectrometers: Solar radio type II bursts serve as early indicators of incoming\ngeo-effective space weather events such as coronal mass ejections (CMEs). In\norder to investigate the origin of high-frequency type II bursts (HF type II\nbursts), we have identified 51 of them (among 180 type II bursts from SWPC\nreports) that are observed by ground-based Compound Astronomical Low-cost\nLow-frequency Instrument for Spectroscopy and Transportable Observatory\n(CALLISTO) spectrometers and whose upper-frequency cutoff (of either\nfundamental or harmonic emission) lies in between 150 MHz-450 MHz during\n2010-2019. We found that 60% of HF type II bursts, whose upper-frequency cutoff\n$\\geq$ 300 MHz originate from the western longitudes. Further, our study finds\na good correlation $\\sim $ 0.73 between the average shock speed derived from\nthe radio dynamic spectra and the corresponding speed from CME data. Also, we\nfound that analyzed HF type II bursts are associated with wide and fast CMEs\nlocated near the solar disk. In addition, we have analyzed the spatio-temporal\ncharacteristics of two of these high-frequency type II bursts and compared the\nderived from radio observations with those derived from multi-spacecraft CME\nobservations from SOHO/LASCO and STEREO coronagraphs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the Variation of Fourier Parameters for Galactic and LMC Cepheids at\n  Optical, Near-Infrared and Mid-Infrared Wavelengths: We present a light curve analysis of fundamental-mode Galactic and Large\nMagellanic Cloud (LMC) Cepheids based on the Fourier decomposition technique.\nWe have compiled light curve data for Galactic and LMC Cepheids in optical\n({\\it VI}), near-infrared ({\\it JHK}$_s$) and mid-infrared (3.6 $\\&$\n4.5-$\\mu$m) bands from the literature and determined the variation of their\nFourier parameters as a function of period and wavelength. We observed a\ndecrease in Fourier amplitude parameters and an increase in Fourier phase\nparameters with increasing wavelengths at a given period. We also found a\ndecrease in the skewness and acuteness parameters as a function of wavelength\nat a fixed period. We applied a binning method to analyze the progression of\nthe mean Fourier parameters with period and wavelength. We found that for\nperiods longer than about 20 days, the values of the Fourier amplitude\nparameters increase sharply for shorter wavelengths as compared to wavelengths\nlonger than the $J$-band. We observed the variation of the Hertzsprung\nprogression with wavelength. The central period of the Hertzsprung progression\nwas found to increase with wavelength in the case of the Fourier amplitude\nparameters and decrease with increasing wavelength in the case of phase\nparameters. We also observed a small variation of the central period of the\nprogression between the Galaxy and LMC, presumably related to metallicity\neffects. These results will provide useful constraints for stellar pulsation\ncodes that incorporate stellar atmosphere models to produce Cepheid light\ncurves in various bands.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Binary Neutron Stars in Quasi-equilibrium: Quasi-equilibrium sequences of binary neutron stars are constructed for a\nvariety of equations of state in general relativity. Einstein's constraint\nequations in the Isenberg-Wilson-Mathews approximation are solved together with\nthe relativistic equations of hydrostationary equilibrium under the assumption\nof irrotational flow. We focus on unequal-mass sequences as well as equal-mass\nsequences, and compare those results. We investigate the behavior of the\nbinding energy and total angular momentum along a quasi-equilibrium sequence,\nthe endpoint of sequences, and the orbital angular velocity as a function of\ntime, changing the mass ratio, the total mass of the binary system, and the\nequation of state of a neutron star. It is found that the orbital angular\nvelocity at the mass-shedding limit can be determined by an empirical formula\nderived from an analytic estimation. We also provide tables for 160 sequences\nwhich will be useful as a guideline of numerical simulations for the inspiral\nand merger performed in the near future.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The search of the stellar clusters in vicinity of YSOs with high and\n  middle masses: The results of the searching on the bases of GPS UKIDSS survey's data of\ndense compact stellar clusters in the vicinity of 20 YSOs with high and middle\nmasses are presented. Totally we have revealed clusters in 13 areas. Around 5\nobjects (IRAS 18151-1208, IRAS 18316-0602, IRAS 19110+1045, IRAS 19213+1723,\nIRAS 20056+3350) they are newly detected. The radii and stellar density have\nsignificant gradient: from 0.2 to 2.7 pc and from 3 to 1000 stars/arcmin^2\nrespectively.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Scattering line polarization in rotating, optically thick disks: To interpret observations of astrophysical disks it is essential to\nunderstand the formation process of the emitted light. If the disk is optically\nthick, scattering dominated and permeated by a Keplerian velocity field,\nNon-Local Thermodynamic Equilibrium radiative transfer modeling must be done to\ncompute the emergent spectrum from a given disk model. We investigate Non-local\nthermodynamic equilibrium polarized line formation in different simple disk\nmodels and aim to demonstrate the importance of both radiative transfer effects\nand scattering as well as the effects of velocity fields. We self-consistently\nsolve the coupled equations of radiative transfer and statistical equilibrium\nfor a two level atom model by means of Jacobi iteration. We compute scattering\npolarization, that is Q/I and U/I line profiles. The degree of scattering\npolarization is significantly influenced by the inclination of the disk with\nrespect to observer, but also by the optical thickness of the disk and the\npresence of rotation. Stokes U shows double-lobed profiles with amplitude which\nincreases with the disk rotation. Our results suggest that the line profiles,\nespecially the polarized ones, emerging from gaseous disks differ significantly\nfrom the profiles predicted by simple approximations. The profiles are diverse\nin shape, but typically symmetric in Stokes Q and antisymmetric in Stokes U. A\nclear indicator of disk rotation is the presence of Stokes U, which might prove\nto be a useful diagnostic tool. We also demonstrate that, for moderate\nrotational velocities, an approximate treatment can be used, where non-local\nthermodynamic equilibrium radiative transfer is done in the velocity field-free\napproximation and Doppler shift is applied in the process of spatial\nintegration over the whole emitting surface.",
        "category": "astro-ph_SR"
    },
    {
        "text": "First ALMA Observation of a Solar Plasmoid Ejection from an X-ray Bright\n  Point: Eruptive phenomena such as plasmoid ejections or jets are an important\nfeature of solar activity with the potential for improving our understanding of\nthe dynamics of the solar atmosphere. Such ejections are often thought to be\nsignatures of the outflows expected in regions of fast magnetic reconnection.\nThe 304 A EUV line of Helium, formed at around 10^5 K, is found to be a\nreliable tracer of such phenomena, but the determination of physical parameters\nfrom such observations is not straightforward. We have observed a plasmoid\nejection from an X-ray bright point simultaneously at millimeter wavelengths\nwith ALMA, at EUV wavelengths with AIA, in soft X-rays with Hinode/XRT. This\npaper reports the physical parameters of the plasmoid obtained by combining the\nradio, EUV and X-ray data. As a result, we conclude that the plasmoid can\nconsist either of (approximately) isothermal 10^5 K plasma that is optically\nthin at 100 GHz, or else a 10^4 K core with a hot envelope. The analysis\ndemonstrates the value of the additional temperature and density constraints\nthat ALMA provides, and future science observations with ALMA will be able to\nmatch the spatial resolution of space-borne and other high-resolution\ntelescopes.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Nuclear constraints on non-Newtonian gravity at femtometer scale: Effects of the non-Newtonian gravity on properties of finite nuclei are\nstudied by consistently incorporating both the direct and exchange contribution\nof the Yukawa potential in the Hartree-Fock approach using a well-tested Skyrme\nforce for the strong interaction. It is shown for the first time that the\nstrength of the Yukawa term in the non-Newtonian gravity is limited to\n$\\log(|\\alpha|)<1.75/[\\lambda(\\rm fm)]^{0.54} + 33.6$ within the length scale\nof $\\lambda=1-10$ fm in order for the calculated properties of finite nuclei\nnot to be in conflict with accurate experimental data available.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Periodic Spectroscopic Variability of FU Orionis: FU Orionis systems are young stars undergoing outbursts of disc accretion and\nwhere the optical spectrum contains lines associated with both the disc\nphotosphere and a wind component. Previous observations of the prototype FU\nOrionis have suggested that the wind lines and the photospheric lines are\nmodulated with periods of 14.54 and 3.54 days respectively (Herbig et al.\n2003). We have re-observed the system at higher spectral resolution, by\nmonitoring variations of optical line profiles over 21 nights in 2007 and have\nfound periods of 13.48 and 3.6 days in the wind and disc components consistent\nwith the above: this implies variability mechanisms that are stable over at\nleast a decade. In addition we have found: i) that the variations in the\nphotospheric absorption lines are confined to the blue wing of the line (around\n-9km/s): we tentatively ascribe this to an orbiting hotspot in the disc which\nis obscured by a disc warp during its receding phase. ii) The wind period is\nmanifested not only in blue-shifted Halpha absorption, but also in red-shifted\nemission of Halpha and Hbeta, as well as in blue-shifted absorption of Na I D,\nLi I and Fe II. iii) We find that the periodic modulation of blue-shifted\nHalpha absorption at around -100km/s, is phase lagged with respect to\nvariations in the other lines by ~1.8days. This is consistent with a picture in\nwhich variations at the wind base first affect chromospheric emission and then\nlow velocity blue-shifted absorption, followed - after a lag equal to the\npropagation time of disturbances across the wind's acceleration region - by a\nresponse in high velocity blue-shifted absorption. Such arguments constrain the\nsize of the acceleration region to ~10^12cm. We discuss possible mechanisms for\nperiodic variations within the innermost 0.1AU of the disc, including the\npossibility that these variations indicate the presence of an embedded hot\nJupiter.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Undetected Binary Stars Cause an Observed Mass Dependent Age Gradient in\n  Upper Scorpius: Young stellar associations represent a key site for the study of star\nformation, but to accurately compare observations to models of stellar\nevolution, the age of an association must be determined. The Upper Scorpius\nregion is the youngest section of the Scorpius-Centaurus OB association, which\nis the largest collection of nearby, young, low-mass stars. The true age of\nUpper Scorpius is not clear, and an observed mass-dependent age gradient in\nUpper Scorpius, as well as in other star-forming regions, complicates age\nmeasurements. The age gradient may indicate a genuine astrophysical feature or\nmay be an artifact of unrecognized systematic effects in stellar age\nmeasurements. We have conducted a synthetic red-optical low-resolution\nspectroscopic survey of a simulated analog to the Upper Scorpius star-forming\nregion to investigate the effects of unresolved binary stars (which have\nmass-dependent demographics) on age measurements of a stellar population. We\nfind that the observed mass-dependent age gradient in Upper Scorpius can be\nexplained by a population of undetected binary stars. For a simulated\npopulation with an age of 10 (RMS = 2) Myr, we measure an age of 10.5 (RMS =\n3.5) Myr for F stars, and 7.5 (RMS = 5.8) Myr for M stars. This discrepancy is\ncaused by the mass-dependent mass ratio distribution and the variable steepness\nof the mass-luminosity relation. Our results support the previously suggested\n10 Myr age for Upper Scorpius, with a small intrinsic age spread.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A novel investigation of the small-scale magnetic activity of the quiet\n  Sun via the Hanle effect in the Sr I 4607 \u00c5 line: One of the key research problems in stellar physics is to decipher the\nsmall-scale magnetic activity of the quiet solar atmosphere. Recent\nmagneto-convection simulations that account for small-scale dynamo action have\nprovided three-dimensional (3D) models of the solar photosphere characterized\nby a high degree of small-scale magnetic activity, similar to that found\nthrough theoretical interpretation of the scattering polarization observed in\nthe Sr I 4607 \\AA\\ line. Here we present the results of a novel investigation\nof the Hanle effect in this resonance line, based on 3D radiative transfer\ncalculations in a high-resolution magneto-convection model having most of the\nconvection zone magnetized close to the equipartition and a surface mean field\nstrength ${\\langle B \\rangle}{\\approx}170$ G. The Hanle effect produced by the\nmodel's magnetic field depolarizes the zero-field scattering polarization\nsignals significantly, to the extent that the center-to-limb variation of the\ncalculated spatially-averaged polarization amplitudes is compatible with the\nobservations. The standard deviation of the horizontal fluctuations of the\ncalculated scattering polarization signals is very sensitive to the model's\nmagnetic field and we find that the predicted spatial variations are\nsufficiently sizable so as to be able to detect them, especially with the next\ngeneration of solar telescopes. We find that at all on-disk positions the\ntheoretical scattering polarization signals are anti-correlated with the\ncontinuum intensity. To facilitate reaching new observational breakthroughs, we\nshow how the theoretically predicted polarization signals and spatial\nvariations are modified when deteriorating the signal-to-noise ratio and the\nspectral and spatial resolutions of the simulated observations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Some glimpses from helioseismology at the dynamics of the deep solar\n  interior: Helioseismology has taught us a great deal about the stratification and\nkinematics of the solar interior, sufficient for us to embark upon dynamical\nstudies more detailed than have been possible before. The most sophisticated\nstudies to date have been the very impressive numerical simulations of the\nconvection zone, from which, especially in recent years, a great deal has been\nlearnt. Those simulations, and the seismological evidence with which they are\nbeing confronted, are reviewed elsewhere in this volume. Our understanding of\nthe global dynamics of the radiative interior of the Sun is in a much more\nprimitive state. Nevertheless, some progress has been made, and seismological\ninference has provided us with evidence of more to come. Some of that I\nsummarize here, mentioning in passing hints that are pointing the way to the\nfuture.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Probing the shape of the mixing profile and of the thermal structure at\n  the convective core boundary through asteroseismology: Aims: We investigate from a theoretical perspective if space asteroseismology\ncan be used to distinguish between different thermal structures and shapes of\nthe near-core mixing profiles for different types of coherent oscillation modes\nin massive stars with convective cores, and if this capacity depends on the\nevolutionary stage of the models along the main sequence. Methods: We compute\n1D stellar structure and evolution models for four different prescriptions of\nthe mixing and temperature gradient in the near-core region. Their effect on\nthe frequencies of dipole prograde gravity modes in both Slowly Pulsating B and\n$\\beta$ Cep stars is investigated, as well as for pressure modes in $\\beta$ Cep\nstars. Results: A comparison between the mode frequencies of the different\nmodels at various stages during the main sequence evolution reveals that they\nare more sensitive to a change in temperature gradient than to the exact shape\nof the mixing profile in the near-core region. Depending on the duration of the\nobserved light curve, one can distinguish between either just the temperature\ngradient, or also between the shapes of the mixing coefficient. The relative\nfrequency differences are in general larger for more evolved models, and are\nlargest for the higher-frequency pressure modes in $\\beta$ Cep stars.\nConclusions:In order to unravel the core boundary mixing and thermal structure\nof the near-core region, one must have asteroseismic masses and radii with\n$\\sim 1\\%$ relative precision for hundreds of stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Horizontal-branch morphology and multiple stellar populations in the\n  anomalous globular cluster M22: M22 is an anomalous globular cluster that hosts two groups of stars with\ndifferent metallicity and s-element abundance. The star-to-star light-element\nvariations in both groups, with the presence of individual Na-O and C-N\nanticorrelations, demonstrates that this Milky-Way satellite has experienced a\ncomplex star-formation history. We have analysed FLAMES/UVES spectra for seven\nstars covering a small color interval, on the reddest horizontal-branch (HB)\nportion of this cluster and investigated possible relations between the\nchemical composition of a star and its location along the HB. Our chemical\nabundance analysis takes into account effects introduced by deviations from the\nlocal-thermodynamic equilibrium (NLTE effects), that are significant for the\nmeasured spectral lines in the atmospheric parameters range spanned by our\nstars. We find that all the analysed stars are barium-poor and sodium-poor,\nthus supporting the idea that the position of a star along the HB is strictly\nrelated to the chemical composition, and that the HB-morphology is influenced\nby the presence of different stellar populations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Automatic Recognition of Sunspots in HSOS Full-Disk Solar Images: A procedure is introduced to recognise sunspots automatically in solar\nfull-disk photosphere images obtained from Huairou Solar Observing Station,\nNational Astronomical Observatories of China. The images are first\npre-processed through Gaussian algorithm. Sunspots are then recognised by the\nmorphological Bot-hat operation and Otsu threshold. Wrong selection of sunspots\nis eliminated by a criterion of sunspot properties. Besides, in order to\ncalculate the sunspots areas and the solar centre, the solar limb is extracted\nby a procedure using morphological closing and erosion operations and setting\nan adaptive threshold. Results of sunspot recognition reveal that the number of\nthe sunspots detected by our procedure has a quite good agreement with the\nmanual method. The sunspot recognition rate is 95% and error rate is 1.2%. The\nsunspot areas calculated by our method have high correlation (95%) with the\narea data from USAF/NOAA.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On Socially Distant Neighbors: Using Binaries to Constrain the Density\n  of Objects in the Galactic Center: Stars often reside in binary configurations. The nuclear star cluster\nsurrounding the supermassive black hole (SMBH) in the Galactic Center (GC) is\nexpected to include a binary population. In this dense environment, a binary\nfrequently encounters and interacts with neighboring stars. These interactions\nvary from small perturbations to violent collisions. In the former case, weak\ngravitational interactions unbind a soft binary over the evaporation timescale,\nwhich depends on the binary properties as well as the density of surrounding\nobjects and velocity dispersion. Similarly, collisions can also unbind a\nbinary, and the collision rate depends on the density. Thus, the detection of a\nbinary with known properties can constrain the density profile in the GC with\nimplications for the number of compact objects, which are otherwise challenging\nto detect. We estimate the density necessary to unbind a binary within its\nlifetime for an orbit of arbitrary eccentricity about the SMBH. We find that\nthe eccentricity has a minimal impact on the density constraint. In this proof\nof concept, we demonstrate that this procedure can probe the density in the GC\nusing hypothetical young and old binaries as examples. Similarly, a known\ndensity profile provides constraints on the binary orbital separation. Our\nresults highlight the need to consider multiple dynamical processes in tandem.\nIn certain cases, often closer to the SMBH, the collision timescale rather than\nthe evaporation timescale gives the more stringent density constraint, while\nother binaries farther from the SMBH provide unreliable density constraints\nbecause they migrate inwards due to mass segregation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Study of the Blazhko type RRc stars in the Stripe 82 region using SDSS\n  and ZTF: RR Lyrae stars are pulsating stars, many of which also show a long-term\nvariation called the Blazhko effect which is a modulation of amplitude and\nphase of the lightcurve. In this work, we searched for the incidence rate of\nthe Blazhko effect in the first-overtone pulsating RR Lyrae (RRc) stars of the\nGalactic halo. The focus was on the Stripe 82 region in the Galactic halo which\nwas studied by Sesar et al using the Sloan Digital Sky Survey (SDSS) data. In\ntheir work, 104 RR Lyrae stars were classified as RRc type. We combined their\nSDSS light curves with Zwicky Transient Facility (ZTF) data, and use them to\ndocument the Blazhko properties of these RRc stars. Our analysis showed that\namong the 104 RRc stars, 8 were rather RRd stars, and were excluded from the\nstudy. Out of remaining 96, 34 were Blazhko type, 62 were non-Blazhko type,\ngiving the incidence rate of 35.42% for Blazhko RRc stars. The shortest Blazhko\nperiod found was 12.808 +/- 0.001 d for SDSS 747380, while the longest was 3100\n+/- 126 d for SDSS 3585856. Combining SDSS and ZTF data sets increased the\nprobability of detecting the small variations due to the Blazhko effect, and\nthus provided a unique opportunity to find longer Blazhko periods. We found\nthat 85% of RRc stars had the Blazhko period longer than 200 d.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Direct formation of millisecond pulsars from rotationally delayed\n  accretion-induced collapse of massive white dwarfs: Millisecond pulsars (MSPs) are believed to be old neutron stars, formed via\nType Ib/c core-collapse supernovae, which have subsequently been spun up to\nhigh rotation rates via accretion from a companion star in a highly\ncircularised low-mass X-ray binary. The recent discoveries of Galactic field\nbinary MSPs in eccentric orbits, and mass functions compatible with that\nexpected for helium white dwarf companions, PSR J2234+06 and PSR J1946+3417,\ntherefore challenge this picture. Here we present a hypothesis for producing\nthis new class of systems, where the MSPs are formed directly from a\nrotationally-delayed accretion-induced collapse of a super-Chandrasekhar mass\nwhite dwarf. We compute the orbital properties of the MSPs formed in such\nevents and demonstrate that our hypothesis can reproduce the observed\neccentricities, masses and orbital periods of the white dwarfs, as well as\nforecasting the pulsar masses and velocities. Finally, we compare this\nhypothesis to a triple star scenario.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Imaging from STIX visibility amplitudes: Aims: To provide the first demonstration of STIX Fourier-transform X-ray\nimaging using semi-calibrated (amplitude-only) visibility data acquired during\nthe Solar Orbiter's cruise phase. Methods: We use a parametric imaging approach\nby which STIX visibility amplitudes are fitted by means of two non-linear\noptimization methods: a fast meta-heuristic technique inspired by social\nbehavior, and a Bayesian Monte Carlo sampling method, which, although slower,\nprovides better quantification of uncertainties. Results: When applied to a set\nof solar flare visibility amplitudes recorded by STIX on November 18, 2020 the\ntwo parametric methods provide very coherent results. The analysis also\ndemonstrates the ability of STIX to reconstruct high time resolution\ninformation and, from a spectral viewpoint, shows the reliability of a\ndouble-source scenario consistent with a thermal versus nonthermal\ninterpretation. Conclusions: In this preliminary analysis of STIX imaging based\nonly on visibility amplitudes, we formulate the imaging problem as a non-linear\nparametric issue we addressed by means of two high-performance optimization\ntechniques that both showed the ability to sample the parametric space in an\neffective fashion, thus avoiding local minima.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Angular Expansion of Nova Shells: Nova shells can provide us with important information on their distance,\ntheir interactions with the circumstellar and interstellar media, and the\nevolution in morphology of the ejecta. We have obtained narrow-band images of a\nsample of five nova shells, namely DQHer, FHSer, TAur, V476Cyg, and V533Her,\nwith ages in the range from 50 to 130 years. These images have been compared\nwith suitable available archival images to derive their angular expansion\nrates. We find that all the nova shells in our sample are still in the free\nexpansion phase, which can be expected, as the mass of the ejecta is 7-45 times\nlarger than the mass of the swept-up circumstellar medium. The nova shells will\nkeep expanding freely for time periods up to a few hundred years, reducing\ntheir time dispersal into the interstellar medium.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Accretion Kinematics in the T Tauri Binary TWA 3A: Evidence for\n  Preferential Accretion onto the TWA 3A Primary: We present time-series, high-resolution optical spectroscopy of the eccentric\nT Tauri binary TWA 3A. Our analysis focuses on variability in the strength and\nstructure of the accretion tracing emission lines H alpha and He I 5876A. We\nfind emission line strengths to display the same orbital-phase dependent\nbehavior found with time-series photometry, namely, bursts of accretion near\nperiastron passages. Such bursts are in good agreement with numerical\nsimulations of young eccentric binaries. During accretion bursts, the emission\nof He I 5876A consistently traces the velocity of the primary star. After\nremoving a model for the system's chromospheric emission, we find the primary\nstar typically emits ~70% of the He I accretion flux. We interpret this result\nas evidence for circumbinary accretion streams that preferentially feed the TWA\n3A primary. This finding is in contrast to most numerical simulations, which\npredict the secondary should be the dominant accretor in a binary system. Our\nresults may be consistent with a model in which the precession of an eccentric\ncircumbinary disk gap alternates between preferentially supplying mass to the\nprimary and secondary.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Properties of quasi-periodic pulsations in solar flares from a single\n  active region: We investigate the properties of a set of solar flares originating from a\nsingle active region (AR) that exhibit QPPs, and look for signs of the QPP\nperiods relating to AR properties. The AR studied, best known as NOAA 12192,\nwas unusually long-lived and produced 181 flares. Data from the GOES, EVE,\nFermi, Vernov and NoRH observatories were used to determine if QPPs were\npresent in the flares. For the soft X-ray GOES and EVE data, the time\nderivative of the signal was used. Power spectra of the time series data\n(without any form of detrending) were inspected, and flares with a peak above\nthe 95% confidence level in the spectrum were labelled as having candidate\nQPPs. The confidence levels were determined taking account of uncertainties and\nthe possible presence of red noise. AR properties were determined using HMI\nline of sight magnetograms. A total of 37 flares (20% of the sample) show good\nevidence of having QPPs, and some of the pulsations can be seen in data from\nmultiple instruments and in different wavebands. The QPP periods show a weak\ncorrelation with the flare amplitude and duration, but this may be due to an\nobservational bias. A stronger correlation was found between the QPP period and\nduration of the QPP signal, which can be partially but not entirely explained\nby observational constraints. No correlations were found with the AR area,\nbipole separation, or average magnetic field strength. The fact that a\nsubstantial fraction of the flare sample showed evidence of QPPs using a strict\ndetection method with minimal processing of the data demonstrates that these\nQPPs are a real phenomenon, which cannot be explained by the presence of red\nnoise or the superposition of multiple unrelated flares. The lack of\ncorrelation between the QPP periods and AR properties implies that the\nsmall-scale structure of the AR is important, and/or that different QPP\nmechanisms act in different cases.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Detailed optical spectroscopy of the B[e] star MWC 17: Based on the data of multiple high-resolution R=60 000 observations obtained\nat the 6-meter telescope BTA in combination with the NES spectrograph, we\nstudied the features of the optical spectrum of the star MWC 17 with the B[e]\nphenomenon. In the wavelength interval 4050-6750 A we identified numerous\npermitted and forbidden emissions, interstellar NaI lines, and diffuse\ninterstellar bands (DIBs). Radial velocities were estimated from lines of\nvarious origin. As the systemic velocity, Vsys, the velocity from the forbidden\nemissions can be accepted: Vr=-47 km/s (relative to the local standard Vlsr=-42\nkm/s). Comparison of the obtained data with the ealier measurements allows us\nto conclude on the absence of considerable variability of spectral details.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Mass and orbit constraints of the gamma-ray binary LS 5039: We present the results of space-based photometric and ground-based\nspectroscopic observing campaigns on the gamma-ray binary LS 5039. The new\norbital and physical parameters of the system are similar to former results,\nexcept we found a lower eccentricity. Our MOST-data show that any broad-band\noptical photometric variability at the orbital period is below the 2 mmag\nlevel. Light curve simulations support the lower value of eccentricity and\nimply that the mass of the compact object is higher than 1.8 solar masses.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Embedded protostellar disks around (sub-)solar stars. II. Disk masses,\n  sizes, densities, temperatures and the planet formation perspective: We present basic properties of protostellar disks in the embedded phase of\nstar formation (EPSF), which is difficult to probe observationally using\navailable observational facilities. We use numerical hydrodynamics simulations\nof cloud core collapse and focus on disks formed around stars in the 0.03-1.0\nMsun mass range. Our obtained disk masses scale near-linearly with the stellar\nmass. The mean and median disk masses in the Class 0 and I phases\n(M_{d,C0}^{mean}=0.12 Msun, M_{d,C0}^{mdn}=0.09 Msun and M_{d,CI}^{mean}=0.18\nMsun, M_{d,CI}^{mdn}=0.15 Msun, respectively) are greater than those inferred\nfrom observations by (at least) a factor of 2--3. We demonstrate that this\ndisagreement may (in part) be caused by the optically thick inner regions of\nprotostellar disks, which do not contribute to millimeter dust flux. We find\nthat disk masses and surface densities start to systematically exceed that of\nthe minimum mass solar nebular for objects with stellar mass as low as\nM_st=0.05-0.1 Msun. Concurrently, disk radii start to grow beyond 100 AU,\nmaking gravitational fragmentation in the disk outer regions possible. Large\ndisk masses, surface densities, and sizes suggest that giant planets may start\nforming as early as in the EPSF, either by means of core accretion (inner disk\nregions) or direct gravitational instability (outer disk regions), thus\nbreaking a longstanding stereotype that the planet formation process begins in\nthe Class II phase.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Calculating energy storage due to topological changes in emerging active\n  region NOAA AR 11112: The Minimum Current Corona (MCC) model provides a way to estimate stored\ncoronal energy using the number of field lines connecting regions of positive\nand negative photospheric flux. This information is quantified by the net flux\nconnecting pairs of opposing regions in a connectivity matrix. Changes in the\ncoronal magnetic field, due to processes such as magnetic reconnection,\nmanifest themselves as changes in the connectivity matrix. However, the\nconnectivity matrix will also change when flux sources emerge or submerge\nthrough the photosphere, as often happens in active regions. We have developed\nan algorithm to estimate the changes in flux due to emergence and submergence\nof magnetic flux sources. These estimated changes must be accounted for in\norder to quantify storage and release of magnetic energy in the corona. To\nperform this calculation over extended periods of time, we must additionally\nhave a consistently labeled connectivity matrix over the entire observational\ntime span. We have therefore developed an automated tracking algorithm to\ngenerate a consistent connectivity matrix as the photospheric source regions\nevolve over time. We have applied this method to NOAA Active Region 11112,\nwhich underwent a GOES M2.9 class flare around 19:00 on Oct.16th, 2010, and\ncalculated a lower bound on the free magnetic energy buildup of ~8.25 x 10^30\nergs over 3 days.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Modelling the occurrence of grand minima in sun-like stars using a\n  dynamo model: In this work, we have studied the variability and frequency of occurrence of\nthe grand minima using kinematic dynamo models of one solar mass star with\ndifferent rotation rates and depths of convection zones. We specify the\nlarge-scale flows (differential rotations and meridional circulations) from\ncorresponding hydrodynamic models. We include stochastic fluctuations in the\nBabcock-Leighton source for the poloidal field to produce variable stellar\ncycles. We observe that the rapidly rotating stars produce highly irregular\ncycles with strong magnetic fields and rarely produce Maunder-like grand\nminima, whereas the slowly rotating stars (Sun and longer rotation period)\nproduce smooth cycles of weaker strength and occasional grand minima. In\ngeneral, the number of the grand minima increases with the decrease in rotation\nrate. These results can be explained by the fact that with the increase of\nrotation period, the supercriticality of the dynamo decreases, and the dynamo\nis more prone to produce extended grand minima in this regime.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Search for Strongly Mg-enhanced Stars from the Sloan Digital Sky\n  Survey: Strongly Mg-enhanced stars with [Mg/Fe] $>$ 1.0 show peculiar abundance\npatterns and hence are of great interest for our understanding of stellar\nformation and chemical evolution of the Galaxy. A systematical search for\nstrongly Mg-enhanced stars based on the low-resolution $(R\\simeq2000)$ spectra\nof the Sloan Digital Sky Survey (SDSS) is carried out by finding the best\nmatched synthetic spectrum to the observed one in the region of Mg I b lines\naround $\\lambda$5170\\AA$~$via a profile matching method. The advantage of our\nmethod is that fitting parameters are refined by reproducing the [Mg/Fe] ratios\nof 47 stars from very precise high-resolution spectroscopic (HRS) analysis by\nNissen and Schuster (2010); and these parameters are crucial to the precision\nand validity of the derived Mg abundances. As a further check of our method, Mg\nabundances are estimated with our method for member stars in four Galactic\nglobular clusters (M92, M13, M3, M71) which cover the same metallicity range as\nour sample, and the results are in good agreement with those of HRS analysis in\nthe literature. The validation of our method is also proved by the agreement of\n[Mg/Fe] between our values and those of HRS analysis by Aoki et al.(2013).\nFinally, 33 candidates of strongly Mg-enhanced stars with [Mg/Fe]$>$1.0 are\nselected from 14850 F and G stars. Follow-up observations will be carried out\non these candidates with high-resolution spectroscopy by large telescopes in\nthe near future, so as to check our selection procedure and to perform a\nprecise and detailed abundance analysis and to explore the origins of these\nstars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Helium abundances and the helium isotope anomaly of sdB stars: Helium abundances and atmospheric parameters have been determined from high\nresolution spectra for a new sample of 46 bright hot subdwarf B (sdB) stars.\nThe helium abundances have been measured with high accuracy. We confirm the\ncorrelation of helium abundance with temperature and the existence of two\ndistinct sequences in helium abundance found previously. We focused on isotopic\nshifts of helium lines and found helium-3 to be strongly enriched in 8 of our\nprogramme stars. Most of these stars cluster in a small temperature range\nbetween 27000 K and 31000 K very similar to the known helium-3-rich main\nsequence B stars, which cluster at somewhat lower temperatures. This phenomenon\nis most probably related to diffusion processes in the atmosphere, but poses a\nchallenge to diffusion models.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the optically-thick winds of Wolf-Rayet stars: (abridged) The strong winds of Wolf-Rayet (WR) stars are important for the\nmechanical and chemical feedback of the most massive stars and determine\nwhether they end their lives as neutron stars or black holes. In this work we\ninvestigate theoretically the mass-loss properties of H-free WR stars of the\nnitrogen sequence (WN stars). We connect stellar structure models for He stars\nwith wind models for optically-thick winds and assess how both types of models\ncan simultaneously fulfill their respective sonic-point conditions. Fixing the\nouter wind law and terminal wind velocity, we obtain unique solutions for the\nmass-loss rates of optically-thick, radiatively-driven winds of WR stars in the\nphase of core He-burning. The resulting mass-loss relations as a function of\nstellar parameters, agree well with previous empirical relations. Furthermore,\nwe encounter stellar mass limits below which no continuous solutions exist.\nWhile these mass limits agree with observations of WR stars in the Galaxy, they\nare in conflict with observations in the LMC. While our results confirm in\nparticular the slope of oft-used empirical mass-loss relations, they imply that\nonly part of the observed WN population can be understood in the framework of\nthe standard assumptions of a smooth transonic flow and compact stellar core.\nThis means that alternative approaches, such as a clumped and inflated wind\nstructure, or deviations from the diffusion limit at the sonic point may have\nto be invoked. Qualitatively, the existence of mass limits for the formation of\nWR-type winds may be relevant for the non-detection of low-mass WR stars in\nbinary systems, which are believed to be progenitors of Type Ib/c supernovae.\nThe sonic-point conditions derived in this work may provide a possibility to\ninclude optically-thick winds in stellar evolution models in a more physically\nmotivated form than in current models.",
        "category": "astro-ph_SR"
    },
    {
        "text": "MOST Observations of our Nearest Neighbor: Flares on Proxima Centauri: We present a study of white light flares from the active M5.5 dwarf Proxima\nCentauri using the Canadian microsatellite MOST. Using 37.6 days of monitoring\ndata from 2014 and 2015, we have detected 66 individual flare events, the\nlargest number of white light flares observed to date on Proxima Cen. Flare\nenergies in our sample range from $10^{29}$-$10^{31.5}$ erg. The flare rate is\nlower than that of other classic flare stars of similar spectral type, such as\nUV Ceti, which may indicate Proxima Cen had a higher flare rate in its youth.\nProxima Cen does have an unusually high flare rate given its slow rotation\nperiod, however. Extending the observed power-law occurrence distribution down\nto $10^{28}$ erg, we show that flares with flux amplitudes of 0.5% occur 63\ntimes per day, while superflares with energies of $10^{33}$ erg occur ~8 times\nper year. Small flares may therefore pose a great difficulty in searches for\ntransits from the recently announced 1.27 M_earth Proxima b, while frequent\nlarge flares could have significant impact on the planetary atmosphere.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The formation of long-period eccentric binaries with a helium white\n  dwarf: The recent discovery of long-period eccentric binaries hosting a He-WD or a\nsdB star has been challenging binary-star modelling. Based on accurate\ndeterminations of the stellar and orbital parameters for IP Eri, a K0 + He-WD\nsystem, we propose an evolutionary path that is able to explain the\nobservational properties of this system and, in particular, to account for its\nhigh eccentricity (0.25). Our scenario invokes an enhanced-wind mass loss on\nthe first red giant branch (RGB) in order to avoid mass transfer by Roche-lobe\noverflow, where tides systematically circularize the orbit. We explore how the\nevolution of the orbital parameters depends on the initial conditions and show\nthat eccentricity can be preserved and even increased if the initial separation\nis large enough. The low spin velocity of the K0 giant implies that accretion\nof angular momentum from a (tidally-enhanced) RGB wind should not be efficient.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Reconnaissance of the HR 8799 Exosolar System II: Astrometry and Orbital\n  Motion: We present an analysis of the orbital motion of the four sub-stellar objects\norbiting HR8799. Our study relies on the published astrometric history of this\nsystem augmented with an epoch obtained with the Project 1640 coronagraph +\nIntegral Field Spectrograph (IFS) installed at the Palomar Hale telescope. We\nfirst focus on the intricacies associated with astrometric estimation using the\ncombination of an Extreme Adaptive Optics system (PALM-3000), a coronagraph and\nan IFS. We introduce two new algorithms. The first one retrieves the stellar\nfocal plane position when the star is occulted by a coronagraphic stop. The\nsecond one yields precise astrometric and spectro-photometric estimates of\nfaint point sources even when they are initially buried in the speckle noise.\nThe second part of our paper is devoted to studying orbital motion in this\nsystem. In order to complement the orbital architectures discussed in the\nliterature, we determine an ensemble of likely Keplerian orbits for HR8799bcde,\nusing a Bayesian analysis with maximally vague priors regarding the overall\nconfiguration of the system. While the astrometric history is currently too\nscarce to formally rule out coplanarity, HR8799d appears to be misaligned with\nrespect to the most likely planes of HR8799bce orbits. This misalignment is\nsufficient to question the strictly coplanar assumption made by various authors\nwhen identifying a Laplace resonance as a potential architecture. Finally, we\nestablish a high likelihood that HR8799de have dynamical masses below 13 M_Jup\nusing a loose dynamical survival argument based on geometric close encounters.\nWe illustrate how future dynamical analyses will further constrain dynamical\nmasses in the entire system.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Sub-Chandrasekhar White Dwarf Mergers as the Progenitors of Type Ia\n  Supernovae: Type Ia supernovae are generally thought to be due to the thermonuclear\nexplosions of carbon-oxygen white dwarfs with masses near the Chandrasekhar\nmass. This scenario, however, has two long-standing problems. First, the\nexplosions do not naturally produce the correct mix of elements, but have to be\nfinely tuned to proceed from sub-sonic deflagration to super-sonic detonation.\nSecond, population models and observations give formation rates of\nnear-Chandrasekhar white dwarfs that are far too small. Here, we suggest that\ntype Ia supernovae instead result from mergers of roughly equal-mass\ncarbon-oxygen white dwarfs, including those that produce sub-Chandrasekhar mass\nremnants. Numerical studies of such mergers have shown that the remnants\nconsist of rapidly rotating cores that contain most of the mass and are hottest\nin the center, surrounded by dense, small disks. We argue that the disks\naccrete quickly, and that the resulting compressional heating likely leads to\ncentral carbon ignition. This ignition occurs at densities for which pure\ndetonations lead to events similar to type Ia supernovae. With this merger\nscenario, we can understand the type Ia rates, and have plausible reasons for\nthe observed range in luminosity and for the bias of more luminous supernovae\ntowards younger populations. We speculate that explosions of white dwarfs\nslowly brought to the Chandrasekhar limit---which should also occur---are\nresponsible for some of the \"atypical\" type Ia supernovae.",
        "category": "astro-ph_SR"
    },
    {
        "text": "AGB and post-AGB objects in the outer Galaxy: We present the results of our search for low- and intermediate mass evolved\nstars in the outer Galaxy using AllWISE catalogue photometry. We show that the\n[3.4]-[12] versus [4.6]-[22] colour-colour diagram is most suitable for\nseparating C-rich/O-rich AGB and post- AGB star candidates. We are able to\nselect 2,510 AGB and 24,821 post-AGB star candidates. However, the latter are\nseverely mixed with the known young stellar objects in this diagram.",
        "category": "astro-ph_SR"
    },
    {
        "text": "New mass-loss rates of B supergiants from global wind models: Massive stars lose a significant fraction of mass during their evolution.\nHowever, the corresponding mass-loss rates are rather uncertain. To improve\nthis, we calculated global line-driven wind models for Galactic B supergiants.\nOur models predict radial wind structure directly from basic stellar\nparameters. The hydrodynamic structure of the flow is consistently determined\nfrom the photosphere in nearly hydrostatic equilibrium to supersonically\nexpanding wind. The radiative force is derived from the solution of the\nradiative transfer equation in the comoving frame. We provide a simple formula\nthat predicts theoretical mass-loss rates as a function of stellar luminosity\nand effective temperature. The mass-loss rate of B supergiants slightly\ndecreases with temperature down to about 22.5 kK, where the region of\nrecombination of Fe IV to Fe III starts to appear. In this region, which is\nabout 5 kK wide, the mass-loss rate gradually increases by a factor of about 6.\nThe increase of the mass-loss rate is associated with a gradual decrease of\nterminal velocities by a factor of about 2. We compared the predicted wind\nparameters with observations. While the observed wind terminal velocities are\nreasonably reproduced by the models, the situation with mass-loss rates is less\nclear. The mass-loss rates derived from observations that are uncorrected for\nclumping are by a factor of 3 to 9 higher than our predictions on cool and hot\nsides of the studied sample, respectively. These observations can be reconciled\nwith theory assuming a temperature-dependent clumping factor. On the other\nhand, the mass-loss rate estimates that are not sensitive to clumping agree\nwith our predictions much better. Our predictions are by a factor of about 10\nlower than the values currently used in evolutionary models appealing for\nreconsideration of the role of winds in the stellar evolution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A helium-flash-induced mixing event can explain the lithium abundances\n  of red clump stars: Observations demonstrate that the surface abundance of $^7{\\rm Li}$ in\nlow-mass stars changes dramatically between the tip of the red giant branch and\nthe red clump. This naturally suggests an association with the helium core\nflash, which occurs between these two stages. Using stellar evolution models\nand a simple, ad hoc mixing prescription, we demonstrate that the $^7{\\rm Li}$\nenhancement can be explained by a brief chemical mixing event that occurs at\nthe time of the first, strongest He sub-flash. The amount of $^7{\\rm Be}$\nalready present above the H-burning shell just before the flash, once it mixes\ninto the cooler envelope and undergoes an electron capture converting it to\n$^7{\\rm Li}$, is sufficient to explain the observed abundance at the red clump.\nWe suggest that the excitation of internal gravity waves by the vigorous\nturbulent convection during the flash may provide a physical mechanism that can\ninduce such mixing.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Externally heated protostellar cores in the Ophiuchus star-forming\n  region: We present APEX 218 GHz observations of molecular emission in a complete\nsample of embedded protostars in the Ophiuchus star-forming region. To study\nthe physical properties of the cores, we calculate H$_2$CO and c-C$_3$H$_2$\nrotational temperatures, both of which are good tracers of the kinetic\ntemperature of the molecular gas. We find that the H$_2$CO temperatures range\nbetween 16 K and 124 K, with the highest H$_2$CO temperatures toward the hot\ncorino source IRAS 16293-2422 (69-124 K) and the sources in the $\\rho$ Oph A\ncloud (23-49 K) located close to the luminous Herbig Be star S 1, which\nexternally irradiates the $\\rho$ Oph A cores. On the other hand, the\nc-C$_3$H$_2$ rotational temperature is consistently low (7-17 K) in all\nsources. Our results indicate that the c-C$_3$H$_2$ emission is primarily\ntracing more shielded parts of the envelope whereas the H$_2$CO emission (at\nthe angular scale of the APEX beam; 3600 au in Ophiuchus) mainly traces the\nouter irradiated envelopes, apart from in IRAS 16293-2422, where the hot corino\nemission dominates. In some sources, a secondary velocity component is also\nseen, possibly tracing the molecular outflow.",
        "category": "astro-ph_SR"
    },
    {
        "text": "$f$-mode interaction with models of sunspot : near-field scattering and\n  multi-frequency effects: We use numerical simulations to investigate the interaction of an $f$-mode\nwave packet with small and large models of a sunspot in a stratified\natmosphere. While a loose cluster model has been largely studied before, we\nfocus in this study on the scattering from an ensemble of tightly compact\ntubes. We showed that the small compact cluster produces a slight distorted\nscattered wave field in the transverse direction, which can be attributed to\nthe simultaneous oscillations of the pairs of tubes within the cluster aligned\nin a perpendicular direction to the incoming wave. However, no signature of a\nmultiple-scattering regime has been observed from this model, while it has been\nclearly observable for the large compact cluster model. Furthermore, we pointed\nout the importance of the geometrical shape of the monolithic model on the\ninteraction of $f$-mode waves with a sunspot in a high frequency range ($\\nu =$\n5 mHz). These results are a contribution to the observational effort to\ndistinguish seismically between different configurations of magnetic flux tubes\nwithin sunspots and plage.",
        "category": "astro-ph_SR"
    },
    {
        "text": "What is the relationship between photospheric flow fields and solar\n  flares?: We estimated photospheric velocities by separately applying the Fourier Local\nCorrelation Tracking (FLCT) and Differential Affine Velocity Estimator (DAVE)\nmethods to 2708 co-registered pairs of SOHO/MDI magnetograms, with nominal\n96-minute cadence and ~2\" pixels, from 46 active regions (ARs) from 1996-1998\nover the time interval t45 when each AR was within 45^o of disk center. For\neach magnetogram pair, we computed the average estimated radial magnetic field,\nB; and each tracking method produced an independently estimated flow field, u.\nWe then quantitatively characterized these magnetic and flow fields by\ncomputing several extensive and intensive properties of each; extensive\nproperties scale with AR size, while intensive properties do not depend\ndirectly on AR size. Intensive flow properties included moments of speeds,\nhorizontal divergences, and radial curls; extensive flow properties included\nsums of these properties over each AR, and a crude proxy for the ideal Poynting\nflux, the total |u| B^2. Several magnetic quantities were also computed,\nincluding: total unsigned flux; a measure of the amount of unsigned flux near\nstrong-field polarity inversion lines, R; and the total B^2. Next, using\ncorrelation and discriminant analysis, we investigated the associations between\nthese properties and flares from the GOES flare catalog, when averaged over\nboth t45 and shorter time windows, of 6 and 24 hours. We found R and total |u|\nB^2 to be most strongly associated with flares; no intensive flow properties\nwere strongly associated with flares.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Origin of Sequential Chromospheric Brightenings: Sequential chromospheric brightenings (SCBs) are often observed in the\nimmediate vicinity of erupting flares and are associated with coronal mass\nejections. Since their initial discovery in 2005, there have been several\nsubsequent investigations of SCBs. These studies have used differing detection\nand analysis techniques, making it difficult to compare results between\nstudies. This work employs the automated detection algorithm of Kirk et al.\n(Solar Phys. 283, 97, 2013) to extract the physical characteristics of SCBs in\n11 flares of varying size and intensity. We demonstrate that the magnetic\nsubstructure within the SCB appears to have a significantly smaller area than\nthe corresponding H-alpha emission. We conclude that SCBs originate in the\nlower corona around 0.1 R_sun above the photosphere, propagate away from the\nflare center at speeds of 35 - 85 km/s, and have peak photosphere magnetic\nintensities of 148 +/- 2.9 G. In light of these measurements, we infer SCBs to\nbe distinctive chromospheric signatures of erupting coronal mass ejections.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The origin of the planetary nebula M 1-16. A morphokinematic and\n  chemical analysis: We investigated the origin of the Planetary Nebula (PN) M 1-16 using\nnarrow-band optical imaging, and high- and low-resolution optical spectra to\nperform a detailed morpho-kinematic and chemical studies. M 1-16 is revealed to\nbe a multipolar PN that predominantly emits in [O III] in the inner part of the\nnebula and [N II] in the lobes. A novel spectral unsharp masking technique was\napplied to the position-velocity (PV) maps to reveal a set of multiple\nstructures at the centre of M 1-16 spanning radial velocities from\n$-$40km$\\,$s$^{-1}$ to 20km$\\,$s$^{-1}$, with respect to the systemic velocity.\nThe morpho-kinematic model indicates that the deprojected velocity of the lobe\noutflows are $\\geq$100km$\\,$s$^{-1}$, and particularly the larger lobes and\nknots have a deprojected velocity of $\\simeq$350km$\\,$s$^{-1}$; the inner\nellipsoidal component has a deprojected velocity of $\\simeq$29km$\\,$s$^{-1}$. A\nkinematical age of $\\sim$8700yr has been obtained from the model assuming a\nhomologous velocity expansion law and a distance of 6.2$\\pm$1.9kpc. The\nchemical analysis indicates that M 1-16 is a Type I PN with a central star of\nPN (CSPN) mass in the range of $\\simeq$0.618-0.713M$_\\odot$ and an initial mass\nfor the progenitor star between 2.0 and 3.0M$_\\odot$ (depending on\nmetallicity). An $T_\\mathrm{eff}\\simeq$140$\\,$000K and log$(L/L_{\\odot})$=2.3\nwas estimated using the 3MdB photoionisation models to reproduce the ionisation\nstage of the PN. All of these results have led us to suggest that M 1-16 is an\nevolved PN, contrary to the scenario of proto-PN suggested in previous studies.\nWe propose that the mechanism responsible for the morphology of M 1-16 is\nrelated to the binary (or multiple star) evolution scenario.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Simultaneous Multiwavelength Observations of Magnetic Activity in\n  Ultracool Dwarfs. IV. The Active, Young Binary NLTT 33370 AB (=2MASS\n  J13142039+1320011): We present multi-epoch simultaneous radio, optical, H{\\alpha}, UV, and X-ray\nobservations of the active, young, low-mass binary NLTT 33370 AB (blended\nspectral type M7e). This system is remarkable for its extreme levels of\nmagnetic activity: it is the most radio-luminous ultracool dwarf (UCD) known,\nand here we show that it is also one of the most X-ray luminous UCDs known. We\ndetect the system in all bands and find a complex phenomenology of both flaring\nand periodic variability. Analysis of the optical light curve reveals the\nsimultaneous presence of two periodicities, 3.7859 $\\pm$ 0.0001 and 3.7130\n$\\pm$ 0.0002 hr. While these differ by only ~2%, studies of differential\nrotation in the UCD regime suggest that it cannot be responsible for the two\nsignals. The system's radio emission consists of at least three components:\nrapid 100% polarized flares, bright emission modulating periodically in phase\nwith the optical emission, and an additional periodic component that appears\nonly in the 2013 observational campaign. We interpret the last of these as a\ngyrosynchrotron feature associated with large-scale magnetic fields and a cool,\nequatorial plasma torus. However, the persistent rapid flares at all rotational\nphases imply that small-scale magnetic loops are also present and reconnect\nnearly continuously. We present an SED of the blended system spanning more than\n9 orders of magnitude in wavelength. The significant magnetism present in NLTT\n33370 AB will affect its fundamental parameters, with the components' radii and\ntemperatures potentially altered by ~+20% and ~-10%, respectively. Finally, we\nsuggest spatially resolved observations that could clarify many aspects of this\nsystem's nature.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Bayesian Asteroseismology of 23 Solar-Like Kepler Targets: We study 23 previously published Kepler targets to perform a consistent\ngrid-based Bayesian asteroseismic analysis and compare our results to those\nobtained via the Asteroseismic Modelling Portal (AMP). We find differences in\nthe derived stellar parameters of many targets and their uncertainties. While\nsome of these differences can be attributed to systematic effects between\nstellar evolutionary models, we show that the different methodologies deliver\nincompatible uncertainties for some parameters. Using non-adiabatic models and\nour capability to measure surface effects, we also investigate the dependency\nof these surface effects on the stellar parameters. Our results suggest a\ndependence of the magnitude of the surface effect on the mixing length\nparameter which also, but only minimally, affects the determination of stellar\nparameters. While some stars in our sample show no surface effect at all, the\nmost significant surface effects are found for stars that are close to the\nSun's position in the HR diagram.",
        "category": "astro-ph_SR"
    },
    {
        "text": "X-ray Irradiation of the LkCa 15 Protoplanetary Disk: LkCa 15 in the Taurus star-forming region has recently gained attention as\nthe first accreting T Tauri star likely to host a young protoplanet. High\nspatial resolution infrared observations have detected the suspected\nprotoplanet within a dust-depleted inner gap of the LkCa 15 transition disk at\na distance of 15 AU from the star. If this object's status as a protoplanet is\nconfirmed, LkCa 15 will serve as a unique laboratory for constraining physical\nconditions within a planet-forming disk. Previous models of the LkCa 15 disk\nhave accounted for disk heating by the stellar photosphere but have ignored the\npotential importance of X-ray ionization and heating. We report here the\ndetection of LkCa 15 as a bright X-ray source with Chandra. The X-ray emission\nis characterized by a cool heavily-absorbed plasma component at kT_cool ~0.3\nkeV and a harder component at kT_hot ~5 keV. We use the observed X-ray\nproperties to provide initial estimates of the X-ray ionization and heating\nrates within the tenuous inner disk. These estimates and the observed X-ray\nproperties of LkCa 15 can be used as a starting point for developing more\nrealistic disk models of this benchmark system.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Impact of Type II Spicules in the Corona: Simulations and Synthetic\n  Observables: The role of type II spicules in the corona has been a much debated topic in\nrecent years. This paper aims to shed light on the impact of type II spicules\nin the corona using novel 2.5D radiative MHD simulations including ion-neutral\ninteraction effects with the Bifrost code. We find that the formation of\nsimulated type II spicules, driven by the release of magnetic tension, impacts\nthe corona in various manners. Associated with the formation of spicules, the\ncorona exhibits 1) magneto-acoustic shocks and flows which supply mass to\ncoronal loops, and 2) transversal magnetic waves and electric currents that\npropagate at Alfv\\'en speeds. The transversal waves and electric currents,\ngenerated by the spicule's driver and lasting for many minutes, are dissipated\nand heat the associated loop. These complex interactions in the corona can be\nconnected with blue shifted secondary components in coronal spectral lines\n(Red-Blue asymmetries) observed with Hinode/EIS and SOHO/SUMER, as well as the\nEUV counterpart of type II spicules and propagating coronal disturbances (PCDs)\nobserved with the 171~\\AA\\ and 193~\\AA\\ SDO/AIA channels.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Coronal temperature profiles obtained from kinetic models and from\n  coronal brightness measurements obtained during solar eclipses: Coronal density, temperature and heat flux distributions for the equatorial\nand polar corona have been deduced by Lemaire [2012] from Saito's model of\naveraged coronal white light (WL) brightness and polarization observations.\nThey are compared with those determined from a kinetic collisionless/exospheric\nmodel of the solar corona. This comparison indicates rather similar\ndistributions at large radial distances (> 7 Rs) in the collisionless region.\nHowever, rather important differences are found close to the Sun in the\nacceleration region of the solar wind. The exospheric heat flux is directed\naway from the Sun, while that inferred from all WL coronal observations is in\nthe opposite direction, i.e., conducting heat from the inner corona toward the\nchromosphere. This could indicate that the source of coronal heating rate\nextends up into the inner corona where it maximizes at r > 1.5 Rs well above\nthe transition region.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Cannibals in the thick disk II -- Radial-velocity monitoring of the\n  young alpha-rich stars: Determining ages of stars for reconstructing the history of the Milky Way\nremains one of the most difficult tasks in astrophysics. This involves knowing\nwhen it is possible to relate the stellar mass with its age and when it is not.\nThe young $\\alpha-$rich (YAR) stars present such a case in which we are still\nnot sure about their ages because they are relatively massive, implying young\nages, but their abundances are $\\alpha-$enhanced, which implies old ages. We\nreport the results from new observations from a long-term\nradial-velocity-monitoring campaign complemented with high-resolution\nspectroscopy, as well as new astrometry and seismology of a sample of 41 red\ngiants from the third version of APOKASC, which includes YAR stars. The aim is\nto better characterize the YAR stars in terms of binarity, mass, abundance\ntrends, and kinematic properties.The radial velocities of HERMES, APOGEE, and\nGaia were combined to determine the binary fraction among YAR stars. In\ncombination with their mass estimate, evolutionary status, chemical\ncomposition, and kinematic properties, it allowed us to better constrain the\nnature of these objects. We found that stars with $\\mathrm{M} < 1\n\\mathrm{M}_\\odot$ were all single, whereas stars with $\\mathrm{M} > 1\n\\mathrm{M}_\\odot$ could be either single or binary. This is in agreement with\ntheoretical predictions of population synthesis models. Studying their [C/N],\n[C/Fe], and [N/Fe], trends with mass, it became clear that many YAR stars do\nnot follow the APOKASC stars, favoring the scenario that most of them are the\nproduct of mass transfer. Abr.",
        "category": "astro-ph_SR"
    },
    {
        "text": "UVMag: stellar formation, evolution, structure and environment with\n  space UV and visible spectropolarimetry: Important insights into the formation, structure, evolution and environment\nof all types of stars can be obtained through the measurement of their winds\nand possible magnetospheres. However, this has hardly been done up to now\nmainly because of the lack of UV instrumentation available for long periods of\ntime. To reach this aim, we have designed UVMag, an M-size space mission\nequipped with a high-resolution spectropolarimeter working in the UV and\nvisible spectral range. The UV domain is crucial in stellar physics as it is\nvery rich in atomic and molecular lines and contains most of the flux of hot\nstars. Moreover, covering the UV and visible spectral domains at the same time\nwill allow us to study the star and its environment simultaneously. Adding\npolarimetric power to the spectrograph will multiply tenfold the capabilities\nof extracting information on stellar magnetospheres, winds, disks, and magnetic\nfields. Examples of science objectives that can be reached with UVMag are\npresented for pre-main sequence, main sequence and evolved stars. They will\ncast new light onto stellar physics by addressing many exciting and important\nquestions. UVMag is currently undergoing a Research and Technology study and\nwill be proposed at the forthcoming ESA call for M-size missions. This\nspectropolarimeter could also be installed on a large UV and visible\nobservatory (e.g. NASA's LUVOIR project) within a suite of instruments.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The hybrid CONe WD + He star scenario for the progenitors of type Ia\n  supernovae: The hybrid CONe white dwarfs (WDs) have been suggested to be possible\nprogenitors of type Ia supernovae (SNe Ia). In this article, we systematically\nstudied the hybrid CONe WD + He star scenario for the progenitors of SNe Ia, in\nwhich a hybrid CONe WD increases its mass to the Chandrasekhar mass limit by\naccreting He-rich material from a non-degenerate He star. According to a series\nof detailed binary population synthesis simulations, we obtained the SN Ia\nbirthrates and delay times for this scenario. The SN Ia birthrates for this\nscenario are ~0.033-0.539*10^(-3)yr^(-1), which roughly accounts for 1-18% of\nall SNe Ia. The estimated delay times are ~28Myr-178Myr, which are the youngest\nSNe Ia predicted by any progenitor model so far. We suggest that SNe Ia from\nthis scenario may provide an alternative explanation of type Iax SNe. We also\npresented some properties of the donors at the point when the WDs reach the\nChandrasekhar mass. These properties may be a good starting point for\ninvestigating the surviving companions of SNe Ia, and for constraining the\nprogenitor scenario studied in this work.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Case AD, AR, and AS binary evolution and their possible connections with\n  W UMa binaries: Close detached binaries were theoretically predicted to evolve into contact\nby three subtypes of case A binary evolution: case AD, AR, and AS, which\ncorrespond to the formation of contact during dynamic-, thermal-, and\nnuclear-timescale mass transfer phase, respectively. It is unclear, however,\nwhat is the difference between contact binaries in these subtypes, and whether\nall of these subtypes can account for the formation of observed W UMa binaries.\nUsing Eggleton's stellar evolution code with the nonconservative assumption, I\nobtained the low-mass contact binaries produced by case AD, AR, and AS at the\nmoment of contact, and their parameter spaces. The results support that the\nprogenitors of low-mass contact binaries are detached binaries with orbital\nperiods shorter than $\\sim2-5\\,$d, and their borderlines depend strongly on the\nprimary mass. In addition, the period-colour relations for case AR and AS can\nbe in better agreement with that for observed W UMa candidates, but case AD\nshows a significantly worse agreement. Moreover, case AR and AS can produce a\nshort-period limit (corresponding to a low-mass limit) at almost any age, e.g.\nfrom young age ($\\sim0.2\\,$Gyr) to old age ($\\sim13\\,$Gyr), agreeing with\nobserved W UMa binaries in star clusters, but no such limit occurs for case AD\nat any age. These results support that case AR and AS, as opposed to case AD,\ncan lead to W UMa binaries (including young W UMa binaries).",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Sun-like star orbiting a boson star: The high-precision astrometric mission GAIA recently reported the remarkable\ndiscovery of a Sun-like star closely orbiting a dark object, with a semi-major\naxis and period of $1.4\\, \\rm{AU}$ and $187.8$ days respectively. While the\nplausible expectation for the central dark object is a black hole, the\nevolutionary mechanism leading to the formation of such a two-body system is\nhighly challenging. Here, we challenge the scenario of a central black hole and\nshow that the observed orbital dynamics can be explained under fairly general\nassumptions if the central dark object is a stable clump of bosonic particles\nof spin-0, or spin-1, known as a boson star. We further explain how future\nastrometric measurements of similar systems will provide an exciting\nopportunity to probe the fundamental nature of compact objects and test compact\nalternatives to black holes.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Multi-scale magnetic field investigation of the M-dwarf eclipsing binary\n  CU Cancri: We aim to characterise the magnetic field of the eclipsing binary CU Cnc. The\ndetermination of magnetic field parameters of this target enables comparisons\nwith both observations of similar stars and theoretical predictions of the\nmagnetic field strength for CU Cnc. The target is therefore providing an\nexcellent opportunity to test our understanding of the generation of magnetic\nfields in low-mass stars and its impact on stellar structure. We use\nspectropolarimetric observations obtained with ESPaDOnS to investigate the\nmagnetic properties of CU Cnc. We generate average line profiles with LSD,\nwhich are used to extract information about the radial velocities of the\ncomponents, expanding the number of radial velocity measurements available and\nallowing for a determination of orbital parameters. Stokes V LSD profiles are\nused with ZDI to obtain large-scale magnetic field structures on both\ncomponents. We also use polarised radiative transfer modelling to investigate\nthe small-scale fields by utilising Zeeman splitting of magnetically sensitive\nTi I lines in non-polarised spectra. The large-scale fields are dominantly\npoloidal and have an average strength of ~100 G on both components. This\nanalysis of the large-scale fields likely suffers from some amount of\nhemisphere degeneracy due to the high inclination of the target. Both\ncomponents also show unusual magnetic field configurations compared to stars\nwith similar parameters, the primary is weakly axisymmetric (~10%) and the\nsecondary has a strong torroidal contribution (~20%). The small-scale fields\nare significantly stronger, at 3.1 and 3.6 kG for the primary and secondary\nrespectively. This measurement is in excellent agreement with surface field\nstrength predictions for CU Cnc from magnetoconvective stellar evolution\nmodels. These results indicates that magnetic fields play a significant role in\nradius inflation of active stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "ALMA and VLA observations of the outflows in IRAS 16293-2422: We present ALMA and VLA observations of the molecular and ionized gas at\n0.1-0.3 arcsec resolution in the Class 0 protostellar system IRAS 16293-2422.\nThese data clarify the origins of the protostellar outflows from the deeply\nembedded sources in this complex region. Source A2 is confirmed to be at the\norigin of the well known large scale north-east--south-west flow. The most\nrecent VLA observations reveal a new ejection from that protostar,\ndemonstrating that it drives an episodic jet. The central compact part of the\nother known large scale flow in the system, oriented roughly east-west, is well\ndelineated by the CO(6-5) emission imaged with ALMA and is confirmed to be\ndriven from within component A. Finally, a one-sided blueshifted bubble-like\noutflow structure is detected here for the first time from source B to the\nnorth-west of the system. Its very short dynamical timescale (~ 200 yr), low\nvelocity, and moderate collimation support the idea that source B is the\nyoungest object in the system, and possibly one of the youngest protostars\nknown.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Anatomy of an Unusual Edge-on Protoplanetary Disk I. Dust Settling\n  in a Cold Disk: As the earliest stage of planet formation, massive, optically thick, and gas\nrich protoplanetary disks provide key insights into the physics of star and\nplanet formation. When viewed edge-on, high resolution images offer a unique\nopportunity to study both the radial and vertical structures of these disks and\nrelate this to vertical settling, radial drift, grain growth, and changes in\nthe midplane temperatures. In this work, we present multi-epoch HST and Keck\nscattered light images, and an ALMA 1.3 mm continuum map for the remarkably\nflat edge-on protoplanetary disk SSTC2DJ163131.2-242627, a young solar-type\nstar in $\\rho$ Ophiuchus. We model the 0.8 $\\mu$m and 1.3 mm images in separate\nMCMC runs to investigate the geometry and dust properties of the disk using the\nMCFOST radiative transfer code. In scattered light, we are sensitive to the\nsmaller dust grains in the surface layers of the disk, while the sub-millimeter\ndust continuum observations probe larger grains closer to the disk midplane. An\nMCMC run combining both datasets using a covariance-based log-likelihood\nestimation was marginally successful, implying insufficient complexity in our\ndisk model. The disk is well characterized by a flared disk model with an\nexponentially tapered outer edge viewed nearly edge-on, though some degree of\ndust settling is required to reproduce the vertically thin profile and lack of\napparent flaring. A colder than expected disk midplane, evidence for dust\nsettling, and residual radial substructures all point to a more complex radial\ndensity profile to be probed with future, higher resolution observations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Science with the ngVLA: Planetary Nebulae: Planetary nebulae (PNe) represent the near endpoints of evolution for stars\nof initial mass $\\sim$1-8 $M_\\odot$, wherein the envelope of an asymptotic\ngiant branch (AGB) star becomes photodissociated and ionized by high-energy\nradiation from a newly emerging white dwarf that was the progenitor star's\ncore. It is increasingly evident that most PNe are descended from binary\nsystems. PNe hence provide unique insight into a diverse range of astrophysical\nphenomena, including the influence of companion stars on the late stages of\nstellar evolution; stellar wind interactions and shocks; the physics and\nchemistry of photoionized plasmas and photon-dominated regions (PDRs); and\nenrichment of the ISM in the products of intermediate-mass stellar\nnucleosynthesis. We describe specific examples of the potential impact of the\nngVLA in each of these areas.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Separable Solution for the Oscillatory Structure of Plasma in\n  Accretion Disks: We provide a new analysis of the system of partial differential equations\ndescribing the radial and vertical equilibria of the plasma in accretion disks.\nIn particular, we show that the partial differential system can be separated\nonce a definite, oscillatory (or hyperbolic) form for the radial dependence of\nthe relevant physical quantities is assumed. The system is thus reduced to an\nordinary differential system in the vertical dimensionless coordinate. The\nresulting equations can be integrated analytically in the limit of small\nmagnetic pressure. We complete our analysis with a direct numerical integration\nof the more general case. The main result is that a ring-like density profile\n(i.e., radial oscillations in the mass density) can appear even in the limit of\nsmall magnetic pressure.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Signature of Collision of Magnetic Flux Tubes in the Quiet Solar\n  Photosphere: Collision of the magnetic flux tubes in the Quiet Sun was proposed as one of\nthe possible sources for the heating of the solar atmosphere (Furusawa and\nSakai, 2000). The solar photosphere was observed using the New Solar Telescope\nad Big Bear Solar Observatory. In TiO spectral line at 705.68 nm we approached\nresolution of 0.1\". The horizontal plasma wave was observed spreading from the\nlarger bright point. Shorty after this wave an increase in the oscillatory\npower appeared at the same location as the observed bright point. This behavior\nmatches some of the results from the simulation of the collision of the two\nflux tubes with a weak current.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Kepler Study of Starspot Lifetimes with Respect to Light Curve\n  Amplitude and Spectral Type: Wide-field high precision photometric surveys such as Kepler have produced\nreams of data suitable for investigating stellar magnetic activity of cooler\nstars. Starspot activity produces quasi-sinusoidal light curves whose phase and\namplitude vary as active regions grow and decay over time. Here we investigate,\nfirstly, whether there is a correlation between the size of starspots - assumed\nto be related to the amplitude of the sinusoid - and their decay timescale and,\nsecondly, whether any such correlation depends on the stellar effective\ntemperature. To determine this, we computed the autocorrelation functions of\nthe light curves of samples of stars from Kepler and fitted them with apodised\nperiodic functions. The light curve amplitudes, representing spot size were\nmeasured from the root-mean-squared scatter of the normalised light curves. We\nused a Monte Carlo Markov Chain to measure the periods and decay timescales of\nthe light curves. The results show a correlation between the decay time of\nstarspots and their inferred size. The decay time also depends strongly on the\ntemperature of the star. Cooler stars have spots that last much longer, in\nparticular for stars with longer rotational periods. This is consistent with\ncurrent theories of diffusive mechanisms causing starspot decay. We also find\nthat the Sun is not unusually quiet for its spectral type - stars with\nsolar-type rotation periods and temperatures tend to have (comparatively)\nsmaller starspots than stars with mid-G or later spectral types.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Seismic detection of acoustic sharp features in the CoRoT target HD49933: The technique of determining the acoustic location of layers of sharp changes\nin the sound speed inside a star from the oscillatory signal in its frequencies\nis applied on a solar-type star, the CoRoT target, HD49933. We are able to\ndetermine the acoustic depth of the second helium ionisation zone of HD49933 to\nbe 794 +55/-68 seconds. The acoustic depth of the base of the convective zone\nis found to be 1855 +173/-412 seconds where the large error bars reflect the\nambiguity in the result, which is difficult to determine with present precision\non the frequencies because of the intrinsically weak nature of the signal. The\npositions of both these layers are consistent with those in a representative\nstellar model of HD49933.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Origin of Major Solar Activity - Collisional Shearing Between\n  Nonconjugated Polarities of Multiple Bipoles Emerging Within Active Regions: Active Regions (ARs) that exhibit compact Polarity Inversion Lines (PILs) are\nknown to be very flare-productive. However, the physical mechanisms behind this\nstatistical inference have not been demonstrated conclusively. We show that\nsuch PILs can occur due to the collision between two emerging flux tubes nested\nwithin the same AR. In such multipolar ARs, the flux tubes may emerge\nsimultaneously or sequentially, each initially producing a bipolar magnetic\nregion (BMR) at the surface. During each flux tube's emergence phase, the\nmagnetic polarities can migrate such that opposite polarities belonging to\ndifferent BMRs collide, resulting in shearing and cancellation of magnetic\nflux. We name this process 'collisional shearing' to emphasize that the\nshearing and flux cancellation develops due to the collision. Collisional\nshearing is a process different from the known concept of flux cancellation\noccurring between polarities of a single bipole, a process that has been\ncommonly used in many numerical models. High spatial and temporal resolution\nobservations from the Solar Dynamics Observatory for two emerging ARs, AR11158\nand AR12017, show the continuous cancellation of up to 40% of the unsigned\nmagnetic flux of the smallest BMR, which occurs at the collisional PIL for as\nlong as the collision persists. The flux cancellation is accompanied by a\nsuccession of solar flares and CMEs, products of magnetic reconnection along\nthe collisional PIL. Our results suggest that the quantification of magnetic\ncancellation driven by collisional shearing needs to be taken into\nconsideration in order to improve the prediction of solar energetic events and\nspace weather.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Carbon, nitrogen and $\u03b1$-element abundances determine the formation\n  sequence of the Galactic thick and thin disks: Using the DR12 public release of APOGEE data, we show that thin and thick\ndisk separate very well in the space defined by [$\\alpha$/Fe], [Fe/H] and\n[C/N]. Thick disk giants have both higher [C/N] and higher [$\\alpha$/Fe] than\ndo thin disk stars with similar [Fe/H]. We deduce that the thick disk is\ncomposed of lower mass stars than the thin disk. Considering the fact that at a\ngiven metallicity there is a one-to-one relation between stellar mass and age,\nwe are then able to infer the chronology of disk formation. Both the thick and\nthe thin disks - defined by [$\\alpha$/Fe] -- converge in their dependance on\n[C/N] and [C+N/Fe] at [Fe/H]$\\approx$-0.7. We conclude that 1) the majority of\nthick disk stars formed earlier than did the thin disk stars 2) the formation\nhistories of the thin and thick disks diverged early on, even when the [Fe/H]\nabundances are similar 3) that the star formation rate in the thin disk has\nbeen lower than in the thick disk, at all metallicities. Although these general\nconclusions remain robust, we also show that current stellar evolution models\ncannot reproduce the observed C/N ratios for thick disk stars. Unexpectedly,\nreduced or inhibited canonical extra-mixing is very common in field stars.\nWhile subject to abundance calibration zeropoint uncertainties, this implies a\nstrong dependence of non canonical extra-mixing along the red giant branch on\nthe initial composition of the star and in particular on the $\\alpha$ elemental\nabundance.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the photometric signature of fast rotators: Rapidly rotating stars have been recently recognized as having a major role\nin the interpretation of colour-magnitude diagrams of young and\nintermediate-age star clusters in the Magellanic Clouds and in the Milky Way.\nIn this work, we evaluate the distinctive spectra and distributions in\ncolour-colour space that follow from the presence of a substantial range in\neffective temperatures across the surface of fast rotators. The calculations\nare inserted in a formalism similar to the one usually adopted for non-rotating\nstars, which allows us to derive tables of bolometric corrections as a function\nnot only of a reference effective temperature, surface gravity and metallicity,\nbut also of the rotational speed with respect to the break-up value, $\\omega$,\nand the inclination angle, $i$. We find that only very fast rotators\n($\\omega>0.95$) observed nearly equator-on ($i>45^\\circ$) present sizable\ndeviations from the colour-colour relations of non-rotating stars. In light of\nthese results, we discuss the photometry of the $\\sim$ 200-Myr-old cluster NGC\n1866 and its split main sequence, which has been attributed to the simultaneous\npresence of slow and fast rotators. The small dispersion of its stars in\ncolour-colour diagrams allow us to conclude that fast rotators in this cluster\neither have rotational velocities $\\omega<0.95$, or are all observed nearly\npole-on. Such geometric colour-colour effects, although small, might be\npotentially detectable in the huge, high-quality photometric samples in the\npost-Gaia era, in addition to the evolutionary effects caused by\nrotation-induced mixing.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Predictive Capabilities of Avalanche Models for Solar Flares: We assess the predictive capabilities of various classes of avalanche models\nfor solar flares. We demonstrate that avalanche models cannot generally be used\nto predict specific events due to their high sensitivity to their embedded\nstochastic process. We show that deterministically driven models can\nnevertheless alleviate this caveat and be efficiently used for large events\npredictions. Our results promote a new approach for large (typically X-class)\nsolar flares predictions based on simple and computationally inexpensive\navalanche models.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Kepler observations of the beaming binary KPD 1946+4340: The Kepler Mission has acquired 33.5d of continuous one-minute photometry of\nKPD 1946+4340, a short-period binary system that consists of an sdB and a white\ndwarf. In the light curve, eclipses are clearly seen, with the deepest\noccurring when the compact white dwarf crosses the disc of the sdB (0.4%) and\nthe more shallow ones (0.1%) when the sdB eclipses the white dwarf. As\nexpected, the sdB is deformed by the gravitational field of the white dwarf,\nwhich produces an ellipsoidal modulation of the light curve. Spectacularly, a\nvery strong Doppler beaming (aka Doppler boosting) effect is also clearly\nevident at the 0.1% level. This originates from the sdB's orbital velocity,\nwhich we measure to be 164.0\\pm1.9 km/s from supporting spectroscopy. We\npresent light curve models that account for all these effects, as well as\ngravitational lensing. We derive system parameters and uncertainties from the\nlight curve using Markov Chain Monte Carlo simulations. Adopting a theoretical\nwhite dwarf mass-radius relation, the mass of the subdwarf is found to be\n0.47\\pm0.03 Msun and the mass of the white dwarf 0.59\\pm0.02 Msun. The\neffective temperature of the white dwarf is 15 900\\pm300K. With a spectroscopic\neffective temperature of Teff = 34 730\\pm250K and a surface gravity of log g =\n5.43\\pm0.04, the sdB is in a shell He burning stage. The detection of Doppler\nbeaming in Kepler light curves potentially allows one to measure radial\nvelocities without the need of spectroscopic data. For the first time, a\nphotometrically observed Doppler beaming amplitude is compared to a\nspectroscopically established value. The sdB's radial velocity amplitude\nderived from the photometry 168\\pm4 km/s is in perfect agreement with the\nspectroscopic value. After subtracting our best model for the orbital effects,\nwe searched the residuals for stellar oscillations but did not find any\nsignificant pulsation frequencies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Araucaria Project: High-precision orbital parallax and masses of the\n  eclipsing binary TZ~Fornacis: Context: Independent distance estimates are particularly useful to check the\nprecision of other distance indicators, while accurate and precise masses are\nnecessary to constrain evolution models. Aim: The goal is to measure the masses\nand distance of the detached eclipsing-binary TZ~For with a precision level\nlower than 1\\,\\% using a fully geometrical and empirical method. Method: We\nobtained the first interferometric observations of TZ~For with the VLTI/PIONIER\ncombiner, which we combined with new and precise radial velocity measurements\nto derive its three-dimensional orbit, masses, and distance. Results: The\nsystem is well resolved by PIONIER at each observing epoch, which allowed a\ncombined fit with eleven astrometric positions. Our derived values are in a\ngood agreement with previous work, but with an improved precision. We measured\nthe mass of both components to be $M_1 = 2.057 \\pm 0.001\\,M_\\odot$ and $M_2 =\n1.958 \\pm 0.001\\,M_\\odot$. The comparison with stellar evolution models gives\nan age of the system of $1.20 \\pm 0.10$\\,Gyr. We also derived the distance to\nthe system with a precision level of 1.1\\,\\%: $d = 185.9 \\pm 1.9$\\,pc. Such\nprecise and accurate geometrical distances to eclipsing binaries provide a\nunique opportunity to test the absolute calibration of the surface\nbrightness-colour relation for late-type stars, and will also provide the best\nopportunity to check on the future Gaia measurements for possible systematic\nerrors.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Multi-wavelength Spectral Analysis of Ellerman Bombs Observed by FISS\n  and IRIS: Ellerman bombs (EBs) are a kind of solar activities that is suggested to\noccur in the lower atmosphere. Recent observations using the Interface Region\nImaging Spectrograph (IRIS) show connections of EBs and IRIS bombs (IBs),\nimplying that EBs might be heated to a much higher temperature ($8\\times10^{4}$\nK) than previous results. Here we perform a spectral analysis of the EBs\nsimultaneously observed by the Fast Imaging Solar Spectrograph (FISS) and IRIS.\nThe observational results show clear evidence of heating in the lower\natmosphere, indicated by the wing enhancement in H$\\alpha$, Ca II 8542 \\r{A}\nand Mg II triplet lines, and also by brightenings in the images of 1700 \\r{A}\nand 2832 \\r{A} ultraviolet continuum channels. Additionally, the Mg II triplet\nline intensity is correlated with that of H$\\alpha$ when the EB occurs,\nindicating the possibility to use the triplet as an alternative way to identify\nEBs. However, we do not find any signal in IRIS hotter lines (C II and Si IV).\nFor further analysis, we employ a two-cloud model to fit the two chromospheric\nlines (H$\\alpha$ and Ca II 8542 \\r{A}) simultaneously, and obtain a temperature\nenhancement of 2300 K for a strong EB. This temperature is among the highest of\nprevious modeling results while still insufficient to produce IB signatures at\nultraviolet wavelengths.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Multi-epoch spectropolarimetry of SN 2009ip: direct evidence for\n  aspherical circumstellar material: We present spectropolarimetry of SN 2009ip throughout the evolution of its\n2012 explosion. During the initial 2012a phase, when the source spectrum\nexhibits broad P-Cygni lines, we measure a V-band polarization of P~0.9% at a\nposition angle of ~166 deg, indicating substantial asphericity for the 2012a\noutflow. Near the subsequent peak of the 2012b phase, when the spectrum shows\nsigns of intense interaction with circumstellar material (CSM), we measure\nP~1.7% at a position angle of 72 deg, indicating a separate physical component\nof polarization, which has a higher degree of asphericity than the 2012a\noutflow and an orthogonal axis of symmetry on the sky. Around 30 days past\npeak, which is coincident with a bump in the declining light curve, we measure\nP~0.7% and significant variations in P across some broad lines, particularly\nHeI/NaI. By 60 days past peak the source appears to be approaching a low value\nof interstellar polarization (P<0.2%). The results are consistent with a\nscenario in which a potentially bipolar explosion during 2012a impacts a\ntoroidal distribution of CSM, thereby causing the 2012b brightening.\nOrthogonality between the 2012a and 2012b geometries is inconsistent with the\nhypothesis that the 2012a event launched the CSM that was hit 40 days later by\nmaterial from the 2012b event. Rather, the aspherical CSM probably has a\nseparate origin associated with the outbursts of the stellar progenitor during\nprior years. Previous calculations that assumed spherical symmetry for the CSM\nhave underestimated the required explosion energy, as the results suggest that\n<10% of the fast SN ejecta participated in strong CSM interaction during the\n2012b phase. In light of the spectropolarimetric results, a kinetic energy of\n1e51 erg for the ejecta is difficult to avoid, supporting the interpretation\nthat the 2012 outburst was the result of a core-collapse supernova explosion\n(abridged)",
        "category": "astro-ph_SR"
    },
    {
        "text": "Heavy Elements Nucleosynthesis On Accreting White Dwarfs: building seeds\n  for the p-process: The origin of the proton-rich trans-iron isotopes in the solar system is\nstill uncertain. Single-degenerate thermonuclear supernovae (SNIa) with\nn-capture nucleosynthesis seeds assembled in the external layers of the\nprogenitor's rapidly accreting white dwarf phase may produce these isotopes. We\ncalculate the stellar structure of the accretion phase of five white dwarf\nmodels with initial masses >~ 0.85Msun using the stellar code MESA. The\nnear-surface layers of the 1, 1.26, 1.32 and 1.38Msun models are most\nrepresentative of the regions in which the bulk of the p nuclei are produced\nduring SNIa explosions, and for these models we also calculate the\nneutron-capture nucleosynthesis in the external layers. Contrary to previous\nrapidly-accreting white dwarf models at lower mass, we find that the H-shell\nashes are the main site of n-capture nucleosynthesis. We find high neutron\ndensities up to several 10^15 cm^-3 in the most massive WDs. Through the\nrecurrence of the H-shell ashes these intermediate neutron densities can be\nsustained effectively for a long time leading to high neutron exposures with a\nstrong production up to Pb. Both the neutron density and the neutron exposure\nincrease with increasing the mass of the accreting WD. Finally, the SNIa\nnucleosynthesis is calculated using the obtained abundances as seeds. We obtain\nsolar to super-solar abundances for p-nuclei with A>96. Our models show that\nSNIa are a viable p-process production site.",
        "category": "astro-ph_SR"
    },
    {
        "text": "An empirical clock to measure the dynamical age of stellar systems: Blue Straggler Stars (BSS) are among the brightest and more massive stars in\nglobular clusters (GCs). For this reason they represent an ideal tool to probe\nthe dynamical evolution of these stellar systems. Here I show, following the\nresults by Ferraro et al. (2012), that the BSS radial distribution can be used\nas a powerful indicator of the cluster dynamical age. In fact on the basis of\ntheir BSS radial distribution shape, GCs can be efficiently grouped in\ndifferent families corresponding to the different dynamical stages reached by\nthe stellar systems. This allows to define a first empirical clock, the\ndynamical clock, able to measure the dynamical age of a stellar system from\npure observational quantities.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Faint warm debris disks around nearby bright stars explored by AKARI and\n  IRSF: Context: Debris disks are important observational clues for understanding\nplanetary-system formation process. In particular, faint warm debris disks may\nbe related to late planet formation near 1 AU. A systematic search of faint\nwarm debris disks is necessary to reveal terrestrial planet formation. Aims:\nFaint warm debris disks show excess emission that peaks at mid-IR wavelengths.\nThus we explore debris disks using the AKARI mid-IR all-sky point source\ncatalog (PSC), a product of the second generation unbiased IR all-sky survey.\nMethods : We investigate IR excess emission for 678 isolated main-sequence\nstars for which there are 18 micron detections in the AKARI mid-IR all-sky\ncatalog by comparing their fluxes with the predicted fluxes of the photospheres\nbased on optical to near-IR fluxes and model spectra. The near-IR fluxes are\nfirst taken from the 2MASS PSC. However, 286 stars with Ks<4.5 in our sample\nhave large flux errors in the 2MASS photometry due to saturation. Thus we have\nmeasured accurate J, H, and Ks band fluxes, applying neutral density (ND)\nfilters for Simultaneous InfraRed Imager for Unbiased Survey (SIRIUS) on IRSF,\nthe \\phi 1.4 m near-IR telescope in South Africa, and improved the flux\naccuracy from 14% to 1.8% on average. Results: We identified 53 debris-disk\ncandidates including eight new detections from our sample of 678 main-sequence\nstars. The detection rate of debris disks for this work is ~8%, which is\ncomparable with those in previous works by Spitzer and Herschel. Conclusion:\nThe importance of this study is the detection of faint warm debris disks around\nnearby field stars. At least nine objects have a large amount of dust for their\nages, which cannot be explained by the conventional steady-state collisional\ncascade model.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The IACOB spectroscopic database of Northern Galactic OB stars: We present the IACOB spectroscopic database, an homogeneous set of high\nquality, high resolution spectra of Galactic O- and B-type stars obtained with\nthe FIES spectrograph attached to the Nordic Optical Telescope. We also present\nsome results from ongoing projects using the IACOB database.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Absolute Properties of the Eclipsing $\u03b3$ Dor Star V404 Lyrae: We present the first high-resolution spectra for the eclipsing binary V404\nLyr showing $\\gamma$ Dor pulsations, which we use to study its absolute\nproperties. By fitting models to the disentangling spectrum of the primary\nstar, we found that it has an effective temperature of $T_{\\rm eff,1}=7,330 \\pm\n150$ K and a rotational velocity of $v_1\\sin$$i=148\\pm18$ km s$^{-1}$. The\nsimultaneous analysis of our double-lined radial velocities and the\npulsation-subtracted ${\\it Kepler}$ data gives us accurate stellar and system\nparameters of V404 Lyr. The masses, radii, and luminosities are $M_1$ =\n2.17$\\pm$0.06 M$_\\odot$, $R_1$ = 1.91$\\pm$0.02 R$_\\odot$, and $L_1$ =\n9.4$\\pm$0.8 L$_\\odot$ for the primary, and $M_2$ = 1.42$\\pm$0.04 M$_\\odot$,\n$R_2$ = 1.79$\\pm$0.02 R$_\\odot$, and $L_2$ = 2.9$\\pm$0.2 L$_\\odot$ for the\nsecondary. The tertiary component orbiting the eclipsing pair has a mass of\n$M_{\\rm 3b}$ = 0.71$\\pm$0.15 $M_\\odot$ in an orbit of $P_{\\rm 3b}$ = 642$\\pm$3\ndays, $e_{\\rm 3b}$ = 0.21$\\pm$0.04, and $a_{\\rm 3b}$ = 509$\\pm$2 R$_\\odot$. The\nthird light of $l_3=4.1\\pm0.2\\%$ could be partly attributable to the K-type\ncircumbinary object. By applying a multiple frequency analysis to the\neclipse-subtracted light residuals, we detected 45 frequencies with signal to\nnoise amplitude ratios larger than 4.0. Identified as independent pulsation\nmodes, seven frequencies ($f_1-f_6$, $f_9$), their new pulsation constants, and\nthe location in the Hertzsprung-Russell diagram indicate that the pulsating\nprimary is a $\\gamma$ Dor-type variable star.",
        "category": "astro-ph_SR"
    },
    {
        "text": "STiC -- A multi-atom non-LTE PRD inversion code for full-Stokes solar\n  observations: The inference of the underlying state of the plasma in the solar chromosphere\nremains extremely challenging because of the nonlocal character of the observed\nradiation and plasma conditions in this layer. Inversion methods allow us to\nderive a model atmosphere that can reproduce the observed spectra by\nundertaking several physical assumptions.\n  The most advanced approaches involve a depth-stratified model atmosphere\ndescribed by temperature, line-of-sight velocity, turbulent velocity, the three\ncomponents of the magnetic field vector, and gas and electron pressure. The\nparameters of the radiative transfer equation are computed from a solid ground\nof physical principles. To apply these techniques to spectral lines that sample\nthe chromosphere, NLTE effects must be included in the calculations.\n  We developed a new inversion code STiC to study spectral lines that sample\nthe upper chromosphere. The code is based the RH synthetis code, which we\nmodified to make the inversions faster and more stable. For the first time,\nSTiC facilitates the processing of lines from multiple atoms in non-LTE, also\nincluding partial redistribution effects. Furthermore, we include a\nregularization strategy that allows for model atmospheres with a complex\nstratification, without introducing artifacts in the reconstructed physical\nparameters, which are usually manifested in the form of oscillatory behavior.\nThis approach takes steps toward a node-less inversion, in which the value of\nthe physical parameters at each grid point can be considered a free parameter.\n  In this paper we discuss the implementation of the aforementioned techniques,\nthe description of the model atmosphere, and the optimizations that we applied\nto the code. We carry out some numerical experiments to show the performance of\nthe code and the regularization techniques that we implemented. We made STiC\npublicly available to the community.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the nature of massive helium star winds and Wolf-Rayet-type mass loss: The mass-loss rates of massive helium stars are one of the major\nuncertainties in modern astrophysics. Regardless of whether they were stripped\nby a binary companion or managed to peel off their outer layers by themselves,\nthe influence and final fate of helium stars -- in particular the resulting\nblack hole mass -- highly depends on their wind mass loss as stripped-envelope\nobjects. While empirical mass-loss constraints for massive helium stars have\nimproved over the last decades, the resulting recipes are limited to\nmetallicities with the observational ability to sufficiently resolve individual\nstars. Yet, theoretical efforts have been hampered by the complexity of\nWolf-Rayet (WR) winds arising from the more massive helium stars. In an\nunprecedented effort, we calculate next-generation stellar atmosphere models\nresembling massive helium main sequence stars with Fe-bump driven winds up to\n$500\\,M_\\odot$ over a wide metallicity range between $2.0$ and $0.02\\,Z_\\odot$.\nWe uncover a complex $\\Gamma_\\text{e}$-dependency of WR-type winds and their\nmetallicity-dependent breakdown. The latter can be related to the onset of\nmultiple scattering, requiring higher $L/M$-ratios at lower metallicity. Based\non our findings, we derive the first ever theoretically-motivated mass-loss\nrecipe for massive helium stars. We also provide estimates for LyC and He II\nionizing fluxes, finding stripped helium stars to contribute considerably at\nlow metallicity. In sharp contrast to OB-star winds, the mass loss for helium\nstars scales with the terminal velocity. While limited to the helium main\nsequence, our study marks a major step towards a better theoretical\nunderstanding of helium star evolution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Photoionization and electron-ion recombination in astrophysical plasmas: Photoionization and its inverse, electron-ion recombination, are key\nprocesses that influence many astrophysical plasmas (and gasses), and the\ndiagnostics that we use to analyse the plasmas. In this review we provide a\nbrief overview of the importance of photoionization and recombination in\nastrophysics. We highlight how the data needed for spectral analyses, and the\nrequired accuracy, varies considerably in different astrophysical environments.\nWe then discuss photoionization processes, highlighting resonances in their\ncross-sections. Next we discuss radiative recombination, and low and high\ntemperature dielectronic recombination. The possible suppression of low\ntemperature dielectronic recombination (LTDR) and high temperature dielectronic\nrecombination (HTDR) due to the radiation field and high densities is\ndiscussed. Finally we discuss a few astrophysical examples to highlight\nphotoionization and recombination processes.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evidence of a Plasmoid-Looptop Interaction and Magnetic Inflows During a\n  Solar Flare/CME Eruptive Event: Observational evidence is presented for the merging of a downward-propagating\nplasmoid with a looptop kernel during an occulted limb event on 2007 January\n25. RHESSI lightcurves in the 9-18 keV energy range, as well as that of the 245\nMHz channel of the Learmonth Solar Observatory, show enhanced nonthermal\nemission in the corona at the time of the merging suggesting that additional\nparticle acceleration took place. This was attributed to a secondary episode of\nreconnection in the current sheet that formed between the two merging sources.\nRHESSI images were used to establish a mean downward velocity of the plasmoid\nof 12 km/s. Complementary observations from the SECCHI suite of instruments\nonboard STEREO-Behind showed that this process occurred during the acceleration\nphase of the associated CME. From wavelet-enhanced EUVI, images evidence of\ninflowing magnetic field lines prior to the CME eruption is also presented. The\nderived inflow velocity was found to be 1.5 km/s. This combination of\nobservations supports a recent numerical simulation of plasmoid formation,\npropagation and subsequent particle acceleration due to the tearing mode\ninstability during current sheet formation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the Distribution of Orbital Eccentricities for Very Low-Mass Binaries: We have compiled a sample of 16 orbits for very low-mass stellar (<0.1 Msun)\nand brown dwarf binaries, enabling the first comprehensive study of the\neccentricity distribution for such objects. We find that very low-mass binaries\nspan a broad range of eccentricities (0.03<e<0.83), with a median eccentricity\nof 0.34. We examine potential observational biases in this sample, and for\nvisual binaries we show through Monte Carlo simulations that appropriate\nselection criteria result in all eccentricities being equally represented (<5%\ndifference between input and output e distributions). The orbits of this sample\nof very low-mass binaries show some significant differences from their\nsolar-type counterparts. They lack a correlation between orbital period and\neccentricity and display a much higher fraction of near-circular orbits (e<0.1)\nthan solar-type stars, which together suggest a different formation mechanism\nor dynamical history for these two populations. Very low-mass binaries also do\nnot follow the e^2 distribution of Ambartsumian (1937), which would be expected\nif their orbits were distributed in phase space according to a function of\nenergy alone (e.g., the Boltzmann distribution). We find that numerical\nsimulations of very low-mass star formation do not completely reproduce the\nobserved properties of our binary sample. The cluster formation model of Bate\n(2009) agrees very well with the overall e distribution, but lack any high-e\n(>0.6) binaries at orbital periods comparable to our sample. In contrast, the\ncircumstellar disk fragmentation model of Stamatellos & Whitworth (2009)\npredicts only high-e binaries and thus is highly inconsistent with our sample.\nThese discrepancies could be explained if multiple formation processes have\nproduced the field population.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The intimate relation between the low T/W instability and the\n  co-rotation point: We study the low T/W instability associated with the f-mode of differentially\nrotating stars. Our stellar models are described by a polytropic equation of\nstate and the rotation profile is given by the standard j-constant law. The\nproperties of the relevant oscillation modes, including the instability growth\ntime, are determined from time evolutions of the linearised dynamical equations\nin Newtonian gravity. In order to analyse the instability we monitor also the\ncanonical energy and angular momentum. Our results demonstrate that the l=m=2\nf-mode becomes unstable as soon as a co-rotation point develops inside the star\n(i.e. whenever there is a point where the mode's pattern speed matches the bulk\nangular velocity). Considering various degrees of differential rotation, we\nshow that the instability grows faster deep inside the co-rotation region and\ndeduce an empirical relation that correlates the mode frequency and the star's\nparameters, which captures the main features of the l=m=2 f-mode growth time.\nThis function is proportional to the product of the kinetic to gravitational\nenergy ratio and the gradient of the star's spin, strengthening further the\nrelationship between the co-rotation point and the low T/W instability. We\nbriefly consider also the l=m=2 r-mode and demonstrate that it never moves far\ninside the co-rotation region even for significant differential rotation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Flux emergence and coronal eruption: Our aim is to study the photospheric flux distribution of a twisted flux tube\nthat emerges from the solar interior. We also report on the eruption of a new\nflux rope when the emerging tube rises into a pre-existing magnetic field in\nthe corona. To study the evolution, we use 3D numerical simulations by solving\nthe time-dependent and resistive MHD equations. We qualitatively compare our\nnumerical results with MDI magnetograms of emerging flux at the solar surface.\nWe find that the photospheric magnetic flux distribution consists of two\nregions of opposite polarities and elongated magnetic tails on the two sides of\nthe polarity inversion line (PIL), depending on the azimuthal nature of the\nemerging field lines and the initial field strength of the rising tube. Their\nshape is progressively deformed due to plasma motions towards the PIL. Our\nresults are in qualitative agreement with observational studies of magnetic\nflux emergence in active regions (ARs). Moreover, if the initial twist of the\nemerging tube is small, the photospheric magnetic field develops an undulating\nshape and does not possess tails. In all cases, we find that a new flux rope is\nformed above the original axis of the emerging tube that may erupt into the\ncorona, depending on the strength of the ambient field.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Exploring Bistability in the Cycles of the Solar Dynamo through Global\n  Simulations: The calling card of solar magnetism is the sunspot cycle, during which\nsunspots regularly reverse their polarity sense every 11 years. However, a\nnumber of more complicated time-dependent behaviors have also been identified.\nIn particular, there are temporal modulations associated with active longitudes\nand hemispheric asymmetry, when sunspots appear at certain solar longitudes or\nelse in one hemisphere preferentially. So far, a direct link between between\nthis asymmetric temporal behavior and the underlying solar dynamo has remained\nelusive. In this work, we present results from global, 3D magnetohydrodynamic\n(MHD) simulations, which for the first time display both behavior reminiscent\nof the sunspot cycle (regular polarity reversals and equatorward migration of\ninternal magnetic field) and asymmetric, irregular behavior that in the\nsimulations we interpret as active longitudes and hemispheric asymmetry. The\nsimulations are thus bistable, in that the turbulent convection can stably\nsupport two distinct flavors of magnetism at different times, in superposition,\nor with smooth transitions from one state to the other. We discuss this new\nfamily of dynamo models in the context of the extensive observations of the\nSun's surface magnetic field with the Solar and Heliospheric Observatory (SOHO)\nand the Solar Dynamics Observatory (SDO), as well as earlier observations of\nsunspot number and synoptic maps. We suggest that the solar dynamo itself may\nbe bistable in nature, exhibiting two types of temporal behavior in the\nmagnetic field.",
        "category": "astro-ph_SR"
    },
    {
        "text": "High spatial resolution imaging of SO and H2CO in AB Auriga: the first\n  SO image in a transitional disk: Transitional disks are structures of dust and gas around young stars with\nlarge inner cavities in which planet formation may occur. Lopsided dust\ndistributions are observed in the dust continuum emission at millimeter\nwavelengths. These asymmetrical structures can be explained as the result of an\nenhanced gas density vortex where the dust is trapped potentially promoting the\nrapid growth to the planetesimal scale. AB Aur hosts a transitional disk with a\nclear horseshoe morphology which strongly suggests the presence of a dust trap.\nOur goal is to investigate its formation and the possible effects on the gas\nchemistry. We used the NOEMA interferometer to image the 1mm continuum dust\nemission and the 13CO J=2->1, C18O J=2->1, SO J=56->45 and H2CO J=303->202\nrotational lines. Line integrated intensity ratio images are built to\ninvestigate the chemical changes within the disk. We have used a single point\n(n,T) chemical model to investigate the lifetime of gaseous CO, H2CO and SO in\nthe dust trap. Our model shows that for densities >10^7 cm^-3, the SO molecules\nare depleted (directly frozen or converted into SO2 and then frozen out) in\nless than 0.1 Myr. The lower SO abundance towards the dust trap could indicate\nthat a larger fraction of the gas is in a high density environment. Gas\ndynamics, grain growth and gas chemistry are coupled in the planet formation\nprocess. Because of the strong dependence of SO abundance on the gas density,\nthe sulfur chemistry can be used as a chemical diagnostic to detect the\nbirthsites of future planets. However, the large uncertainties inherent to\nchemical models and the limited knowledge of the disk physical structure and\ninitial conditions are important drawbacks.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Driving solar coronal MHD simulations on high-performance computers: The quality of today's research is often tightly limited to the available\ncomputing power and scalability of codes to many processors. For example,\ntackling the problem of heating the solar corona requires a most realistic\ndescription of the plasma dynamics and the magnetic field. Numerically solving\nsuch a magneto-hydrodynamical (MHD) description of a small active region (AR)\non the Sun requires millions of computation hours on current high-performance\ncomputing (HPC) hardware. The aim of this work is to describe methods for an\nefficient parallelization of boundary conditions and data input/output (IO)\nstrategies that allow for a better scaling towards thousands of processors\n(CPUs). The Pencil Code is tested before and after optimization to compare the\nperformance and scalability of a coronal MHD model above an AR. We present a\nnovel boundary condition for non-vertical magnetic fields in the photosphere,\nwhere we approach the realistic pressure increase below the photosphere. With\nthat, magnetic flux bundles become narrower with depth and the flux density\nincreases accordingly. The scalability is improved by more than one order of\nmagnitude through the HPC-friendly boundary conditions and IO strategies. This\nwork describes also the necessary nudging methods to drive the MHD model with\nobserved magnetic fields from the Sun's photosphere. In addition, we present\nthe upper and lower atmospheric boundary conditions (photospheric and towards\nthe outer corona), including swamp layers to diminish perturbations before they\nreach the boundaries. Altogether, these methods enable more realistic 3D MHD\nsimulations than previous models regarding the coronal heating problem above an\nAR -- simply because of the ability to use a large amount of CPUs efficiently\nin parallel.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Hydrogen Balmer Line Broadening in Solar and Stellar Flares: The broadening of the hydrogen lines during flares is thought to result from\nincreased charge (electron, proton) density in the flare chromosphere. However,\ndisagreements between theory and modeling prescriptions have precluded an\naccurate diagnostic of the degree of ionization and compression resulting from\nflare heating in the chromosphere. To resolve this issue, we have incorporated\nthe unified theory of electric pressure broadening of the hydrogen lines into\nthe non-LTE radiative transfer code RH. This broadening prescription produces a\nmuch more realistic spectrum of the quiescent, A0 star Vega compared to the\nanalytic approximations used as a damping parameter in the Voigt profiles. We\ntest recent radiative-hydrodynamic (RHD) simulations of the atmospheric\nresponse to high nonthermal electron beam fluxes with the new broadening\nprescription and find that the Balmer lines are over-broadened at the densest\ntimes in the simulations. Adding many simultaneously heated and cooling model\nloops as a \"multithread\" model improves the agreement with the observations. We\nrevisit the three-component phenomenological flare model of the YZ CMi\nMegaflare using recent and new RHD models. The evolution of the broadening,\nline flux ratios, and continuum flux ratios are well-reproduced by a\nmultithread model with high-flux nonthermal electron beam heating, an extended\ndecay phase model, and a \"hot spot\" atmosphere heated by an ultrarelativistic\nelectron beam with reasonable filling factors: 0.1%, 1%, and 0.1% of the\nvisible stellar hemisphere, respectively. The new modeling motivates future\nwork to understand the origin of the extended gradual phase emission.",
        "category": "astro-ph_SR"
    },
    {
        "text": "An AMR Study of the Common Envelope Phase of Binary Evolution: The hydrodynamic evolution of the common envelope phase of a low mass binary\ncomposed of a 1.05 Msun red giant and a 0.6 Msun companion has been followed\nfor five orbits of the system using a high resolution method in three spatial\ndimensions. During the rapid inspiral phase, the interaction of the companion\nwith the red giant's extended atmosphere causes about 25% of the common\nenvelope to be ejected from the system, with mass continuing to be lost at the\nend of the simulation at a rate ~ 2 Msun/yr. In the process the resulting loss\nof angular momentum and energy reduces the orbital separation by a factor of\nseven. After this inspiral phase the eccentricity of the orbit rapidly\ndecreases with time. The gravitational drag dominates hydrodynamic drag at all\ntimes in the evolution, and the commonly-used Bondi-Hoyle-Lyttleton\nprescription for estimating the accretion rate onto the companion significantly\noverestimates the true rate. On scales comparable to the orbital separation,\nthe gas flow in the orbital plane in the vicinity of the two cores is subsonic\nwith the gas nearly corotating with the red giant core and circulating about\nthe red giant companion. On larger scales, 90% of the outflow is contained\nwithin 30 degrees of the orbital plane, and the spiral shocks in this material\nleave an imprint on the density and velocity structure. Of the energy released\nby the inspiral of the cores, only about 25% goes toward ejection of the\nenvelope.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Physical and chemical properties of Wolf-Rayet planetary nebulae: Wolf-Rayet ([WR]) and weak emission-line ($wels$) central stars of planetary\nnebulae (PNe) have hydrogen-deficient atmospheres, whose origins are not well\nunderstood. In the present study, we have conducted plasma diagnostics and\nabundance analyses of 18 Galactic PNe surrounding [WR] and $wels$ nuclei, using\ncollisionally excited lines (CELs) and optical recombination lines (ORLs)\nmeasured with the Wide Field Spectrograph on the ANU 2.3-m telescope at the\nSiding Spring Observatory complemented with optical archival data. Our plasma\ndiagnostics imply that the electron densities and temperatures derived from\nCELs are correlated with the intrinsic nebular H$\\beta$ surface brightness and\nexcitation class, respectively. Self-consistent plasma diagnostics of heavy\nelement ORLs of N${}^{2+}$ and O${}^{2+}$ suggest that a small fraction of cool\n($\\lesssim 7000$ K), dense ($\\sim 10^4-10^5$ cm$^{-3}$) materials may be\npresent in some objects, though with large uncertainties. Our abundance\nanalyses indicate that the abundance discrepancy factors (ADF$\\equiv$ORLs/CELs)\nof O${}^{2+}$ are correlated with the dichotomies between forbidden-line and He\nI temperatures. Our results likely point to the presence of a tiny fraction of\ncool, oxygen-rich dense clumps within the diffuse warm ionized nebulae.\nMoreover, our elemental abundances derived from CELs are mostly consistent with\nAGB models in the range of initial masses from 1.5 to 5M$_{\\odot}$. Further\nstudies are necessary to understand better the origins of abundance\ndiscrepancies in PNe around [WR] and $wels$ stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Submillimeter continuum observations of Sagittarius B2 at subarcsecond\n  spatial resolution: We report the first high spatial resolution submillimeter continuum\nobservations of the Sagittarius B2 cloud complex using the Submillimeter Array\n(SMA). With the subarcsecond resolution provided by the SMA, the two massive\nstar-forming clumps Sgr B2(N) and Sgr B2(M) are resolved into multiple compact\nsources. In total, twelve submillimeter cores are identified in the Sgr B2(M)\nregion, while only two components are observed in the Sgr B2(N) clump. The gas\nmass and column density are estimated from the dust continuum emission. We find\nthat most of the cores have gas masses in excess of 100 M$_{\\odot}$ and column\ndensities above 10$^{25}$ cm$^{-2}$. The very fragmented appearance of Sgr\nB2(M), in contrast to the monolithic structure of Sgr B2 (N), suggests that the\nformer is more evolved. The density profile of the Sgr B2(N)-SMA1 core is well\nfitted by a Plummer density distribution. This would lead one to believe that\nin the evolutionary sequence of the Sgr B2 cloud complex, a massive star forms\nfirst in an homogeneous core, and the rest of the cluster forms subsequently in\nthe then fragmenting structure.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Collision strengths for FIR and UV transtions in PIII and the phosphorus\n  abundance: Phosphorus abundance is crucial for DNA-based extraterrestrial life in\nexoplanets. Atomic data for observed spectral lines of P-ions are needed for\nits accurate determination. We present the first calculations for collision\nstrengths for the forbidden PIII fine structure transition $3s^23p\n(^2P^o_{1/2-3/2})$ within the ground state at 17.9 $\\mu$m, as well as allowed\nUV transitions in the $3s^23p (^2P^o_{1/2,3/2}) \\rightarrow 3s3p^2\n(^2D_{3/2,5/2}, ^2S_{1/2}, ^2P_{1/2,3/2})$ multiplets between 915-1345 $\\AA$.\nCollision strengths are computed using the Breit-Pauli R-Matrix method\nincluding the first 18 levels, and they exhibit extensive auto-ionizing\nresonance structures. In particular, the Maxwellian averaged effective\ncollision strength for the FIR 17.9 $\\mu$m transition shows a factor 3\ntemperature variation broadly peaking at typical nebular temperatures. Its\ntheoretical emissivity with solar phosphorus abundance is computed relative to\nH$\\beta$ and found to be similar to observed intensties from planetary nebulae;\nthe abundances derived in earlier works are 3-5 times sub-solar. The results\npertain to the reported paucity of phosphorus from preferred production sites\nin supernovae, and abundances in planetary nebulae and supernova remnants.",
        "category": "astro-ph_SR"
    },
    {
        "text": "MAVKA: Software for Statistically Optimal Determination of Extrema: We introduce the program MAVKA for determination of characteristics of\nextrema using observations in the adjacent data intervals, with intended\napplications to variable stars, but it may be used for signals of arbitrary\nnature. We have used a dozen of basic functions, some of them use the interval\nnear extremum without splitting the interval (algebraic polynomial in general\nform, \"Symmetrical\" algebraic polynomial using only even degrees of time\n(phase) deviation from the position of symmetry argument), others split the\ninterval into 2 subintervals (a Taylor series of the \"New Algol Variable\", \"the\nfunction of Prof. Z. Mikul\\'a\\v{s}ek\"), or even 3 parts (\"Asymptotic Parabola\",\n\"Wall-Supported Parabola\", \"Wall-Supported Line\", \"Wall-Supported Asymptotic\nParabola\", \"Parabolic Spline of defect 1\"). The variety of methods allows to\nchoose the \"best\" (statistically optimal) approximation for a given data\nsample. As the criterion, we use the accuracy of determination of the extremum.\nFor all parameters, the statistical errors are determined. The methods are\nillustrated by applications to observations of pulsating and eclipsing variable\nstars, as well as to the exoplanet transits. They are used for the\ninternational campaigns \"Inter-Longitude Astronomy\", \"Virtual Observatory\" and\n\"AstroInformatics\". The program may be used for studies of individual objects,\nalso using ground-based (NSVS, ASAS, WASP, CRTS et al.) and space (GAIA,\nKEPLER, HIPPARCOS/TYCHO, WISE et al.) surveys.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Inferring properties of small convective cores in main-sequence\n  solar-like pulsators: This work concerns the study of the properties of convective cores in\nmain-sequence models of solar-like pulsators and what information they may hold\nabout stellar ages. We verified that the maximum absolute frequency derivative\nof particular combinations of frequencies, which we name \"the slopes\", provides\ninformation on the relative size of the discontinuity in the sound-speed\nprofile at the border of the convectively mixed region. Since the latter is\nrelated to the evolutionary state of stars, we show that for models with masses\nabove $1.3\\,\\rm M_\\odot$, it may be possible to estimate the fraction of\nstellar main-sequence evolution from the slopes. Moreover, for models with\nmasses below $1.2\\,\\rm M_\\odot$ we verified that it may be possible to use the\nslopes to discriminate against models with small amounts of core overshoot.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Primordial triples and collisions of massive stars: Massive stars are known to have a high multiplicity, with examples of higher\norder multiples among the nearest and best studied objects. In this paper we\nstudy hierarchical multiple systems (an inner binary as a component of a wider\nbinary) of massive stars in a clustered environment, in which a system with a\nsize of 100--1000 au will undergo many close encounters during the short\nlifetime of a massive star. Using two types of N-body experiment we determine\nthe post-formation collision probabilities of these massive hierarchies. We\nfind that, depending on the specifics of the environment, the hierarchy, and\nthe amount of time that is allowed to pass, tens of percent of hierarchies will\nexperience a collision, typically between the two stars of the inner binary. In\naddition to collisions, clusters hosting a hierarchical massive system produce\nhigh velocity runaways at an enhanced rate. The primordial multiplicity\nspecifics of massive stars appear to play a key role in the generation of these\nrelatively small number events in cluster simulations, complicating their use\nas diagnostics of a cluster's history.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Epicyclic frequencies of spheroidal stars with non-uniform density: We consider the gravitational potential of a rotating star with non-uniform\ndensity to derive the orbital and epicyclic frequencies of the particles\norbiting the star. We assume that the star is composed of concentric spheroids\nof constant density, with a global power-law distribution of density inside the\nstar. At the lowest order approximation, we recover the known result for the\nMaclaurin spheroid that the maximum in the radial epicyclic frequency occurs at\n$r=\\sqrt{2}ae$, for eccentricities $\\geq 1/\\sqrt{2}$. We find that the nature\nof these characteristic frequencies differs based on the geometry of the\nrotating star. For an oblate spheroid, the orbits resemble retrograde-Kerr\norbits and the location of the radial epicyclic maximum approaches the stellar\nsurface as the density variation inside the star becomes steeper. On the\ncontrary, orbits around a prolate spheroid resemble prograde-Kerr orbits, but\nthe marginally stable orbit does not exist for prolate-shaped stars. The\norbital frequency is larger (smaller) than the Keplerian value for an oblate\n(prolate) star with the equality attained as $e \\rightarrow 0$ or $r\n\\rightarrow \\infty$. The radial profiles of the angular velocity and the\nangular momentum allow for a stable accreting disc around any nature of\noblate/prolate spheroid.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Images of unclassified and supergiant B[e] stars disks with\n  interferometry: B[e] stars are among the most peculiar objects in the sky. This spectral\ntype, characterised by allowed and forbidden emission lines, and a large\ninfrared excess, does not represent an homogenous class of objects, but\ninstead, a mix of stellar bodies seen in all evolutionary status. Among them,\none can find Herbig stars, planetary nebulae central stars, interacting\nbinaries, supermassive stars, and even \"unclassified\" B[e] stars: systems\nsharing properties of several of the above. Interferometry, by resolving the\ninnermost regions of these stellar systems, enables us to reveal the true\nnature of these peculiar stars among the peculiar B[e] stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The kinematic characteristics of magnetic O-type stars: Although magnetic fields have been discovered in ten massive O-type stars\nduring the last years, the origin of their magnetic fields remains unknown.\nAmong the magnetic O-type stars, two stars, HD36879 and HD57682, were\nidentified as candidate runaway stars in the past, and theta^1 Ori C was\nreported to move rapidly away from its host cluster. We search for an\nexplanation for the occurrence of magnetic fields in O-type stars by examining\nthe assumption of their runaway status. We use the currently best available\nastrometric, spectroscopic, and photometric data to calculate the kinematical\nstatus of seven magnetic O-type stars with previously unknown space velocities.\nThe results of the calculations of space velocities suggest that five out of\nthe seven magnetic O-type stars can be considered as candidate runaway stars.\nOnly two stars, HD155806 and HD164794, with the lowest space velocities, are\nlikely members of Sco OB4 and NGC6530, respectively. However, the non-thermal\nradio emitter HD164794 is a binary system with colliding winds, for which the\ndetected magnetic field has probably a different origin in comparison to other\nmagnetic O-type stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Dynamical investigation of the multiple star ADS 9173 AB: Star ADS 9173=WDS 14135+5147=Hip 69483 is a complex system. The B component\nhas a spectroscopic companion, whose orbit with a period of 4.9 years has been\nknown since 1986. The Gaia telescope has detected a distant faint pair over 100\narcsec away from the bright AB pair. In our article, we study the movement in a\nbright pair based on long-term observations with the 26-inch refractor of the\nPulkovo Observatory. The AB pair orbit with a period of 6306 years was\ncalculated using the apparent motion parameters (AMP) method. The astrometric\norbit of the component B was determined on the basis of the residuals of the\nhomogeneous CCD observations up to 2023 with the 26-inch refractor. It is in\nagreement with the spectroscopic one. The remaining secondary residuals show a\nwave with a period of approximately 20 years, the reasons for which are\ndiscussed.",
        "category": "astro-ph_SR"
    },
    {
        "text": "EUV Emission and Scattered Light Diagnostics of Equatorial Coronal Holes\n  as Seen by Hinode/EIS: Spectroscopic diagnostics of solar coronal plasmas critically depends on the\nuncertainty in the measured line intensities. One of the main sources of\nuncertainty is instrumental scattered light, which is potentially most\nimportant in low-brightness areas. In the solar corona, such areas include\npolar and equatorial coronal holes, which are the source regions of the solar\nwind; instrument-scattered light must thus pose a significant obstacle to\nstudies of the source regions of the solar wind. In this paper we investigate\nthe importance of instrument-scattered light on observations of equatorial\ncoronal holes made by the Hinode/EIS spectrometer in two different phases of\nthe solar cycle. We find that the instrument-scattered light is significant at\nall temperatures, and in both regions it amounts to approximately 10% of the\naverage intensity of the neighboring quiet Sun regions. Such contribution\ndominates the measured intensity for spectral lines formed at temperatures\nlarger than Log T = 6.15 K, and has deep implications for spectroscopic\ndiagnostics of equatorial coronal hole plasmas and studies of the source\nregions of a large portion of the solar wind which reaches Earth. Our results\nsuggest that the high temperature tail of in the coronal hole plasma\ndistribution with temperature, however small, is an artifact due to the\npresence of scattered light.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Achieving Fast Reconnection in Resistive MHD Models via Turbulent Means: Astrophysical fluids are generally turbulent and this preexisting turbulence\nmust be taken into account for the models of magnetic reconnection which are\nattepmted to be applied to astrophysical, solar or heliospheric environments.\nIn addition, reconnection itself induces turbulence which provides an important\nfeedback on the reconnection process. In this paper we discuss both theoretical\nmodel and numerical evidence that magnetic reconnection gets fast in the\napproximation of resistive MHD. We consider the relation between the Lazarian &\nVishniac turbulent reconnection theory and Lapenta's numerical experiments\ntestifying of the spontaneous onset of turbulent reconnection in systems which\nare initially laminar.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Characterization of solar-cycle induced frequency shift of medium- and\n  high-degree acoustic modes: Although it is well known that the solar acoustic mode frequency increases as\nthe solar activity increases, the mechanism behind it is still unknown. Mode\nfrequencies with 20 < l < 900 obtained by applying spherical harmonic\ndecomposition to MDI full-disk observations were used. First, the dependence of\nsolar acoustic mode frequency with solar activity was examined and evidence of\na quadratic relation was found indicating a saturation effect at high solar\nactivity. Then, the frequency dependence of frequency differences between the\nactivity minimum and maximum was analyzed. The frequency shift scaled by the\nnormalized mode inertia follows a simple power law where the exponent for the p\nmodes decreases by 37% for modes with frequency larger than 2.5 mHz.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Effect of latitudinal differential rotation on solar Rossby waves:\n  Critical layers, eigenfunctions, and momentum fluxes in the equatorial\n  $\u03b2$ plane: Retrograde-propagating waves of vertical vorticity with longitudinal\nwavenumbers between 3 and 15 have been observed on the Sun with a dispersion\nrelation close to that of classical sectoral Rossby waves. The observed\nvorticity eigenfunctions are symmetric in latitude, peak at the equator, switch\nsign near $20^\\circ$-$30^\\circ$, and decrease at higher latitudes. We search\nfor an explanation that takes into account solar latitudinal differential\nrotation. In the equatorial $\\beta$ plane, we study the propagation of linear\nRossby waves (phase speed $c <0$) in a parabolic zonal shear flow, $U = -\n\\overline{U}\\ \\xi^2<0$, where $\\overline{U} = 244$ m/s and $\\xi$ is the sine of\nlatitude. In the inviscid case, the eigenvalue spectrum is real and continuous\nand the velocity stream functions are singular at the critical latitudes where\n$U = c$. We add eddy viscosity in the problem to account for wave attenuation.\nIn the viscous case, the stream functions are solution of a fourth-order\nmodified Orr-Sommerfeld equation. Eigenvalues are complex and discrete. For\nreasonable values of the eddy viscosity corresponding to supergranular scales\nand above (Reynolds number $100 \\le Re \\le 700$), all modes are stable. At\nfixed longitudinal wavenumber, the least damped mode is a symmetric mode with a\nreal frequency close to that of the classical Rossby mode, which we call the R\nmode. For $Re \\approx 300$, the attenuation and the real part of the\neigenfunction is in qualitative agreement with the observations (unlike the\nimaginary part of the eigenfunction, which has a larger amplitude in the model.\nConclusion: Each longitudinal wavenumber is associated with a latitudinally\nsymmetric R mode trapped at low latitudes by solar differential rotation. In\nthe viscous model, R modes transport significant angular momentum from the\ndissipation layers towards the equator.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Calibration of the mixing-length parameter $\u03b1$ for the MLT and FST\n  models by matching with CO$^5$BOLD models: The CoRoT and Kepler missions provided a wealth of high-quality data for\nsolar-like oscillations. To make the best of such data for seismic inferences,\nwe need theoretical models with precise near-surface structure, which has\nsignificant influence on solar-like oscillation frequencies. The mixing-length\nparameter, $\\alpha$, is a key factor for the near-surface structure. In the\nconvection formulations used in evolution codes, the $\\alpha$ is a free\nparameter that needs to be properly specified. We calibrated $\\alpha$ values by\nmatching entropy profiles of 1D envelope models with those of 3D CO$^5$BOLD\nmodels. For such calibration, previous works concentrated on the classical\nmixing-length theory (MLT). Here we also analyzed the full spectrum turbulence\n(FST) models. For the atmosphere part in the 1D models, we use the Eddington\ngrey $T(\\tau)$ relation and the one with the solar-calibrated Hopf-like\nfunction. For both the MLT and FST models with a mixing length $l=\\alpha H_p$,\ncalibrated $\\alpha$ values increase with increasing $g$ or decreasing $T_{\\rm\neff}$. For the FST models, we also calibrated values of $\\alpha^*$ defined as\n$l=r_{\\rm top}-r+\\alpha^*H_{p,{\\rm top}}$. $\\alpha^*$ is found to increase with\n$T_{\\rm eff}$ and $g$. As for the correspondence to the 3D models, the solar\nHopf-like function gives a photospheric-minimum entropy closer to a 3D model\nthan the Eddington $T(\\tau)$. The structure below the photosphere depends on\nthe convection model. However, not a single convection model gives the best\ncorrespondence since the averaged 3D quantities are not necessarily related via\nan EOS. Although the FST models with $l=r_{\\rm top}-r+\\alpha^*H_{p,{\\rm top}}$\nare found to give the frequencies closest to the solar observed ones, a more\nappropriate treatment of the top part of the 1D convective envelope is\nnecessary.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The nonisothermal stage of magnetic star formation. II. Results: In a previous paper we formulated the problem of the formation and evolution\nof fragments (or cores) in magnetically-supported, self-gravitating molecular\nclouds in axisymmetric geometry, accounting for the effects of ambipolar\ndiffusion and Ohmic dissipation, grain chemistry and dynamics, and radiative\ntransfer. Here we present results of star formation simulations that accurately\ntrack the evolution of a protostellar fragment over eleven orders of magnitude\nin density (from 300 cm^-3 to \\approx 10^14 cm^-3), i.e., from the early\nambipolar-diffusion--initiated fragmentation phase, through the magnetically\nsupercritical, dynamical-contraction phase and the subsequent magnetic\ndecoupling stage, to the formation of a protostellar core in near hydrostatic\nequilibrium. As found by Fiedler & Mouschovias (1993), gravitationally-driven\nambipolar diffusion leads to the formation and subsequent dynamic contraction\nof a magnetically supercritical core. Moreover, we find that ambipolar\ndiffusion, not Ohmic dissipation, is responsible for decoupling all the species\nexcept the electrons from the magnetic field, by a density \\approx 3 x 10^12\ncm^-3. Magnetic decoupling precedes the formation of a central stellar object\nand ultimately gives rise to a concentration of magnetic flux (a `magnetic\nwall') outside the hydrostatic core --- as also found by Tassis & Mouschovias\n(2005a,b) through a different approach. At approximately the same density at\nwhich Ohmic dissipation becomes more important than ambipolar diffusion\n(\\gtrsim 7 x 10^12 cm^-3), the grains carry most of the electric charge as well\nas the electric current. The prestellar core remains disclike down to radii ~\n10 AU, inside which thermal pressure becomes important. The magnetic flux\nproblem of star formation is resolved for at least strongly magnetic newborn\nstars by this stage of the evolution, i.e., by a central density \\approx 10^14\ncm^-3. The hydrostatic core has radius \\approx 2 AU, density \\approx 10^14\ncm^-3, temperature \\approx 300 K, magnetic field strength \\approx 0.2 G,\nmagnetic flux \\approx 5 x 10^18 Wb, luminosity ~ 10^-3 L_\\odot, and mass ~\n10^-2 M_\\odot.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Multiscale statistical analysis of coronal solar activity: Multi-filter images from the solar corona are used to obtain temperature maps\nwhich are analyzed using techniques based on proper orthogonal decomposition\n(POD) in order to extract dynamical and structural information at various\nscales. Exploring active regions before and after a solar flare and comparing\nthem with quiet regions we show that the multiscale behavior presents distinct\nstatistical properties for each case that can be used to characterize the level\nof activity in a region. Information about the nature of heat transport is also\nbe extracted from the analysis.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The orbital and superhump periods of the dwarf nova SDSS\n  J093249.57+472523.0: We report unfiltered CCD photometry of the eclipsing dwarf nova SDSS\nJ093249.57+472523.0 obtained during its first confirmed outburst in 2011 March.\nThe outburst amplitude was at least 3.0 magnitudes above mean quiescence and it\nlasted at least 11 days, although we missed the beginning of the outburst.\nSuperhumps having peak-to-peak amplitude up to 0.3 magnitudes were present\nduring the outburst, thereby establishing it to be a member of the SU UMa\nfamily. The mean superhump period was Psh = 0.06814(11) d. Analysis of our\nmeasurements of eclipse times of minimum, supplemented with data from other\nresearchers, allowed us to measure the orbital period as Porb = 0.06630354(5)\nd. The superhump period excess was epsilon = 0.028(1) which is consistent with\nof SU UMa systems of similar Porb. The FWHM eclipse duration varied between 6\nand 13 mins and the eclipse depth was up to 1.6 magnitudes.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Progress of Solar Cycle 24 at High Latitudes: The \"extended\" solar cycle 24 began in 1999 near 70 degrees latitude,\nsimilarly to cycle 23 in 1989 and cycle 22 in 1979. The extended cycle is\nmanifested by persistent Fe XIV coronal emission appearing near 70 degrees\nlatitude and slowly migrating towards the equator, merging with the latitudes\nof sunspots and active regions (the \"butterfly diagram\") after several years.\nCycle 24 began its migration at a rate 40% slower than the previous two solar\ncycles, thus indicating the possibility of a peculiar cycle. However, the onset\nof the \"Rush to the Poles\" of polar crown prominences and their associated\ncoronal emission, which has been a precursor to solar maximum in recent cycles\n(cf. Altrock 2003), has just been identified in the northern hemisphere.\nPeculiarly, this \"Rush\" is leisurely, at only 50% of the rate in the previous\ntwo cycles. The properties of the current \"Rush to the Poles\" yields an\nestimate of 2013 or 2014 for solar maximum.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Bipolar jets produced by a spectroscopic binary: We present evidence that the spectroscopically identified bipolar jets of the\npre-main sequence binary KH 15D are a common product of the whole binary\nsystem, rather than being launched from either star individually. They may be\nlaunched from the innermost part of the circumbinary disk (CBD) or may result\nfrom the merging of two outflows driven by the individual stars. This evidence\nis based on high-resolution H-alpha and [OI] 6300A line profiles obtained\nduring eclipse phases of this nearly edge-on system. The occultation of star A\n(the only currently visible star) by the disk strongly suppresses the stellar\nH-alpha and continuum emission and allows one to study the faint redshifted and\nblueshifted emission components of the bipolar jets. The strongest evidence for\njet production by the whole binary system comes from the observed radial\nvelocity symmetry of the two jet components relative to the systemic velocity\nof the binary, in combination with current accretion models from the CBD onto a\nbinary system.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Massive Binary System 9 Sgr Revisited: New Insights into\n  Disentangling Methods: Disentangling techniques are often needed to obtain the spectra of the\nindividual components of binary or multiple systems. A thorough analysis of the\nshift-and-add algorithm of Marchenko, Moffat, & Eenens (1998) reveals that in\nmany cases the line fluxes are poorly reproduced and spurious wings appear. The\ncauses of these discrepancies are discussed and a new disentangling package,\nQER20, is presented which significantly reduces these errors and vastly\nincreases the performance. When applied to the massive binary 9 Sgr, our new\ncode yields line fluxes which are notably different from those previously\npublished and lead us to revise the spectral classification to slightly earlier\nsubtypes: O3V((f +)) for the primary and O5V((f)) for the secondary. We show\nthat with the MME98 algorithm the classification of massive stars in binaries\ncan be off by several subtypes whilst there are no such errors when the QER20\npackage is used.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Magnetohydrodynamic Turbulent Cascade of Coronal Loop Magnetic Fields: The Parker model for coronal heating is investigated through a high\nresolution simulation. An inertial range is resolved where fluctuating magnetic\nenergy E_M (k_perp) \\propto k_\\perp^{-2.7} exceeds kinetic energy E_K (k_\\perp)\n\\propto k_\\perp^{-0.6}. Increments scale as \\delta b_\\ell \\simeq \\ell^{-0.85}\nand \\delta u_\\ell \\simeq \\ell^{+0.2} with velocity increasing at small scales,\nindicating that magnetic reconnection plays a prime role in this turbulent\nsystem. We show that spectral energy transport is akin to standard\nmagnetohydrodynamic (MHD) turbulence even for a system of reconnecting current\nsheets sustained by the boundary. In this new MHD turbulent cascade, kinetic\nenergy flows are negligible while cross-field flows are enhanced, and through a\nseries of \"reflections\" between the two fields, cascade more than half of the\ntotal spectral energy flow.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Statistical Study and Live Catalogue of Multi-Spacecraft 3He-Rich Time\n  Periods over Solar Cycles 23, 24, and 25: Using ion measurements from Ultra-Low-Energy Isotope Spectrometer (ULEIS)\nobservations onboard Advanced Composition Explorer (ACE) and Solar Isotope\nSpectrometer (SIS) observations onboard the Solar Terrestrial Observatory\n(STEREO)-A and STEREO-B spacecraft, we have identified 854 3He-rich time\nperiods between 1997 September and 2021 March. We include all event types with\nobserved 3He enhancements such as corotating interaction regions (CIRs),\ngradual solar energetic particle (SEP) events, interplanetary shocks, and\nimpulsive SEP events. We employ two different mass separation techniques to\nobtain 3He, 4He, Fe, and O fluences for each event, and we determine the\n3He/4He and Fe/O abundance ratios between 0.32 to 0.45 MeV/nucleon and 0.64 to\n1.28 MeV/nucleon. We find a clear correlation in the 3He/4He and Fe/O abundance\nratios between both energy ranges. We find two distinct trends in the 3He/4He\nvs. Fe/O relation. For low 3He/4He values, there is a positive linear\ncorrelation between 3He/4He and Fe/O. However, at 3He/4He ~ 0.3, Fe/O appears\nto reach a limit and the correlation weakens significantly. We provide a live\ncatalogue of 3He rich time periods that includes the robust determination of\nthe onset and end times of the 3He enhancements in SEP-associated periods for\ndifferent types of events observed my multiple spacecraft. This catalogue is\navailable for public use. New releases will follow after major additions such\nas adding new periods from new missions (e.g., Parker Solar Probe and Solar\nOrbiter), identifying event types (impulsive SEP events, etc.), or adding new\nparameters such as remote observations detailing characteristics of the active\nregions.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spiral arms and instability within the AFGL 4176 mm1 disc: We present high-resolution (30 mas or 130 au at 4.2 kpc) Atacama Large\nMillimeter/submillimeter Array observations at 1.2 mm of the disc around the\nforming O-type star AFGL 4176 mm1. The disc (AFGL 4176 mm1-main) has a radius\nof ~1000 au and contains significant structure, most notably a spiral arm on\nits redshifted side. We fitted the observed spiral with logarithmic and\nArchimedean spiral models. We find that both models can describe its structure,\nbut the Archimedean spiral with a varying pitch angle fits its morphology\nmarginally better. As well as signatures of rotation across the disc, we\nobserve gas arcs in CH$_3$CN that connect to other millimetre continuum sources\nin the field, supporting the picture of interactions within a small cluster\naround AFGL 4176 mm1-main. Using local thermodynamic equilibrium modelling of\nthe CH$_3$CN K-ladder, we determine the temperature and velocity field across\nthe disc, and thus produce a map of the Toomre stability parameter. Our results\nindicate that the outer disc is gravitationally unstable and has already\nfragmented or is likely to fragment in the future, possibly producing further\ncompanions. These observations provide evidence that disc fragmentation is one\npossible pathway towards explaining the high fraction of multiple systems\naround high-mass stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Multi-Channel Three-Dimensional SOLA Inversion for Local Helioseismology: Inversions for local helioseismology are an important and necessary step for\nobtaining three-dimensional maps of various physical quantities in the solar\ninterior. Frequently, the full inverse problems that one would like to solve\nprove intractable because of computational constraints. Due to the enormous\nseismic data sets that already exist and those forthcoming, this is a problem\nthat needs to be addressed. To this end, we present a very efficient linear\ninversion algorithm for local helioseismology. It is based on a subtractive\noptimally localized averaging (SOLA) scheme in the Fourier domain, utilizing\nthe horizontal-translation invariance of the sensitivity kernels. In Fourier\nspace the problem decouples into many small problems, one for each horizontal\nwave vector. This multi-channel SOLA method is demonstrated for an example\nproblem in time-distance helioseismology that is small enough to be solved both\nin real and Fourier space. We find that both approaches are successful in\nsolving the inverse problem. However, the multi-channel SOLA algorithm is much\nfaster and can easily be parallelized.",
        "category": "astro-ph_SR"
    },
    {
        "text": "GW Librae: Still Hot Eight Years Post-Outburst: We report continued Hubble Space Telescope (HST) ultraviolet spectra and\nground-based optical photometry and spectroscopy of GW Librae eight years after\nits largest known dwarf nova outburst in 2007. This represents the longest\ncooling timescale measured for any dwarf nova. The spectra reveal that the\nwhite dwarf still remains about 3000 K hotter than its quiescent value. Both\nultraviolet and optical light curves show a short period of 364-373 s, similar\nto one of the non-radial pulsation periods present for years prior to the\noutburst, and with a similar large UV/optical amplitude ratio. A large\nmodulation at a period of 2 h (also similar to that observed prior to outburst)\nis present in the optical data preceding and during the HST observations, but\nthe satellite observation intervals did not cover the peaks of the optical\nmodulation so it is not possible to determine its corresponding UV amplitude.\nThe similarity of the short and long periods to quiescent values implies the\npulsating, fast spinning white dwarf in GW Lib may finally be nearing its\nquiescent configuration.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Analytical expressions for the envelope binding energy of giants as a\n  function of basic stellar parameters: The common-envelope (CE) phase is an important stage in the evolution of\nbinary stellar populations. The most common way to compute the change in\norbital period during a CE is to relate the binding energy of the envelope of\nthe Roche-lobe filling giant to the change in orbital energy. Especially in\npopulation-synthesis codes, where the evolution of millions of stars must be\ncomputed and detailed evolutionary models are too expensive computationally,\nsimple approximations are made for the envelope binding energy. In this study,\nwe present accurate analytic prescriptions based on detailed stellar-evolution\nmodels that provide the envelope binding energy for giants with metallicities\nbetween Z = 10-4 and Z = 0.03 and masses between 0.8 Msun and 100 Msun, as a\nfunction of the metallicity, mass, radius and evolutionary phase of the star.\nOur results are also presented in the form of electronic data tables and\nFortran routines that use them. We find that the accuracy of our fits is better\nthan 15% for 90% of our model data points in all cases, and better than 10% for\n90% of our data points in all cases except the asymptotic giant branches for\nthree of the six metallicities we consider. For very massive stars (M > 50\nMsun), when stars lose more than ~20% of their initial mass due to stellar\nwinds, our fits do not describe the models as accurately. Our results are more\nwidely applicable - covering wider ranges of metallicity and mass - and are of\nhigher accuracy than those of previous studies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Predicting convective blueshift and radial-velocity dispersion due to\n  granulation for FGK stars: To detect Earth-mass planets using the Doppler method, a major obstacle is to\ndifferentiate the planetary signal from intrinsic stellar variability (e.g.,\npulsations, granulation, spots and plages). Convective blueshift, which results\nfrom small-scale convection at the surface of Sun-like stars, is relevant for\nEarth-twin detections as it exhibits Doppler noise on the order of 1 m/s. Here,\nwe present a simple model for convective blueshift based on fundamental\nequations of stellar structure. Our model successfully matches observations of\nconvective blueshift for FGK stars. Based on our model, we also compute the\nintrinsic noise floor for stellar granulation in the radial velocity\nobservations. We find that for a given mass range, stars with higher\nmetallicities display lower radial-velocity dispersion due to granulation, in\nagreement with MHD simulations. We also provide a set of formulae to predict\nthe amplitude of radial-velocity dispersion due to granulation as a function of\nstellar parameters. Our work is vital in identifying the most amenable stellar\ntargets for EPRV surveys and radial velocity follow-up programmes for TESS,\nCHEOPS, and the upcoming PLATO mission.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The dependence of the evolution of SN type Ia progenitors on the C\n  burning rate uncertainty and parameters of convective boundary mixing: Evolution of a supernova type Ia progenitor requires formation of a CO white\ndwarf, which implies a dependence on the C burning rate (CBR). It can also be\naffected by the recently identified possibility of C flame quenching by\nconvective boundary mixing. We present first results of our study of the\ncombined effect of these two potential sources of uncertainty on the SN Ia\nprogenitor evolution. We consider the possibility that the CBR is higher than\nits currently recommended value by as much as a factor of 1000 if unidentified\nresonances are important, or that it is significantly lower because of the\nhindrance effect. For stellar models that assume the Schwarzschild boundary for\nconvection, the maximum initial mass for the formation of CO WDs increases from\nM_i ~ 5.5 Msun for the CBR factor of 1000 to M_i > 7.0 Msun for the CBR factor\nof 0.01. For C-flame quenching models, hybrid C-O-Ne WDs form for a range of\ninitial mass of Delta M_i ~ 1 Msun, which increases a fraction of stars that\nform WDs capable of igniting C in a thermonuclear runaway. The most extreme\ncase is found for the CBR factor of 0.1 that is supported by the hindrance\nmodel. This nuclear physics assumption, combined with C flame quenching, leads\nto the formation of a hybrid C-O-Ne WD with a mass of 1.3 Msun. Such WDs do not\nneed to accrete much mass to reach the Chandrasekhar limit.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A resolved debris disk around the candidate planet-hosting star HD95086: Recently, a new planet candidate was discovered on direct images around the\nyoung (10-17 Myr) A-type star HD95086. The strong infrared excess of the system\nindicates that, similarly to HR8799, {\\ss} Pic, and Fomalhaut, the star harbors\na circumstellar disk. Aiming to study the structure and gas content of the\nHD95086 disk, and to investigate its possible interaction with the newly\ndiscovered planet, here we present new optical, infrared and millimeter\nobservations. We detected no CO emission, excluding the possibility of an\nevolved gaseous primordial disk. Simple blackbody modeling of the spectral\nenergy distribution suggests the presence of two spatially separate dust belts\nat radial distances of 6 and 64 AU. Our resolved images obtained with the\nHerschel Space Observatory reveal a characteristic disk size of ~6.0x5.4 arcsec\n(540x490 AU) and disk inclination of ~25 degree. Assuming the same inclination\nfor the planet candidate's orbit, its re-projected radial distance from the\nstar is 62 AU, very close to the blackbody radius of the outer cold dust ring.\nThe structure of the planetary system at HD95086 resembles the one around\nHR8799. Both systems harbor a warm inner dust belt and a broad colder outer\ndisk and giant planet(s) between the two dusty regions. Modelling implies that\nthe candidate planet can dynamically excite the motion of planetesimals even\nout to 270 AU via their secular perturbation if its orbital eccentricity is\nlarger than about 0.4. Our analysis adds a new example to the three known\nsystems where directly imaged planet(s) and debris disks co-exist.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Exploring the RCrA environment with SPHERE: Discovery of a new stellar\n  companion: Aims. R Coronae Australis (R CrA) is the brightest star of the Coronet nebula\nof the Corona Australis (CrA) star forming region. It has very red colors,\nprobably due to dust absorption and it is strongly variable. High contrast\ninstruments allow for an unprecedented direct exploration of the immediate\ncircumstellar environment of this star. Methods. We observed R CrA with the\nnear-IR channels (IFS and IRDIS) of SPHERE at VLT. In this paper, we used four\ndifferent epochs, three of them from open time observations while one is from\nthe SPHERE guaranteed time. The data were reduced using the DRH pipeline and\nthe SPHERE Data Center. On the reduced data we implemented custom IDL routines\nwith the aim to subtract the speckle halo.We have also obtained pupil-tracking\nH-band (1.45-1.85 micron) observations with the VLT/SINFONI near-infrared\nmedium-resolution (R~3000) spectrograph. Results. A companion was found at a\nseparation of 0.156\" from the star in the first epoch and increasing to 0.18400\nin the final one. Furthermore, several extended structures were found around\nthe star, the most noteworthy of which is a very bright jet-like structure\nNorth-East from the star. The astrometric measurements of the companion in the\nfour epochs confirm that it is gravitationally bound to the star. The SPHERE\nphotometry and the SINFONI spectrum, once corrected for extinction, point\ntoward an early M spectral type object with a mass between 0.3 and 0.55 M?. The\nastrometric analyis provides constraints on the orbit paramenters: e~0.4,\nsemi-major axis at 27-28 au, inclination of ~ 70{\\deg} and a period larger than\n30 years. We were also able to put constraints of few MJup on the mass of\npossible other companions down to separations of few tens of au.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Intense bipolar structures from stratified helical dynamos: We perform direct numerical simulations of the equations of\nmagnetohydrodynamics with external random forcing and in the presence of\ngravity. The domain is divided into two parts: a lower layer where the forcing\nis helical and an upper layer where the helicity of the forcing is zero with a\nsmooth transition in between. At early times, a large-scale helical dynamo\ndevelops in the bottom layer. At later times the dynamo saturates, but the\nvertical magnetic field continues to develop and rises to form dynamic bipolar\nstructures at the top, which later disappear and reappear. Some of the\nstructures look similar to $\\delta$ spots observed in the Sun. This is the\nfirst example of magnetic flux concentrations, owing to strong density\nstratification, from self-consistent dynamo simulations that generate bipolar,\nsuper-equipartition strength, magnetic structures whose energy density can\nexceeds the turbulent kinetic energy by even a factor of ten.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Molecules with ALMA at Planet-forming Scales (MAPS) XIII: HCO$^+$ and\n  disk ionization structure: We observed HCO$^+$ $J=1-0$ and H$^{13}$CO$^+$ $J=1-0$ emission towards the\nfive protoplanetary disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480\nas part of the MAPS project. HCO$^+$ is detected and mapped at\n0.3\\arcsec\\,resolution in all five disks, while H$^{13}$CO$^+$ is detected\n(SNR$>6 \\sigma$) towards GM Aur and HD 163296 and tentatively detected (SNR$>3\n\\sigma$) towards the other disks by a matched filter analysis. Inside a radius\nof $R\\sim 100$ au, the HCO$^+$ column density is flat or shows a central dip.\nAt outer radii ($\\gtrsim 100$ au), the HCO$^+$ column density decreases\noutwards, while the column density ratio of HCO$^+$/CO is mostly in the range\nof $\\sim 10^{-5}-10^{-4}$. We derived the HCO$^+$ abundance in the warm CO-rich\nlayer, where HCO$^+$ is expected to be the dominant molecular ion. At $R\\gtrsim\n100$ au, the HCO$^+$ abundance is $\\sim 3 \\times 10^{-11} - 3\\times 10^{-10}$,\nwhich is consistent with a template disk model with X-ray ionization. At the\nsmaller radii, the abundance decreases inwards, which indicates that the\nionization degree is lower in denser gas, especially inside the CO snow line,\nwhere the CO-rich layer is in the midplane. Comparison of template disk models\nwith the column densities of HCO$^+$, N$_2$H$^+$, and N$_2$D$^+$ indicates that\nthe midplane ionization rate is $\\gtrsim 10^{-18}$ s$^{-1}$ for the disks\naround IM Lup, AS 209, and HD 163296. We also find hints of an increased\nHCO$^+$ abundance around the location of dust continuum gaps in AS 209, HD\n163296, and MWC 480. This paper is part of the MAPS special issue of the\nAstrophysical Journal Supplement.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The diversity of light curve variations of Blazhko stars: The ground-based Konkoly Blazhko Survey I and II aim to collect accurate,\nextended, multicolour light curves of galactic field RRab stars in order to\ndetermine the incidence rate of the modulation in the sample, to study the\nmodulation in details, to study long-term changes in the modulation properties\nand to find changes in the mean global physical parameters of the stars with\nBlazhko phase. Here the diversity of light curve variations of Blazhko stars is\ndemonstrated through a sub-sample of the modulated RRab stars found by the\nfirst part of this survey.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The single-sided pulsator CO~Camelopardalis: CO~Cam (TIC 160268882) is the second ``single-sided pulsator'' to be\ndiscovered. These are stars where one hemisphere pulsates with a significantly\nhigher amplitude than the other side of the star. CO~Cam is a binary star\ncomprised of an Am $\\delta$~Sct primary star with $T_{\\rm eff} = 7070 \\pm\n150$\\,K, and a spectroscopically undetected G main-sequence secondary star. The\ndominant pulsating side of the primary star is centred on the L$_1$ point. We\nhave modelled the spectral energy distribution combined with radial velocities,\nand independently the {\\em TESS} light curve combined with radial velocities.\nBoth of these give excellent agreement and robust system parameters for both\nstars. The $\\delta$~Sct star is an oblique pulsator with at least four low\nradial overtone (probably) f~modes with the pulsation axis coinciding with the\ntidal axis of the star, the line of apsides. Preliminary theoretical modelling\nindicates that the modes must produce much larger flux perturbations near the\nL$_1$ point, although this is difficult to understand because the pulsating\nstar does not come near to filling its Roche lobe. More detailed models of\ndistorted pulsating stars should be developed. These newly discovered\nsingle-sided pulsators offer new opportunities for astrophysical inference from\nstars that are oblique pulsators in close binary stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Temporal and Spatial Evolutions of a Large Sunspot Group and Great\n  Auroral Storms around the Carrington Event in 1859: The Carrington event is considered to be one of the most extreme space\nweather events in observational history within a series of magnetic storms\ncaused by extreme interplanetary coronal mass ejections (ICMEs) from a large\nand complex active region (AR) emerged on the solar disk. In this article, we\nstudy the temporal and spatial evolutions of the source sunspot active region\nand visual aurorae, and compare this storm with other extreme space weather\nevents on the basis of their spatial evolution. Sunspot drawings by Schwabe,\nSecchi, and Carrington describe the position and morphology of the source AR at\nthat time. Visual auroral reports from the Russian Empire, Iberia, Ireland,\nOceania, and Japan fill the spatial gap of auroral visibility and revise the\ntime series of auroral visibility in mid to low magnetic latitudes (MLATs). The\nreconstructed time series is compared with magnetic measurements and shows the\ncorrespondence between low to mid latitude aurorae and the phase of magnetic\nstorms. The spatial evolution of the auroral oval is compared with those of\nother extreme space weather events in 1872, 1909, 1921, and 1989 as well as\ntheir storm intensity, and contextualizes the Carrington event, as one of the\nmost extreme space weather events, but likely not unique.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Gaps in the Main-Sequence of Star Cluster Hertzsprung Russell Diagrams: The presence of gaps or regions of small numbers of stars in the main\nsequence of the Hertzsprung Russell Diagram (HRD) of star clusters has been\nreported in literature. This is interesting and significant as it could be\nrelated to star formation and/or rapid evolution or instabilities. In this\npaper, using Gaia DR3 photometry and confirmed membership data, we explore the\nHRD of nine open clusters with reported gaps, identify them and assess their\nimportance and spectral types.",
        "category": "astro-ph_SR"
    },
    {
        "text": "NIKA2 observations around LBV stars: emission from stars and\n  circumstellar material: Luminous Blue Variable (LBV) stars are evolved massive objects, previous to\ncore-collapse supernova. LBVs are characterized by photometric and\nspectroscopic variability, produced by strong and dense winds, mass-loss events\nand very intense UV radiation. LBVs strongly disturb their surroundings by\nheating and shocking, and produce important amounts of dust. The study of the\ncircumstellar material is therefore crucial to understand how these massive\nstars evolve, and also to characterize their effects onto the interstellar\nmedium. The versatility of NIKA2 is a key in providing simultaneous\nobservations of both the stellar continuum and the extended, circumstellar\ncontribution. The NIKA2 frequencies (150 and 260 GHz) are in the range where\nthermal dust and free-free emission compete, and hence NIKA2 has the capacity\nto provide key information about the spatial distribution of circumstellar\nionized gas, warm dust and nearby dark clouds; non-thermal emission is also\npossible even at these high frequencies. We show the results of the first NIKA2\nsurvey towards five LBVs. We detected emission from four stars, three of them\nimmersed in tenuous circumstellar material. The spectral indices show a complex\ndistribution and allowed us to separate and characterize different components.\nWe also found nearby dark clouds, with spectral indices typical of thermal\nemission from dust. Spectral indices of the detected stars are negative and\nhard to be explained only by free-free processes. In one of the sources,\nG79.29+0.46, we also found a strong correlation of the 1mm and 2mm continuum\nemission with respect to nested molecular shells at 0.1 pc from the LBV. The\nspectral index in this region clearly separates four components: the LBV star,\na bubble characterized by free-free emission, and a shell interacting with a\nnearby infrared dark cloud.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On WD-WD Mergers in Triple Systems: The Role of Kozai Resonance with\n  Tidal Friction: White dwarf-white dwarf (WD-WD) mergers may lead to type Ia supernovae\nevents. Thompson (2011) suggested that many such binaries are produced in\nhierarchical triple systems. The tertiary induces eccentricity oscillations in\nthe inner binary via the Kozai-Lidov mechanism, driving the binary to high\neccentricities, and significantly reducing the gravitational wave merger\ntimescale (T_GW) over a broad range of parameter space. Here, we investigate\nthe role of tidal forces in these systems. We show that tidal effects are\nimportant in the regime of moderately high initial relative inclination between\nthe inner binary and the outer tertiary. For 85 < i_0 < 90 degrees (prograde)\nand 97 < i_0 < 102 degrees (retrograde), tides combine with GW radiation to\ndramatically decrease T_GW. In the regime of high inclinations between 91 < i_0\n< 96 degrees, the inner binary likely suffers a direct collision, as in the\nwork of Katz & Dong (2012) and tidal effects do not play an important role.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Variable stars in Palomar 13; an evaporating globular cluster: We present new CCD $VI$ photometry of the distant globular cluster Pal 13.\nFourier decomposition of the light curves of the three cluster member RRab\nstars lead to estimations of [Fe/H]=-1.65, and a distance of 23.67$\\pm$0.57\nkpc. Light and colour near minimum phases for RRab stars leads to an estimate\nof $E(B-V)$=0.104 $\\pm$ 0.001. A $V/(V-I)$ colour-magnitude diagram, built\nexclusively with likely star members, shows consistency with the above\nparameters and an age of 12 Gyrs. A search of variable stars in the field of\nview of our images revealed the variability of a red giant cluster member and\nof three probably non-member stars; two RRab stars and one W Virginis star or\nCW. The GAIA proper motions of member stars in Pal 13 show a significant\nscatter, consistent with the scenario of the cluster being tidally stripped.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Outburst of the Young Star Gaia19bey: We report photometry and spectroscopy of the outburst of the young stellar\nobject Gaia19bey. We have established the outburst light curve with archival\nGaia G, ATLAS Orange, ZTF r-band and Pan-STARRS rizy-filter photometry, showing\nan outburst of approximately 4 years duration, longer than typical EXors but\nshorter than FUors. Its pre-outburst SED shows a flat far-infrared spectrum,\nconfirming the early evolutionary state of Gaia19bey and its similarity to\nother deeply embedded young stars experiencing outbursts. A lower limit to the\npeak outburst luminosity is approximately 182 L_sun at an assumed distance of\n1.4 kpc, the minimum plausible distance. Infrared and optical spectroscopy near\nmaximum light showed an emission line spectrum, including HI lines, strong red\nCaII emission, other metal emission lines, infrared CO bandhead emission, and a\nstrong infrared continuum. Towards the end of the outburst, the emission lines\nhave all but disappeared and the spectrum has changed into an almost pure\ncontinuum spectrum. This indicates a cessation of magnetospheric accretion\nactivity. The near-infrared colors have become redder as Gaia19bey has faded,\nindicating a cooling of the continuum component. Near the end of the outburst,\nthe only remaining strong emission lines are forbidden shock-excited emission\nlines. Adaptive optics integral field spectroscopy shows the H_2 1--0 S(1)\nemission with the morphology of an outflow cavity and the extended emission in\nthe [FeII] line at 1644 nm with the morphology of an edge-on disk. However, we\ndo not detect any large-scale jet from Gaia19bey.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Sun Remains Relatively Refractory Depleted: Elemental Abundances for\n  17,412 Gaia RVS Solar Analogs and 50 Planet Hosts: The elemental abundances of stars, particularly the refractory elements\n(e.g., Fe, Si, Mg), play an important role in connecting stars to their\nplanets. Most Sun-like stars do not have refractory abundance measurements\nsince obtaining a large sample of high-resolution spectra is difficult with\noversubscribed observing resources. In this work we infer abundances for C, N,\nO, Na, Mn, Cr, Si, Fe, Ni, Mg, V, Ca, Ti, Al, and Y for solar analogs with Gaia\nRVS spectra (R=11,200) using the Cannon, a data-driven method. We train a\nlinear model on a reference set of 34 stars observed by Gaia RVS with precise\nabundances measured from previous high resolution spectroscopic efforts (R >\n30,000--110,000). We then apply this model to several thousand Gaia RVS solar\nanalogs. This yields abundances with average upper limit precisions of\n0.04--0.1 dex for 17,412 stars, 50 of which are identified planet (candidate)\nhosts. We subsequently test the relative refractory depletion of these stars\nwith increasing element condensation temperature compared to the Sun. The Sun\nremains refractory depleted compared to other Sun-like stars regardless of our\ncurrent knowledge of the planets they host. This is inconsistent with theories\nof various types of planets locking up or sequestering refractories.\nFurthermore, we find no significant abundance differences between identified\nclose-in giant planet hosts, giant planet hosts, and terrestrial/small planet\nhosts and the rest of the sample within our precision limits. This work\ndemonstrates the utility of data-driven learning for future exoplanet\ncomposition and demographics studies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Origin of the CEMP-no Group Morphology in the Milky Way: The elemental-abundance signatures of the very first stars are imprinted on\nthe atmospheres of CEMP-no stars, as various evidence suggests they are\nbona-fide second-generation stars. It has recently been recognized that the\nCEMP-no stars can be sub-divided into at least two groups, based on their\ndistinct morphology in the $A$(C)-[Fe/H] space, indicating the likely existence\nof multiple pathways for their formation. In this work, we compare the halo\nCEMP-no group morphology with that of stars found in satellite dwarf galaxies\nof the Milky Way -- a very similar $A$(C)-[Fe/H] pattern is found, providing\nclear evidence that halo CEMP-no stars were indeed accreted from their host\nmini-halos, similar in nature to those that formed in presently observed\nultra-faint dwarfs (UFDs) and dwarf spheroidal (dSph) galaxies. We also infer\nthat the previously noted \"anomalous\" CEMP-no halo stars (with high $A$(C) and\nlow [Ba/Fe] ratios) that otherwise would be associated with Group I may have\nthe same origin as the Group III CEMP-no halo stars, by analogy with the\nlocation of several Group III CEMP-no stars in the UFDs and dSphs and their\ndistinct separation from that of the CEMP-$s$ stars in the $A$(Ba)-$A$(C)\nspace. Interestingly, CEMP-no stars associated with UFDs include both Group II\nand Group III stars, while the more massive dSphs appear to have only Group II\nstars. We conclude that understanding the origin of the CEMP-no halo stars\nrequires knowledge of the masses of their parent mini-halos, which is related\nto the amount of carbon dilution prior to star formation, in addition to the\nnature of their nucleosynthetic origin.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Chemical Compositions of a sample of candidate post-AGB stars: We have derived elemental abundances for a sample of nine IRAS sources with\ncolours similar to those of post-AGB stars. For IRAS 01259+6823, IRAS\n05208-2035, IRAS 04535+3747 and IRAS 08187-1905 this is the first detailed\nabundance analysis based upon high resolution spectra. Mild indication of\ns-processing for IRAS 01259+6823, IRAS 05208-2035 and IRAS 08187-1905 have been\nfound and a more comprehensive study of s-process enhanced objects IRAS\n17279-1119 and IRAS 22223+4327 have been carried out. We have also made a\ncontemporary abundance analysis of the high galactic latitude supergiants BD+39\n4926 and HD 107369. The former is heavily depleted in refractories and\nestimated [Zn/H] of -0.7 dex most likely gives initial metallicity of the star.\nFor HD 107369 the abundances of alpha and Fe-peak elements are similar to those\nof halo objects and moderate deficiency of s-process elements is seen. IRAS\n07140-2321 despite being a short period binary with circumstellar shell does\nnot exhibit selective depletion of refractory elements. We have compiled the\nstellar parameters and abundances for post-AGB stars with s-process\nenhancement, those showing significant depletion of condensable elements and\nthose showing neither. The compilation shows that the s-process enhanced group\ncontains very small number of binaries, and observed [alpha/Fe] are generally\nsimilar to thick disc values. It is likely that they represent AGB evolution of\nsingle stars.The compilation of depleted group contains larger fraction of\nbinaries and generally supports the hypothesis of dusty discs surrounding\nbinary post-AGB stars inferred via the shape of their SED and mid IR\ninterferometry. IRAS 07140-2321 and BD+39 4926 are difficult to explain with\nthis scenario and indicate the existence of additional parameter/condition\nneeded to explain the depletion phenomenon.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Ensemble Learning for CME Arrival Time Prediction: The Sun constantly releases radiation and plasma into the heliosphere.\nSporadically, the Sun launches solar eruptions such as flares and coronal mass\nejections (CMEs). CMEs carry away a huge amount of mass and magnetic flux with\nthem. An Earth-directed CME can cause serious consequences to the human system.\nIt can destroy power grids/pipelines, satellites, and communications.\nTherefore, accurately monitoring and predicting CMEs is important to minimize\ndamages to the human system. In this study we propose an ensemble learning\napproach, named CMETNet, for predicting the arrival time of CMEs from the Sun\nto the Earth. We collect and integrate eruptive events from two solar cycles,\n#23 and #24, from 1996 to 2021 with a total of 363 geoeffective CMEs. The data\nused for making predictions include CME features, solar wind parameters and CME\nimages obtained from the SOHO/LASCO C2 coronagraph. Our ensemble learning\nframework comprises regression algorithms for numerical data analysis and a\nconvolutional neural network for image processing. Experimental results show\nthat CMETNet performs better than existing machine learning methods reported in\nthe literature, with a Pearson product-moment correlation coefficient of 0.83\nand a mean absolute error of 9.75 hours.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Shape-based Feature Engineering for Solar Flare Prediction: Solar flares are caused by magnetic eruptions in active regions (ARs) on the\nsurface of the sun. These events can have significant impacts on human\nactivity, many of which can be mitigated with enough advance warning from good\nforecasts. To date, machine learning-based flare-prediction methods have\nemployed physics-based attributes of the AR images as features; more recently,\nthere has been some work that uses features deduced automatically by deep\nlearning methods (such as convolutional neural networks). We describe a suite\nof novel shape-based features extracted from magnetogram images of the Sun\nusing the tools of computational topology and computational geometry. We\nevaluate these features in the context of a multi-layer perceptron (MLP) neural\nnetwork and compare their performance against the traditional physics-based\nattributes. We show that these abstract shape-based features outperform the\nfeatures chosen by the human experts, and that a combination of the two feature\nsets improves the forecasting capability even further.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A new way to infer variations of the seismic solar radius: We show that the mean phase of waves propagating all the way from the far\nside of the Sun to the front side, as measured by seismic holography, varies\nwith time. The change is highly anticorrelated with solar cycle activity and is\nconsistent with other recent results on the variation of the seismic radius of\nthe Sun. The phase change that we observe corresponds to a few kilometers\ndifference in the seismic solar radius from solar maximum to solar minimum in\nagreement with inferrences from global helioseismology studies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Multiple Star Systems in the Orion Nebula: This work presents an interferometric study of the massive-binary fraction in\nthe Orion Trapezium Cluster with the recently comissioned GRAVITY instrument.\nWe observe a total of 16 stars of mainly OB spectral type. We find three\npreviously unknown companions for $\\theta ^1$ Ori B, $\\theta ^2$ Ori B, and\n$\\theta ^2$ Ori C. We determine a separation for the previously suspected\ncompanion of NU Ori. We confirm four companions for $\\theta ^1$ Ori A, $\\theta\n^1$ Ori C, $\\theta ^1$ Ori D, and $\\theta ^2$ Ori A, all with substantially\nimproved astrometry and photometric mass estimates. We refine the orbit of the\neccentric high-mass binary $\\theta ^1$ Ori C and we are able to derive a new\norbit for $\\theta ^1$ Ori D. We find a system mass of 21.7 $M_{\\odot}$ and a\nperiod of $53$ days. Together with other previously detected companions seen in\nspectroscopy or direct imaging, eleven of the 16 high-mass stars are multiple\nsystems. We obtain a total number of 22 companions with separations up to 600\nAU. The companion fraction of the early B and O stars in our sample is about 2,\nsignificantly higher than in earlier studies of mostly OB associations. The\nseparation distribution hints towards a bimodality. Such a bimodality has been\npreviously found in A stars, but rarely in OB binaries, which up to this point\nhave been assumed to be mostly compact with a tail of wider companions. We also\ndo not find a substantial population of equal-mass binaries. The observed\ndistribution of mass ratios declines steeply with mass, and like the direct\nstar counts, indicates that our companions follow a standard power law initial\nmass function. Again, this is in contrast to earlier findings of flat mass\nratio distributions in OB associations. We exclude collision as a dominant\nformation mechanism but find no clear preference for core accretion or\ncompetitive accretion.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Measuring stellar rotation periods with Kepler: We measure rotation periods for 12151 stars in the Kepler field, based on the\nphotometric variability caused by stellar activity. Our analysis returns stable\nrotation periods over at least six out of eight quarters of Kepler data. This\nlarge sample of stars enables us to study the rotation periods as a function of\nspectral type. We find good agreement with previous studies and vsini\nmeasurements for F, G and K stars. Combining rotation periods, B-V color, and\ngyrochronology relations, we find that the cool stars in our sample are\npredominantly younger than ~1Gyr.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Rotational Splitting of Pulsational Modes: Mode splittings produced by uniform rotation and a particular form of\ndifferential rotation are computed for two-dimensional rotating 10 Mo ZAMS\nstellar models. The change in the character of the mode splitting is traced as\na function of uniform rotation rate, and it is found that only relatively slow\nrotation rates are required before the mode splitting becomes asymmetric about\nthe azimuthally symmetric (m=0) mode. Increased rotation produces a\nprogressively altered pattern of the individual modes with respect to each\nother. Large mode splittings begin to overlap with the mode splittings produced\nby different radial and latitudinal modes at relatively low rotation rates. The\nmode splitting pattern for the differentially rotating stars we model is\ndifferent than that for uniformly rotating stars, making the mode splitting a\npossible discriminant of the internal angular momentum distribution if one\nassumes the formidable challenge of mode identification can be overcome.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The HST Large Programme on NGC6752. II. Multiple populations at the\n  bottom of the main sequence probed in NIR: Historically, multiple populations in Globular Clusters (GCs) have been\nmostly studied from ultraviolet and optical filters down to stars that are more\nmassive than ~0.6 solar masses. Here we exploit deep near-infrared (NIR)\nphotometry from the Hubble Space Telescope to investigate multiple populations\namong M-dwarfs in the GC NGC6752. We discovered that the three main populations\n(A, B and C), previously observed in the brightest part of the color-magnitude\ndiagram, define three distinct sequences that run from the main-sequence (MS)\nknee towards the bottom of the MS (~0.15 solar masses). These results, together\nwith similar findings on NGC2808, M4, and omega Centauri, demonstrate that\nmultiple sequences of M-dwarfs are common features of the color-magnitude\ndiagrams of GCs. The three sequences of low-mass stars in NGC6752 are\nconsistent with stellar populations with different oxygen abundances. The range\nof [O/Fe] needed to reproduce the NIR CMD of NGC6752 is similar to the oxygen\nspread inferred from high-resolution spectroscopy of red-giant branch (RGB)\nstars. The relative numbers of stars in the three populations of M-dwarfs are\nsimilar to those derived among RGB and MS stars more massive than ~0.6 solar\nmasses. As a consequence, the evidence that the properties of multiple\npopulations do not depend on stellar mass is a constraint for the formation\nscenarios.",
        "category": "astro-ph_SR"
    },
    {
        "text": "2M17091769+3127589: a mass-transfer binary with an extreme mass ratio: We present the orbital solution of a peculiar double-lined spectroscopic and\neclipsing binary system, 2M17091769+3127589. This solution was obtained by a\nsimultaneous fit of both APOGEE radial velocities and TESS and ASAS-SN light\ncurves to determine masses and radii. This system consists of an\n$M=0.256^{+0.010}_{-0.006}$ $M_\\odot$, $R=3.961^{+0.049}_{-0.032}$ $R_{\\odot}$\nred giant and a hotter $M=1.518 ^{+0.057}_{-0.031}$ $M_\\odot$,\n$R=2.608^{+0.034}_{-0.321}$ $R_{\\odot}$ subgiant. Modelling with the MESA\nevolutionary codes indicates that the system likely formed 5.26 Gyrs ago, with\na $M=1.2$ $M_\\odot$ primary that is now the system's red giant and a $M=1.11$\n$M_\\odot$ secondary that is now a more massive subgiant. Due to Roche-lobe\noverflow as the primary ascends the red giant branch, the more evolved\n\"primary\" (i.e., originally the more massive star of the pair) is now only\none-sixth as massive as the \"secondary\". Such a difference between the initial\nand the current mass ratio is one of the most extreme detected so far.\nEvolutionary modelling suggests the system is still engaged in mass transfer,\nat a rate of $\\dot{M} \\sim 10^{-9}$ $M_\\odot$ yr$^{-1}$, and it provides an\nexample of a less evolved precursor to some of the systems that consist of\nwhite dwarfs and blue stragglers.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Physical conditions around 6.7 GHz methanol masers-I: Ammonia: Methanol masers at 6.7 GHz are known to be tracers of high-mass star\nformation in our Galaxy. In this paper, we study the large scale physical\nconditions in the star forming clumps/cores associated with 6.7 GHz methanol\nmasers using observations of the (1,1), (2,2) and (3,3) inversion transitions\nof ammonia with the Effelsberg telescope. The gas kinetic temperature is found\nto be higher than in infrared dark clouds, highlighting the relatively evolved\nnature of the maser sources. Other than a weak correlation between maser\nluminosity and the ammonia line width, we do not find any differences between\nlow and high luminosity methanol masers.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Comprehensive Radiative Magnetohydrodynamics Simulation of Active\n  Region Scale Flux Emergence from the Convection Zone to the Corona: We present a comprehensive radiative magnetohydrodynamic simulation of the\nquiet Sun and large solar active regions. The 197 Mm wide simulation domain\nspans from 18 (10) Mm beneath the photosphere to 113 Mm in the solar corona.\nRadiative transfer assuming local thermal equilibrium, optically-thin radiative\nlosses, and anisotropic conduction transport provide the necessary realism for\nsynthesizing observables to compare with remote sensing observations of the\nphotosphere and corona. This model self-consistently reproduces observed\nfeatures of the quiet Sun, emerging and developed active regions, and solar\nflares up to M class. Here, we report an overview of the first results. The\nsurface magnetoconvection yields an upward Poynting flux that is dissipated in\nthe corona and heats the plasma to over one million K. The quiescent corona\nalso presents ubiquitous propagating waves, jets, and bright points with sizes\ndown to 2 Mm. Magnetic flux bundles emerge into the photosphere and give rise\nto strong and complex active regions with over $10^{23}$ Mx magnetic flux. The\ncoronal free magnetic energy, which is approximately 18\\% of the total magnetic\nenergy, accumulates to approximately $10^{33}$ erg. The coronal magnetic field\nis clearly non-force-free, as the Lorentz force needs to balance the pressure\nforce and viscous stress as well as drive magnetic field evolution. The\nemission measure from $\\log_{10}T{=}4.5$ to $\\log_{10}T{>}7$ provides a\ncomprehensive view of the active region corona, such as coronal loops of\nvarious lengths and temperatures, mass circulation by evaporation and\ncondensation, and eruptions from jets to large-scale mass ejections.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Joint Milli-Arcsecond Pathfinder Survey (JMAPS): Mission Overview\n  and Attitude Sensing Applications: The Joint Milliarcsecond Pathfinder Survey (JMAPS) is a Department of Navy\nbright star astrometric all-sky survey scheduled for launch in the 2012\ntimeframe. Mission objectives include a complete update of star positions for\nthe 2015 epoch to accuracy levels of 1 milliarcsecond (5 nano-radians) for\nbright stars, as well as demonstration of 10 milliarcsecond attitude\ndetermination capability and 50 milli-arcsecond attitude control on-orbit. In\nthe following paper, we describe the general instrument design and expected\nperformance. We also discuss the new mission capabilities enabled by the\nunprecedented attitude determination accuracy of such an instrument, and focus\nspecifically on the application to long distance (50,000-100,00 km) formation\nflying and solar system navigation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Differential photometry of delta Scorpii during 2011 periastron: Hundred observations of Delta Scorpii over 200 days, from April 2 to October\n16, 2011, have been made for AAVSO visually and digitally from Rio de Janeiro,\nRome and Paris. The three most luminous pixels either of the target star and\nthe two reference stars are used to evaluate the magnitude through differential\nphotometry. The main sources of errors are outlined. The system of Delta\nScorpii, a spectroscopic double star, has experienced a close periastron in\nJuly 2011 within the outer atmospheres of the two giant components. The whole\nluminosity of Delta Scorpii system increased from about Mv=1.8 to 1.65 peaking\naround 5 to 15 July 2011, but there are significant rapid fluctuations of 0.2 -\n0.3 magnitudes occurring in 20 days that seem to be real, rather than a\nconsequence of systematic errors due to the changes of reference stars and\nobserving conditions. This method is promising for being applied to other\nbright variable stars like Betelgeuse and Antares. After August the magnitude\nremained constant at Mv=1.8 until the last observation on October 16 made in\ntwilight from Rome.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Optical flare activity in the low-mass eclipsing binary GJ~3236: We present our observations of the low-mass eclipsing binary GJ~3236. We have\nanalyzed a phased $R_C$ light-curve and confirmed previously determined\nfundamental parameters of the components. We detected evolution of the spot(s)\nand found that there exists a large spot near a polar region of the primary\ncomponent and another spot either on the primary or the secondary component. We\nalso observed 7 flare events and determined a flare rate of about 0.1 flares\nper hour. We observed two high energy, long-term flares with a complex light\ncurve and possibly four weak short-term flaring events. A majority of the\nflares was detected in the $R_C$ filter, which indicate their high energy.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A first eROSITA view of ultracool dwarfs: We present the first X-ray detections of ultracool dwarfs (UCDs) from the\nfirst all-sky survey of the extended ROentgen Survey with an Imaging Telescope\nArray (eROSITA) onboard the Russian Spektrum-Roentgen-Gamma (SRG) mission. We\nuse three publicly available input catalogs of spectroscopically confirmed UCDs\nand Gaia-selected UCD candidates that together comprise nearly 20000 objects.\nWe first extracted all X-ray sources from the catalog of the first eROSITA\nsurvey, eRASS1, that have a UCD or candidate within three times their\npositional uncertainty. Then we examined all Gaia objects in the vicinity of\nthese 96 X-ray sources and we associated them to the most plausible counterpart\non the basis of their spatial separation to the X-ray position and their\nmultiwavelength properties. This way we find 40 UCDs that have a secure\nidentification with an X-ray source and 18 plausible UCD X-ray emitters.\nTwenty-one of these X-ray emitting UCDs have a spectroscopic confirmation,\nwhile the others have been selected based on Gaia photometry and we computed\nspectral types from the G-J color. The spectral types of the X-ray emitting\nUCDs and candidates range between M5 and M9, and the distances range from 3.5\nto 190 pc. The majority of the UCDs from the eRASS1 sample show a ratio of\nX-ray to bolometric luminosity well above the canonical saturation limit of log\n(Lx/Lbol) ~ -3. For the two most extreme outliers, we showed through an\nanalysis of the eRASS1 light curve that these high values are due to flaring\nactivity. The X-ray spectra of the two brightest objects both reveal an\nemission-measure weighted plasma temperature of kT ~ 0.75 keV. These\nobservations demonstrate the potential of eROSITA for advancing our knowledge\non the faint coronal X-ray emission from UCDs by building statistical samples\nfor which the average X-ray brightness, flares, and coronal temperatures can be\nderived.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Temporal and latitudinal variations of Ca-K plage and network area: An\n  implication to meridional flows: The Ca-K spectroheliograms obtained at the Kodaikanal observatory (KO) are\nused to generate a uniform time series using the equal contrast technique (ECT)\nto study the long and short-term variation in the solar chromosphere. The\npercentage of plage, Enhanced network (EN), Active network (AN), and Quiet\nnetwork (QN) area at various latitudes is compared with the activity at\n35$^{\\circ}$ latitude and also with the sunspot number for the period of 1907\n-- 1984. The values of phase differences indicate that the activity begins at\n$\\sim$45$^{\\circ}$ latitude and shift progressively to the lower latitude at a\nspeed of $\\sim$~9.4~m~sec$^{-1}$ . The shift speed slows down gradually and\nreaches $\\sim$~3~m~sec$^{-1}$ at $\\sim$5$^{\\circ}$ latitude. No phase\ndifference between the variations of Ca-K activity at 55$^{\\circ}$,\n65$^{\\circ}$, and 75$^{\\circ}$ latitude belts implies that changes in the\nactivity are happening simultaneously. The analysis shows that the activity at\npolar latitude belts is anti-correlated with the sunspot number. This study\nindicates that a multi-cell meridional flow pattern could exist in the solar\nconvection zone. One type of cell could transport the magnetic elements from\nmid-latitude to low-latitude belts through meridional flows, and the other one\ncould be operating in the polar region.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Discovery of possible molecular counterparts to the infrared Double\n  Helix Nebula in the Galactic center: We have discovered two molecular features at radial velocities of -35 km/s\nand 0 km/s toward the infrared Double Helix Nebula (DHN) in the Galactic center\nwith NANTEN2. The two features show good spatial correspondence with the DHN.\nWe have also found two elongated molecular ridges at these two velocities\ndistributed vertically to the Galactic plane over 0.8 degree. The two ridges\nare linked by broad features in velocity and are likely connected physically\nwith each other. The ratio between the 12CO J=2-1 and J=1-0 transitions is 0.8\nin the ridges which is larger than the average value 0.5 in the foreground gas,\nsuggesting the two ridges are in the Galactic center. An examination of the K\nband extinction reveals a good coincidence with the CO 0 km/s ridge and is\nconsistent with a distance of 8 +/-2 kpc. We discuss the possibility that the\nDHN was created by a magnetic phenomenon incorporating torsional Alfv\\'en waves\nlaunched from the circumnuclear disk (Morris, Uchida & Do 2006) and present a\nfirst estimate of the mass and energy involved in the DHN.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Transients Obscured by Dusty Disks: Dust absorption is invoked in a number of contexts for hiding a star that has\nsurvived some sort of transient event from view. Dust formed in a transient is\nexpanding away from the star and, in spherical models, the mass and energy\nbudgets implied by a high optical depth at late times make such models\nuntenable. Concentrating the dust in a disk or torus can in principle hide a\nsource from an equatorial observer using less mass and so delay this problem.\nHowever, using axisymmetric dust radiation transfer models with a range of\nequatorial dust concentrations, we find that this is quite difficult to achieve\nin practice. The polar optical depth must be either low or high to avoid\nscattering optical photons to equatorial observers. Most of the emission\nremains at wavelengths easily observed by JWST, and the equatorial brightness\nis reduced by at most a factor of ~2 compared to isotropic emission even for\nequatorial (visual) optical depths of 1000. It is particularly difficult to\nhide a source with silicate dusts because the absorption feature near 10\\\nmicrons frequently leads to the emission being concentrated just bluewards of\nthe feature, near 8 microns.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Principle of Minimum Energy in Magnetic Reconnection in a Self-Organized\n  Critical Model for Solar Flares: Solar flares are an abrupt release of magnetic energy in the Sun's atmosphere\ndue to reconnection of the coronal magnetic field. This occurs in response to\nturbulent flows at the photosphere which twist the coronal field. Similar to\nearthquakes, solar flares represent the behavior of a complex system, and\nexpectedly their energy distribution follows a power law. We present a\nstatistical model based on the principle of minimum energy in a coronal loop\nundergoing magnetic reconnection, which is described as an avalanche process.\nWe show the distribution of peaks for the flaring events in this self-organized\ncritical system is scale-free. The obtained power law index 1.84 $ \\pm $ 0.02\nfor the peaks is in good agreement with satellite observations of soft X-ray\nflares. The principle of minimum energy can be applied for general avalanche\nmodels to describe many other phenomena.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The 1600 Angstrom Emission Bump in Protoplanetary Disks: A Spectral\n  Signature of H$_{2}$O Dissociation: The FUV continuum spectrum of many accreting pre-main sequence stars,\nClassical T Tauri Stars (CTTSs), does not continue smoothly from the\nwell-studied Balmer continuum emission in the NUV, suggesting that additional\nprocesses contribute to the short-wavelength emission in these objects. The\nmost notable spectral feature in the FUV continuum of some CTTSs is a broad\nemission approximately centered at 1600~\\AA, which has been referred to as the\n\"1600 A Bump\". The origin of this feature remains unclear. We have assembled\narchival FUV spectra of 37 disk-hosting systems observed by the {\\it Hubble\nSpace Telescope}. Clear 1600 A Bump emission is observed above the smooth,\nunderlying 1100-1800 A continuum spectrum in 19/37 Classical T Tauri disks in\nthe HST sample, with the detection rate in transition disks (8/8) being much\nhigher than in primordial or non-transition sources (11/29). We describe a\nspectral deconvolution analysis to separate the Bump (spanning 1490~--~1690 A)\nfrom the underlying FUV continuum, finding an average Bump luminosity,\n$L$(Bump7 x 10^{29} erg s^{-1}. We find that the 1600 A Bump is characterized\nby a peak wavelength of 1598.6 +/- 3.3 A.\n  Contrary to previous studies, we find that this feature is inconsistent with\nmodels of H2 excited by electron-impact. We show that this Bump emits roughly\n10-80% of the total fluorescent H2 luminosity for stars with well-defined Bump\nfeatures. Energetically, this suggests that the carrier of the 1600 A Bump\nemission is powered by Ly-a photons. We argue that the most likely mechanism is\nLy-a-driven dissociation of H2O in the inner disk, r < 2 AU. We demonstrate\nthat non-thermally populated H2O fragments can qualitatively account for the\nobserved emission (discrete and continuum), and find that the average\nLy-a-driven H2O dissociation rate is 1.7x 10^{42} water molecules s^{-1}.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Discovery of periodic class II methanol masers associated with\n  G339.986-0.425 region: Ten new class II methanol masers from the 6.7-GHz Methanol Multibeam survey\ncatalogues III and IV were selected for a monitoring programme at both 6.7 and\n12.2 GHz with the 26m Hartebeesthoek Radio Astronomy Observatory (HartRAO)\nradio telescope for two years and nine months, from August 2012 to May 2015. In\nthe sample, only masers associated with G339.986-0.425 were found to show\nperiodic variability at both 6.7 and 12.2 GHz. The existence of periodic\nvariation was tested with four independent methods. The analytical method gave\nthe best estimation of the period, which was 246 $\\pm$ 1 days. The time series\nof G339.986-0.425 show strong correlations across velocity channels and between\nthe 6.7 and 12.2 GHz masers. The time delay was also measured across channels\nand shows structure across the spectrum which is continuous between different\nmaser components.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Chromospheric Response to the Sunquake generated by the X9.3 Flare\n  of NOAA 12673: Active region NOAA 12673 was extremely volatile in 2017 September, producing\nmany solar flares, including the largest of solar cycle 24, an X9.3 flare of 06\nSeptember 2017. It has been reported that this flare produced a number of\nsunquakes along the flare ribbon (Sharykin & Kosovichev 2018; Zhao & Chen\n2018). We have used co-temporal and co-spatial Helioseismic and Magnetic Imager\n(HMI) line-of-sight (LOS) and Swedish 1-m Solar Telescope observations to show\nevidence of the chromospheric response to these sunquakes. Analysis of the Ca\nII 8542 \\AA\\space line profiles of the wavefronts revealed that the crests\nproduced a strong blue asymmetry, whereas the troughs produced at most a very\nslight red asymmetry. We used the combined HMI, SST datasets to create\ntime-distance diagrams and derive the apparent transverse velocity and\nacceleration of the response. These velocities ranged from 4.5 km s$^{-1}$ to\n29.5 km s$^{-1}$ with a constant acceleration of 8.6 x 10$^{-3}$ km s$^{-2}$.\nWe employed NICOLE inversions, in addition to the Center-of-Gravity (COG)\nmethod to derive LOS velocities ranging 2.4 km s$^{-1}$ to 3.2 km s$^{-1}$.\nBoth techniques show that the crests are created by upflows. We believe that\nthis is the first chromospheric signature of a flare induced sunquake.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Chemical Yields from Supernovae and Hypernovae: We review the final stages of stellar evolution, supernova properties, and\nchemical yields as a function of the progenitor's mass M. (1) 8 - 10 Ms stars\nare super-AGB stars when the O+Ne+Mg core collapses due to electron capture.\nThese AGB-supernovae may constitute an SN 2008S-like sub-class of Type IIn\nsupernovae. These stars produce little alpha-elements and Fe-peak elements, but\nare important sources of Zn and light p-nuclei. (2) 10 - 90 Ms stars undergo\nFe-core collapse. Nucleosynthesis in aspherical explosions is important, as it\ncan well reproduce the abundance patterns observed in extremely metal-poor\nstars. (3) 90 - 140 Ms stars undergo pulsational nuclear instabilities at\nvarious nuclear burning stages, including O and Si-burning. (4) 140 - 300 Ms\nstars become pair-instability supernovae, if the mass loss is small enough. (5)\nStars more massive than 300 Ms undergo core-collapse to form intermediate mass\nblack holes.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Discovery of 34 low-mass comoving systems using NOIRLab Source Catalog\n  DR2: We present the discovery of 34 comoving systems containing an ultra-cool\ndwarf found by means of the NOIRLab Source Catalog (NSC) DR2. NSC's angular\nresolution of $\\sim$1\" allows for the detection of small separation binaries\nwith significant proper motions. We used the catalog's accurate proper motion\nmeasurements to identify the companions by cross-matching a previously compiled\nlist of brown dwarf candidates with NSC DR2. The comoving pairs consist of\neither a very low-mass star and an ultra-cool companion, or a white dwarf and\nan ultra-cool companion. The estimated spectral types of the primaries are in\nthe K and M dwarf regimes, those of the secondaries in the M, L and T dwarf\nregimes. We calculated angular separations between $\\sim$2 and $\\sim$56\",\nparallactic distances between $\\sim$43 and $\\sim$261 pc and projected physical\nseparations between $\\sim$169 and $\\sim$8487 AU. The lowest measured total\nproper motion is 97 mas yr$^{-1}$, the highest 314 mas yr$^{-1}$. Tangential\nvelocities range from $\\sim$23 to $\\sim$187 km s$^{-1}$. We also determined\ncomoving probabilities, estimated mass ratios and calculated binding energies\nfor each system. We found no indication of possible binarity for any component\nof the 34 systems in the published literature. The discovered systems can\ncontribute to the further study of the formation and evolution of low-mass\nsystems as well as to the characterization of cool substellar objects.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Miras around the Galactic Center: We report results of our near-IR survey for variables in a field of view of\n20 arcmin by 30 arcmin towards the Galactic Center (GC), where we detected 1364\nlong-period variables. We have established a method for the simultaneous\nestimation of distances and extinctions using the period-luminosity relations\nfor the JHKs bands. Our method is applicable to Miras with periods in the range\n100-350 days and mean magnitudes available in two or more filters. Here we\ndiscuss 143 Miras whose distances and extinctions were obtained based on their\nperiods and H- and Ks-band magnitudes. We find that almost all of them are\nlocated at the same distance to within our accuracy, and the distance modulus\nof the GC is estimated to be 14.58+-0.02+-0.11 mag. The former error\ncorresponds to the statistical error and the latter to the systematic one which\nincludes the uncertainty of our assumed distance modulus of the LMC\n(18.45+-0.05 mag). We also discuss the large and highly variable extinction\ntowards the GC.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Flickering in AGB stars: Probing the nature of accreting companions: Binary companions to asymptotic giant branch (AGB) stars are an important\naspect of their evolution. Few AGB companions have been detected, and in most\ncases it is difficult to distinguish between main-sequence and white dwarf\ncompanions. Detection of photometric flickering, a tracer of compact accretion\ndisks around white dwarfs, can help identify the nature of these companions. In\nthis work, we searched for flickering in four AGB stars suggested to have\nlikely accreting companions. We found no signs for flickering in two targets:\nR~Aqr and V1016 Cyg. Flickering was detected in the other two stars: Mira and Y\nGem. We investigated the true nature of Mira's companion using three different\napproaches. Our results for Mira strongly suggest that its companion is a white\ndwarf.",
        "category": "astro-ph_SR"
    },
    {
        "text": "MHD waves in sunspots: The review addresses the spatial frequency morphology of sources of sunspot\noscillations and waves, including their localization, size, oscillation\nperiods, height localization with the mechanism of cut-off frequency that forms\nthe observed emission variability. Dynamic of sunspot wave processes, provides\nthe information about the structure of wave fronts and their time variations,\ninvestigates the oscillation frequency transformation depending on the wave\nenergy is shown. The initializing solar flares caused by trigger agents like\nmagnetoacoustic waves, accelerated particle beams, and shocks are discussed.\nSpecial attention is paid to the relation between the flare reconnection\nperiodic initialization and the dynamics of sunspot slow magnetoacoustic waves.\nA short review of theoretical models of sunspot oscillations is provided.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Simultaneous Multiwavelength Observations of Magnetic Activity in\n  Ultracool Dwarfs. IV. The Active, Young Binary NLTT 33370 AB (=2MASS\n  J13142039+1320011): We present multi-epoch simultaneous radio, optical, H{\\alpha}, UV, and X-ray\nobservations of the active, young, low-mass binary NLTT 33370 AB (blended\nspectral type M7e). This system is remarkable for its extreme levels of\nmagnetic activity: it is the most radio-luminous ultracool dwarf (UCD) known,\nand here we show that it is also one of the most X-ray luminous UCDs known. We\ndetect the system in all bands and find a complex phenomenology of both flaring\nand periodic variability. Analysis of the optical light curve reveals the\nsimultaneous presence of two periodicities, 3.7859 $\\pm$ 0.0001 and 3.7130\n$\\pm$ 0.0002 hr. While these differ by only ~2%, studies of differential\nrotation in the UCD regime suggest that it cannot be responsible for the two\nsignals. The system's radio emission consists of at least three components:\nrapid 100% polarized flares, bright emission modulating periodically in phase\nwith the optical emission, and an additional periodic component that appears\nonly in the 2013 observational campaign. We interpret the last of these as a\ngyrosynchrotron feature associated with large-scale magnetic fields and a cool,\nequatorial plasma torus. However, the persistent rapid flares at all rotational\nphases imply that small-scale magnetic loops are also present and reconnect\nnearly continuously. We present an SED of the blended system spanning more than\n9 orders of magnitude in wavelength. The significant magnetism present in NLTT\n33370 AB will affect its fundamental parameters, with the components' radii and\ntemperatures potentially altered by ~+20% and ~-10%, respectively. Finally, we\nsuggest spatially resolved observations that could clarify many aspects of this\nsystem's nature.",
        "category": "astro-ph_SR"
    },
    {
        "text": "6.7 GHz variability characteristics of new periodic methanol maser\n  sources: Discovery of periodic maser emission was an unexpected result from monitoring\nobservations of methanol transitions in high-mass young stellar objects. We\nreport on the detection of five new periodic sources from a monitoring program\nwith the Torun 32 m telescope. Variability with a period of 149 to 540 d and\ndifferent patterns from sinusoidal-like to intermittent was displayed. Three\ndimensional structure of G59.633$-$0.192 determined from the time delays of\nburst peaks of the spectral features and high angular resolution map implies\nthat the emission traces a disk. For this source the 6.7 GHz light curve\nfollowed the infrared variability supporting a radiative scheme of pumping. An\nunusual time delay of $\\sim$80 d occurred in G30.400$-$0.296 could not be\nexplained by the light travel time and may suggest a strong differentiation of\nphysical conditions and excitation in this deeply embedded source. Our\nobservations suggest the intermittent variability may present a simple response\nof maser medium to the underlying variability induced by the accretion\nluminosity while other variability patterns may reflect more complex changes in\nthe physical conditions.",
        "category": "astro-ph_SR"
    },
    {
        "text": "LMC Blue Supergiant Stars and the Calibration of the Flux-weighted\n  Gravity--Luminosity Relationship: High quality spectra of 90 blue supergiant stars in the Large Magellanic\nCloud are analyzed with respect to effective temperature, gravity, metallicity,\nreddening, extinction and extinction law. An average metallicity, based on Fe\nand Mg abundances, relative to the Sun of [Z] = -0.35 +/- 0.09 dex is obtained.\nThe reddening distribution peaks at E(B-V) = 0.08 mag, but significantly larger\nvalues are also encountered. A wide distribution of the ratio of extinction to\nreddening is found ranging from Rv = 2 to 6. The results are used to\ninvestigate the blue supergiant relationship between flux-weighted gravity, and\nabsolute bolometric magnitude. The existence of a tight relationship, the FGLR,\nis confirmed. However, in contrast to previous work the observations reveal\nthat the FGLR is divided into two parts with a different slope. For\nflux-weighted gravities larger than 1.30 dex the slope is similar as found in\nprevious work, but the relationship becomes significantly steeper for smaller\nvalues of the flux-weighted gravity. A new calibration of the FGLR for\nextragalactic distance determinations is provided.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Two circumstellar nebulae discovered with the Wide-field Infrared Survey\n  Explorer and their massive central stars: We report the discovery of two mid-infrared nebulae in the northern\nhemisphere with the Wide-field Infrared Survey Explorer and the results of\noptical spectroscopy of their central stars, BD+60 2668 (composed of two\ncomponents, separated from each other by \\approx 3 arcsec) and ALS 19653, with\nthe Calar Alto 3.5-m telescope and the Southern African Large Telescope (SALT),\nrespectively. We classify the components of BD+60 2668 as stars of spectral\ntypes B0.5 II and B1.5 III. ALS 19653 is indicated in the SIMBAD data base as a\nplanetary nebula, while our observations show that it is a massive B0.5 Ib\nstar, possibly in a binary system. Using the stellar atmosphere code FASTWIND,\nwe derived fundamental parameters of the three stars as well as their surface\nelement abundances, implying that all of them are either on the main sequence\nor only recently left it. This provides further evidence that massive stars can\nproduce circumstellar nebulae while they are still relatively unevolved. We\nalso report the detection of optical counterparts to the mid-infrared nebulae\nand a second, more extended optical nebula around ALS 19653, and present the\nresults of SALT spectroscopy of both nebulae associated with this star. The\npossible origin of the nebulae is discussed.",
        "category": "astro-ph_SR"
    },
    {
        "text": "High-precision abundances of Sc, Mn, Cu, and Ba in solar twins. Trends\n  of element ratios with stellar age: A previous study of correlations between element abundance ratios, [X/Fe],\nand ages of solar twin stars is extended to include Sc, Mn, Cu, and Ba. HARPS\nspectra with S/N > 600 are used to derive very precise (+/- 0.01 dex)\ndifferential abundances, and stellar ages with internal errors less than 1 Gyr\nare obtained by interpolation in the logg - Teff diagram between isochrones\ncalculated with the Aarhus Stellar Evolution Code. For stars younger than 6\nGyr, [X/Fe] is tightly correlated with stellar age for all elements. For ages\nbetween 6 and 9 Gyr, the [X/Fe] - age correlations break down and the stars\nsplit up into two groups having respectively high and low [X/Fe] for the odd-Z\nelements. It is concluded that while stars in the solar neighborhood younger\nthan about 6 Gyr were formed from interstellar gas with a smooth chemical\nevolution, older stars have originated from regions enriched by supernovae with\ndifferent neutron excesses. Furthermore, the correlations between abundance\nratios and stellar age suggest that: i) Sc is made in Type II supernovae along\nwith the alpha-capture elements, ii) the Type II to Ia SNe yield ratio is about\nthe same for Mn and Fe, iii) Cu is mainly made by the weak s-process in massive\nstars, iv) the Ba/Y yield ratio for AGB stars increases with decreasing stellar\nmass, v) [Y/Mg] and [Y/Al] can be used as \"chemical clocks\" when determining\nages of solar metallicity stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The DDO Close Binary Spectroscopic Program: The survey of radial velocity orbits for short period (P < 1 day), bright (V\n< 10, with a few fainter stars) conducted at the David Dunlap Observatory in\nthe last 9 years before its closure in 2008 included 162 binaries and resulted\nin 150 SB2 orbits and 5 SB1 spectroscopic orbits thus becoming one of the main\nlegacies of DDO. The paper summarizes the main results from the survey.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Measurement of the core-collapse progenitor mass distribution of the\n  Small Magellanic Cloud: The physics of core-collapse (CC) supernovae (SNe) and how the explosions\ndepend on progenitor properties are central questions in astronomy. For only a\nhandful of SNe, the progenitor star has been identified in pre-explosion\nimages. Supernova remnants (SNRs), which are observed long after the original\nSN event, provide a unique opportunity to increase the number of progenitor\nmeasurements. Here, we systematically examine the stellar populations in the\nvicinities of 23 known SNRs in the Small Magellanic Cloud (SMC) using the star\nformation history (SFH) maps of Harris & Zaritsky (2004). We combine the\nresults with constraints on the SNR metal abundances and environment from X-ray\nand optical observations. We find that 22 SNRs in the SMC have local SFHs and\nproperties consistent with a CC explosion, several of which are likely to have\nbeen high-mass progenitors. This result supports recent theoretical findings\nthat high-mass progenitors can produce successful explosions. We estimate the\nmass distribution of the CC progenitors and find that this distribution is\nsimilar to a Salpeter IMF (within the uncertainties), while this result is\nshallower than the mass distribution found in M31 and M33 by Jennings et al.\n(2014) and D\\'{\\i}az-Rodr\\'{\\i}guez et al. (2018) using a similar approach.\nAdditionally, we find that a number of the SMC SNRs exhibit a burst of star\nformation between 50-200 Myr ago. As these sources are likely CC, this\nsignature may be indicative of massive stars undergoing delayed CC as a\nconsequence of binary interaction, rapid rotation, or low metallicity. In\naddition, the lack of Type Ia SNRs in the SMC is possibly a result of the short\nvisibility times of these sources as they may fall below the sensitivity limits\nof current radio observations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Image patch analysis of sunspots and active regions. I. Intrinsic\n  dimension and correlation analysis: The flare-productivity of an active region is observed to be related to its\nspatial complexity. Mount Wilson or McIntosh sunspot classifications measure\nsuch complexity but in a categorical way, and may therefore not use all the\ninformation present in the observations. Moreover, such categorical schemes\nhinder a systematic study of an active region's evolution for example. We\npropose fine-scale quantitative descriptors for an active region's complexity\nand relate them to the Mount Wilson classification. We analyze the local\ncorrelation structure within continuum and magnetogram data, as well as the\ncross-correlation between continuum and magnetogram data. We compute the\nintrinsic dimension, partial correlation, and canonical correlation analysis\n(CCA) of image patches of continuum and magnetogram active region images taken\nfrom the SOHO-MDI instrument. We use masks of sunspots derived from continuum\nas well as larger masks of magnetic active regions derived from the magnetogram\nto analyze separately the core part of an active region from its surrounding\npart. We find the relationship between complexity of an active region as\nmeasured by Mount Wilson and the intrinsic dimension of its image patches.\nPartial correlation patterns exhibit approximately a third-order Markov\nstructure. CCA reveals different patterns of correlation between continuum and\nmagnetogram within the sunspots and in the region surrounding the sunspots.\nThese results also pave the way for patch-based dictionary learning with a view\ntowards automatic clustering of active regions.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Close detached white dwarf + brown dwarf binaries: further evidence for\n  low values of the common envelope efficiency: Common envelope evolution is a fundamental ingredient in our understanding of\nthe formation of close binary stars containing compact objects which includes\nthe progenitors of type Ia supernovae, short gamma ray bursts and most stellar\ngravitational wave sources. To predict the outcome of common envelope evolution\nwe still rely to a large degree on a simplified energy conservation equation.\nUnfortunately, this equation contains a theoretically rather poorly constrained\nefficiency parameter ($\\alpha_{\\mathrm{CE}}$) and, even worse, it is unclear if\nenergy sources in addition to orbital energy (such as recombination energy)\ncontribute to the envelope ejection process. In previous works we reconstructed\nthe evolution of observed populations of post common envelope binaries (PCEBs)\nconsisting of white dwarfs with main sequence star companions and found\nindications that the efficiency is rather small\n($\\alpha_{\\mathrm{CE}}\\simeq0.2-0.3$) and that extra energy sources are only\nrequired in very few cases. Here we used the same reconstruction tool to\ninvestigate the evolutionary history of a sample of observed PCEBs with brown\ndwarf companions. In contrast to previous works, we found that the evolution of\nobservationally well characterized PCEBs with brown dwarf companions can be\nunderstood assuming a low common envelope efficiency\n($\\alpha_{\\mathrm{CE}}=0.24-0.41$), similar to that required to understand\nPCEBs with main sequence star companions, and that contributions from\nrecombination energy are not required. We conclude that the vast majority of\nPCEBs form from common envelope evolution that can be parameterized with a\nsmall efficiency and without taking into account additional energy sources.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evolution of massive population III stars with rotation and magnetic\n  fields: [Abridged] We present a new grid of massive population III star models\nincluding the effects of rotation on the stellar structure and chemical mixing,\nand magnetic torques for the transport of angular momentum. Based on the grid,\nwe also present a phase diagram for the expected final fates of rotating\nmassive Pop III stars. Our non-rotating models become redder than the previous\nmodels in the literature, given the larger overshooting parameter adopted in\nthis study. In particular, convective dredge-up of the helium core material\ninto the hydrogen envelope is observed in our non-rotating very massive star\nmodels (>~200 Msun), which is potentially important for the chemical yields. On\nthe other hand, the stars become bluer and more luminous with a higher\nrotational velocity. With the Spruit-Tayler dynamo, our models with a\nsufficiently high initial rotational velocity can reach the critical rotation\nearlier and lose more mass as a result, compared to the previous models without\nmagnetic fields. The most dramatic effect of rotation is found with the\nso-called chemically homogeneous evolution (CHE), which is observed for a\nlimited mass and rotational velocity range. CHE has several important\nconsequences: 1) Both primary nitrogen and ionizing photons are abundantly\nproduced. 2) Conditions for gamma-ray burst progenitors are fulfilled for an\ninitial mass range of 13 - 84 Msun. 3) Pair instability supernovae of type Ibc\nare expected for 84 -190 Msun and 4) Both a pulsational pair instability\nsupernova and a GRB may occur from the same progenitor of about 56 - 84 Msun,\nwhich might significantly influence the consequent GRB afterglow. We find that\nCHE does not occur for very massive stars (> 190 Msun), in which case the\nhydrogen envelope expands to the red-supergiant phase and the final angular\nmomentum is too low to make any explosive event powered by rotation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Age and The Mass of The \u03b1Herculis Triple-Star System From A\n  MESA Grid of rotating stars with 1.3 <= M/Msun <= 8.0: \\alpha^1 Her is the second closest Asymptotic Giant Branch (AGB) star to the\nSun, and the variable luminous M5 Ib-II member of a triple stellar system\ncontaining G8 III and A9 IV-V components. However, the mass of this important\nstar was previously uncertain with published values ranging from ~2 - 15 Msun.\nAs shown by this study, its fortuitous membership in a nearby resolved triple\nstar system, makes it possible to determine its fundamental properties\nincluding its mass and age. We present over twenty years of VRI photometry of\n\\alpha^1 Her as well as Wing intermediate-band near-IR TiO and NIR continuum\nphotometry. We introduce a new photometry-based calibration technique, and\nextract the effective temperature and luminosity of \\alpha^1 Her, in agreement\nwith recent interferometric measures. We find, Teff=3280 +/- 87 K and\nlog(L/Lsun)=3.92 +/- 0.14.\n  With the MESA code, we calculate a dense grid of evolutionary tracks for\nGalactic low- to intermediate-mass (1.3 to 8 Msun) rotating stars from the\npre-main sequence phase to the advanced AGB phase. We include atomic diffusion\nand rotation mechanisms to treat the effects of extra elemental mixing. Based\non the observed properties of the \\alpha Herculis stars, we constrain the age\nof the system to lie in the range 0.41 to 1.25 Gyr. Thus, the mass of \\alpha^1\nHer lies in the range 2.175 <= M/Msun <= 3.250. We compare our model-based age\ninference with recent tracks of the Geneva and STAREVOL codes, and show their\nagreement. In the prescribed mass range for \\alpha^1 Her, the observed 12C/13C\nand 16O/17O ratios are consistent (within 2\\sigma) with the ratios predicted by\nthe MESA, Geneva and STAREVOL codes.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Optical photometric GTC/OSIRIS observations of the young massive\n  association Cygnus OB2: In order to fully understand the gravitational collapse of molecular clouds,\nthe star formation process and the evolution of circumstellar disks, these\nphenomena must be studied in different Galactic environments with a range of\nstellar contents and positions in the Galaxy. The young massive association\nCygnus OB2, in the Cygnus-X region, is an unique target to study how star\nformation and the evolution of circumstellar disks proceed in the presence of a\nlarge number of massive stars. We present a catalog obtained with recent\noptical observations in r,i,z filters with OSIRIS, mounted on the $10.4\\,m$ GTC\ntelescope, which is the deepest optical catalog of Cyg OB2 to date.\n  The catalog consist of 64157 sources down to M=0.15 solar masses at the\nadopted distance and age of Cyg OB2. A total of 38300 sources have good\nphotometry in all three bands. We combined the optical catalog with existing\nX-ray data of this region, in order to define the cluster locus in the optical\ndiagrams. The cluster locus in the r-i vs. i-z diagram is compatible with an\nextinction of the optically selected cluster members in the 2.64<AV<5.57 range.\nWe derive an extinction map of the region, finding a median value of AV=4.33 in\nthe center of the association, decreasing toward the north-west. In the\ncolor-magnitude diagrams, the shape of the distribution of main sequence stars\nis compatible with the presence of an obscuring cloud in the foreground at\nabout 850+/-25 pc from the Sun.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Brightness of the Sun's small scale magnetic field: proximity effects: The net effect of the small scale magnetic field on the Sun's (bolometric)\nbrightness is studied with realistic 3D MHD simulations. The direct effect of\nbrightening within the magnetic field itself is consistent with measurements in\nhigh-resolution observations. The high 'photometric accuracy' of the\nsimulations, however, reveal compensating brightness effects that are hard to\ndetect observationally. The influence of magnetic concentrations on the\nsurrounding nonmagnetic convective flows (a 'proximity effect') reduces the\nbrightness by an amount exceeding the brightening by the magnetic\nconcentrations themselves. The net photospheric effect of the small scale field\n(~ -0.34% at a mean flux density of 50 G) is thus negative. We conclude that\nthe main contribution to the observed positive correlation between the magnetic\nfield and total solar irradiance must be magnetic dissipation in layers around\nthe temperature minimum and above (not included in the simulations). This\nagrees with existing inferences from observations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Block-induced complex structures building the flare-productive solar\n  active region 12673: Solar active region (AR) 12673 produced 4 X-class, 27 M-class, and numerous\nlower class flares during its passage across the visible solar disk in\nSeptember 2017. Our study is to answer the questions why this AR was so\nflare-productive and how the X9.3 flare, the largest one of the last decade,\ntook place. We find that there was a sunspot in the initial several days, and\nthen two bipolar regions emerged nearby it successively. Due to the standing of\nthe pre-existing sunspot, the movement of the bipoles was blocked, while the\npre-existing sunspot maintained its quasi-circular shaped umbra only with the\ndisappearance of a part of penumbra. Thus, the bipolar patches were\nsignificantly distorted, and the opposite polarities formed two semi-circular\nshaped structures. After that, two sequences of new bipolar regions emerged\nwithin the narrow semi-circular zone, and the bipolar patches separated along\nthe curved channel. The new bipoles sheared and interacted with the previous\nones, forming a complex topological system, during which numerous flares\noccurred. At the highly sheared region, a great deal of free energy was\naccumulated. On September 6, one negative patch near the polarity inversion\nline began to rapidly rotate and shear with the surrounding positive fields,\nand consequently the X9.3 flare erupted. Our results reveal that the\nblock-induced complex structures built the flare-productive AR and the X9.3\nflare was triggered by an erupting filament due to the kink instability. To\nbetter illustrate this process, a block-induced eruption model is proposed for\nthe first time.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Properties of simulated sunspot umbral dots: Realistic 3D radiative MHD simulations reveal the magneto-convective\nprocesses underlying the formation of the photospheric fine structure of\nsunspots, including penumbral filaments and umbral dots. Here we provide\nresults from a statistical analysis of simulated umbral dots and compare them\nwith reports from high-resolution observations. A multi-level segmentation and\ntracking algorithm has been used to isolate the bright structures in synthetic\nbolometric and continuum brightness images. Areas, brightness, and lifetimes of\nthe resulting set of umbral dots are found to be correlated: larger umbral dots\ntend to be brighter and live longer. The magnetic field strength and velocity\nstructure of umbral dots on surfaces of constant optical depth in the continuum\nat 630 nm indicate that the strong field reduction and high velocities in the\nupper parts of the upflow plumes underlying umbral dots are largely hidden from\nspectro-polarimetric observations. The properties of the simulated umbral dots\nare generally consistent with the results of recent high-resolution\nobservations. However, the observed population of small, short-lived umbral\ndots is not reproduced by the simulations, possibly owing to insufficient\nspatial resolution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "BANYAN. VII. A New Population of Young Substellar Candidate Members of\n  Nearby Moving Groups from the BASS Survey: [Abbreviated] We present the results of a near-infrared (NIR) spectroscopic\nfollow-up survey of 182 M4-L7 low-mass stars and brown dwarfs (BDs) from the\nBANYAN All-Sky Survey (BASS) for candidate members of nearby, young moving\ngroups (YMGs). We confirm signs of low-gravity for 42 new BD discoveries with\nestimated masses between 8-75 $M_{Jup}$ and identify previously unrecognized\nsigns of low gravity for 24 known BDs. This allows us to refine the fraction of\nlow-gravity dwarfs in the high-probability BASS sample to $\\sim$82%. We use\nthis unique sample of 66 young BDs, supplemented with 22 young BDs from the\nliterature, to construct new empirical NIR absolute magnitude and color\nsequences for low-gravity BDs. We obtain a spectroscopic confirmation of\nlow-gravity for 2MASS J14252798-3650229, which is a new $\\sim$27 $M_{Jup}$, L4\n$\\gamma$ bona fide member of AB Doradus. We identify a total of 19 new\nlow-gravity candidate members of YMGs with estimated masses below 13 $M_{Jup}$,\nseven of which have kinematically estimated distances within 40 pc. These\nobjects will be valuable benchmarks for a detailed atmospheric characterization\nof planetary-mass objects with the next generation of instruments. We find 16\nstrong candidate members of the Tucana-Horologium association with estimated\nmasses between 12.5-14 $M_{Jup}$, a regime where our study was particularly\nsensitive. This would indicate that for this association there is at least one\nisolated object in this mass range for every $17.5_{-5.0}^{+6.6}$ main-sequence\nstellar member, a number significantly higher than expected based on standard\nlog-normal initial mass function, however in the absence of radial velocity and\nparallax measurements for all of them, it is likely that this over-density is\ncaused by a number of young interlopers from other moving groups. We identify\n12 new L0-L5 field BDs, seven of which display peculiar properties.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Radio emission as a stellar activity indicator: Radio observations of stars trace the plasma conditions and magnetic field\nproperties of stellar magnetospheres and coronae. Depending on the plasma\nconditions at the emitter site, radio emission in the metre- and decimetre-wave\nbands is generated via different mechanisms such as gyrosynchrotron, electron\ncyclotron maser instability, and plasma radiation processes. The ongoing LOFAR\nTwo-metre Sky Survey (LoTSS) and VLA Sky Survey (VLASS) are currently the most\nsensitive wide-field radio sky surveys ever conducted. Because these surveys\nare untargeted, they provide an opportunity to study the statistical properties\nof the radio-emitting stellar population in an unbiased manner. Here, we\nperform an untargeted search for stellar radio sources down to sub-mJy level\nusing these radio surveys. We find that the population of radio-emitting\nstellar systems is mainly composed of two distinct categories:\nchromospherically active stellar (CAS) systems and M dwarfs. We also seek to\nidentify signatures of a gradual transition within the M-dwarf population from\nchromospheric/coronal acceleration close to the stellar surface similar to that\nobserved on the Sun, to magnetospheric acceleration occurring far from the\nstellar surface similar to that observed on Jupiter. We determine that radio\ndetectability evolves with spectral type, and we identify a transition in radio\ndetectability around spectral type M4, where stars become fully convective.\nFurthermore, we compare the radio detectability vs spectra type with X-ray and\noptical flare (observed by TESS) incidence statistics. We find that the radio\nefficiency of X-ray/optical flares, which is the fraction of flare energy\nchannelled into radio-emitting charges, increases with spectral type. These\nresults motivate us to conjecture that the emergence of large-scale magnetic\nfields in CAS systems and later M dwarfs leads to an increase in radio\nefficiency.",
        "category": "astro-ph_SR"
    },
    {
        "text": "IC 4663: The first unambiguous [WN] Wolf-Rayet central star of a\n  planetary nebula: We report on the serendipitous discovery of the first central star of a\nplanetary nebula (PN) that mimics the helium- and nitrogen-rich WN sequence of\nmassive Wolf-Rayet (WR) stars. The central star of IC 4663 (PN G346.2-08.2) is\ndominated by broad He II and N V emission lines which correspond to a [WN3]\nspectral type. Unlike previous [WN] candidates, the surrounding nebula is\nunambiguously a PN. At an assumed distance of 3.5 kpc, corresponding to a\nstellar luminosity of 4000 Lsun, the V=16.9 mag central star remains 4-6 mag\nfainter than the average luminosity of massive WN3 stars even out to an\nimprobable d=8 kpc. The nebula is typical of PNe with an elliptical morphology,\na newly discovered Asymptotic Giant Branch (AGB) halo, a relatively low\nexpansion velocity (v_exp=30 km/s) and a highly ionised spectrum with an\napproximately Solar chemical abundance pattern. The [WN3] star is hot enough to\nshow Ne VII emission (T_*=140+/-20 kK) and exhibits a fast wind (v_infty=1900\nkm/s), which at d=3.5 kpc would yield a clumped mass loss rate of Mdot = 1.8 x\n10^-8 Msun/yr with a small stellar radius (R_*=0.11 Rsun). Its atmosphere\nconsists of helium (95%), hydrogen (<2%), nitrogen (0.8%), neon (0.2%) and\noxygen (0.05%) by mass. Such an unusual helium-dominated composition cannot be\nproduced by any extant scenario used to explain the H-deficiency of post-AGB\nstars. The O(He) central stars share a similar composition and the discovery of\nIC 4663 provides the first evidence for a second He-rich/H-deficient post-AGB\nevolutionary sequence [WN]->O(He). This suggests there is an alternative\nmechanism responsible for producing the majority of H-deficient post-AGB stars\nthat may possibly be expanded to include other He-rich/H-deficient stars such\nas R Coronae Borealis stars and AM Canum Venaticorum stars. The origin of the\nunusual composition of [WN] and O(He) central stars remains unexplained.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Multi-epoch spectropolarimetry of SN 2009ip: direct evidence for\n  aspherical circumstellar material: We present spectropolarimetry of SN 2009ip throughout the evolution of its\n2012 explosion. During the initial 2012a phase, when the source spectrum\nexhibits broad P-Cygni lines, we measure a V-band polarization of P~0.9% at a\nposition angle of ~166 deg, indicating substantial asphericity for the 2012a\noutflow. Near the subsequent peak of the 2012b phase, when the spectrum shows\nsigns of intense interaction with circumstellar material (CSM), we measure\nP~1.7% at a position angle of 72 deg, indicating a separate physical component\nof polarization, which has a higher degree of asphericity than the 2012a\noutflow and an orthogonal axis of symmetry on the sky. Around 30 days past\npeak, which is coincident with a bump in the declining light curve, we measure\nP~0.7% and significant variations in P across some broad lines, particularly\nHeI/NaI. By 60 days past peak the source appears to be approaching a low value\nof interstellar polarization (P<0.2%). The results are consistent with a\nscenario in which a potentially bipolar explosion during 2012a impacts a\ntoroidal distribution of CSM, thereby causing the 2012b brightening.\nOrthogonality between the 2012a and 2012b geometries is inconsistent with the\nhypothesis that the 2012a event launched the CSM that was hit 40 days later by\nmaterial from the 2012b event. Rather, the aspherical CSM probably has a\nseparate origin associated with the outbursts of the stellar progenitor during\nprior years. Previous calculations that assumed spherical symmetry for the CSM\nhave underestimated the required explosion energy, as the results suggest that\n<10% of the fast SN ejecta participated in strong CSM interaction during the\n2012b phase. In light of the spectropolarimetric results, a kinetic energy of\n1e51 erg for the ejecta is difficult to avoid, supporting the interpretation\nthat the 2012 outburst was the result of a core-collapse supernova explosion\n(abridged)",
        "category": "astro-ph_SR"
    },
    {
        "text": "Synthetic Light Curves for Born Again Events: Preliminary Results: The development of surveys which will be able to cover a large region of the\nsky several times per year will allow the massive detection of transient events\ntaking place in timescales of years. In addition, the projected full\ndigitalization of the Harvard plate collection will open a new window to\nidentify slow transients taking place in timescales of centuries. In\nparticular, these projects will allow the detection of stars undergoing slow\neruptions as those expected during late helium flashes in the post-AGB\nevolution. In order to identify those transients which correspond with late\nhelium flashes the development of synthetic light curves of those events is\nmandatory. In this connection we present preliminary results of a project aimed\nat computing grids of theoretical light curves of born again stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spectroscopic survey of Kepler stars. II. FIES/NOT observations of A-\n  and F-type stars: We have analysed high-resolution spectra of 28 A and 22 F stars in the Kepler\nfield, observed with the FIES spectrograph at the Nordic Optical Telescope. We\nprovide spectral types, atmospheric parameters and chemical abundances for 50\nstars. Balmer, Fe I, and Fe II lines were used to derive effective\ntemperatures, surface gravities, and microturbulent velocities. We determined\nchemical abundances and projected rotational velocities using a spectrum\nsynthesis technique. Effective temperatures calculated by spectral energy\ndistribution fitting are in good agreement with those determined from the\nspectral line analysis. The stars analysed include chemically peculiar stars of\nthe Am and Lambda Boo types, as well as stars with approximately solar chemical\nabundances. The wide distribution of projected rotational velocity, Vsini, is\ntypical for A and F stars. The microturbulence velocities obtained are typical\nfor stars in the observed temperature and surface gravity ranges. Moreover, we\naffirm the results of Niemczura et al., that Am stars do not have\nsystematically higher microturbulent velocities than normal stars of the same\ntemperature.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Observations of Reconnection Flows in a Flare on the Solar Disk: Magnetic reconnection is a well-accepted part of the theory of solar eruptive\nevents, though the evidence is still circumstantial. Intrinsic to the\nreconnection picture of a solar eruptive event, particularly in the standard\nmodel for two-ribbon flares (\"CSHKP\" model), are an advective flow of\nmagnetized plasma into the reconnection region, expansion of field above the\nreconnection region as a flux rope erupts, retraction of heated\npost-reconnection loops, and downflows of cooling plasma along those loops. We\nreport on a unique set of SDO/AIA imaging and Hinode/EIS spectroscopic\nobservations of the disk flare SOL2016-03-23T03:54 in which all four flows are\npresent simultaneously. This includes spectroscopic evidence for a plasma\nupflow in association with large-scale expanding closed inflow field. The\nreconnection inflows are symmetric, and consistent with fast reconnection, and\nthe post-reconnection loops show a clear cooling and deceleration as they\nretract. Observations of coronal reconnection flows are still rare, and most\nevents are observed at the solar limb, obscured by complex foregrounds, making\ntheir relationship to the flare ribbons, cusp field and arcades formed in the\nlower atmosphere difficult to interpret. The disk location and favorable\nperspective of this event have removed these ambiguities giving a clear picture\nof the reconnection dynamics.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Using the Sun to estimate Earth-like planets detection capabilities. V.\n  Parameterizing the impact of solar activity components on radial velocities: Stellar activity induced by active structures (eg, spots, faculae) is known\nto strongly impact the radial velocity time series. It then limits the\ndetection of small planetary RV signals (eg, an Earth-mass planet in the\nhabitable zone of a solar-like star). In previous papers, we studied the\ndetectability of such planets around the Sun seen as an edge-on star. For that\npurpose, we computed the RV and photometric variations induced by solar\nmagnetic activity, using all active structures observed over one entire cycle.\nOur goal is to perform similar studies on stars with different physical and\ngeometrical properties. As a first step, we focus on Sun-like stars seen with\nvarious inclinations, and on estimating detection capabilities with forthcoming\ninstruments. To do so, we first parameterize the solar active structures with\nthe most realistic pattern so as to obtain results consistent with the observed\nones. We simulate the growth, evolution and decay of solar spots, faculae and\nnetwork, using parameters and empiric laws derived from solar observations and\nliterature. We generate the corresponding structure lists over a full solar\ncycle. We then build the resulting spectra and deduce the RV and photometric\nvariations for a `Sun' seen with various inclinations. The produced RV signal\ntakes into account the photometric contribution of structures as well as the\nattenuation of the convective blueshift. The comparison between our simulated\nactivity pattern and the observed one validates our model. We show that the\ninclination of the stellar rotation axis has a significant impact on the time\nseries. RV long-term amplitudes as well as short-term jitters are significantly\nreduced when going from edge-on to pole-on configurations. Assuming spin-orbit\nalignment, the optimal configuration for planet detection is an inclined star\n(i~45{\\deg}).",
        "category": "astro-ph_SR"
    },
    {
        "text": "A method to deconvolve stellar rotational velocities III. The\n  probability distribution function via Maximum Likelihood utilizing Finite\n  Distribution Mixtures: The study of accurate methods to estimate the distribution of stellar\nrotational velocities is important for understanding many aspects of stellar\nevolution. From such observations we obtain the projected rotational speed v\nsin(i) in order to recover the true distribution of the rotational velocity. To\nthat end, we need to solve a difficult inverse problem that can be posed as a\nFredholm integral of the first kind. n this work we have used a novel approach\nbased on Maximum likelihood (ML) estimation to obtain an approximation of the\ntrue rotational velocity probability density function expressed as a sum of\nknown distribution families. In our proposal, the measurements have been\ntreated as random variables drawn from the projected rotational velocity\nprobability density function. We analyzed the case of Maxwellian sum\napproximation, where we estimated the parameters that define the sum of\ndistributions. The performance of the proposed method is analyzed using Monte\nCarlo simulations considering two theoretical cases for the probability density\nfunction of the true rotational stellar velocities: i) an unimodal Maxwellian\nprobability density distribution and ii) a bimodal Maxwellian probability\ndensity distribution. The results show that the proposed method yielded more\naccurate estimates in comparison with the Tikhonov regularization method,\nespecially for small sample length N=50. Our proposal was evaluated using real\ndata from three sets of measurements, and our findings were validated using\nthree statistical tests. The ML approach with Maxwellian sum approximation is a\naccurate method to deconvolve the rotational velocity probability density\nfunction, even when the sample length is small (N= 50)",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the Doppler Shift and Asymmetry of Stokes Profiles of Photospheric\n  FeI and Chromospheric MgI Lines: We analyzed the full Stokes spectra using simultaneous measurements of the\nphotospheric (FeI 630.15 and 630.25 nm) and chromospheric (MgI b2 517.27 nm)\nlines. The data were obtained with the HAO/NSO Advanced Stokes Polarimeter,\nabout a near disc center sunspot region, NOAA AR 9661. We compare the\ncharacteristics of Stokes profiles in terms of Doppler shifts and asymmetries\namong the three spectral lines, which helps us to better understand the\nchromospheric lines and the magnetic and flow fields in different magnetic\nregions. The main results are: (1) For penumbral area observed by the\nphotospheric FeI lines, Doppler velocities derived from Stokes I (Vi) are very\nclose to those derived from linear polarization profiles (Vlp) but\nsignificantly different from those derived from Stokes V profiles (Vzc), which\nprovides direct and strong evidence that the penumbral Evershed flows are\nmagnetized and mainly carried by the horizontal magnetic component. (2) The\nrudimentary inverse Evershed effect observed by the MgI b2 line provides a\nqualitative evidence on its formation height that is around or just above the\ntemperature minimum region. (3) Vzc and Vlp in penumbrae and Vzc in pores\ngenerally approach their Vi observed by the chromospheric MgI line, which is\nnot the case for the photospheric FeI lines. (4) Outer penumbrae and pores show\nsimilar behavior of the Stokes V asymmetries that tend to change from positive\nvalues in the photosphere (FeI lines) to negative values in the low\nchromosphere (MgI line). (5) The Stokes V profiles in plage regions are highly\nasymmetric in the photosphere and more symmetric in the low chromosphere. (6)\nStrong red shifts and large asymmetries are found around the magnetic polarity\ninversion line within the common penumbra of the Delta spot. This study thus\nemphasizes the importance of spectro-polarimetry using chromospheric lines.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Role of Inverse Compton Scattering in Solar Coronal Hard X-ray and\n  Gamma-ray Sources: Coronal hard X-ray (HXR) and continuum gamma-ray sources associated with the\nimpulsive phase of solar flares have been the subject of renewed interest in\nrecent years. They have been interpreted in terms of thin-target, nonthermal\nbremsstrahlung emission. This interpretation has led to rather extreme physical\nrequirements in some cases. For example, in one case, essentially all of the\nelectrons in the source must be accelerated to nonthermal energies to account\nfor the coronal HXR source. In other cases, the extremely hard photon spectra\nof the coronal continuum gamma-ray emission suggest that the low energy cutoff\nof the electron energy distribution lies in the MeV energy range. Here we\nconsider the role of inverse Compton scattering (ICS) as an alternate emission\nmechanism in both the ultra- and mildly relativistic regimes. It is known that\nrelativistic electrons are produced during powerful flares; these are capable\nof up-scattering soft photospheric photons to HXR and gamma-ray energies.\nPreviously overlooked is the fact that mildly relativistic electrons, generally\nproduced in much greater numbers in flares of all sizes, can up-scatter EUV/SXR\nphotons to HXR energies. We also explore ICS on anisotropic electron\ndistributions and show that the resulting emission can be significantly\nenhanced over an isotropic electron distribution for favorable viewing\ngeometries. We briefly review results from bremsstrahlung emission and\nreconsider circumstances under which nonthermal bremsstrahlung or ICS would be\nfavored. Finally, we consider a selection of coronal HXR and gamma-ray events\nand find that in some cases the ICS is a viable alternative emission mechanism.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Planetary Nebulae in the Small Magellanic Cloud: We analyse the planetary nebulae (PNe) population of the Small Magellanic\nCloud (SMC), based on evolutionary models of stars with metallicities in the\nrange $10^{-3} \\leq Z \\leq 4\\times 10^{-3}$ and mass $0.9 M\\odot < M <\n8M\\odot$, evolved through the asymptotic giant branch (AGB) phase. The models\nused account for dust formation in the circumstellar envelope. To characterise\nthe PNe sample of the SMC, we compare the observed abundances of the various\nspecies with the final chemical composition of the AGB models: this study\nallows us to identify the progenitors of the PNe observed, in terms of mass and\nchemical composition. According to our interpretation, most of the PNe descend\nfrom low-mass ($M < 2 M\\odot$) stars, which become carbon rich, after\nexperiencing repeated third dredge-up episodes, during the AGB phase. A\nfraction of the PNe showing the signature of advanced CNO processing are\ninterpreted as the progeny of massive AGB stars, with mass above $\\sim 6\nM\\odot$, undergoing strong hot bottom burning. The differences with the\nchemical composition of the PNe population of the Large Magellanic Cloud (LMC)\nis explained on the basis of the diverse star formation history and\nage-metallicity relation of the two galaxies. The implications of the present\nstudy for some still highly debated points regarding the AGB evolution are also\ncommented.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The galactic unclassified B[e] star HD 50138 III. The short-term line\n  profile variability of its photospheric lines: HD 50138 presents the B[e] phenomenon, but its nature is not clear yet. This\nstar is known to present spectral variations, which have been associated with\noutbursts and shell phases. We analyze the line profile variability of HD 50138\nand its possible origin, which provide possible hints to its evolutionary\nstage, so far said to be close to the end of (or slightly beyond) the main\nsequence. New high-resolution spectra of HD 50138 obtained with the HERMES\nspectrograph over several nights (five of them consecutively) were analyzed,\nallowing us to confirm short-term line profile variability. Our new data show\nshort-term variations in the photospheric lines. On the other hand, purely\ncircumstellar lines (such as [O I] lines) do not show such rapid variability.\nThe rotational velocity of HD 50138, V_rot = 90.3 +- 4.3 km/s, and the rotation\nperiod, P = 3.64 +- 1.16 d, were derived from the He II 4026A photospheric\nline. Based on the moment method, we confirm that the origin of this short-term\nline profile variability is not stellar spots, and it may be caused by\npulsations. In addition, we show that macroturbulence may affect the profiles\nof photospheric lines, as is seen for B supergiants. The location of HD 50138\nat the end of (or slightly beyond) the main sequence, the newly detected\npresence of line profile variability resembling pulsating stars, and\nmacroturbulence make this star a fascinating object.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Verification of Asymptotic Relation for Mixed Modes in Red Giant Stars: High-precision space observations, such as made by the \\kepler\\ and \\corot\\\nmissions, allow us to detect mixed modes for $l = 1$ modes in their high\nsignal-to-noise photometry data. By means of asteroseismology, the inner\nstructure of red giant (RG) stars is revealed the first time with the help of\nmixed modes. We analyse these mixed modes of a 1.3 M$_{\\sun}$ RG model\ntheoretically from the approximate asymptotic descriptions of oscillations.\nWhile fitting observed frequencies with the eigenvalue condition for mixed\nmodes, a good estimate of period spacing and coupling strength is also acquired\nfor more evolved models. We show that the behaviour of the mode inertia in a\ngiven mode varies dramatically when the coupling is strong. An approximation of\nperiod spacings is also obtained from the asymptotic dispersion relation, which\nprovides a good estimate of the coupling strength as well as period spacing\nwhen g-mode-like mixed modes are sufficiently dense. By comparing the\ntheoretical coupling strength from the integral expression with the ones from\nfitting methods, we confirmed that the theoretical asymptotic equation is\nproblematic in the evanescent region due to the potential singularities as well\nas the use of the Cowling approximation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Age-Rotation-Activity Relation: From Myrs to Gyrs: Over the past 40 years, observational surveys have established the existence\nof a tight relationship between a star's age, rotation period, and magnetic\nactivity. This age-rotation-activity relation documents the interplay between a\nstar's magnetic dynamo and angular momentum evolution, and provides a valuable\nage estimator for isolated field stars. While the age-rotation-activity\nrelation has been studied extensively in clusters younger than 500 Myr,\nempirically measured rotation periods are scarce for older ages. Using the\nPalomar Transient Factory (PTF), we have begun a survey of stellar rotation to\nmap out the late-stage evolution of the age-rotation-activity relation: the\nColumbia/Cornell/Caltech PTF (CCCP) survey of open clusters. The first CCCP\ntarget is the nearby ~600 Myr Hyades-analog Praesepe, where PTF has produced\nlight curves spanning more than 3 months and containing >150 measurements for\n~650 cluster members. Analyzing these light curves, we have measured rotation\nperiods for 40 K & M cluster members, filling the gap between the periods\npreviously reported for solar-type Hyads (Radick et al. 1987, Prosser et al.\n1995) and for a handful of low-mass Praesepe members (Scholz et al. 2007). Our\nmeasurements indicate that Praesepe's period-color relation undergoes at\ntransition at a characteristic spectral type of ~M1 --- from a well-defined\nsingular relation at higher mass, to a more scattered distribution of both fast\nand slow-rotators at lower masses. The location of this transition is broadly\nconsistent with expectations based on observations of younger clusters and the\nassumption that stellar-spin down is the dominant mechanism influencing angular\nmomentum evolution at ~600 Myr. In addition to presenting the results of our\nphotometric monitoring of Praesepe, we summarize the status and future of the\nCCCP survey.",
        "category": "astro-ph_SR"
    },
    {
        "text": "H-alpha as a Luminosity Class Diagnostic for K- and M-type Stars: We have identified the H-alpha absorption feature as a new spectroscopic\ndiagnostic of luminosity class in K- and M-type stars. From high-resolution\nspectra of 19 stars with well-determined physical properties (including\neffective temperatures and stellar radii), we measured equivalent widths for\nH-alpha and the Ca II triplet and examined their dependence on both luminosity\nclass and stellar radius. H-alpha shows a strong relation with both luminosity\nclass and radius that extends down to late M spectral types. This behavior in\nH-alpha has been predicted as a result of the density-dependent overpopulation\nof the metastable 2S level in hydrogen, an effect that should become dominant\nfor Balmer line formation in non-LTE conditions. We conclude that this new\nmetallicity-insensitive diagnostic of luminosity class in cool stars could\nserve as an effective means of discerning between populations such as Milky Way\ngiants and supergiant members of background galaxies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The (B0+?)+O6 system FN CMa: A case for tidal-pulsational interaction?: FN CMa is visually double with a separation of about 0.6arcsec. Sixty\nhigh-cadence VLT/UVES spectra permit the A and B components to be disentangled,\nas the relative contribution of each star to the total light entering the\nspectrograph fluctuates between exposures due to changes in seeing. Component A\nexhibits rapid line-profile variations, leading us to attribute the photometric\nvariability seen by HIPPARCOS (with a derived P=0.08866d) to this component.\nFrom a total of 122 archival and new echelle spectra it is shown that component\nA is an SB1 binary with an orbital period of 117.55 days. The eccentricity of\n0.6 may result in tidal modulation of the pulsation(s) of component Aa.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Atmospheric parameters of Cepheids from flux ratios with ATHOS: I. The\n  temperature scale: Context: The effective temperature is a key parameter governing the\nproperties of a star. For stellar chemistry, it has the strongest impact on the\naccuracy of the abundances derived. Since Cepheids are pulsating stars,\ndetermining their effective temperature is more complicated that in the case of\nnon-variable stars. Aims: We want to provide a new temperature scale for\nclassical Cepheids, with a high precision and full control of the systematics.\nMethods: Using a data-driven machine learning technique employing observed\nspectra, and taking great care to accurately phase single-epoch observations,\nwe have tied flux ratios to (label) temperatures derived using the infrared\nsurface brightness method. Results: We identified 143 flux ratios that allow us\nto determine the effective temperature with a precision of a few K and an\naccuracy better than 150 K, which is in line with the most accurate temperature\nmeasures available to date. The method does not require a normalization of the\ninput spectra and provides homogeneous temperatures for low- and\nhigh-resolution spectra, even at the lowest signal-to-noise ratios. Due to the\nlack of a dataset of sufficient sample size for Small Magellanic Cloud\nCepheids, the temperature scale does not extend to Cepheids with [Fe/H] < -0.6\ndex but nevertheless provides an exquisite, homogeneous means of characterizing\nGalactic and Large Magellanic Cloud Cepheids. Conclusions: The temperature\nscale will be extremely useful in the context of spectroscopic surveys for\nMilky Way archaeology with the WEAVE and 4MOST spectrographs. It paves the way\nfor highly accurate and precise metallicity estimates, which will allow us to\nassess the possible metallicity dependence of Cepheids' period-luminosity\nrelations and, in turn, to improve our measurement of the Hubble constant H0.",
        "category": "astro-ph_SR"
    },
    {
        "text": "High-Precision Radio and Infrared Astrometry of LSPM J1314+1320AB - II:\n  Testing Pre--Main-Sequence Models at the Lithium Depletion Boundary with\n  Dynamical Masses: We present novel tests of pre$-$main-sequence models based on individual\ndynamical masses for the M7 binary LSPM J1314+1320AB. Joint analysis of our\nKeck adaptive optics astrometric monitoring along with Very Long Baseline Array\nradio data from a companion paper yield component masses of $0.0885\\pm0.0006$\n$M_{\\odot}$ and $0.0875\\pm0.0010$ $M_{\\odot}$ and a parallactic distance of\n$17.249\\pm0.013$ pc. We also derive component luminosities that are consistent\nwith the system being coeval at an age of $80.8\\pm2.5$ Myr, according to BHAC15\nevolutionary models. The presence of lithium is consistent with model\npredictions, marking the first time the theoretical lithium depletion boundary\nhas been tested with ultracool dwarfs of known mass. However, we find that the\naverage evolutionary model-derived effective temperature ($2950\\pm5$ K) is 180\nK hotter than we derive from a spectral type$-$$T_{\\rm eff}$ relation based on\nBT-Settl models ($2770\\pm100$ K). We suggest that the dominant source of this\ndiscrepancy is model radii being too small by $\\approx$13%. In a test that\nmimics the typical application of evolutionary models by observers, we derive\nmasses on the H-R diagram using the luminosity and BT-Settl temperature. The\nestimated masses are $46^{+16}_{-19}$% (2.0$\\sigma$) lower than we measure\ndynamically and would imply that this is a system of $\\approx$50 $M_{\\rm Jup}$\nbrown dwarfs, highlighting the large systematic errors possible when inferring\nmasses from the H-R diagram. This is first time masses have been measured for\nultracool ($\\geq$M6) dwarfs displaying spectral signatures of low gravity.\nBased on features in the infrared, LSPM J1314+1320AB appears higher gravity\nthan typical Pleiades and AB Dor members, opposite the expectation given its\nyounger age. The components of LSPM J1314+1320AB are now the nearest, lowest\nmass pre$-$main-sequence stars with direct mass measurements.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Kinematic evidence for feedback-driven star formation in NGC 1893: OB associations are the prevailing star forming sites in the Galaxy. Up to\nnow, the process of how OB associations were formed remained a mystery. A\npossible process is self-regulating star formation driven by feedback from\nmassive stars. However, although a number of observational studies uncovered\nvarious signposts of feedback-driven star formation, the effectiveness of such\nfeedback has been questioned. Stellar and gas kinematics is a promising tool to\ncapture the relative motion of newborn stars and gas away from ionizing\nsources. We present high-resolution spectroscopy of stars and gas in the young\nopen cluster NGC 1893. Our findings show that newborn stars and the tadpole\nnebula Sim 130 are moving away from the central cluster containing two O-type\nstars, and that the timescale of sequential star formation is about 1 Myr\nwithin a 9 parsec distance. The newborn stars formed by feedback from massive\nstars account for at least 18 per cent of the total stellar population in the\ncluster, suggesting that this process can play an important role in the\nformation of OB associations. These results support the self-regulating star\nformation model.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Kepler and Hale observations of V523 Lyr: We present new observations of the cataclysmic variable V523 Lyr, a member of\nthe open cluster NGC 6791. The Kepler Space telescope obtained photometric\nobservations of this source and we examine the nearly 3 year long light curve.\nThe observations show numerous small amplitude outbursts recurring on average\nevery 33 days, intermittent quasi-periodic oscillations, and a significant\nfully coherent period of ~3.8 hr which we identify as the orbital period of the\nbinary. Contemporaneous optical spectroscopy of V523 Lyr reveals a faint blue\nsource with broad Balmer absorption lines containing narrow emission cores.\nH$\\alpha$ is in emission above the continuum. The low amplitude of the\nphotometric signal and no detected velocity motion suggest a low orbital\ninclination. We discuss the properties of V523 Lyr and show that it is a member\nof the growing group of anomalous Z Cam type CVs, systems which show stunted\noutbursts, light curve standstills, and occasional deep drops in brightness.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Why variable AGB stars with Long Secondary Periods aren't binaries, but\n  are dusty: Roughly 30% of variable AGB stars show a Long Secondary Period, or LSP. These\nLSPs have posed something of a problem in recent years and their cause remains\na mystery. By combining VLT-derived velocity curves with MACHO and OGLE light\ncurves we were able to examine many properties of these stars and test the\ntheory that LSPs are caused by binarity. We show why we concluded that the\nbinary model for LSPs is unlikely. Examining mid-infrared SAGE observations for\nstars with LSPs shows that these stars are surrounded by a significant amount\nof cool dust in a nonspherical distribution, e.g. a disk or clumps. The\nunlikeliness of binarity in these stars forces us to conclude that the dust is\nnot in a disk. We are left without an acceptable explanation for Long Secondary\nPeriods in AGB stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Towards understanding dynamo action in M dwarfs: Recent progress in observational studies of magnetic activity in M dwarfs\nurgently requires support from ideas of stellar dynamo theory. We propose a\nstrategy to connect observational and theoretical studies. In particular, we\nsuggest four magnetic configurations that appear relevant to dwarfs from the\nviewpoint of the most conservative version of dynamo theory, and discuss\nobservational tests to identify the configurations observationally. As\nexpected, any such identification contains substantial uncertainties. However\nthe situation in general looks less pessimistic than might be expected. Several\nidentifications between the phenomenology of individual stars and dynamo models\nare suggested. Remarkably, all models discussed predict substantial surface\nmagnetic activity at rather high stellar latitudes. This prediction looks\nunexpected from the viewpoint of our experience observing the Sun (which of\ncourse differs in some fundamental ways from these late-type dwarfs). We stress\nthat a fuller understanding of the topic requires a long-term (at least 15\nyears) monitoring of M dwarfs by Zeeman-Doppler imaging.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Detection of the Central Star of the Planetary Nebula NGC 6302: NGC 6302 is one of the highest ionization planetary nebulae known and shows\nemission from species with ionization potential >300eV. The temperature of the\ncentral star must be >200,000K to photoionize the nebula, and has been\nsuggested to be up to ~ 400,000K. On account of the dense dust and molecular\ndisc, the central star has not convincingly been directly imaged until now. NGC\n6302 was imaged in six narrow band filters by Wide Field Camera 3 on HST as\npart of the Servicing Mission 4 Early Release Observations. The central star is\ndirectly detected for the first time, and is situated at the nebula centre on\nthe foreground side of the tilted equatorial disc. The magnitudes of the\ncentral star have been reliably measured in two filters(F469N and F673N).\nAssuming a hot black body, the reddening has been measured from the\n(4688-6766\\AA) colour and a value of c=3.1, A_v=6.6 mag determined. A G-K main\nsequence binary companion can be excluded. The position of the star on the HR\ndiagram suggests a fairly massive PN central star of about 0.64,M_sun close to\nthe white dwarf cooling track. A fit to the evolutionary tracks for\n(T,L,t)=(200,000K, 2000L_sun, 2200yr), where t is the nebular age, is obtained;\nhowever the luminosity and temperature remain uncertain. The model tracks\npredict that the star is rapidly evolving, and fading at a rate of almost 1 %\nper year. Future observations could test this prediction.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evidence of a complex structure within the 2013 August 19 coronal mass\n  ejection. Radial and longitudinal evolution in the inner heliosphere: Context: Late on 2013 August 19, a coronal mass ejection (CME) erupted from\nan active region located near the far-side central meridian from Earth's\nperspective. The event and its accompanying shock were remotely observed by the\nSTEREO-A, STEREO-B and SOHO spacecraft. The interplanetary counterpart (ICME)\nwas intercepted by MESSENGER near 0.3 au, and by both STEREO-A and STEREO-B,\nnear 1 au, which were separated by 78{\\deg} in heliolongitude. The main\nobjective of this study is to follow the radial and longitudinal evolution of\nthe ICME throughout the inner heliosphere, and to examine possible scenarios\nfor the different magnetic flux-rope configuration observed on the solar disk,\nand measured in situ at the locations of MESSENGER and STEREO-A, separated by\n15{\\deg} in heliolongitude, and at STEREO-B, which detected the ICME flank.\nResults: We find that the magnetic flux-rope structure detected at STEREO-B\nbelongs to the same ICME detected at MESSENGER and STEREO-A. The opposite\nhelicity deduced at STEREO-B, might be due to the spacecraft intercepting one\nof the legs of the structure far from the flux-rope axis, while STEREO-A and\nMESSENGER are crossing through the core of the magnetic flux rope. The\ndifferent flux-rope orientations measured at MESSENGER and STEREO-A arise\nprobably because the two spacecraft measure a curved, highly distorted and\nrather complex magnetic flux-rope topology. The ICME may have suffered\nadditional distortion in its evolution in the inner heliosphere, such as the\nwest flank is propagating faster than the east flank when arriving 1 au.\nConclusions: This work illustrates how the ambient conditions can significantly\naffect the expansion and propagation of the CME/ICME, introducing additional\nirregularities to the already asymmetric eruption, and how these complex\nstructures cannot be directly reconstructed with the current models available.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Be Star HD 215227: A Candidate Gamma-ray Binary: The emission-line Be star HD 215227 lies within the positional error circle\nof the newly identified gamma-ray source AGL J2241+4454. We present new blue\nspectra of the star, and we point out the morphological and variability\nsimilarities to other Be binaries. An analysis of the available optical\nphotometry indicates a variation with a period of 60.37 +/- 0.04 d, which may\ncorrespond to an orbital modulation of the flux from the disk surrounding the\nBe star. The distance to the star of 2.6 kpc and its relatively large Galactic\nlatitude suggest that the binary was ejected from the plane by a supernova\nexplosion that created the neutron star or black hole companion. The binary and\nrunaway properties of HD 215227 make it an attractive candidate as the optical\ncounterpart of AGL J2241+4454 and as a new member of the small class of\ngamma-ray emitting binaries.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Formation of the helium EUV resonance lines: Context: While classical models successfully reproduce intensities of many\ntransition region lines, they predict helium EUV line intensities roughly an\norder of magnitude lower than the observed value.\n  Aims: To determine the relevant formation mechanism(s) of the helium EUV\nresonance lines, capable of explaining the high intensities under quiet sun\nconditions.\n  Methods: We synthesise and study the emergent spectra from a 3D\nradiation-magnetohydrodynamics simulation model. The effects of coronal\nillumination and non-equilibrium ionisation of hydrogen and helium are included\nself-consistently in the numerical simulation.\n  Results: Radiative transfer calculations result in helium EUV line\nintensities that are an order of magnitude larger than the intensities\ncalculated under the classical assumptions. The enhanced intensity of He I 584\nis primarily caused by He II recombination cascades. The enhanced intensity of\nHe II 304 and He II 256 is caused primarily by non-equilibrium helium\nionisation.\n  Conclusion: The analysis shows that the long standing problem of the high\nhelium EUV line intensities disappears when taking into account optically thick\nradiative transfer and non-equilibrium ionisation effects.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The solar wind in time: a change in the behaviour of older winds?: In the present paper, we model the wind of solar analogues at different ages\nto investigate the evolution of the solar wind. Recently, it has been suggested\nthat winds of solar type stars might undergo a change in properties at old\nages, whereby stars older than the Sun would be less efficient in carrying away\nangular momentum than what was traditionally believed. Adding to this, recent\nobservations suggest that old solar-type stars show a break in coronal\nproperties, with a steeper decay in X-ray luminosities and temperatures at\nolder ages. We use these X-ray observations to constrain the thermal\nacceleration of winds of solar analogues. Our sample is based on the stars from\nthe `Sun in time' project with ages between 120-7000 Myr. The break in X-ray\nproperties leads to a break in wind mass-loss rates ($\\dot{M}$) at roughly 2\nGyr, with $\\dot{M}$ (t < 2 Gyr) $\\propto t^{-0.74}$ and $\\dot{M}$ (t > 2 Gyr)\n$\\propto$ $t^{-3.9}$. This steep decay in $\\dot{M}$ at older ages could be the\nreason why older stars are less efficient at carrying away angular momentum,\nwhich would explain the anomalously rapid rotation observed in older stars. We\nalso show that none of the stars in our sample would have winds dense enough to\nproduce thermal emission above 1-2 GHz, explaining why their radio emissions\nhave not yet been detected. Combining our models with dynamo evolution models\nfor the magnetic field of the Earth we find that, at early ages ($\\approx$100\nMyr) our Earth had a magnetosphere that was 3 or more times smaller than its\ncurrent size.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Physical structure of the envelopes of intermediate-mass protostars: Context: Intermediate mass protostars provide a bridge between low- and\nhigh-mass protostars. Furthermore, they are an important component of the UV\ninterstellar radiation field. Despite their relevance, little is known about\ntheir formation process. Aims: We present a systematic study of the physical\nstructure of five intermediate mass, candidate Class 0 protostars. Our two\ngoals are to shed light on the first phase of intermediate mass star formation\nand to compare these protostars with low- and high-mass sources. Methods: We\nderived the dust and gas temperature and density profiles of the sample. We\nanalysed all existing continuum data on each source and modelled the resulting\nSED with the 1D radiative transfer code DUSTY. The gas temperature was then\npredicted by means of a modified version of the code CHT96. Results: We found\nthat the density profiles of five out of six studied intermediate mass\nenvelopes are consistent with the predictions of the \"inside-out\" collapse\ntheory.We compared several physical parameters, like the power law index of the\ndensity profile, the size, the mass, the average density, the density at 1000\nAU and the density at 10 K of the envelopes of low-, intermediate, and\nhigh-mass protostars. When considering these various physical parameters, the\ntransition between the three groups appears smooth, suggesting that the\nformation processes and triggers do not substantially differ.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Activity on a Li-rich giant: DI Psc revisited: We present a new Doppler imaging study for the Li-rich single K-giant DI Psc.\nSurface temperature maps are reconstructed for two subsequent rotation cycles.\nFrom the time evolution of the spot distribution antisolar-type differential\nrotation pattern is revealed. We show marks of non-uniform Li-abundance as\nwell. The possible connection between the current evolutionary phase of the\nstar and its magnetic activity is briefly discussed.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Abundance ratios in GALAH DR2 and their implications for nucleosynthesis: Using a sample of 70 924 stars from the second data release of the GALAH\noptical spectroscopic survey, we construct median sequences of [X/Mg] vs.\n[Mg/H] for 21 elements, separating the high-$\\alpha$/``low-Ia'' and\nlow-$\\alpha$/``high-Ia'' stellar populations through cuts in [Mg/Fe]. Previous\nwork with the near-IR APOGEE survey has shown that such sequences are nearly\nindependent of location in the Galactic disk, implying that they are determined\nby stellar nucleosynthesis yields with little sensitivity to other chemical\nevolution aspects. The separation between the two [X/Mg] sequences indicates\nthe relative importance of prompt and delayed enrichment mechanisms, while the\nsequences' slopes indicate metallicity dependence of the yields. GALAH and\nAPOGEE measurements agree for some of their common elements, but differ in\nsequence separation or metallicity trends for others. GALAH offers access to\nnine new elements. We infer that about $75\\%$ of solar C comes from core\ncollapse supernovae and $25\\%$ from delayed mechanisms. We find core collapse\nfractions of $60-80\\%$ for the Fe-peak elements Sc, Ti, Cu, and Zn, with strong\nmetallicity dependence of the core collapse Cu yield. For the neutron capture\nelements Y, Ba, and La, we infer large delayed contributions with non-monotonic\nmetallicity dependence. The separation of the [Eu/Mg] sequences implies that at\nleast $\\sim30\\%$ of Eu enrichment is delayed with respect to star formation. We\ncompare our results to predictions of several supernova and AGB yield models;\nC, Na, K, Mn, and Ca all show discrepancies with models that could make them\nuseful diagnostics of nucleosynthesis physics.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Characterising the observational properties of \u03b4 Sct stars in the\n  era of space photometry from the Kepler mission: The {\\delta} Sct stars are a diverse group of intermediate-mass pulsating\nstars located on and near the main sequence within the classical instability\nstrip in the Hertzsprung-Russell diagram. Many of these stars are hybrid stars\npulsating simultaneously with pressure and gravity modes that probe the physics\nat different depths within a star's interior. Using two large ensembles of\n{\\delta} Sct stars observed by the Kepler Space Telescope, the instrumental\nbiases inherent to Kepler mission data and the statistical properties of these\nstars are investigated. An important focus of this work is an analysis of the\nrelationships between the pulsational and stellar parameters, and their\ndistribution within the classical instability strip. It is found that a\nnon-negligible fraction of main sequence {\\delta} Sct stars exist outside\ntheoretical predictions of the classical instability boundaries, which\nindicates the necessity of a mass-dependent mixing length parameter to\nsimultaneously explain low- and high-radial order pressure modes in {\\delta}\nSct stars within the Hertzsprung-Russell diagram. Furthermore, a search for\nregularities in the amplitude spectra of these stars is also presented,\nspecifically the frequency difference between pressure modes of consecutive\nradial order. In this work, it is demonstrated that an ensemble-based approach\nusing space photometry from the Kepler mission is not only plausible for\n{\\delta} Sct stars, but that it is a valuable method for identifying the most\npromising stars for mode identification and asteroseismic modelling. The full\nscientific potential of studying {\\delta} Sct stars is as yet unrealised. The\nensembles discussed in this paper represent a high-quality data set for future\nstudies of rotation and angular momentum transport inside A and F stars using\nasteroseismology.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Collisional relaxation of electrons in a warm plasma and accelerated\n  nonthermal electron spectra in solar flares: Extending previous studies of nonthermal electron transport in solar flares\nwhich include the effects of collisional energy diffusion and thermalization of\nfast electrons, we present an analytic method to infer more accurate estimates\nof the accelerated electron spectrum in solar flares from observations of the\nhard X-ray spectrum. Unlike for the standard cold-target model, the spatial\ncharacteristics of the flaring region, especially the necessity to consider a\nfinite volume of hot plasma in the source, need to be taken into account in\norder to correctly obtain the injected electron spectrum from the\nsource-integrated electron flux spectrum (a quantity straightforwardly obtained\nfrom hard X-ray observations). We show that the effect of electron\nthermalization can be significant enough to nullify the need to introduce an\n{\\it ad hoc} low-energy cutoff to the injected electron spectrum in order to\nkeep the injected power in non-thermal electrons at a reasonable value. Rather\nthe suppression of the inferred low-energy end of the injected spectrum\ncompared to that deduced from a cold-target analysis allows the inference from\nhard X-ray observations of a more realistic energy in injected non-thermal\nelectrons in solar flares.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Absolute Parameters of Young Stars: PU Pup: We present combined photometric and spectroscopic analyses of the southern\nbinary star PU Pup. High-resolution spectra of this system were taken at the\nUniversity of Canterbury Mt. John Observatory in the years 2008 and again in\n2014-15. We find the light contribution of the secondary component to be only\n$\\sim$2\\% of the total light of the system in optical wavelengths, resulting in\na single-lined spectroscopic binary. Recent TESS data revealed grazing eclipses\nwithin the light minima, though the tidal distortion, examined also from\nHIPPARCOS data, remains the predominating light curve effect. Our model shows\nPU Pup to have the more massive primary relatively close to filling its Roche\nlobe. PU Pup is thus approaching the rare `fast phase' of interactive (Case B)\nevolution. Our adopted absolute parameters are as follows: $M_1$ = 4.10\n($\\pm$0.20) M$_{\\odot}$, $M_2$ = 0.65 ($\\pm$0.05) M$_{\\odot}$, $R_{1}$ = 6.60\n($\\pm$0.30) R$_{\\odot}$, $R_2$ = 0.90 ($\\pm$0.10) R$_{\\odot}$; $T_{1}$ = 11500\n($\\pm$500) K, $T_{2}$ = 5000 ($\\pm$350) K; photometric distance = 186 ($\\pm$20)\npc, age = 170 ($\\pm$20) My. The less-massive secondary component is found to be\nsignificantly oversized and overluminous compared to standard Main Sequence\nmodels. We discuss this discrepancy referring to heating from the reflection\neffect.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Automatic Identification and Tracking of Coronal Flux Ropes -- Part\n  I: Footpoints and Fluxes: Investigating the early-stage evolution of an erupting flux rope from the Sun\nis important to understand the mechanisms of how it looses its stability and\nits space weather impacts. Our aim is to develop an efficient scheme for\ntracking the early dynamics of erupting solar flux ropes and use the algorithm\nto analyse its early-stage properties. The algorithm is tested on a data-driven\nsimulation of an eruption that took place in active region AR12473. We\ninvestigate the modelled flux rope's footpoint movement and magnetic flux\nevolution and compare with observational data from the Solar Dynamics\nObservatory's Atmospheric Imaging Assembly in the 211 $\\unicode{x212B}$ and\n1600 $\\unicode{x212B}$ channels. To carry out our analysis, we use the\ntime-dependent data-driven magnetofrictional model (TMFM). We also perform\nanother modelling run, where we stop the driving of the TMFM midway through the\nflux rope's rise through the simulation domain and evolve it instead with a\nzero-beta magnetohydrodynamic (MHD) approach. The developed algorithm\nsuccessfully extracts a flux rope and its ascend through the simulation domain.\nWe find that the movement of the modelled flux rope footpoints showcases\nsimilar trends in both TMFM and relaxation MHD run: they recede from their\nrespective central location as the eruption progresses and the positive\npolarity footpoint region exhibits a more dynamic behaviour. The ultraviolet\nbrightenings and extreme ultraviolet dimmings agree well with the models in\nterms of their dynamics. According to our modelling results, the toroidal\nmagnetic flux in the flux rope first rises and then decreases. In our\nobservational analysis, we capture the descending phase of toroidal flux. In\nconclusion, the extraction algorithm enables us to effectively study the flux\nrope's early dynamics and derive some of its key properties such as footpoint\nmovement and toroidal magnetic flux.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Gravitational settling in pulsating subdwarf B stars and their\n  progenitors: Diffusion of atoms can be important during quiescent phases of stellar\nevolution. Particularly in the very thin inert envelopes of subdwarf B stars,\ndiffusive movements will considerably change the envelope structure and the\nsurface abundances on a short timescale. Also, the subdwarfs will inherit the\neffects of diffusion in their direct progenitors, namely giants near the tip of\nthe red giant branch. This will influence the global evolution and the\npulsational properties of subdwarf B stars. We investigate the impact of\ngravitational settling, thermal diffusion and concentration diffusion on the\nevolution and pulsations of subdwarf B stars. Our diffusive stellar models are\ncompared with models evolved without diffusion. We constructed subdwarf B\nmodels with a mass of 0.465 Msun from a 1 and 3 Msun ZAMS progenitor. The low\nmass star ignited helium in an energetic flash, while the intermediate mass\nstar started helium fusion gently. For each progenitor type we computed series\nwith and without atomic diffusion. Atomic diffusion in red giants causes the\nhelium core mass at the onset of helium ignition to be larger. We find an\nincrease of 0.0015 Msun for the 1 Msun model and 0.0036 Msun for the 3 Msun\nmodel. The effects on the red giant surface abundances are small after the\nfirst dredge up. The evolutionary tracks of the diffusive subdwarf B models are\nshifted to lower surface gravities and effective temperatures due to outward\ndiffusion of hydrogen. This affects both the frequencies of the excited modes\nand the overall frequency spectrum. Especially the structure and pulsations of\nthe post-non-degenerate sdB star are drastically altered, proving that atomic\ndiffusion cannot be ignored in these stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Ages of Stars: The age of an individual star cannot be measured, only estimated through\nmostly model-dependent or empirical methods, and no single method works well\nfor a broad range of stellar types or for a full range in age. This review\npresents a summary of the available techniques for age-dating stars and\nensembles of stars, their realms of applicability, and their strengths and\nweaknesses. My emphasis is on low-mass stars because they are present from all\nepochs of star formation in the Galaxy and because they present both special\nopportunities and problems. The ages of open clusters are important for\nunderstanding the limitations of stellar models and for calibrating empirical\nage indicators. For individual stars, a hierarchy of quality for the available\nage-dating methods is described. Although our present ability to determine the\nages of even the nearest stars is mediocre, the next few years hold great\npromise as asteroseismology probes beyond stellar surfaces and starts to\nprovide precise interior properties of stars and as models continue to improve\nwhen stressed by better observations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Magnetic Fields of New CP Stars Discovered with Kepler Mission Data: The paper presents the first results of the ongoing spectropolarimetric\nmonitoring of magnetic fields of stars, whose chemically peculiar nature has\nbeen previously revealed with the 1-m SAO RAS telescope. We selected the sample\ncandidates using the photometric data of the Kepler and TESS space missions.\nThe efficiency of the method of searching for new CP stars based on photometric\nlight curves has been confirmed. We present the magnetic field measurements and\nestimate the atmospheric parameters of the objects under study.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Response of Granulation to Small Scale Bright Features in the Quiet Sun: We detected 2.8 bright points (BPs) per Mm$^2$ in the Quiet Sun (QS) with the\nNew Solar Telescope (NST) at Big Bear Solar Observatory; using the TiO 705.68\nnm spectral line, at an angular resolution ~ 0.1'' to obtain 30 min data\nsequence. Some BPs formed knots that were stable in time and influenced the\nproperties of the granulation pattern around them. The observed granulation\npattern within ~ 3'' of knots presents smaller granules than those observed in\na normal granulation pattern; i.e., around the knots a suppressed convection is\ndetected. Observed BPs covered ~ 5% of the solar surface and were not\nhomogeneously distributed. BPs had an average size of 0.22'', they were\ndetectable for 4.28 min in average, and had an averaged contrast of 0.1% in the\ndeep red TiO spectral line.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Solar Neighborhood XXIV. Parallax Results from the CTIOPI 0.9-m\n  Program: Stars with $\u03bc$ $\\ge$ 1\\farcs0 yr$^{-1}$ (MOTION Sample) and\n  Subdwarfs: We present 41 trigonometric parallaxes of 37 stellar systems, most of which\nhave proper motions greater than 1\\farcs0 yr$^{-1}$. These are the first\ntrigonometric parallaxes for 24 systems. Overall, there are 15 red dwarf\nsystems and 22 red subdwarf systems in the sample. Five of the systems are\nmultiples with directly detected companions, and we have discovered\nperturbations caused by unseen companions in two additional cases, the dwarf\nLHS 501 and the subdwarf LHS 440. The latter system may eventually provide\nimportant dynamical mass points on the subdwarf mass-luminosity relation. Two\nadditional stars of note are LHS 272, the third closest M-type subdwarf at a\ndistance of only 13.6 pc, and LHS 2734AB, a high velocity subdwarf binary with\n$V_{tan}>$ 700 km/sec, which likely exceeds the escape velocity of the Milky\nWay. We also report the first long term variability study of cool subdwarfs\nindicating that cool subdwarfs are less photometrically variable than their\nmain sequence counterparts.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Gas and dust from solar metallicity AGB stars: We study the asymptotic giant branch (AGB) evolution of stars with masses\nbetween $1~M_{\\odot} - 8.5~M_{\\odot}$. We focus on stars with a solar chemical\ncomposition, which allows us to interpret evolved stars in the Galaxy. We\npresent a detailed comparison with models of the same chemistry, calculated\nwith a different evolution code and based on a different set of physical\nassumptions. We find that stars of mass $\\ge 3.5~M_{\\odot}$ experience hot\nbottom burning at the base of the envelope. They have AGB lifetimes shorter\nthan $\\sim 3\\times 10^5$ yr and eject into their surroundings gas contaminated\nby proton-capture nucleosynthesis, at an extent sensitive to the treatment of\nconvection. Low mass stars with $1.5~M_{\\odot} \\le M \\le 3~M_{\\odot}$ become\ncarbon stars. During the final phases the C/O ratio grows to $\\sim 3$. We find\na remarkable agreement between the two codes for the low-mass models and\nconclude that predictions for the physical and chemical properties of these\nstars, and the AGB lifetime, are not that sensitive to the modelling of the AGB\nphase. The dust produced is also dependent on the mass: low-mass stars produce\nmainly solid carbon and silicon carbide dust, whereas higher mass stars produce\nsilicates and alumina dust. Possible future observations potentially able to\nadd more robustness to the present results are also discussed.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Stellar granulation as seen in disk-integrated intensity. II.\n  Theoretical scaling relations compared with observations: A large set of stars observed by CoRoT and Kepler shows clear evidence for\nthe presence of a stellar background, which is interpreted to arise from\nsurface convection, i.e., granulation. These observations show that the\ncharacteristic time-scale (tau_eff) and the root-mean-square (rms) brightness\nfluctuations (sigma) associated with the granulation scale as a function of the\npeak frequency (nu_max) of the solar-like oscillations. We aim at providing a\ntheoretical background to the observed scaling relations based on a model\ndeveloped in the companion paper. We computed for each 3D model the theoretical\npower density spectrum (PDS) associated with the granulation as seen in\ndisk-integrated intensity on the basis of the theoretical model. For each PDS\nwe derived tau_eff and sigma and compared these theoretical values with the\ntheoretical scaling relations derived from the theoretical model and the Kepler\nmeasurements. We derive theoretical scaling relations for tau_eff and sigma,\nwhich show the same dependence on nu_max as the observed scaling relations. In\naddition, we show that these quantities also scale as a function of the\nturbulent Mach number (Ma) estimated at the photosphere. The theoretical\nscaling relations for tau_eff and sigma match the observations well on a global\nscale. Our modelling provides additional theoretical support for the observed\nvariations of sigma and tau_eff with nu_m max. It also highlights the important\nrole of Ma in controlling the properties of the stellar granulation. However,\nthe observations made with Kepler on a wide variety of stars cannot confirm the\ndependence of our scaling relations on Ma. Measurements of the granulation\nbackground and detections of solar-like oscillations in a statistically\nsufficient number of cool dwarf stars will be required for confirming the\ndependence of the theoretical scaling relations with Ma.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Solar LImb Prominence CAtcher and Tracker (SLIPCAT): An Automated System\n  and Its Preliminary Statistical Results: In this paper, we present an automated system, which has the capability to\ncatch and track solar limb prominences based on observations from EUV 304\npassband. The characteristic parameters and their evolution, including height,\nposition angle, area, length and brightness, are obtained without manual\ninterventions. By applying the system to the STEREO-B/SECCHI/EUVI 304 data\nduring 2007 April -2009 October, we obtain a total of 9477 well-tracked\nprominences and a catalog of these events available online at\nhttp://space.ustc.edu.cn/dreams/slipcat/. A detailed analysis of these\nprominences suggests that the system has a rather good performance. We have\nobtained several interesting statistical results based on the catalog. Most\nprominences appear below the latitude of 60 degrees and at the height of about\n26 Mm above the solar surface. Most of them are quite stable during the period\nthey are tracked. Nevertheless, some prominences have an upward speed of more\nthan 100 km/s, and some others show significant downward and/or azimuthal\nspeeds. There are strong correlations among the brightness, area and height.\nThe expansion of a prominence is probably one major cause of its fading during\nthe rising or erupting process.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Survey of Stellar Families: Multiplicity of Solar-Type Stars: We present the results of a comprehensive assessment of companions to\nsolar-type stars. A sample of 454 stars, including the Sun, was selected from\nthe Hipparcos catalog with {\\pi} > 40 mas, {\\sigma}_{\\pi}/{\\pi} < 0.05, 0.5 < B\n- V < 1.0 (~ F6-K3), and constrained by absolute magnitude and color to exclude\nevolved stars. New observational aspects of this work include surveys for (1)\nvery close companions with long-baseline interferometry at the CHARA Array, (2)\nclose companions with speckle interferometry, and (3) wide proper motion\ncompanions identified by blinking multi-epoch archival images. In addition, we\ninclude the results from extensive radial-velocity monitoring programs and\nevaluate companion information from various catalogs.\n  The overall observed fractions of single, double, triple, and higher order\nsystems are 56% \\pm 2%, 33% \\pm 2%, 8% \\pm 1%, and 3% \\pm 1%, respectively,\ncounting all confirmed stellar and brown dwarf companions. Our completeness\nanalysis indicates that only a few undiscovered companions remain in this\nwell-studied sample, implying that the majority (54% \\pm 2%) of solar-type\nstars are single, in contrast to the results of prior multiplicity studies. The\norbital-period distribution of companions is unimodal and roughly log-normal\nwith a peak of about 300 years. The period-eccentricity relation shows a\nroughly flat distribution beyond the expected circularization for periods below\n12 days. The mass-ratio distribution shows a preference for like-mass pairs,\nwhich occur more frequently in relatively close pairs. The fraction of planet\nhosts among single, binary, and multiple systems are statistically\nindistinguishable, suggesting that planets are as likely to form around single\nstars as they are around components of binary or multiple systems with\nsufficiently wide separations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Sigmoid-to-Flux-Rope Transition Leading to A Loop-Like Coronal Mass\n  Ejection: Sigmoids are one of the most important precursor structures for solar\neruptions. In this Letter, we study a sigmoid eruption on 2010 August 1 with\nEUV data obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar\nDynamic Observatory (SDO). In AIA 94 \\AA\\ (Fe XVIII; 6 MK), topological\nreconfiguration due to tether-cutting reconnection is unambiguously observed\nfor the first time, i.e., two opposite J-shaped loops reconnect to form a\ncontinuous S-shaped loop, whose central portion is dipped and aligned along the\nmagnetic polarity inversion line (PIL), and a compact loop crossing the PIL. A\ncausal relationship between photospheric flows and coronal tether-cutting\nreconnections is evidenced by the detection of persistent converging flows\ntoward the PIL using line-of-sight magnetograms obtained by the Helioseismic\nand Magnetic Imager (HMI) on board SDO. The S-shaped loop remains in\nquasi-equilibrium in the lower corona for about 50 minutes, with the central\ndipped portion rising slowly at ~10 km s-1. The speed then increases to ~60 km\ns-1 about 10 minutes prior to the onset of a GOES-class C3.2 flare, as the\nS-shaped loop speeds up its transformation into an arch-shaped loop, which\neventually leads to a loop-like coronal mass ejection (CME). The AIA\nobservations combined with H? filtergrams as well as hard X-ray (HXR) imaging\nand spectroscopy are consistent with most flare loops being formed by\nreconnection of the stretched legs of less-sheared J-shaped loops that\nenvelopes the rising flux rope, in agreement with the standard tether-cutting\nscenario.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Subsonic structure and optically thick winds from Wolf--Rayet stars: Wolf-Rayet star's winds can be so dense and so optically thick that the\nphotosphere appears in the highly supersonic part of the outflow, veiling the\nunderlying subsonic part of the star, and leaving the initial acceleration of\nthe wind inaccessible to observations. We investigate the conditions and the\nstructure of the subsonic part of the outflow of Galactic WR stars, in\nparticular of the WNE subclass; our focus is on the conditions at the sonic\npoint. We compute 1D hydrodynamic stellar structure models for massive helium\nstars adopting outer boundaries at the sonic point. We find that the outflows\nof our models are accelerated to supersonic velocities by the radiative force\nfrom opacity bumps either at temperatures of the order of 200kK by the Fe\nopacity bump or of the order of 50kK by the HeII opacity bump. For a given\nmass-loss rate, the conditions in the subsonic part of the outflow are\nindependent from the detailed physical conditions in the supersonic part. The\nclose proximity to the Eddington limit at the sonic point allows us to\nconstruct a Sonic HR diagram, relating the sonic point temperature to the L/M\nratio and the stellar mass-loss rate, thereby constraining the sonic point\nconditions, the subsonic structure, and the stellar wind mass-loss rates from\nobservations. The minimum mass-loss rate necessary to have the flow accelerated\nto supersonic velocities by the Fe opacity bump is derived. A comparison of the\nobserved parameters of Galactic WNE stars to this minimum mass-loss rate\nindicates that their winds are launched to supersonic velocities by the\nradiation pressure arising from the Fe-bump. Conversely, models which do not\nshow transonic flows from the Fe opacity bump form inflated envelopes. We\nderive an analytic criterion for the appearance of envelope inflation in the\nsubphotospheric layers.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Formation of Transient Coronal Holes during Eruption of a Quiescent\n  Filament and its Overlying Sigmoid: By using H$\\alpha$, He I 10830, EUV and soft X-ray (SXR) data, we examined a\nfilament eruption that occurred on a quiet-sun region near the center of the\nsolar disk on 2006 January 12, which disturbed a sigmoid overlying the filament\nchannel observed by the $\\emph{GOES-12}$ SXR Imager (SXI), and led to the\neruption of the sigmoid. The event was associated with a partial halo coronal\nmass ejection (CME) observed by the Large Angle and Spectrometric Coronagraphs\n(LASCO) on board the Solar and Heliospheric Observatory ($\\emph{SOHO}$), and\nresulted in the formation of two flare-like ribbons, post-eruption coronal\nloops, and two transient coronal holes (TCHs), but there were no significantly\nrecorded $\\emph{GOES}$ or H$\\alpha$ flares corresponding to the eruption. The\ntwo TCHs were dominated by opposite magnetic polarities and were located on the\ntwo ends of the eruptive sigmoid. They showed similar locations and shapes in\nHe I 10830, EUV and SXR observations. During the early eruption phase,\nbrightenings first appeared on the locations of the two subsequent TCHs, which\ncould be clearly identified on He I 10830, EUV and SXR images. This eruption\nevent could be explained by the magnetic flux rope model, and the two TCHs were\nlikely to be the feet of the flux rope.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Anisotropic turbulent transport in stably stratified rotating stellar\n  radiation zones: Rotation is one of the key physical mechanisms that deeply impact the\nevolution of stars. Helio- and asteroseismology reveal a strong extraction of\nangular momentum from stellar radiation zones over the whole\nHertzsprung-Russell diagram. Turbulent transport in differentially rotating\nstably stratified stellar radiation zones should be carefully modeled and its\nstrength evaluated. Stratification and rotation imply that this turbulent\ntransport is anisotropic. Only phenomenological prescriptions have been\nproposed for the transport in the horizontal direction, which however\nconstitutes a cornerstone in current theoretical formalisms for stellar\nhydrodynamics in evolution codes. We derive a new theoretical prescription for\nthe anisotropy of the turbulent transport in radiation zones using a spectral\nformalism for turbulence that takes simultaneously stable stratification,\nrotation, and a radial shear into account. Then, the horizontal turbulent\ntransport resulting from 3D turbulent motions sustained by the instability of\nthe radial differential rotation is derived. We implement this framework in the\nstellar evolution code STAREVOL and quantify its impact on the rotational and\nstructural evolution of low-mass stars from the pre-main-sequence to the red\ngiant branch. The anisotropy of the turbulent transport scales as\n$N^4\\tau^2/\\left(2\\Omega^2\\right)$, $N$ and $\\Omega$ being the buoyancy and\nrotation frequencies respectively and $\\tau$ a time characterizing the source\nof turbulence. This leads to a horizontal turbulent transport of similar\nstrength in average that those obtained with previously proposed prescriptions\neven if it can be locally larger below the convective envelope. As a\nconsequence, a complementary transport mechanism like internal gravity waves or\nmagnetic fields is still needed to explain the observed strong transport of\nangular momentum along stellar evolution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Nonradial Oscillations in Classical Pulsating Stars. Predictions and\n  Discoveries: After a brief historical introduction and recalling basic concepts of stellar\noscillation theory, I focus my review on interpretation of secondary\nperiodicities found in RR Lyrae stars and Cepheids as a manifestation of\nnonradial mode excitation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Ejection of globular cluster interstellar media through ionization by\n  white dwarfs: UV radiation from white dwarfs can efficiently clear Galactic globular\nclusters (GCs) of their intra-cluster medium (ICM). This solves the problem of\nthe missing ICM in clusters, which is otherwise expected to build up to easily\nobservable quantities. To show this, we recreate the ionizing flux in 47 Tuc,\nfollowing randomly generated stars through their AGB, post-AGB and white dwarf\nevolution. Each white dwarf can ionize all the material injected into the\ncluster by stellar winds for ~3 Myr of its evolution: ~40 such white dwarfs\nexist at any point. Every GC's ICM should be ionized. The neutral cloud in M15\nshould be caused by a temporary overdensity. A pressure-supported ICM will\nexpand over the cluster's tidal radius, where it will be truncated, allowing\nJeans escape. The modelled Jeans mass-loss rate approximates the total stellar\nmass-loss rate, allowing efficient clearing of ICM. Any cluster's ICM mass\nshould equal the mass injected by its stars over the sound-travel time between\nthe cluster core and tidal radius. We predict ~11.3 solar masses of ICM within\n47 Tuc, cleared over ~4 Myr, compared to a dynamical timescale of 4.3 Myr. We\npresent a new mass hierarchy, discussing the transition between globular\nclusters dwarf galaxies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Multiple and changing cycles of active stars II. Results: We study the time variations of the cycles of 20 active stars based on\ndecades-long photometric or spectroscopic observations. A method of\ntime-frequency analysis, as discussed in a companion paper, is applied to the\ndata. Fifteen stars definitely show multiple cycles; the records of the rest\nare too short to verify a timescale for a second cycle. The cycles typically\nshow systematic changes. For three stars, we found two cycles in each of them\nthat are not harmonics, and which vary in parallel, indicating that a common\nphysical mechanism arising from a dynamo construct. The positive relation\nbetween the rotational and cycle periods is confirmed for the inhomogeneous set\nof active stars. Stellar activity cycles are generally multiple and variable.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Type II supernovae from the Carnegie Supernova Project-I. I. Bolometric\n  light curves of 74 SNe II using uBgVriYJH photometry: The present study is the first of a series of three papers where we\ncharacterise the type II supernovae (SNe~II) from the Carnegie Supernova\nProject-I to understand their diversity in terms of progenitor and explosion\nproperties. In this first paper, we present bolometric light curves of 74\nSNe~II. We outline our methodology to calculate the bolometric luminosity,\nwhich consists of the integration of the observed fluxes in numerous\nphotometric bands ($uBgVriYJH$) and black-body (BB) extrapolations to account\nfor the unobserved flux at shorter and longer wavelengths. BB fits were\nperformed using all available broadband data except when line blanketing\neffects appeared. Photometric bands bluer than $r$ that are affected by line\nblanketing were removed from the fit, which makes near-infrared (NIR)\nobservations highly important to estimate reliable BB extrapolations to the\ninfrared. BB fits without NIR data produce notably different bolometric light\ncurves, and therefore different estimates of SN~II progenitor and explosion\nproperties when data are modelled. We present two methods to address the\nabsence of NIR observations: (a) colour-colour relationships from which NIR\nmagnitudes can be estimated using optical colours, and (b) new prescriptions\nfor bolometric corrections as a function of observed SN~II colours. Using our\n74 SN~II bolometric light curves, we provide a full characterisation of their\nproperties based on several observed parameters. We measured magnitudes at\ndifferent epochs, as well as durations and decline rates of different phases of\nthe evolution. An analysis of the light-curve parameter distributions was\nperformed, finding a wide range and a continuous sequence of observed\nparameters which is consistent with previous analyses using optical light\ncurves.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Multiplicity Census of Intermediate-Mass Stars in Scorpius-Centaurus: Stellar multiplicity properties have been studied for much of the range from\nthe lowest to the highest stellar masses, but intermediate-mass stars from\nF-type to late A-type have received relatively little attention. Here we report\non a Gemini/NICI snapshot imaging survey of 138 such stars in the young\nScorpius-Centaurus (Sco-Cen) region, for the purpose of studying multiplicity\nwith sensitivity down to planetary masses at wide separations. In addition to\ntwo brown dwarfs and a companion straddling the hydrogen burning limit we\nreported previously, here we present 26 new stellar companions and determine a\nmultiplicity fraction within 0.1\"--5.0\" of 21+/-4%. Depending on the adopted\nsemi-major axis distribution, our results imply a total multiplicity in the\nrange of ~60--80%, which further supports the known trend of a smoothly\ncontinuous increase in the multiplicity fraction as a function of primary\nstellar mass. A surprising feature in the sample is a distinct lack of nearly\nequal-mass binaries, for which we discuss possible reasons. The survey yielded\nno additional companions below or near the deuterium-burning limit, implying\nthat their frequency at >200 AU separations is not quite as high as might be\ninferred from previous detections of such objects within the Sco-Cen region.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Rossby Wave Instability with Self-Gravity: The Rossby wave instability (RWI) in non-self-gravitating discs can be\ntriggered by a bump at a radius $r_0$ in the disc surface mass-density (which\nis proportional to the inverse potential vorticity). It gives rise to a growing\nnon-axisymmetric perturbation [$\\propto \\exp(im\\phi)$, $m=1,2..$] in the\nvicinity of $r_0$ consisting of anticyclonic vortices which may facilitate\nplanetesimal growth in protoplanetary discs. Here, we analyze a continuum of\nthin disc models ranging from self-gravitating to non-selfgravitating. The key\nquantities determining the stability/instability are: (1) the parameters of the\nbump (or depression) in the disc surface density, (2) the Toomre $Q$ parameter\nof the disc (a non-self-gravitating disc has $Q\\gg1$), and (3) the\ndimensionless azimuthal wavenumber of the perturbation $\\bar{k}_\\phi =mQh/r_0$,\nwhere $h$ is the half-thickness of the disc. For discs stable to axisymmetric\nperturbations ($Q>1$), the self-gravity has a significant role for\n$\\bar{k}_\\phi < \\pi/2$ or $m<(\\pi/2) (r_0/h)Q^{-1}$; instability may occur for\na depression or groove in the surface density if $Q\\lesssim 2$. For\n$\\bar{k}_\\phi > \\pi/2$ the self-gravity is not important, and instability may\noccur at a bump in the surface density. Thus, for all mode numbers $m \\ge 1$,\nthe self-gravity is unimportant for $Q > (\\pi/2)(r_0/h)$. We suggest that the\nself-gravity be included in simulations for cases where $Q< (r_0/h)$.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spectroscopic and Photometric Behaviour of LP Ori: We performed spectroscopic and photometric analyses on the early B-type LP\nOri young stellar object located in the Orion Nebula. The high-resolution\nspectra of LP Ori was obtained at the Ankara University Kreiken Observatory in\n2023, while all spectra recorded over the past 19 years were extracted from the\nESO and ESPaDOnS archives. In these spectra of LP Ori, there is typically an\nemission observed in the core of the Balmer profile. This structure is\naccompanied by a $\\sim$14-year interval inverse P-Cygni repetition superimposed\non the Balmer profile. Additionally, an emission in the $\\lambda$5875 He~I line\nis visible in the spectra in the year 2023. When this emission is considered\ntogether with the inverse P-Cygni structures, these observations suggest that\nLP Ori is a Herbig Ae/Be star. The abundance pattern of LP Ori is close to\nsolar with the exception of a slightly rich helium and slightly poor Al\nabundance. Additionally, the spectral energy distribution of LP Ori was\nconstructed to confirm the infrared excess caused by its circumstellar disk.\nFurthermore, the photometric analysis performed on the TESS observations of LP\nOri shows significant photometric variability and the frequency analysis\nreveals a $\\beta$~Cephei star in its forthcoming evolution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Coronal response to magnetically-suppressed CME events in M-dwarf stars: We report the results of the first state-of-the-art numerical simulations of\nCoronal Mass Ejections (CMEs) taking place in realistic magnetic field\nconfigurations of moderately active M-dwarf stars. Our analysis indicates that\na clear, novel, and observable, coronal response is generated due to the\ncollapse of the eruption and its eventual release into the stellar wind.\nEscaping CME events, weakly suppressed by the large-scale field, induce a\nflare-like signature in the emission from coronal material at different\ntemperatures due to compression and associated heating. Such flare-like\nprofiles display a distinctive temporal evolution in their Doppler shift signal\n(from red to blue), as the eruption first collapses towards the star and then\nperturbs the ambient magnetized plasma on its way outwards. For stellar fields\nproviding partial confinement, CME fragmentation takes place, leading to rise\nand fall flow patterns which resemble the solar coronal rain cycle. In strongly\nsuppressed events, the response is better described as a gradual brightening,\nin which the failed CME is deposited in the form of a coronal rain cloud\nleading to a much slower rise in the ambient high-energy flux by relatively\nsmall factors ($\\sim2-3$). In all the considered cases (escaping/confined) a\nfractional decrease in the emission from mid-range coronal temperature plasma\noccurs, similar to the coronal dimming events observed on the Sun. Detection of\nthe observational signatures of these CME-induced features requires a sensitive\nnext generation X-ray space telescope.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Dynamics of the photosphere along the solar cycle from SDO/HMI: As the global magnetic field of the Sun has an activity cycle, one expects to\nobserve some variation of the dynamical properties of the flows visible in the\nphotosphere. We investigate the flow field during the solar cycle by analysing\nSDO/HMI observations of continuum intensity, Doppler velocity and longitudinal\nmagnetic field. We first picked data at disk center during 6 years along the\nsolar cycle with a 48-hour time step in order to study the overall evolution of\nthe continuum intensity and magnetic field. Then we focused on thirty 6-hour\nsequences of quiet regions without any remnant of magnetic activity separated\nby 6 months, in summer and winter, when disk center latitude B0 is close to\nzero. The horizontal velocity was derived from the local correlation tracking\ntechnique over a field of view of 216.4Mm x 216.4Mm located at disk center. Our\nmeasurements at disk center show the stability of the flow properties between\nmeso- and supergranular scales along the solar cycle. The network magnetic\nfield, produced locally at disk center independently from large scale dynamo,\ntogether with continuum contrast, vertical and horizontal flows, seem to remain\nconstant during the solar cycle.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evolution of the Radial Size and Expansion of Coronal Mass Ejections\n  Investigated by Combining Remote and In-Situ Observations: A fundamental property of coronal mass ejections (CMEs) is their radial\nexpansion, which determines the increase in the CME radial size and the\ndecrease in the CME magnetic field strength as the CME propagates. CME radial\nexpansion can be investigated either by using remote observations or by in-situ\nmeasurements based on multiple spacecraft in radial conjunction. However, there\nhave been only few case studies combining both remote and in-situ observations.\nIt is therefore unknown if the radial expansion estimated remotely in the\ncorona is consistent with that estimated locally in the heliosphere. To address\nthis question, we first select 22 CME events between the years 2010 and 2013,\nwhich were well observed by coronagraphs and by two or three spacecraft in\nradial conjunction. We use the graduated cylindrical shell model to estimate\nthe radial size, radial expansion speed, and a measure of the dimensionless\nexpansion parameter of CMEs in the corona. The same parameters and two\nadditional measures of the radial-size increase and magnetic-field-strength\ndecrease with heliocentric distance of CMEs based on in-situ measurements are\nalso calculated. For most of the events, the CME radial size estimated by\nremote observations is inconsistent with the in-situ estimates. We further\nstatistically analyze the correlations of these expansion parameters estimated\nusing remote and in-situ observations, and discuss the potential reasons for\nthe inconsistencies and their implications for the CME space weather\nforecasting.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Ionization ratios and elemental abundances in the atmosphere of 68 Tauri: We have derived the ionization ratios of twelve elements in the atmosphere of\nthe star 68 Tauri (HD 27962) using an ATLAS9 model atmosphere with 72 layers\ncomputed for the effective temperature and surface gravity of the star. We then\ncomputed a grid of synthetic spectra generated by SYNSPEC49 based on an ATLAS9\nmodel atmosphere in order to model one high resolution spectrum secured by one\nof us (RM) with the echelle spectrograph SOPHIE at Observatoire de Haute\nProvence. We could determine the abundances of several elements in their\ndominant ionization stage, including those defining the Am phenomenon. We thus\nprovide new abundance determinations for 68 Tauri using updated accurate atomic\ndata retrieved from the NIST database which extend previous abundance works.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Asteroseismology of red giants from the first four months of Kepler\n  data: Fundamental parameters: Clear power excess in a frequency range typical for solar-type oscillations\nin red giants has been detected in more than 1000 stars, which have been\nobserved during the first 138 days of the science operation of the NASA Kepler\nsatellite. This sample includes stars in a wide mass and radius range with\nspectral types G and K, extending in luminosity from the bottom of the giant\nbranch up to high-luminous red giants. The high-precision asteroseismic\nobservations with Kepler provide a perfect source for testing stellar structure\nand evolutionary models, as well as investigating the stellar population in our\nGalaxy. We fit a global model to the observed frequency spectra, which allows\nus to accurately estimate the granulation background signal and the global\noscillation parameters, such as the frequency of maximum oscillation power. We\nfind regular patterns of radial and non-radial oscillation modes and use a new\ntechnique to automatically identify the mode degree and the characteristic\nfrequency separations between consecutive modes of the same spherical degree.\nIn most cases, we can also measure the small separation. The seismic parameters\nare used to estimate stellar masses and radii and to place the stars in an H-R\ndiagram by using an extensive grid of stellar models that covers a wide\nparameter range. Using Bayesian techniques throughout our analysis allows us to\ndetermine reliable uncertainties for all parameters. We provide accurate\nseismic parameters and their uncertainties for a large sample of red giants and\ndetermine their asteroseismic fundamental parameters. We investigate the\ninfluence of the stars' metallicities on their positions in the H-R diagram. We\nstudy the red-giant populations in the red clump and bump and compare them to a\nsynthetic population and find a mass and metallicity gradient in the red clump\nand clear evidence of a secondary-clump population.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Search for Alignment of Disk Orientations in Nearby Star-Forming\n  Regions: Lupus, Taurus, Upper Scorpius, $\u03c1$ Ophiuchi, and Orion: Spatial correlations among proto-planetary disk orientations carry unique\ninformation on physics of multiple star formation processes. We select five\nnearby star-forming regions that comprise a number of proto-planetary disks\nwith spatially-resolved images with ALMA and HST, and search for the mutual\nalignment of the disk axes. Specifically, we apply the Kuiper test to examine\nthe statistical uniformity of the position angle (PA: the angle of the major\naxis of the projected disk ellipse measured counter-clockwise from the north)\ndistribution. The disks located in the star-forming regions, except the Lupus\nclouds, do not show any signature of the alignment, supporting the random\norientation. Rotational axes of 16 disks with spectroscopic measurement of PA\nin the Lupus III cloud, a sub-region of the Lupus field, however, exhibit a\nweak and possible departure from the random distribution at a $2\\sigma$ level,\nand the inclination angles of the 16 disks are not uniform as well.\nFurthermore, the mean direction of the disk PAs in the Lupus III cloud is\nparallel to the direction of its filament structure, and approximately\nperpendicular to the magnetic field direction. We also confirm the robustness\nof the estimated PAs in the Lupus clouds by comparing the different\nobservations and estimators based on three different methods including sparse\nmodeling. The absence of the significant alignment of the disk orientation is\nconsistent with the turbulent origin of the disk angular momentum. Further\nobservations are required to confirm/falsify the possible disk alignment in the\nLupus III cloud.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spatio-kinematic models of five nova remnants: correlations between nova\n  shell axial ratio, expansion velocity, and speed class: We present long-slit intermediate-dispersion spectroscopic observations and\nnarrow-band direct imaging of four classical nova shells, namely TAur, HRDel,\nDQHer and QUVul, and the nova-like source CKVul. These are used to construct\nmodels of their nebular remnants using the morpho-kinematic modelling tool\nShape to reveal their 3D shape. All these nova remnants but CKVul can be\ndescribed by prolate ellipsoidal shells with different eccentricity degree,\nfrom the spherical QUVul to the highly elongated shell with an equatorial\ncomponent HRDel. On the other hand, CKVul shows a more complex structure, with\ntwo pairs of nested bipolar lobes. The spatio-kinematic properties of the\nellipsoidal nova shells derived from our models include their true axial\nratios. This parameter is expected to correlate with the expansion velocity and\ndecline time C3 (i.e., their speed class) of a nova as the result the\ninteraction of the ejecta with the circumstellar material and rotation speed\nand magnetic field of the white dwarf. We have compared these three parameters\nincluding data available in the literature for another two nova shells, V533\nHer and FH Ser. There is an anti-correlation between the expansion velocity and\nthe axial ratio and decline time C3 for nova remnants with ellipsoidal\nmorphology, and a correlation between their axial ratios and decline times C3,\nconfirming theoretical expectations that the fastest expanding novae have the\nsmallest axial ratios. We note that the high expansion velocity of the nova\nshell HRDel of 615 km/s is inconsistent with its long decline time C3 of 250\ndays.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Herschel-HIFI detections of hydrides towards AFGL 2591 (Envelope\n  emission versus tenuous cloud absorption): The Heterodyne Instrument for the Far Infrared (HIFI) onboard the Herschel\nSpace Observatory allows the first observations of light diatomic molecules at\nhigh spectral resolution and in multiple transitions. Here, we report deep\nintegrations using HIFI in different lines of hydrides towards the high-mass\nstar forming region AFGL 2591. Detected are CH, CH+, NH, OH+, H2O+, while NH+\nand SH+ have not been detected. All molecules except for CH and CH+ are seen in\nabsorption with low excitation temperatures and at velocities different from\nthe systemic velocity of the protostellar envelope. Surprisingly, the CH(JF,P =\n3/2_2,- - 1/2_1,+) and CH+(J = 1 - 0, J = 2 - 1) lines are detected in emission\nat the systemic velocity. We can assign the absorption features to a foreground\ncloud and an outflow lobe, while the CH and CH+ emission stems from the\nenvelope. The observed abundance and excitation of CH and CH+ can be explained\nin the scenario of FUV irradiated outflow walls, where a cavity etched out by\nthe outflow allows protostellar FUV photons to irradiate and heat the envelope\nat larger distances driving the chemical reactions that produce these\nmolecules.",
        "category": "astro-ph_SR"
    },
    {
        "text": "General model of depolarization and transfer of polarization of singly\n  ionized atoms by collisions with hydrogen atoms: Simulations of the generation of the atomic polarization is necessary for\ninterpreting the second solar spectrum. For this purpose, it is important to\nrigorously determine the effects of the isotropic collisions with neutral\nhydrogen on the atomic polarization of the neutral atoms, ionized atoms and\nmolecules. Our aim is to treat in generality the problem of depolarizing\nisotropic collisions between singly ionized atoms and neutral hydrogen in its\nground state. Using our numerical code, we computed the collisional\ndepolarization rates of the $p$-levels of ions for large number of values of\nthe effective principal quantum number $n^{*}$ and the Uns\\\"old energy $E_p$.\nThen, genetic programming has been utilized to fit the available depolarization\nrates. As a result, strongly non-linear relationships between the collisional\ndepolarization rates, $n^{*}$ and $E_p$ are obtained, and are shown to\nreproduce the original data with accuracy clearly better than 10\\%. These\nrelationships allow quick calculations of the depolarizing collisional rates of\nany simple ion which is very useful for the solar physics community. In\naddition, the depolarization rates associated to the complex ions and to the\nhyperfine levels can be easily derived from our results. In this work we have\nshown that by using powerful numerical approach and our collisional method,\ngeneral model giving the depolarization of the ions can be obtained to be\nexploited for solar applications.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the Temporal Evolution of the Disc Counterpart of Type II Spicules in\n  Quiet Sun: The type II spicule has been speculated to provide enough hot plasma to play\nan important role in the mass loading and heating of the corona. We analyse the\ndisc counterpart to type II spicules, RBEs, in three high quality datasets from\nCRISP at the SST. In a quiet Sun region at disc centre we find lower Doppler\nvelocities, 15-40km/s, and Doppler widths, 2-15km/s, of RBEs than in earlier\ncoronal hole studies, 30-50km/s and 7-23km/s, respectively. We examine the\nspatial dependence of Doppler velocities and widths along RBE axes and conclude\nthat there is no clear trend over the FOV or in individual RBEs in quiet Sun at\ndisc centre. These differences with previous coronal hole studies are\nattributed to the more varying magnetic field configuration in quiet Sun\nconditions. Using an extremely high cadence dataset allowed us to improve\ngreatly on the determination of lifetimes of RBEs, found to range from 5 to 60s\nwith an average of 30s, as well as the transverse motions in RBEs, with\ntransverse velocities up to 55km/s and averaging 12km/s. Furthermore, our\nmeasurements of the recurrence rates of RBEs provide important new constraints\non coronal heating by spicules. We also see many examples of a sinusoidal wave\npattern in the transverse motion with periods averaging 54s and amplitudes from\n21.5 to 129km, agreeing well with previous studies of wave motion in limb\nspicules. We interpret the appearance of RBEs over their full length within a\nfew seconds as the result of a combination of three kinds of motions as\nreported earlier for spicules. Finally, we look at the temporal connection\nbetween Ha and Ca 8542 RBEs and find Ca 8542 in addition to being located\ncloser to the footpoint also appear before the Ha RBE. This connection supports\nthe idea that heating occurs in spicules and contribute more weight to the\nprominence of spicules as a source for heating and mass loading of the corona.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Korg: a modern 1D LTE spectral synthesis package: We present Korg, a new package for 1D LTE (local thermodynamic equilibrium)\nspectral synthesis of FGK stars, which computes theoretical spectra from the\nnear-ultraviolet to the near-infrared, and implements both plane-parallel and\nspherical radiative transfer. We outline the inputs and internals of Korg, and\ncompare synthetic spectra from Korg, MOOG, Turbospectrum, and SME. The\ndisagreements between Korg and the other codes are no larger than those between\nthe other codes, although disagreement between codes is substantial. We examine\nthe case of a C$_2$ band in detail, finding that uncertainties on physical\ninputs to spectral synthesis account for a significant fraction of the\ndisagreement. Korg is 1-100 times faster than other codes in typical use,\ncompatible with automatic differentiation libraries, and easily extensible,\nmaking it ideal for statistical inference and parameter estimation applied to\nlarge data sets. Documentation and installation instructions are available at\nhttps://ajwheeler.github.io/Korg.jl/stable/.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A review of the disc instability model for dwarf novae, soft X-ray\n  transients and related objects: I review the basics of the disc instability model (DIM) for dwarf novae and\nsoft-X-ray transients and its most recent developments, as well as the current\nlimitations of the model, focusing on the dwarf nova case. Although the DIM\nuses the Shakura-Sunyaev prescription for angular momentum transport, which we\nknow now to be at best inaccurate, it is surprisingly efficient in reproducing\nthe outbursts of dwarf novae and soft X-ray transients, provided that some\ningredients, such as irradiation of the accretion disc and of the donor star,\nmass transfer variations, truncation of the inner disc, etc., are added to the\nbasic model. As recently realized, taking into account the existence of winds\nand outflows and of the torque they exert on the accretion disc may\nsignificantly impact the model. I also discuss the origin of the superoutbursts\nthat are probably due to a combination of variations of the mass transfer rate\nand of a tidal instability. I finally mention a number of unsolved problems and\ncaveats, among which the most embarrassing one is the modelling of the low\nstate. Despite significant progresses in the past few years both on our\nunderstanding of angular momentum transport, the DIM is still needed for\nunderstanding transient systems.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Learning about AGB stars by studying the stars polluted by their\n  outflows: A rich zoo of peculiar objects forms when Asymptotic Giant Branch (AGB)\nstars, undergo interactions in a binary system. For example, Barium (Ba) stars\nare main-sequence and red-giant stars that accreted mass from the outflows of a\nformer AGB companion, which is now a dim white dwarf (WD). Their orbital\nproperties can help us constrain AGB binary interaction mechanisms, and their\nchemical abundances are a tracer of the nucleosynthesis processes that took\nplace inside the former AGB star. The observational constraints concerning the\norbital and stellar properties of Ba stars have increased in the past years,\nbut important uncertainties remained concerning their WD companions. In this\ncontribution, we used HD76225 to demonstrate that by combining radial-velocity\ndata with Hipparcos and Gaia astrometry, one can accurately constrain the\norbital inclinations of these systems and obtain the absolute masses of these\nWDs, getting direct information about their AGB progenitors via initial-final\nmass relationships.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The variable 6307A emission line in the spectrum of Eta Carinae:\n  blueshifted [S III] 6313A from the interacting winds: The 6307A emission line in the spectrum of Eta Car (Martin et al. 2006) is\nblue-shifted [S III] 6313A emission originating from the outer wind structures\nof the massive binary system. We realized the identification while analyzing\nmultiple forbidden emission lines not normally seen in the spectra of massive\nstars. The high spatial and moderate spectral resolutions of HST/STIS resolve\nforbidden lines of Fe+, N+, Fe++, S++, Ne++ and Ar++ into spatially and\nvelocity-resolved rope-like features originating from collisionally-excited\nions photo-ionized by UV photons or collisions. While the [Fe II] emission\nextends across a velocity range of +/-500 km/s out to 0.\"7, more highly ionized\nforbidden emissions are systematically blue-shifted (-500 to +200 km/s) and\nextend only to 0.\"4. The [Fe II] defines the outer regions of the massive\nprimary wind. The [N II], [Fe III] emission define the the outer wind\ninteraction regions directly photo-ionized by far-UV radiation. Variations in\nemission of [S III] 9533A, 9071A and 6313A suggest density ranges of 10^6 to\n10^10 cm^-3 for electron temperatures ranging from 8,000 to 13,000K. Mapping\nthe temporal changes of the emission structure at critical phases of the\n5.54-year period will provide important diagnostics of the interacting winds.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Time-resolved optical observations of five cataclysmic variables\n  detected by INTEGRAL: The ESA gamma-ray telescope, INTEGRAL, is detecting relatively more\nintrinsically rare cataclysmic variables (CVs) than were found by surveys at\nlower energies. Specifically, a large fraction of the CVs that are INTEGRAL\nsources consists of asynchronous polars and intermediate polars (IPs). IP\nclassifications have been proposed for the majority of CVs discovered by\nINTEGRAL, but, in many cases, there is very little known about these systems.\nIn order to address this, I present time-resolved optical data of five CVs\ndiscovered through INTEGRAL observations. The white dwarf spin modulation is\ndetected in high-speed photometry of three of the new CVs (IGR J15094-6649, IGR\nJ16500-3307, and IGR J17195-4100), but two others (XSS J12270-4859 and IGR\nJ16167-4957) show no evidence of magnetism, and should be considered\nunclassified systems. Spectroscopic orbital period (P_orb) measurements are\nalso given for IGR J15094-6649, IGR J16167-4957, IGR J16500-3307, and IGR\nJ17195-4100.",
        "category": "astro-ph_SR"
    },
    {
        "text": "NOEMA maps the CO $J = 2-1$ environment of the red supergiant $\u03bc$ Cep: Red supergiant stars are surrounded by a gaseous and dusty circumstellar\nenvironment created by their mass loss which spreads heavy elements into the\ninterstellar medium. The structure and the dynamics of this envelope are\ncrucial to understand the processes driving the red supergiant mass loss and\nthe shaping of the pre-supernova ejecta. We have observed the emission from the\nCO $J = 2-1$ line from the red supergiant star $\\mu$~Cep with the NOEMA\ninterferometer. In the line the synthesized beam was $0.92 \\times 0.72$~arcsec\n($590 \\times 462$~au at 641~pc). The continuum map shows only the unresolved\ncontribution of the free-free emission of the star chromosphere. The\ncontinuum-subtracted channel maps reveal a very inhomogeneous and clumpy\ncircumstellar environment. In particular, we detected a bright CO clump, as\nbright as the central source in the line, at 1.80~arcsec south-west from the\nstar, in the blue channel maps. After a deprojection of the radial velocity\nassuming two different constant wind velocities, the observations were modelled\nusing the 3D radiative transfer code \\textsc{lime} to derive the\ncharacteristics of the different structures. We determine that the gaseous\nclumps observed around $\\mu$~Cep are responsible for a mass loss rate of $(4.9\n\\pm 1.0) \\times 10^{-7}~{\\rm M}_\\odot\\,{\\rm yr}^{-1}$, in addition to a\nspatially unresolved wind component with an estimated mass-loss rate of $2.0\n\\times 10^{-6}~{\\rm M}_\\odot\\,{\\rm yr}^{-1}$. Therefore, the clumps have a\nsignificant role in $\\mu$~Cep's mass loss ($\\ge 25 \\%$). We cannot exclude that\nthe unresolved central outflow may be made of smaller unresolved clumps.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A kinematic study of the disc-outflow system around a high-mass\n  protostar G59.783+0.065 probed by methanol and water masers: Class II CH3OH masers are used as a convenient tracer of disc-like structures\nin high-mass star formation. However, more than half of them show a complex\ndistribution in Very Long Baseline Interferometry (VLBI) maps. The origin of\nsuch a complex distribution is still unknown. We conducted VLBI monitoring\nobservations to unveil the origin of a complex class II CH3OH maser in the\nhigh-mass star-forming region G59.783+0.065. We observed the CH3OH maser at 6.7\nGHz and the H2O maser at 22 GHz to probe detailed circumstellar kinematics and\nstructures by the Japanese VLBI network and the VLBI Exploration of Radio\nAstrometry. We found similar bipolar distributions in both masers, specifically\ntwo clusters located 2000 au apart along the East-West direction. We detected a\nlinear distribution of CH3OH masers in the Western cluster. A position-velocity\ndiagram shows that the Western CH3OH masers trace a rotating disc-wind or\ninfalling component inside an edge-on disc-like structure. In contrast to the\nsimple bipolar expanding motions of the H2O masers, the CH3OH masers exhibited\ncomplex motions despite their spatial coincidence. Some of the Eastern CH3OH\nmasers showed bipolar expansions similar to the H2O masers, while others\ndisplayed random or even inward motions. Such complex kinematics and their\nclose association with the H2O maser could occur at the boundary between\noutflow and inflow. We suggest that the complex distribution of class II CH3OH\nmasers, like G59.783+0.065 arises from several distinct circumstellar\nstructures that simultaneously achieve maser excitation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Extreme Ultraviolet Late-Phase Flares: Before and During the Solar\n  Dynamics Observatory Mission: The SDO EUV observations have revealed interesting characteristics of warm\ncoronal emissions, which peak soon after the hot coronal X-ray emissions peak\nduring a flare and then sometimes peak for a second time hours after the X-ray\nflare peak. This flare type, with two warm coronal emission peaks but only one\nX-ray peak, has been named the EUV late phase. These flares have the distinct\nproperties of i) having a complex magnetic field structure with two initial\nsets of coronal loops, with one upper set overlaying a lower set, ii) having an\neruptive flare initiated in the lower set and disturbing both loop sets, iii)\nhaving the hot coronal emissions emitted only from the lower set, and iv)\nhaving the first peak of the warm coronal emissions associated with the lower\nset and its second peak emitted from the upper set much later. The disturbance\nof the coronal loops by the eruption is at about the same time, but the\nrelaxation and cooling down of the heated coronal loops during the post-flare\nreconnections have different time scales with the longer, upper loops being\nsignificantly delayed from the lower loops. The difference in these cooling\ntime scales is related to the difference between the two peak times of the warm\ncoronal emission and is also apparent in the decay profile of the X-ray\nemissions having two distinct decays, with the first decay slope being steeper\nand the delayed decay slope being smaller during the time of the warm coronal\nemission second peak. The frequency and relationship of the EUV late-phase\ndecay times between the Fe XVI two flare peaks and X-ray decay slopes are\nexamined using three years of SDO/EVE data, and the X-ray dual-decay character\nis then exploited to estimate the frequency of EUV late-phase flares during the\npast four solar cycles. This study indicates that the frequency of EUV\nlate-phase flares peaks before and after each solar cycle minimum.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Stellar magnetism: empirical trends with age and rotation: We investigate how the observed large-scale surface magnetic fields of\nlow-mass stars (~0.1 -- 2 Msun), reconstructed through Zeeman-Doppler imaging\n(ZDI), vary with age t, rotation and X-ray emission. Our sample consists of 104\nmagnetic maps of 73 stars, from accreting pre-main sequence to main-sequence\nobjects (1 Myr < t < 10 Gyr). For non-accreting dwarfs we empirically find that\nthe unsigned average large-scale surface field <|Bv|> is related to age as\n$t^{-0.655 \\pm 0.045}$. This relation has a similar dependence to that\nidentified by Skumanich (1972), used as the basis for gyrochronology. Likewise,\nour relation could be used as an age-dating method (\"magnetochronology\"). The\ntrends with rotation we find for the large-scale stellar magnetism are\nconsistent with the trends found from Zeeman broadening measurements (sensitive\nto large- and small-scale fields). These similarities indicate that the fields\nrecovered from both techniques are coupled to each other, suggesting that\nsmall- and large-scale fields could share the same dynamo field generation\nprocesses. For the accreting objects, fewer statistically significant relations\nare found, with one being a correlation between the unsigned magnetic flux and\nrotation period. We attribute this to a signature of star-disc interaction,\nrather than being driven by the dynamo.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Constraints on the distance moduli, helium and metal abundances, and\n  ages of globular clusters from their RR Lyrae and non-variable\n  horizontal-branch stars. II. Multiple stellar populations in 47Tuc, M3, and\n  M13: We present a new set of horizontal-branch (HB) models computed with the MESA\nstellar evolution code. The models adopt $\\alpha$-enhanced \\cite{ags09} metals\nmixtures and include the gravitational settling of He. They are used in our HB\npopulation synthesis tool to generate theoretical distributions of HB stars in\norder to describe the multiple stellar populations in the globular clusters\n47Tuc, M3, and M13. The observed HB in 47Tuc is reproduced very well by our\nsimulations for [Fe/H] $= -0.70$ and [$\\alpha$/Fe] $= +0.4$ if the initial\nhelium mass fraction varies by $\\Delta Y_0 \\sim 0.03$ and approximately 21%,\n37%, and 42% of the stars have $Y_0 = 0.257$, 0.270, and 0.287, respectively.\nThese simulations yield $(m-M)_V = 13.27$, implying an age near 13.0 Gyr. In\nthe case of M3 and M13, our synthetic HBs for [Fe/H] $= -1.55$ and\n[$\\alpha$/Fe] $= 0.4$ match the observed ones quite well if M3 has $\\Delta Y_0\n\\sim 0.01$ and $(m-M)_V = 15.02$, resulting in an age of 12.6 Gyr, whereas M13\nhas $\\Delta Y_0 \\sim 0.08$ and $(m-M)_V = 14.42$, implying an age of 12.9 Gyr.\nMass loss during giant-branch evolution and $\\Delta Y_0$ appear to be the\nprimary second parameters for M3 and M13. New observations for 7 of the 9 known\nRR Lyrae in M13 are also reported. Surprisingly, periods predicted for the\n$c$-type variables tend to be too high (by up to $\\sim 0.1$~d).",
        "category": "astro-ph_SR"
    },
    {
        "text": "Multi-epoch high-resolution spectroscopy of SN2011fe - Linking the\n  progenitor to its environment: This study attempts to establish a link between the reasonably well known\nnature of the progenitor of SN2011fe and its surrounding environment. This is\ndone with the aim of enabling the identification of similar systems in the vast\nmajority of the cases, when distance and epoch of discovery do not allow a\ndirect approach. To study the circumstellar environment of SN2011fe we have\nobtained high-resolution spectroscopy of SN2011fe on 12 epochs, from 8 to 86\ndays after the estimated date of explosion, targeting in particular at the time\nevolution of CaII and NaI. Three main absorption systems are identified from\nCaII and NaI, one associated to the Milky Way, one probably arising within a\nhigh-velocity cloud, and one most likely associated to the halo of M101. The\nGalactic and host galaxy reddening, deduced from the integrated equivalent\nwidths (EW) of the NaI lines are E(B-V)=0.011+/-0.002 and E(B-V)=0.014+/-0.002\nmag, respectively. The host galaxy absorption is dominated by a component\ndetected at the same velocity measured from the 21-cm HI line at the projected\nSN position (~180 km/s). During the ~3 months covered by our observations, its\nEW changed by 15.6+/-6.5 mA. This small variation is shown to be compatible\nwith the geometric effects produced by therapid SN photosphere expansion\ncoupled to the patchy fractal structure of the ISM. The observed behavior is\nfully consistent with ISM properties similar to those derived for our own\nGalaxy, with evidences for structures on scales <100 AU. SN2011fe appears to be\nsurrounded by a \"clean\" environment. The lack of blue-shifted, time-variant\nabsorption features is fully consistent with the progenitor being a binary\nsystem with a main-sequence, or even another degenerate star.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The two-dimensional internal rotation of KIC11145123: The two-dimensional internal rotation of KIC11145123 has been inferred via\nasteroseismology. Based on the Optimally Localized Averaging method and a\nsimple three-zone modeling of the internal rotation, we have found evidence for\na contrast between the internal rotation of the radiative region and that of\nthe convective core; the radiative region rotates almost uniformly throughout\nthe region, but the convective core may be rotating about 6 times faster than\nthe radiative region above. We have also found a marginally significant\nevidence of latitudinal differential rotation in the outer envelope. These\nnewly indicated features of the internal rotation of the star can help us\nfurther constrain the theory of angular momentum transport inside stars as well\nas understand the complex physical properties of the star, which was once\nthought to be a main-sequence A-type star but recently has been proposed to be\na blue straggler, based on spectroscopy.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Multicolor Photoelectric WBVR-Observations of the CBS HZ~Her=Her~X-1 in\n  1986--1988: We present results of four-color WBVR- photoelectric observations of the\nclose binary HZ~Her=HER~X-1 in 1986--1988. As a rule, the duration of the\nobservations exceeded two 35-day X-ray orbital periods in the 1986--1988\nobserving seasons. The accuracy and length of the photoelectric observations\nfacilitated multi-faceted studies, which enabled us to define several fine\nphotometric effects in the light curves of the binary more precisely and\nattempt to interpret them in a model for the matter flow from the optical\ncomponent to the accretion disk around the neutron star. This model provides a\nsatisfactory explanation for the inhomogeneity of the gas flow and \"hot spot\",\nas well as the existence of distinct \"splashes\" moving in their own Keplerian\norbits around the outer parts of the Keplerian disk. We present series of light\ncurves for all the observing seasons, as well as color--color diagrams that\nreflect the physics of various photometric effects. The transformation\ncoefficients for each of the instrumental systems for the three observatories\nat which the observations were carried out are given. Atmospheric extinction\nwas taken into account during multi-color observations of the object, with\nsubsequent correction for atmospheric effects with accuracies ranging from\n0.003^m to 0.005^m for air masses up to M(z)=2.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Comparison of solar wind speed estimates from nearly simultaneous IPS\n  observations at 327 and 111 MHz: Results are presented of a comparison between solar wind speed estimates made\nusing the time delays between 3 pairs of 327 MHz antennas at ISEE and estimates\nmade by modeling the temporal power spectra observed with the 111 MHz BSA\nantenna at LPI. The observations were made for 6 years in the descending phase\nof solar cycle 24. More than 100 individual records were obtained for the\ncompact source 3C48 and the extended and anisotropic source 3C298. The\ncorrelation between the daily speed estimates from 3C48 is 50%. Their annual\naverages agree within the error estimates and show the expected solar cycle\nvariation. However the correlation between speeds from 3C298 is only 25% and\ntheir annual averages do not agree well. We investigate possible causes of this\nbias in the 3C298 estimated speeds.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Post-flare UV light curves explained with thermal instability of loop\n  plasma: In the present work we study the C8 flare occurred on September 26, 2000 at\n19:49 UT and observed by the SOHO/SUMER spectrometer from the beginning of the\nimpulsive phase to well beyond the disappearance in the X-rays. The emission\nfirst decayed progressively through equilibrium states until the plasma reached\n2-3 MK. Then, a series of cooler lines, i.e. Ca x, Ca vii, Ne vi, O iv and Si\niii (formed in the temperature range log T = 4.3 - 6.3 under equilibrium\nconditions), are emitted at the same time and all evolve in a similar way. Here\nwe show that the simultaneous emission of lines with such a different formation\ntemperature is due to thermal instability occurring in the flaring plasma as\nsoon as it has cooled below ~ 2 MK. We can qualitatively reproduce the relative\nstart time of the light curves of each line in the correct order with a simple\n(and standard) model of a single flaring loop. The agreement with the observed\nlight curves is greatly improved, and a slower evolution of the line emission\nis predicted, if we assume that the model loop consists of an ensemble of\nsubloops or strands heated at slightly different times. Our analysis can be\nuseful for flare observations with SDO/EVE.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Evolution of Protoplanetary Disks: Probing the Inner Disk of Very\n  Low Accretors: We report FUV, optical, and NIR observations of three T Tauri stars in the\nOrion OB1b subassociation with H$\\alpha$ equivalent widths consistent with low\nor absent accretion and various degrees of excess flux in the mid-infrared. We\naim to search for evidence of gas in the inner disk in HST ACS/SBC spectra, and\nto probe the accretion flows onto the star using H$\\alpha$ and He I\n$\\lambda$10830 in spectra obtained at the Magellan and SOAR telescopes. At the\ncritical age of 5 Myr, the targets are at different stages of disk evolution.\nOne of our targets is clearly accreting, as shown by redshifted absorption at\nfree-fall velocities in the He I line and wide wings in H$\\alpha$; however, a\nmarginal detection of FUV H$_2$ suggests that little gas is present in the\ninner disk, although the spectral energy distribution indicates that small dust\nstill remains close to the star. Another target is surrounded by a transitional\ndisk, with an inner cavity in which little sub-micron dust remains. Still, the\ninner disk shows substantial amounts of gas, accreting onto the star at a\nprobably low, but uncertain rate. The third target lacks both a He I line or\nFUV emission, consistent with no accretion or inner gas disk; its very weak IR\nexcess is consistent with a debris disk. Different processes occurring in\ntargets with ages close to the disk dispersal time suggest that the end of\naccretion phase is reached in diverse ways.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Discovery of a Highly Eccentric, Chromospherically Active Binary:\n  ASASSN-V J192114.84+624950.8: As part of an All-Sky Automated Survey for SuperNovae (ASAS-SN) search for\nsources with large flux decrements, we discovered a transient where the\nquiescent, stellar source, ASASSN-V J192114.84+624950.8, rapidly decreased in\nflux by $\\sim55\\%$ ($\\sim0.9$ mag) in the g-band. The \\textit{TESS} light curve\nrevealed that the source is a highly eccentric, eclipsing binary. Fits to the\nlight curve using \\textsc{phoebe} find the binary orbit to have $e=0.79$,\n$P_{\\rm orb}=18.462~\\text{days}$, and $i=88.6^{\\circ}$ and the ratios of the\nstellar radii and temperatures to be $R_2/R_1 = 0.71$ and $T_{e,2}/T_{e,1} =\n0.82$. Both stars are chromospherically active, allowing us to determine their\nrotational periods of $P_1=1.52$ days and $P_2=1.79$ days, respectively. A\nLBT/MODS spectrum shows that the primary is a late-G or early-K type dwarf.\nFits to the SED show that the luminosities and temperatures of the two stars\nare $L_1 = 0.48~L_{\\sun}$, $T_1= 5050~K$, $L_2 = 0.12~L_{\\sun}$, and $T_{2} =\n4190~K$. We conclude that ASASSN-V J192114.84+624950.8 consists of two\nchromospherically active, rotational variable stars in a highly elliptical\neclipsing orbit.",
        "category": "astro-ph_SR"
    },
    {
        "text": "CNO abundances in the globular clusters NGC 1851 and NGC 6752: We measure the C+N+O abundance sum in red giant stars in two Galactic\nglobular clusters, NGC 1851 and NGC 6752. NGC 1851 has a split subgiant branch\nwhich could be due to different ages or C+N+O content while NGC 6752 is\nrepresentative of the least complex globular clusters. For NGC 1851 and NGC\n6752, we obtain average values of A(C+N+O) = 8.16 +/- 0.10 (\\sigma = 0.34) and\n7.62 +/- 0.02 (\\sigma = 0.06), respectively. When taking into account the\nmeasurement errors, we find a constant C+N+O abundance sum in NGC 6752. The\nC+N+O abundance dispersion is only 0.06 dex, and such a result requires that\nthe source of the light element abundance variations does not increase the\nC+N+O sum in this cluster. For NGC 1851, we confirm a large spread in C+N+O. In\nthis cluster, the anomalous RGB has a higher C+N+O content than the canonical\nRGB by a factor of four (~0.6 dex). This result lends further support to the\nidea that the two subgiant branches in NGC 1851 are roughly coeval, but with\ndifferent CNO abundances.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Populations of rotating stars. - I. Models from 1.7 to 15 Msun at Z =\n  0.014, 0.006, and 0.002 with \u03a9/\u03a9crit between 0 and 1: B-type stars are known to rotate at various velocities, including very fast\nrotators near the critical velocity as the Be stars. In this paper, we provide\nstellar models covering the mass range between 1.7 to 15 Msun, which includes\nthe typical mass of known Be stars, at Z = 0.014, 0.006, and 0.002 and for an\nextended range of initial velocities on the zero-age main sequence. We used the\nGeneva stellar-evolution code, including the effects of shellular rotation,\nwith a numerical treatment that has been improved so the code can precisely\ntrack the variation in the angular momentum content of the star as it changes\nunder the influence of radiative winds and/or mechanical mass loss. We discuss\nthe impact of the initial rotation rate on the tracks in the\nHertzsprung-Russell diagram, the main-sequence (MS) lifetimes, the evolution of\nthe surface rotation and abundances, as well as on the ejected masses of\nvarious isotopes. Among the new results obtained from the present grid we find\nthat 1) fast-rotating stars with initial masses around 1.7 Msun present at the\nbeginning of the core hydrogen-burning phase quite small convective cores with\nrespect to their slowly rotating counterparts. This fact may be interesting to\nkeep in mind in the framework of the asteroseismic studies of such stars. 2)\nThe contrast between the core and surface angular velocity is higher in slower\nrotating stars. The values presently obtained are in agreement with the very\nfew values obtained for B-type stars from asteroseismology. 3) At Z = 0.002,\nthe stars in the mass range of 1.7 to 3 Msun with a mean velocity on the MS of\nthe order of 150 km/s show N/H enhancement superior to 0.2 dex at mid-MS, and\nsuperior to 0.4 dex at the end of the MS phase. At solar metallicity the\ncorresponding values are below 0.2 dex at any time in the MS.",
        "category": "astro-ph_SR"
    },
    {
        "text": "SDSS/SEGUE Spectral Feature Analysis For Stellar Atmospheric Parameter\n  Estimation: Large-scale and deep sky survey missions are rapidly collecting a large\namount of stellar spectra, which necessitate the estimation of atmospheric\nparameters directly from spectra and makes it feasible to statistically\ninvestigate latent principles in a large dataset. We present a technique for\nestimating parameters $T_{eff}$, log$~g$ and [Fe/H] from stellar spectra. With\nthis technique, we first extract features from stellar spectra using the LASSO\nalgorithm; then, the parameters are estimated from the extracted features using\nthe SVR. On a subsample of 20~000 stellar spectra from SDSS with reference\nparameters provided by SDSS/SEGUE Pipeline SSPP, estimation consistency are\n0.007458 dex for log$~T_{eff}$ (101.609921 K for $T_{eff}$), 0.189557 dex for\nlog$~g$ and 0.182060 for [Fe/H], where the consistency is evaluated by mean\nabsolute error. Prominent characteristics of the proposed scheme are\nsparseness, locality, and physical interpretability. In this work, every\nspectrum consists of 3821 fluxes, and 10, 19, and 14 typical wavelength\npositions are detected respectively for estimating $T_{eff}$, log$~g$ and\n[Fe/H]. It is shown that the positions are related to typical lines of stellar\nspectra. This characteristic is important in investigating physical indications\nfrom analysis results. Then, stellar spectra can be described by the individual\nfluxes on the detected positions (PD) or local integration of fluxes near them\n(LI). The abovementioned consistency is the result based on features described\nby LI. If features are described by PD, consistency are 0.009092 dex for\nlog$~T_{eff}$ (124.545075 K for $T_{eff}$), 0.198928 dex for log$~g$, and\n0.206814 dex for [Fe/H].",
        "category": "astro-ph_SR"
    },
    {
        "text": "PolarBase: a data base of high resolution spectropolarimetric stellar\n  observations: PolarBase is an evolving data base that contains all stellar data collected\nwith the ESPaDOnS and NARVAL high-resolution spectropolarimeters, in their\nreduced form, as soon as they become public. As of early 2014, observations of\n2,000 stellar objects throughout the Hertzsprung-Russell diagram are available.\nIntensity spectra are available for all targets, and the majority of the\nobservations also include simultaneous spectra in circular or linear\npolarization, with the majority of the polarimetric measurements being\nperformed only in circularly polarized light (Stokes V). Observations are\nassociated with a cross-correlation pseudo-line profile in all available Stokes\nparameters, greatly increasing the detectability of weak polarized signatures.\nStokes V signatures are detected for more than 300 stars of all masses and\nevolutionary stages, and linear polarization is detected in 35 targets. The\ndetection rate in Stokes V is found to be anti-correlated with the stellar\neffective temperature. This unique set of Zeeman detections offers the first\nopportunity to run homogeneous magnetometry studies throughout the H-R diagram.\nThe web interface of PolarBase is available at http://polarbase.irap.omp.eu.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Precise mass measurements for the double neutron star system J1829+2456: PSR J1829+2456 is a radio pulsar in a relativistic binary system with another\nneutron star. It has a rotational period of 41 ms and a mildly eccentric ($e =\n0.14$) 28-hr orbit. We have continued its observations with the Arecibo radio\ntelescope and have now measured the individual neutron star masses of this\nsystem. The pulsar and companion masses are $1.306\\,\\pm\\,0.007\\,M_{\\odot}$ and\n$1.299\\,\\pm\\,0.007\\,M_{\\odot}$ (2$\\sigma$ - 95% confidence, unless stated\notherwise), respectively. We have also measured the proper motion for this\nsystem and used it to estimate a space velocity of 49$^{+77}_{-30}$ km s$^{-1}$\nwith respect to the local standard of rest. The relatively low values for\ncompanion mass, space velocity and orbital eccentricity in this system make it\nsimilar to other double neutron star systems in which the second-formed neutron\nstar is thought to have formed in a low-kick, low mass-loss, symmetric\nsupernova.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Effects of Space Weather on Flight Delays: Although the sun is really far away from us, some solar activities could\nstill influence the performance and reliability of space-borne and ground-based\ntechnological systems on Earth. Those time-varying conditions in space caused\nby the sun are also called space weather, as the atmospheric conditions that\ncan affect weather on the ground. It is known that aviation activities can be\naffected during space weather events, but the exact effects of space weather on\naviation are still unclear. Especially how the flight delays, the top topic\nconcerned by most people, will be affected by space weather has never been\nthoroughly researched. By analyzing huge amount of flight data (~5X106\nrecords), for the first time, we demonstrate that space weather events could\nhave systematically modulating effects on flight delays. The average arrival\ndelay time and 30-minute delay rate during space weather events are\nsignificantly increased by 81.34% and 21.45% respectively compared to those\nduring quiet periods. The evident negative correlation between the yearly\nflight regularity rate and the yearly mean total sunspot number during 22 years\nalso confirms such delay effects. Further studies indicate that the\ninterference in communication and navigation caused by geomagnetic field\nfluctuations and ionospheric disturbances associated with the space weather\nevents will increase the flight delay time and delay rate. These results expand\nthe traditional field of space weather research and could also provide us with\nbrand new views for improving the flight delay predications.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Extensive photometry of the intermediate polar V1033 Cas (IGR\n  J00234+6141): To measure the spin period of the white dwarf in V1033 Cas with high\nprecision, we performed extensive photometry. Observations were obtained over\n34 nights in 2017. The total duration of the observations was 143 h. We found\nthat the spin period of the white dwarf is equal to 563.11633+/-0.00010 s.\nUsing this period, we derived the oscillation ephemeris with a long validity of\n100 years. The spin oscillation semi-amplitude was stable and was equal to\n95.5+/-1.3 mmag. This is a very large semi-amplitude of the spin oscillation\namong intermediate polars, which have similar and lesser spin periods. This\nlarge semi-amplitude suggests that the system is noticeably inclined. The spin\npulse profile was sinusoidal with high accuracy. This may mean that the spin\noscillation is produced by a single accretion curtain whereas the second\naccretion curtain may be obscured by the accretion disc. Despite the large\namount of our observations, we did not detect sidebands. The semi-amplitudes of\nthe undetected sideband oscillations do not exceed 10 mmag. The absence of\nsideband oscillations seems puzzling. We detected the orbital variability of\nV1033 Cas with a period of 4.0243+/-0.0028 h and with a semi-amplitude of\n55+/-4 mmag. The orbital variability semi-amplitude seems large and also\nsuggests that the system is noticeably inclined. Using our oscillation\nephemeris and the times of spin pulse maximum obtained in the past, we found\nthat the spin period is very stable. dP/dt is most probably less than -4 X\n10^(-12). This contradicts the assumption that the white dwarf in V1033 Cas is\nnot spinning at equilibrium. Our spin period and our oscillation ephemeris can\nbe used for further investigations of the stability of the spin period in V1033\nCas.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Discovery of a ~23 Mjup Brown Dwarf Orbiting ~700 AU from the Massive\n  Star HIP 78530 in Upper Scorpius: We present the discovery of a substellar companion on a wide orbit around the\n~2.5 Msun star HIP 78530, which is a member of the 5 Myr-old Upper Scorpius\nassociation. We have obtained follow-up imaging over two years and show that\nthe companion and primary share common proper motion. We have also obtained JHK\nspectroscopy of the companion and confirm its low surface gravity, in\naccordance with the young age of the system. A comparison with Drift-Phoenix\nsynthetic spectra indicates an effective temperature of 2800+/-200 K and a\ncomparison with template spectra of young and old dwarfs indicates a spectral\ntype of M8+/-1. The mass of the companion is estimated to be 19-26 Mjup based\non its bolometric luminosity and the predictions of evolutionary models. The\nangular separation of the companion is 4.5\", which at the distance of the\nprimary star, 156.7 pc, corresponds to a projected separation of ~710 AU. This\ncompanion features one of the lowest mass ratios (~0.009) of any known\ncompanion at separations greater than 100 AU.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Statistical Study of Emerging Flux Regions and the Upper Atmosphere\n  Response: We statistically study the property of emerging flux regions (EFRs) and the\nupper solar atmosphere response to the flux emergence by using data from the\nHelioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly\n(AIA) on board the Solar Dynamics Observatory (SDO). Parameters including the\ntotal emerged flux, the flux growth rate, the maximum area, the duration of the\nemergence and the separation speed of the opposite polarities are adopted to\ndelineate the property of the EFRs. The response of the upper atmosphere is\naddressed by the response of the atmosphere at different wavelengths (and thus\nat different temperatures). According to our results, the total emerged fluxes\nare in the range of (0.44 -- 11.2)$\\times10^{19}$ Mx while the maximum area\nranges from 17 to 182 arcsec$^2$. The durations of the emergence are between 1\nand 12 hours, which are positively correlated to both the total emerged flux\nand the maximum area. The maximum distances between the opposite polarities are\n7 -- 25 arcsec and are also correlated to the duration positively. The\nseparation speeds are from 0.05 to 1.08 km s$^{-1}$, negatively correlated to\nthe duration. The derived flux growth rates are (0.1 -- 1.3)$\\times10^{19}$ Mx\nhr$^{-1}$, which are positively correlated to the total emerging flux. The\nupper atmosphere responds to the flux emergence in the 1600\\AA\\ chromospheric\nline first, and then tens and hundreds of seconds later, in coronal lines, such\nas the 171\\AA\\ (T=10$^{5.8}$ K) and 211\\AA\\ (T=10$^{6.3}$ K) lines almost\nsimultaneously, suggesting the successively heating of atmosphere from the\nchromosphere to the corona.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The quiet Sun at mm Wavelengths as Seen by ALMA: Solar observations at sub-mm, mm and cm wavelengths offer a straightforward\ndiagnostic of physical conditions in the solar atmosphere because they yield\nmeasurement of brightness temperature which, for optically thick features,\nequals intrinsic temperature - much unlike solar diagnostics in other spectral\nranges. The Atacama Large Millimeter and sub-millimeter Array (ALMA) has\ntherefore opened a new, hitherto underexplored, spectral window for studying\nthe enigmatic solar chromosphere. In this review we discuss initial ALMA\nstudies of the quiet chromosphere that used both single-dish and compact-array\ninterferometric observing modes. We present results on the temperature\nstructure of the chromosphere, comparison with classic empirical models of the\nchromosphere, and observations of the chromospheric network and spicules.\nFurthermore, we discuss what may be expected in the future, since the ALMA\ncapabilities continuously expand and improve towards higher angular resolution,\nwavelength coverage, and polarization measurement for magnetometry.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Gravitational fragmentation and formation of giant protoplanets on\n  tens-of-au orbits: Migration of dense gaseous clumps that form in young protostellar disks via\ngravitational fragmentation is investigated to determine the likelihood of\ngiant planet formation. High-resolution numerical hydrodynamics simulations in\nthe thin-disk limit are employed to compute the formation and long-term\nevolution of a gravitationally unstable protostellar disk around a solar-mass\nstar. We show that gaseous clumps that form in the outer regions of the disk\n(>100 AU) through disk fragmentation are often perturbed by other clumps or\ndisk structures, such as spiral arms, and migrate toward the central star on\ntimescales from a few 10^3 to few 10^4 yr. The migration timescale is slowest\nwhen stellar motion in response to the disk gravity is considered. When\napproaching the star, the clumps first gain mass (up to several tens of M_Jup),\nbut then quickly lose most of their diffuse envelopes through tidal torques.\nPart of the clump envelope can be accreted on the central star causing an\nFU-Ori-type accretion and luminosity outburst. The tidal mass loss helps the\nclumps to significantly slow down or even halt their inward migration at a\ndistance of a few tens of AU from the protostar. The resulting clumps are\nheavily truncated both in mass and size compared to their wider-orbit\ncounterparts, keeping only a dense and hot nucleus. During the inward\nmigration, the temperature in the clump interiors may exceed the molecular\nhydrogen dissociation limit (2000 K) and the central region of the clump can\ncollapse into a gas giant protoplanet. We argue that FU-Orionis-type luminosity\noutbursts may be the end product of disk fragmentation and clump inward\nmigration, ushering the formation of giant protoplanets on tens-of-au orbits in\nsystems such as HR~8799.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Detection of the 69 \u03bcm band of crystalline forsterite in the\n  Herschel MESS-program: In this article we present the detection of the 69 {\\mu}m band of the\ncrystalline olivine forsterite within the MESS key program of Herschel. We\ndetermine the temperature of the forsterite grains by fitting the 69 {\\mu}m\nband.",
        "category": "astro-ph_SR"
    },
    {
        "text": "WIRED for EC: New White Dwarfs with $\\textit{WISE}$ Infrared Excesses\n  and New Classification Schemes from the Edinburgh-Cape Blue Object Survey: We present a simple method for identifying candidate white dwarf systems with\ndusty exoplanetary debris based on a single temperature blackbody model fit to\nthe infrared excess. We apply this technique to a sample of Southern Hemisphere\nwhite dwarfs from the recently completed Edinburgh-Cape Blue Object Survey and\nidentify four new promising dusty debris disk candidates. We demonstrate the\nefficacy of our selection method by recovering three of the four\n$\\textit{Spitzer}$ confirmed dusty debris disk systems in our sample. Further\ninvestigation using archival high resolution imaging shows $\\textit{Spitzer}$\ndata of the un-recovered fourth object is likely contaminated by a\nline-of-sight object that either led to a mis-classification as a dusty disk in\nthe literature or is confounding our method. Finally, in our diagnostic plot we\nshow that dusty white dwarfs which also host gaseous debris lie along a\nboundary of our dusty debris disk region, providing clues to the origin and\nevolution of these especially interesting systems.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Radii of Rapidly-Rotating Stars, with Application to Transiting-Planet\n  Hosts: The currently favored method for estimating radii and other parameters of\ntransiting-planet host stars is to match theoretical models to observations of\nthe stellar mean density rho_*, the effective temperature T_eff, and the\ncomposition parameter [Z]. This explicitly model-dependent approach is based on\nreadily-available observations, and results in small formal errors. Here I use\ntwo calibration samples of stars (eclipsing binaries and stars for which\nasteroseismic analyses are available) having well-determined masses and radii\nto estimate the accuracy and systematic errors inherent in the rho_* method.\nWhen matching to the Yonsei-Yale stellar evolution models, I find the most\nimportant systematic error results from selection bias favoring\nrapidly-rotating (hence probably magnetically active) stars among the eclipsing\nbinary sample. If unaccounted for, this bias leads to a mass-dependent\nunderestimate of stellar radii by as much as 4% for stars of 0.4 M_sun,\ndecreasing to zero for masses above about 1.4 M_sun. The asteroseismic sample\nsuggests (albeit with significant uncertainty) that systematic errors are small\nfor slowly-rotating, inactive stars. Systematic errors arising from failings of\nthe Yonsei-Yale models of inactive stars probably exist, but are difficult to\nassess because of the small number of well-characterized comparison stars\nhaving low mass and slow rotation. Poor information about [Z] is an important\nsource of random error, and may be a minor source of systematic error as well.\nWith suitable corrections for rotation, it is likely that systematic errors in\nthe rho_* method can be comparable to or smaller than the random errors,\nyielding radii that are accurate to about 2% for most stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Long-term forcing of Sun's coronal field, open flux and cosmic ray\n  modulation potential during grand minima, maxima and regular activity phases\n  by the solar dynamo mechanism: Magnetic fields generated in the Sun's interior by the solar dynamo mechanism\ndrive solar activity over a range of time-scales. While space-based\nobservations of the Sun's corona exist only for few decades, direct sunspot\nobservations exist for a few centuries, solar open flux and cosmic ray flux\nvariations can be reconstructed through studies of cosmogenic isotopes over\nthousands of years. While such reconstructions indicate the presence of extreme\nsolar activity fluctuations in the past, causal links between millennia scale\ndynamo activity, consequent coronal field, solar open flux and cosmic ray\nmodulation remain elusive. By utilizing a stochastically forced solar dynamo\nmodel we perform long-term simulations to illuminate how the dynamo generated\nmagnetic fields govern the structure of the solar corona and the state of the\nheliosphere -- as indicated by variations in the open flux and cosmic ray\nmodulation potential. We establish differences in the nature of the large-scale\nstructuring of the solar corona during grand maximum, minimum, and regular\nsolar activity phases and simulate how the open flux and cosmic ray modulation\npotential varies over time scales encompassing these different phases of solar\nactivity. We demonstrate that the power spectrum of simulated and reconstructed\nsolar open flux are consistent with each other. Our study provides the\ntheoretical basis for interpreting long-term solar cycle variability based on\nreconstructions relying on cosmogenic isotopes and connects solar internal\nvariations to the forcing of the state of the heliosphere.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Interactions of the magnetospheres of stars and close-in giant planets: Since the first discovery of an extrasolar planetary system more than a\ndecade ago, hundreds more have been discovered. Surprisingly, many of these\nsystems harbor Jupiter-class gas giants located close to the central star, at\ndistances of 0.1 AU or less. Observations of chromospheric 'hot spots' that\nrotate in phase with the planetary orbit, and elevated stellar X-ray\nluminosities,suggest that these close-in planets significantly affect the\nstructure of the outer atmosphere of the star through interactions between the\nstellar magnetic field and the planetary magnetosphere. Here we carry out the\nfirst detailed three-dimensional MagnetoHydroHynamics (MHD) simulation\ncontaining the two magnetic bodies and explore the consequences of such\ninteractions on the steady-state coronal structure. The simulations reproduce\nthe observable features of 1) increase in the total X-ray luminosity, 2)\nappearance of coronal hot spots, and 3) phase shift of these spots with respect\nto the direction of the planet. The proximate cause of these is an increase in\nthe density of coronal plasma in the direction of the planet, which prevents\nthe corona from expanding and leaking away this plasma via a stellar wind. The\nsimulations produce significant low temperature heating. By including dynamical\neffects, such as the planetary orbital motion, the simulation should better\nreproduce the observed coronal heating.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Helioseismology with Solar Orbiter: The Solar Orbiter mission, to be launched in July 2017, will carry a suite of\nremote sensing and in-situ instruments, including the Polarimetric and\nHelioseismic Imager (PHI). PHI will deliver high-cadence images of the Sun in\nintensity and Doppler velocity suitable for carrying out novel helioseismic\nstudies. The orbit of the Solar Orbiter spacecraft will reach a solar latitude\nof up to 21 deg (up to 34 deg by the end of the extended mission) and thus will\nenable the first local helioseismology studies of the polar regions. Here we\nconsider an array of science objectives to be addressed by helioseismology\nwithin the baseline telemetry allocation (51 Gbit per orbit, current baseline)\nand within the science observing windows (baseline 3 x 10 days per orbit). A\nparticularly important objective is the measurement of large-scale flows at\nhigh latitudes (rotation and meridional flow), which are largely unknown but\nplay an important role in flux transport dynamos. The full range of\nEarth-Sun-spacecraft angles provided by the orbit will enable helioseismology\nfrom two vantage points by combining PHI with another instrument: stereoscopic\nhelioseismology will allow the study of the deep solar interior and a better\nunderstanding of the physics of solar oscillations in both quiet Sun and\nsunspots. We have used a model of the PHI instrument to study its performance\nfor helioseismology applications. As input we used a 6 hr time-series of\nrealistic solar magneto-convection simulation (Stagger code) and the SPINOR\nradiative transfer code to synthesize the observables. The simulated power\nspectra of solar oscillations show that the instrument is suitable for\nhelioseismology. In particular, the specified point spread function, image\njitter, and photon noise are no obstacle to a successful mission.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Of?p stars of the Magellanic Clouds: Are they strongly magnetic?: All known Galactic Of?p stars have been shown to host strong, organized,\nmagnetic fields. Recently, five Of?p stars have been discovered in the\nMagellanic Clouds. They posses photometric \\citep{Naze} and spectroscopic\n\\citep{Walborn} variability compatible with the Oblique Rotator Model (ORM).\nHowever, their magnetic fields have yet to be directly detected. We have\ndeveloped an algorithm allowing for the synthesis of photometric observables\nbased on the Analytic Dynamical Magnetosphere (ADM) model of \\citet{Owocki}. We\napply our model to OGLE photometry in order to constrain their magnetic\ngeometries and surface dipole strengths. We predict that the field strengths\nfor some of these candidate extra-Galactic magnetic stars may be within the\ndetection limits of the FORS2 instrument.",
        "category": "astro-ph_SR"
    },
    {
        "text": "COBRaS: The e-MERLIN 21 cm Legacy survey of Cygnus OB2: The role of massive stars is central to an understanding of galactic ecology.\nIt is important to establish the details of how massive stars provide\nradiative, chemical, and mechanical feedback in galaxies. Central to these\nissues is an understanding of the evolution of massive stars, and the critical\nrole of mass loss via strongly structured winds and stellar binarity.\nUltimately, massive stellar clusters shape the structure and energetics of\ngalaxies. We aim to conduct high-resolution, deep field mapping at 21cm of the\ncore of the massive Cygnus OB2 association and to characterise the properties\nof the massive stars and colliding winds at this waveband. We used seven\nstations of the e-MERLIN radio facility, with its upgraded bandwidth and\nenhanced sensitivity to conduct a 21cm census of Cygnus OB2. Based on 42 hours\nof observations, seven overlapping pointings were employed over multiple epochs\nduring 2014 resulting in 1 sigma sensitivities down to ~21microJy and a\nresolution of ~180mas. A total of 61 sources are detected at 21cm over a\n~0.48deg x 0.48deg region centred on the heart of the Cyg OB2 association. Of\nthese 61 sources, 33 are detected for the first time. We detect a number of\npreviously identified sources including four massive stellar binary systems,\ntwo YSOs, and several known X-ray and radio sources. We also detect the LBV\ncandidate (possible binary system) and blue hypergiant (BHG) star of Cyg OB2\n#12. The 21cm observations secured in the COBRaS Legacy project provide data to\nconstrain conditions in the outer wind regions of massive stars; determine the\nnon-thermal properties of massive interacting binaries; examine evidence for\ntransient sources, including those associated with young stellar objects; and\nprovide unidentified sources that merit follow-up observations. The 21cm data\nare of lasting value and will serve in combination with other key surveys of\nCyg OB2.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The characteristics of flare- and CME-productive solar active regions: Solar flares and coronal mass ejections (CMEs) cause immediate and adverse\neffects on the interplanetary space and geospace. The deeper understanding of\nthe mechanisms that produce them and the construction of efficient prediction\nschemes are of paramount importance. The source regions of flares and CMEs\nexhibit some common morphological characteristics associated with strongly\nsheared magnetic polarity inversion lines, indicative of the complex magnetic\nconfigurations that store huge amounts of free magnetic energy and helicity.\nThis knowledge is transformed into parameters that can help us distinguish\nefficiently between quiet, flare-, and CME-productive active regions.\nNonetheless, flare and CME prediction still faces a number of challenges. The\nmagnetic field information is constrained at the photosphere and accessed only\nfrom one vantage point of observation; the dynamic behavior of active regions\nis still not fully incorporated into predictions; the stochasticity of flares\nand CMEs renders their prediction probabilistic. To meet these challenges, new\nproperties have been put forward to describe different aspects of magnetic\nenergy storage mechanisms in active regions and offer the opportunity of\nparametric studies for over an entire solar cycle. This inventory of predictors\nnow includes information from flow fields, transition region/coronal\nspectroscopy, data-driven modeling of the coronal magnetic field, as well as\nparameterizations of dynamic effects from time series. Further work towards\nthese directions may help alleviate the current limitations in observing the\nmagnetic field of higher atmospheric layers. This paper reviews these efforts\nas well as the importance of transforming new knowledge into more efficient\npredictors and including new types of data.",
        "category": "astro-ph_SR"
    },
    {
        "text": "CCD photometric study of the W UMa-type binary II CMa in the field of\n  Berkeley 33: The CCD photometric data of the EW-type binary, II CMa, which is a contact\nstar in the field of the middle-aged open cluster Berkeley 33, are presented.\nThe complete R light curve was obtained. In the present paper, using the five\nCCD epochs of light minimum (three of them are calculated from Mazur et al.\n(1993)'s data and two from our new data), the orbital period P was revised to\n0.22919704 days. The complete R light curve was analyzed by using the 2003\nversion of W-D (Wilson-Devinney) program. It is found that this is a contact\nsystem with a mass ratio $q=0.9$ and a contact factor $f=4.1%$. The high mass\nratio ($q=0.9$) and the low contact factor ($f=4.1%$) indicate that the system\njust evolved into the marginal contact stage.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Transition region contribution to AIA observations in the context of\n  coronal heating: We investigate the relative contributions from the transition region and\ncorona of coronal loops observed by the Atmospheric Imaging Assembly (AIA) on\nthe Solar Dynamics Observatory (SDO). Using EBTEL (Enthalpy-Based Thermal\nEvolution of Loops) hydrodynamic simulations, we model loops with multiple\nlengths and energy fluxes heated randomly by events drawn from power-law\ndistributions with different slopes and minimum delays between events to\ninvestigate how each of these parameters influences observable loop properties.\nWe generate AIA intensities from the corona and transition region for each\nrealization. The variations within and between models generated with these\ndifferent parameters illustrate the sensitivity of narrowband imaging to the\ndetails of coronal heating. We then analyze the transition region and coronal\nemission from a number of observed active regions and find broad agreement with\nthe trends in the models. In both models and observations, the transition\nregion brightness is significant, often greater than the coronal brightness in\nall six \"coronal\" AIA channels. We also identify an inverse relationship,\nconsistent with heating theories, between the slope of the differential\nemission measure (DEM) coolward of the peak temperature and the observed ratio\nof coronal to transition region intensity. These results highlight the use of\nnarrowband observations and the importance of properly considering the\ntransition region in investigations of coronal heating.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Transition region contribution to AIA observations in the context of\n  coronal heating: We investigate the relative contributions from the transition region and\ncorona of coronal loops observed by the Atmospheric Imaging Assembly (AIA) on\nthe Solar Dynamics Observatory (SDO). Using EBTEL (Enthalpy-Based Thermal\nEvolution of Loops) hydrodynamic simulations, we model loops with multiple\nlengths and energy fluxes heated randomly by events drawn from power-law\ndistributions with different slopes and minimum delays between events to\ninvestigate how each of these parameters influences observable loop properties.\nWe generate AIA intensities from the corona and transition region for each\nrealization. The variations within and between models generated with these\ndifferent parameters illustrate the sensitivity of narrowband imaging to the\ndetails of coronal heating. We then analyze the transition region and coronal\nemission from a number of observed active regions and find broad agreement with\nthe trends in the models. In both models and observations, the transition\nregion brightness is significant, often greater than the coronal brightness in\nall six \"coronal\" AIA channels. We also identify an inverse relationship,\nconsistent with heating theories, between the slope of the differential\nemission measure (DEM) coolward of the peak temperature and the observed ratio\nof coronal to transition region intensity. These results highlight the use of\nnarrowband observations and the importance of properly considering the\ntransition region in investigations of coronal heating.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Rapid Changes of Photospheric Magnetic Field after Tether-Cutting\n  Reconnection and Magnetic Implosion: The rapid, irreversible change of the photospheric magnetic field has been\nrecognized as an important element of the solar flare process. This Letter\nreports such a rapid change of magnetic fields during the 2011 February 13 M6.6\nflare in NOAA AR 11158 that we found from the vector magnetograms of the\nHelioseismic and Magnetic Imager with 12-min cadence. High-resolution\nmagnetograms of Hinode that are available at ~-5.5, -1.5, 1.5, and 4 hrs\nrelative to the flare maximum are used to reconstruct three-dimensional coronal\nmagnetic field under the nonlinear force-free field (NLFFF) assumption. UV and\nhard X-ray images are also used to illuminate the magnetic field evolution and\nenergy release. The rapid change is mainly detected by HMI in a compact region\nlying in the center of the magnetic sigmoid, where the mean horizontal field\nstrength exhibited a significant increase by 28%. The region lies between the\ninitial strong UV and hard X-ray sources in the chromosphere, which are\ncospatial with the central feet of the sigmoid according to the NLFFF model.\nThe NLFFF model further shows that strong coronal currents are concentrated\nimmediately above the region, and that more intriguingly, the coronal current\nsystem underwent an apparent downward collapse after the sigmoid eruption.\nThese results are discussed in favor of both the tether-cutting reconnection\nproducing the flare and the ensuing implosion of the coronal field resulting\nfrom the energy release.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Red giants seismology: The space-borne missions CoRoT and Kepler are indiscreet. With their\nasteroseismic programs, they tell us what is hidden deep inside the stars.\nWaves excited just below the stellar surface travel throughout the stellar\ninterior and unveil many secrets: how old is the star, how big, how massive,\nhow fast (or slow) its core is dancing. This paper intends to \\emph{paparazze}\nthe red giants according to the seismic pictures we have from their interiors.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Characterization of mid-infrared polarization due to scattering in\n  protoplanetary disks: It is generally assumed that magnetic fields play an important role in the\nformation and evolution of protoplanetary disks. One way of observationally\nconstraining magnetic fields is to measure polarized emission and absorption\nproduced by magnetically aligned elongated dust grains. The fact that radiation\nalso becomes linearly polarized by light scattering at optical to millimeter\nwavelengths complicates magnetic field studies. We characterize the linear\npolarization of mid-infrared radiation due to scattering of the stellar\nradiation and dust thermal re-emission radiation (self-scattering). We find\nthat the thermal re-emission radiation is stronger than the scattered stellar\nradiation for disks with inner holes smaller than 10 au within the considered\nparameter range. The mid-infrared polarization due to scattering shows several\nclear trends: For scattered stellar radiation only, the linear polarization\ndegree decreases slightly with increasing radial distance, while it increases\nwith radial distance for thermal re-emission radiation only and for a\ncombination of scattered stellar radiation and thermal re-emission radiation.\nThe linear polarization degree decreases with increasing disk flaring and\nluminosity of the central star. An increasing inner radius shifts the increase\nof the linear polarization degree further outside, while a larger scale height\nincreases the linear polarization degree for small radial distances and\ndecreases this degree further outside. For longer wavelengths, the linear\npolarization degree converges more slowly.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Mass loss from inhomogeneous hot star winds I. Resonance line formation\n  in 2D models: Small-scale clumping in the winds of hot, massive stars is conventionally\nincluded in spectral analyses by assuming optically thin clumps, a void\ninter-clump medium, and a smooth velocity field. To reconcile investigations of\ndifferent diagnostics within such models, a highly clumped wind with very low\nmass-loss rates needs to be invoked. Particularly, unsaturated UV resonance\nlines seem to indicate rates an order of magnitude (or even more) lower than\npreviously accepted values. We investigate resonance line formation in\ninhomogeneous hot star winds with non-monotonic velocity fields by means of 2D\nstochastic and pseudo-2D radiation-hydrodynamic wind models. A Monte-Carlo\nradiative transfer code is presented and used to produce synthetic line\nspectra. Results: The optically thin clumping limit is only valid for very weak\nlines. For intermediate strong lines, the velocity spans of the clumps are of\ncentral importance. Current hydrodynamical models predict spans that are too\nlarge to reproduce observed profiles unless a very low mass-loss rate is\ninvoked. By simulating lower spans in 2D stochastic models, the profile\nstrengths become drastically reduced, and are consistent with higher mass-loss\nrates. To simultaneously meet the constraints from strong lines, the\ninter-clump medium must be non-void. A first comparison to the observed PV\ndoublet in the O6 supergiant lam Cep confirms that a stochastic 2D model\nreproduces observations with a mass-loss rate roughly ten times higher than\nthat derived from the same lines but assuming optically thin clumping.\nTentatively this may resolve discrepancies between theoretical predictions,\nevolutionary constraints, and recent derived mass-loss rates, and suggests a\nre-investigation of the structure predicted by current hydrodynamical models.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Intermediate Resolution Near-Infrared Spectroscopy of 36 late-M Dwarfs: We present observations of 36 late-M dwarfs obtained with the KeckII/NIRSPEC\nin the J-band at a resolution of \\sim20,000. We have measured projected\nrotational velocities, absolute radial velocities, and pseudo-equivalent widths\nof atomic lines. 12 of our targets did not have previous measurements in the\nliterature.\n  For the other 24 targets, we confirm previously reported measurements. We\nfind that 13 stars from our sample have vsini below our measurement threshold\n(12 km/s) whereas four of our targets are fast rotators (vsini > 30 km/s). As\nfast rotation causes spectral features to be washed out, stars with low\nprojected rotational velocities are sought for radial velocity surveys.\n  At our intermediate spectral resolution we have confirmed the identification\nof neutral atomic lines reported in Mclean et al. 2007. We also calculated\npseudo-equivalent widths (p-EW) of 12 atomic lines. Our results confirm that\nthe p-EW of K I lines are strongly dependent on spectral types. We observe that\nthe p-EW of Fe I and Mn I lines remain fairly constant with later spectral\ntype. We suggest that those lines are particularly suitable for deriving\nmetallicities for late-M dwarfs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Rotational studies in the Orion Nebula Cluster: from solar mass stars to\n  brown dwarfs: Rotational studies at a variety of ages and masses are important for\nconstraining the angular momentum evolution of young stellar objects (YSO). Of\nparticular interest are the very low mass (VLM) stars and brown dwarfs (BDs),\nbecause of the significant lack of known rotational periods in that mass range.\nWe provide for the first time information on rotational periods for a large\nsample of young VLM stars and BDs. This extensive rotational period study in\nthe 1 Myr old Orion Nebula Cluster (ONC) is based on a deep photometric\nmonitoring campaign using the Wide Field Imager (WFI) camera on the ESO/MPG\n2.2m telescope on La Silla, Chile. Accurate I-band photometry of 2908 stars was\nobtained, extending three magnitudes deeper than previous studies in the ONC.\nWe found 487 periodic variables with estimated masses between 0.5 Msun and\n0.015 Msun, 124 of which are BD candidates. This is by far the most extensive\nand complete rotational period data set for young VLM stars and BDs. In\naddition, 808 objects show non-periodic brightness variations. We study the\ndependence of the period distribution on mass and variability level and compare\nthis with known higher mass objects in the ONC (Herbst et al. 2002) and with\nthe 2 Myr old cluster NGC 2264 (Lamm et al., 2004). We find that substellar\nobjects rotate on average faster than the VLM stars. Our rotational data also\nsuggest a dependence of the rotational periods on position within the field,\nwhich can be explained by a possible age spread in the ONC. In addition,\nperiodic variables with larger peak-to-peak amplitudes rotate on average slower\nthan those with small peak-to-peak amplitude variations, which can possibly be\nexplained by different magnetic field topologies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Empirical Estimate of the Shape of the Upstream Heliopause from IBEX-Lo\n  Helium Measurements: Preliminary Results: We present a simplified model of the outer heliosheath to help interpret the\nobservations of interstellar neutral helium by the IBEX-Lo instruments. We\nassume that the measured particles are composed of the superposition of a\nprimary beam population, with the properties of the local interstellar medium,\nand a secondary population, created by charge exchange between the primary beam\nneutrals and the ions that have been deflected as they approach the heliopause.\nWe extract information on the large-scale shape of the heliopause by comparing\nthe helium flux measured at IBEX along four different look directions with\nsimple models of deflected plasma flow around hypothetical obstacles of\ndifferent aspect ratios to the flow. As a first step in this paper, we model\nthe deflected plasma flow with the analytical solutions for compressible gas\nflow around a series of oblate ellipsoidal obstacles. Our comparisons between\nthe model results and the observations indicate that the heliopause is very\nblunt in the vicinity of the heliospheric nose, especially compared to a\nRankine half-body or cometary shape. The upstream heliopause seems to be highly\nelongated in the directions parallel to the interstellar magnetic field, and\nrelatively more compact and symmetric in the directions transverse to that\nfield. The IBEX-Lo helium observations are not consistent with a heliopause\nelongated in directions parallel to the solar rotation axis.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Intensity contrast from MHD simulations and from HINODE observations: Changes in the solar surface area covered by small-scale magnetic elements\nare thought to cause long-term changes in the solar spectral irradiance, which\nare important for determining the impact on Earth's climate. To study the\neffect of small-scale magnetic elements on total and spectral irradiance, we\nderive their contrasts from 3-D MHD simulations of the solar atmosphere. Such\ncalculations are necessary since measurements of small-scale flux tube\ncontrasts are confined to a few wavelengths and suffer from scattered light and\ninstrument defocus, even for space observations. To test the contrast\ncalculations, we compare rms contrasts from simulations with those obtained\nwith the broad-band filter imager mounted on the Solar Optical Telescope (SOT)\nonboard the Hinode satellite and also analyse centre-to-limb variations (CLV).\nThe 3-D MHD simulations include the interaction between convection and magnetic\nflux tubes. They have been run with non-grey radiative transfer using the MURaM\ncode. Simulations have an average vertical magnetic field of 0G, 50G, and 200G.\nEmergent intensities are calculated with the spectral synthesis code ATLAS9 and\nare convolved with a theoretical point-spread function to account for the\nproperties of the observations' optical system. We find reasonable agreement\nfor simulated and observed intensity distributions in the visible continuum\nbands. Agreement is poorer for the CN and G-Bands. The analysis of the\nsimulations exhibits a potentially more realistic centre-to-limb behaviour than\ncalculations based on 1-D model atmospheres. We conclude that starting from 3-D\nMHD simulations represents a powerful approach to obtaining intensity contrasts\nfor a wide wavelength coverage and for different positions on the solar disk.\nThis also paves the way for future calculations of facular and network contrast\nas a function of magnetic fluxes.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spectral Classification; Old and Contemporary: Beginning with a historical account of the spectral classification, its\nrefinement through additional criteria is presented. The line strengths and\nratios used in two dimensional classifications of each spectral class are\ndescribed. A parallel classification scheme for metal-poor stars and the\nstandards used for classification are presented. The extension of spectral\nclassification beyond M to L and T and spectroscopic classification criteria\nrelevant to these classes are described. Contemporary methods of\nclassifications based upon different automated approaches are introduced.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The dynamical importance of binary systems in young massive star\n  clusters: Characterization of the binary fractions in star clusters is of fundamental\nimportance for many fields in astrophysics. Observations indicate that the\nmajority of stars are found in binary systems, while most stars with masses\ngreater than $0.5 M_\\odot$ are formed in star clusters. In addition, since\nbinaries are on average more massive than single stars, in resolved star\nclusters these systems are thought to be good tracers of (dynamical) mass\nsegregation. Over time, dynamical evolution through two-body relaxation will\ncause the most massive objects to migrate to the cluster center, while the\nrelatively lower-mass objects remain in or migrate to orbits at greater radii.\nThis process will globally dominate a cluster's stellar distribution. However,\nclose encounters involving binary systems may disrupt `soft' binaries. This\nprocess will occur more frequently in a cluster's central, dense region than in\nits periphery, which may mask the effects of mass segregation. Using high\nresolution Hubble Space Telescope observations, combined with sophisticated\n$N$-body simulations, we investigate the radial distributions of the\nmain-sequence binary fractions in massive young Large Magellanic Cloud star\nclusters. We show that binary disruption may play an important role on very\nshort timescales, depending on the environmental conditions in the cluster\ncores. This may lead to radial binary fractions that initially decline in the\ncluster centers, which is contrary to the effects expected from dynamical mass\nsegregation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Mouse that Squeaked: A small flare from Proxima Cen observed in the\n  millimeter, optical, and soft X-ray with Chandra and ALMA: We present millimeter, optical, and soft X-ray observations of a stellar\nflare with an energy squarely in the regime of typical X1 solar flares. The\nflare was observed from Proxima Cen on 2019 May 6 as part of a larger\nmulti-wavelength flare monitoring campaign and was captured by Chandra, LCOGT,\ndu Pont, and ALMA. Millimeter emission appears to be a common occurrence in\nsmall stellar flares that had gone undetected until recently, making it\ndifficult to interpret these events within the current multi-wavelength picture\nof the flaring process. The May 6 event is the smallest stellar millimeter\nflare detected to date. We compare the relationship between the soft X-ray and\nmillimeter emission to that observed in solar flares. The X-ray and optical\nflare energies of 10$^{30.3\\pm0.2}$ and 10$^{28.9\\pm0.1}$ erg, respectively,\nthe coronal temperature of T=11.0$\\pm$2.1 MK, and the emission measure of\n9.5$\\pm$2.2 X 10$^{49}$ cm$^{-3}$ are consistent with M-X class solar flares.\nWe find the soft X-ray and millimeter emission during quiescence are consistent\nwith the Gudel-Benz Relation, but not during the flare. The millimeter\nluminosity is >100X higher than that of an equivalent X1 solar flare and lasts\nonly seconds instead of minutes as seen for solar flares.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The evolutionary state of the chemically peculiar members of the open\n  cluster NGC2516: We aim at establishing safe membership and evolutionary status of 11\nchemically peculiar (CP) stars that are residing in the domain of the open\ncluster NGC2516 and are frequently referred to as cluster members. We queried\nthe Gaia EDR3 catalogue in an area with a radius of 1deg and selected 37508\nstars brighter than G=19mag. The cluster membership was determined in\nparallax-proper motion-space and 719 probable and 764 possible members were\nfound. The obtained average astrometric and photometric parameters of the\ncluster are in good agreement with the most recent literature data. The\nevolutionary status of the target stars was determined with respect to Padova\nisochrones. After minor adjustments including the metallicity, the reddening,\nand the transformation scale variation, a perfect fit of the model to the\nobservations over the whole observed magnitude range was achieved. Only 5 of\nthe 11 considered CP stars could be classified as highly probable cluster\nmembers. Among the Ap/Bp stars with previously detected magnetic fields HD65987\nand HD65712 have a high membership probability and the magnetic star CPD-60\n944B is a possible cluster member. Further we discuss the blue straggler nature\nof HD66194 and the magnetic star HD65987. To our knowledge, HD65987 is\ncurrently the only known blue straggler, with a field of the order of a few\nhundred Gauss. The most striking result of our study is that the strongly\nmagnetic A0p star HD66318 with previously reported very low fractional age does\nnot belong to the NGC2516 cluster at a high level of confidence.",
        "category": "astro-ph_SR"
    },
    {
        "text": "KIC 4142768: An Evolved Gamma Doradus/Delta Scuti Hybrid Pulsating\n  Eclipsing Binary with Tidally Excited Oscillations: We present the characterization of KIC 4142768, an eclipsing binary with two\nevolved A-type stars in an eccentric orbit with a period of 14 days. We measure\nthe fundamental parameters of the two components ($M_1=2.05M_{\\odot},\nR_1=2.96R_{\\odot}$ and $M_2=2.05M_{\\odot}, R_2=2.51R_{\\odot}$) by combining\n{\\it Kepler} photometry and spectra from {\\it Keck} HIRES. The measured surface\nrotation rates are only one-fifth of the pseudo-synchronous rate of the\neccentric orbit. Fourier spectrum of the light curve reveals hybrid pulsations\nof $\\delta$ Scuti and $\\gamma$ Doradus type, with pulsation frequencies at\nabout $15-18$ day$^{-1}$ for p modes and about $0.2-1.2$ day$^{-1}$ for\nlow-frequency g modes. Some of the g modes are exact orbital harmonics and are\nlikely tidally excited. Their pulsation amplitudes and phases both agree with\npredictions from the linear tidal theory for $l=2, m=2$ prograde modes. We\nexamine the period spacing patterns in the free oscillating g modes and\nidentify them mostly as prograde sectoral dipole modes. The unstable frequency\nrange and frequency spacing of p modes and the inferred asymptotic g-mode\nperiod spacings both agree with the stellar model for the primary star evolved\nto a late stage of the main sequence. The inferred rotation rate of the\nconvective core boundary is very slow, similar to the small surface rotation\nrate inferred from the spectroscopy. The measured surface and near-core\nrotation rates provide constraints for testing the mechanism of angular\nmomentum transfer and tidal synchronization in evolved eccentric binary star\nsystems.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Amplitude and lifetime of radial modes in red giant star spectra\n  observed by Kepler: Context: the space-borne missions CoRoT and Kepler have provided photometric\nobservations of unprecedented quality. The study of solar-like oscillations\nobserved in red giant stars by these satellites allows a better understanding\nof the different physical processes occurring in their interiors. In\nparticular, the study of the mode excitation and damping is a promising way to\nimprove our understanding of stellar physics that has, so far, been performed\nonly on a limited number of targets. Aims: the recent asteroseismic\ncharacterization of the evolutionary status for a large number of red giants\nallows us to study the physical processes acting in the interior of red giants\nand how they are modify during stellar evolution. In this work, we aim to\nobtain information on the excitation and damping of pressure modes through the\nmeasurement of the stars' pressure mode widths and amplitudes and to analyze\nhow they are modified with stellar evolution. The objective is to bring\nobservational constraints on the modeling of the physical processes behind mode\nexcitation and damping. Methods: we fit the frequency spectra of red giants\nwith well defined evolutionary status using Lorentzians functions to derive the\npressure mode widths and amplitudes. To strengthen our conclusions, we used two\ndifferent fitting techniques. Results: pressure mode widths and amplitudes were\ndetermined for more than 5000 red giants. With a stellar sample two orders of\nmagnitude larger than previous results, we confirmed that the mode width\ndepends on stellar evolution and varies with stellar effective temperature. In\naddition, we discovered that the mode width depends on stellar mass. We also\nconfirmed observationally the influence of the stellar metallicity on the mode\namplitudes, as predicted by models.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Solar Simulations for the Atacama Large Millimeter Observatory Network: The Atacama Large Millimeter/submillimeter Array (ALMA) will be a valuable\ntool for observing the chromosphere of our Sun at (sub-)millimeter wavelengths\nat high spatial, temporal and spectral resolution and as such has great\npotential to address long-standing scientific questions in solar physics. In\norder to make the best use of this scientific opportunity, the Solar\nSimulations for the Atacama Large Millimeter Observatory Network has been\ninitiated. A key goal of this international collaboration is to support the\npreparation and interpretation of future observations of the Sun with ALMA.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The FERRUM project: laboratory-measured transition probabilities for Cr\n  II: Aims: We measure transition probabilities for Cr II transitions from the z\n^4H_J, z ^2D_J, y ^4F_J, and y ^4G_J levels in the energy range 63000 to 68000\ncm^{-1}. Methods: Radiative lifetimes were measured using time-resolved\nlaser-induced fluorescence from a laser-produced plasma. In addition, branching\nfractions were determined from intensity-calibrated spectra recorded with a UV\nFourier transform spectrometer. The branching fractions and radiative lifetimes\nwere combined to yield accurate transition probabilities and oscillator\nstrengths. Results: We present laboratory measured transition probabilities for\n145 Cr II lines and radiative lifetimes for 14 Cr II levels. The\nlaboratory-measured transition probabilities are compared to the values from\nsemi-empirical calculations and laboratory measurements in the literature.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The polluted atmosphere of the white dwarf NLTT 25792 and the diversity\n  of circumstellar environments: We present an analysis of X-Shooter spectra of the polluted, hydrogen-rich\nwhite dwarf NLTT 25792. The spectra show strong lines of calcium (Ca H&K,\nnear-infrared calcium triplet, and Ca I 4226 and numerous lines of iron along\nwith magnesium and aluminum lines from which we draw the abundance pattern.\nMoreover, the photospheric Ca H&K lines are possibly blended with a\ncircumstellar component shifted by -20 km/s relative to the photosphere. A\ncomparison with a sample of four white dwarfs with similar parameters show\nconsiderable variations in their abundance patterns, particularly in the\ncalcium to magnesium abundance ratio that varies by a factor of five within\nthis sample. The observed variations, even after accounting for diffusion\neffects, imply similar variations in the putative accretion source. Also, we\nfind that silicon and sodium are significantly underabundant in the atmosphere\nof NLTT 25792, a fact that may offer some clues on the nature of the accretion\nsource.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evolution and chemical yields of AGB stars: effects of low-temperature\n  opacities: The studies focused on the Thermally-Pulsing Asymptotic Giant Branch phase\nexperienced by low- and intermediate-mass stars are extremely important in many\nastrophysical contexts. In particular, a detailed computation of their chemical\nyields is essential for several issues, ranging from the chemical evolution of\ngalaxies, to the mechanisms behind the formation of globular clusters. Among\nall the uncertainties affecting the theoretical modelling of this phase, and\ndescribed in the literature, it remains to be fully clarified which results are\nseverely affected by the use of inadequate low-temperature opacities, that are\nin most cases calculated on the basis of the original chemical composition of\nthe stars, and do not consider the changes in the surface chemistry due to the\noccurrence of the third dredge-up and hot-bottom burning. Our investigation is\naimed at investigating this point. By means of full evolutionary models\nincluding new set of molecular opacities computed specifically with the AESOPUS\ntool, we highlight which stellar models, among those present in the literature,\nneed a substantial revision, mainly in relation to the predicted chemical\nyields. The interplay among convection, hot bottom burning and the\nlow-temperature opacity treatment is also discussed",
        "category": "astro-ph_SR"
    },
    {
        "text": "Pulsations, eruptions, and evolution of four yellow hypergiants: We aim to explore the variable photometric and stellar properties of four\nyellow hypergiants (YHGs), HR8752, HR 5171A, $\\rho$ Cas, and HD 179821, and\ntheir pulsations of hundreds of days, and long-term variations (LTVs) of years.\nWe tackled multi-colour and visual photometric data sets, looked for\nphotometric indications betraying eruptions or enhanced mass-loss episodes,\ncalculated stellar properties mainly using a published temperature calibration,\nand investigated the nature of LTVs and their influence on quasi-periods and\nstellar properties. The $BV$ photometry revealed a high-opacity layer in the\natmospheres. When the temperature rises the mass loss increases as well,\nconsequently, as the density of the high-opacity layer. As a result, the\nabsorption in $B$ and $V$ grow. The absorption in $B$, presumably of the order\nof one to a few 0\\fm1, is always higher than in $V$. This difference renders\nredder and variable $(B-V)$ colour indexes, but the absorption law is unknown.\nThis property of YHGs is unpredictable and explains why spectroscopic\ntemperatures are always higher than photometric ones. We propose shorter\ndistances for $\\rho$ Cas and HR 5171A than the accepted ones. Therefore, a\ncorrection to decrease the blue luminescence of HR 5171A by polycyclic aromatic\nhydrocarbon (PAH) molecules is necessary, and HR 5171A would no longer be a\nmember of the cluster Gum48d. HR 5171A is only subject to one source of light\nvariation, not by two as the literature suggests. Eruptive episodes of YHGs\nprefer relatively cool circumstances when a red evolutionary loop (RL) has\nshifted the star to the red on the HR diagram. After the eruption, a blue loop\nevolution (BL) is triggered lasting one to a few decades. The reddening episode\nof HR 5171A between 1960 and 1974 was most likely due to a red loop evolution,\nand the reddening after the 1975 eruption was likely due to a shell ejection.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The potential of combining MATISSE and ALMA observations: Constraining\n  the structure of the innermost region in protoplanetary discs: In order to study the initial conditions of planet formation, it is crucial\nto obtain spatially resolved multi-wavelength observations of the innermost\nregion of protoplanetary discs. We evaluate the advantage of combining\nobservations with MATISSE/VLTI and ALMA to constrain the radial and vertical\nstructure of the dust in the innermost region of circumstellar discs in nearby\nstar-forming regions. Based on a disc model with a parameterized dust density\ndistribution, we apply 3D radiative-transfer simulations to obtain ideal\nintensity maps. These are used to derive the corresponding wavelength-dependent\nvisibilities we would obtain with MATISSE as well as ALMA maps simulated with\nCASA. Within the considered parameter space, we find that constraining the dust\ndensity structure in the innermost $5\\,$au around the central star is\nchallenging with MATISSE alone, whereas ALMA observations with reasonable\nintegration times allow us to derive significant constraints on the disc\nsurface density. However, we find that the estimation of the different disc\nparameters can be considerably improved by combining MATISSE and ALMA\nobservations. For example, combining a 30-minute ALMA observation (at 310 GHz\nwith an angular resolution of 0.03$^{\\prime\\prime}$) for MATISSE observations\nin the L and M bands (with visibility accuracies of about $3\\,\\%$) allows the\nradial density slope and the dust surface density profile to be constrained to\nwithin $\\Delta \\alpha=0.3$ and $\\Delta (\\alpha-\\beta)=0.15$, respectively. For\nan accuracy of ${\\sim 1\\%}$ even the disc flaring can be constrained to within\n$\\Delta \\beta=0.1$. To constrain the scale height to within $5\\,$au, M band\naccuracies of $0.8\\,\\%$ are required. While ALMA is sensitive to the number of\nlarge dust grains settled to the disc midplane we find that the impact of the\nsurface density distribution of the large grains on the observed quantities is\nsmall.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Statistical Study of Coronal Mass Ejection Source Locations: II. Role of\n  Active Regions in CME Production: This is the second paper of the statistical study of coronal mass ejection\n(CME) source locations, in which the relationship between CMEs and active\nregions (ARs) is statistically studied on the basis of the information of CME\nsource locations and the ARs automatically extracted from magnetic synoptic\ncharts of Michelson Doppler Imager (MDI) during 1997 -- 1998. It is found that\nabout 63% of the CMEs are related with ARs, at least about 53% of the ARs\nproduced one or more CMEs, and particularly about 14% of ARs are CME-rich (3 or\nmore CMEs were generated) during one transit across the visible disk. Several\nissues are then tried to clarify: whether or not the CMEs originating from ARs\nare distinct from others, whether or not the CME kinematics depend on AR\nproperties, and whether or not the CME productivity depends on AR properties.\nThe statistical results suggest that (1) there is no evident difference between\nAR-related and non-AR-related CMEs in terms of CME speed, acceleration and\nwidth, (2) the size, strength and complexity of ARs do little with the\nkinematic properties of CMEs, but have significant effects on the CME\nproductivity, and (3) the sunspots in all the most productive ARs at least\nbelong to $\\beta\\gamma$ type, whereas 90% of those in CME-less ARs are $\\alpha$\nor $\\beta$ type only. A detailed analysis on CME-rich ARs further reveals that\n(1) the distribution of the waiting time of same-AR CMEs, consists of two parts\nwith a separation at about 15 hours, which implies that the CMEs with a waiting\ntime shorter than 15 hours are probably truly physical related, and (2) an AR\ntends to produce such related same-AR CMEs at a pace of 8 hours, but cannot\nproduce two or more fast CMEs (>800 km/s) within a time interval of 15 hours.\nThis interesting phenomenon is particularly discussed.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A critical analysis of three near-infrared photometric methods of\n  estimating distances to cataclysmic variables: A critical analysis of three methods of estimating distances to cataclysmic\nvariables (CVs) is performed. These methods, by Ak et al., Beuermann, and\nKnigge, all use near-infrared (JHK or Ks) magnitudes and the Barnes-Evans\nrelation. We compare all three methods to distances measured by trigonometric\nparallax by Thorstensen, with Hubble Space Telescope, and with the HIPPARCOS\nspacecraft.\n  We find that the method of Ak et al. works best overall for all CVs,\npredicting distances on the average 4% less than those measured by\ntrigonometric parallaxes. The method of Beuermann overestimates distances by\n52%. The method of Knigge underestimates distances by 26%, although this was\nonly ever meant as a lower limit, since it assumes all light comes from the\nsecondary star.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The $UBV$ Color Evolution of Classical Novae. III. Time-Stretched\n  Color-Magnitude Diagram of Novae in Outburst: We propose a modified color-magnitude diagram for novae in outburst, i.e.,\n$(B-V)_0$ versus $(M_V-2.5 \\log f_{\\rm s})$, where $f_{\\rm s}$ is the\ntimescaling factor of a (target) nova against a comparison (template) nova,\n$(B-V)_0$ is the intrinsic $B-V$ color, and $M_V$ is the absolute $V$\nmagnitude. We dub it the time-stretched color-magnitude diagram. We carefully\nreanalyzed 20 novae based on the time-stretching method and revised their\nextinctions $E(B-V)$, distance moduli in the $V$ band $(m-M)_V$, distances $d$,\nand timescaling factors $f_{\\rm s}$ against the template nova LV Vul. We have\nfound that these 20 nova outburst tracks broadly follow one of the two template\ntracks, LV Vul/V1668 Cyg or V1500 Cyg/V1974 Cyg group, in the time-stretched\ncolor-magnitude diagram. In addition, we estimate the white dwarf masses and\n$(m-M)_V$ of the novae by directly fitting the absolute $V$ model light curves\n($M_V$) with observational apparent $V$ magnitudes ($m_V$). A good agreement in\nthe two estimates of $(m-M)_V$ confirms the consistency of the time-stretched\ncolor-magnitude diagram. Our distance estimates are in good agreement with the\nresults of Gaia Data Release 2.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Fundamental Parameters of Main-Sequence Stars in an Instant with Machine\n  Learning: Owing to the remarkable photometric precision of space observatories like\nKepler, stellar and planetary systems beyond our own are now being\ncharacterized en masse for the first time. These characterizations are pivotal\nfor endeavors such as searching for Earth-like planets and solar twins,\nunderstanding the mechanisms that govern stellar evolution, and tracing the\ndynamics of our Galaxy. The volume of data that is becoming available, however,\nbrings with it the need to process this information accurately and rapidly.\nWhile existing methods can constrain fundamental stellar parameters such as\nages, masses, and radii from these observations, they require substantial\ncomputational efforts to do so.\n  We develop a method based on machine learning for rapidly estimating\nfundamental parameters of main-sequence solar-like stars from classical and\nasteroseismic observations. We first demonstrate this method on a\nhare-and-hound exercise and then apply it to the Sun, 16 Cyg A & B, and 34\nplanet-hosting candidates that have been observed by the Kepler spacecraft. We\nfind that our estimates and their associated uncertainties are comparable to\nthe results of other methods, but with the additional benefit of being able to\nexplore many more stellar parameters while using much less computation time. We\nfurthermore use this method to present evidence for an empirical diffusion-mass\nrelation. Our method is open source and freely available for the community to\nuse.\n  The source code for all analyses and for all figures appearing in this\nmanuscript can be found electronically at\nhttps://github.com/earlbellinger/asteroseismology",
        "category": "astro-ph_SR"
    },
    {
        "text": "CoRoT-TESS eclipsing binaries with light-travel-time effect: Identifying long-period eclipsing binaries with space-based photometry is\nstill a challenge even in the century of space telescopes due to the relatively\nshort observation sequences and short lifetime of these missions. The\nTransiting Exoplanet Survey Satellite (TESS) space telescope is an appropriate\ntool to supplement previous space-based observations. In this paper we report\nthe first results of the eclipse timing variation (ETV) analyses of eclipsing\nbinaries (EBs) measured by CoRoT and TESS space telescopes. Among the 1428 EB\ncandidates we found 4 new potential triple candidates, for which ETV was\nanalysed and fitted by the well-known light-travel-time effect (LTTE). One of\nthem shows significant phase shift in its folded light curve which required\nextra care. In this paper we also present some other systems showing\nsignificant ETV signals that could be explained by mass transfer or apsidal\nmotion.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Numerical Investigation of a Coronal Mass Ejection from an Anemone\n  Active Region: Reconnection and Deflection of the 2005 August 22 Eruption: We present a numerical investigation of the coronal evolution of a coronal\nmass ejection (CME) on 2005 August 22 using a 3-D thermodynamics\nmagnetohydrodynamic model, the SWMF. The source region of the eruption was\nanemone active region (AR) 10798, which emerged inside a coronal hole. We\nvalidate our modeled corona by producing synthetic extreme ultraviolet (EUV)\nimages, which we compare to EIT images. We initiate the CME with an\nout-of-equilibrium flux rope with an orientation and chirality chosen in\nagreement with observations of a H-alpha filament. During the eruption, one\nfootpoint of the flux rope reconnects with streamer magnetic field lines and\nwith open field lines from the adjacent coronal hole. It yields an eruption\nwhich has a mix of closed and open twisted field lines due to interchange\nreconnection and only one footpoint line-tied to the source region. Even with\nthe large-scale reconnection, we find no evidence of strong rotation of the CME\nas it propagates. We study the CME deflection and find that the effect of the\nLorentz force is a deflection of the CME by about 3 deg/Rsun towards the East\nduring the first 30 minutes of the propagation. We also produce coronagraphic\nand EUV images of the CME, which we compare with real images, identifying a\ndimming region associated with the reconnection process. We discuss the\nimplication of our results for the arrival at Earth of CMEs originating from\nthe limb and for models to explain the presence of open field lines in magnetic\nclouds.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Acceleration and propagation of Solar Energetic Particles: Solar Energetic Particles (SEPs) are an important component of Space Weather,\nincluding radiation hazard to humans and electronic equipment, and the\nionisation of the Earth's atmosphere. We review the key observations of SEPs,\nour current understanding of their acceleration and transport, and discuss how\nthis knowledge is incorporated within Space Weather forecasting tools.\nMechanisms for acceleration during solar flares and at shocks driven by Coronal\nMass Ejections are discussed, as well as the timing relationships between\nsignatures of solar eruptive events and the detection of SEPs in interplanetary\nspace. Evidence on how the parameters of SEP events are related to those of the\nparent solar activity is reviewed and transport effects influencing SEP\npropagation to near-Earth locations are examined. Finally, the approaches to\nforecasting Space Weather SEP effects are discussed. We conclude that both\nflare and CME shock acceleration contribute to Space Weather relevant SEP\npopulations and need to be considered within forecasting tools.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Metric Radio Bursts and Fine Structures Observed on January 20, 2005: A major radio event, associated with an X7.1/2B flare in AR720 and a fast CME\nwas observed on January 20, 2005 with the radio-spectrograph ARTEMIS-IV; it was\nparticularly intense and with a complex radio signature with rich fine\nstructure which was recorded in the 270-420 MHz range at high resolution (100\nsamples/sec). The fine structure is compared with similar results in the\ndecimetric and microwave frequency range. It was found to approximately match\nthe comprehensive Ondrejov Classification in the spectral range 0.8-2 GHz.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Lorentz Force Evolution Reveals the Energy Buildup Processes during\n  Recurrent Eruptive Solar Flares: The energy release and build-up processes in the solar corona have\nsignificant implications in particular for the case of large recurrent flares,\nwhich pose challenging questions about the conditions that lead to the episodic\nenergy release processes. It is not yet clear whether these events occur due to\nthe continuous supply of free magnetic energy to the solar corona or because\nnot all of the available free magnetic energy is released during a single major\nflaring event. In order to address this question, we report on the evolution of\nphotospheric magnetic field and the associated net Lorentz force changes in ARs\n11261 and 11283, each of which gave rise to recurrent eruptive M- and X-class\nflares. Our study reveals that after the abrupt downward changes during each\nflare, the net Lorentz force increases by (2-5)x10^22 dyne in between the\nsuccessive flares. This distinct rebuild-up of net Lorentz forces is the first\nobservational evidence found in the evolution of any non-potential parameter of\nsolar active regions (ARs), which suggests that new energy was supplied to the\nARs in order to produce the recurrent large flares. The rebuild-up of magnetic\nfree energy of the ARs is further confirmed by the observations of continuous\nshearing motion of moving magnetic features of opposite polarities near the\npolarity inversion line. The evolutionary pattern of the net Lorentz force\nchanges reported in this study has significant implications, in particular, for\nthe forecasting of recurrent large eruptive flares from the same AR and hence\nthe chances of interaction between the associated CMEs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A New Transition Wolf-Rayet WN/C Star in the Milky Way: We report the discovery of a new transition type Wolf-Rayet (WR) WN/C star in\nthe Galaxy. According to its coordinates (R.A., Dec)J2000 = 18h51m39.7s,\n-05d34m51.1s, and the distance (7.11 kpc away from Earth) inferred from the\nsecond Gaia, data release, it's found that WR 121-16 is located in the Far 3\nkpc Arm, and it is 3.75 kpc away from the Galactic Center. The optical spectra\nobtained by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope\n(LAMOST) and the 2.16 m telescope, both located at the Xinglong Observatory in\nChina, indicate that this is a WR star of the transitional WN7o/WC subtype. A\ncurrent stellar mass of about 7.1 M_solar, a mass-loss rate of M_dot =\n10^(-4.97) M_solar/yr, a bolometric luminosity of log L/L_solar = 4.88, and a\nstellar temperature of T_* = 47 kK are derived, by fitting the observed\nspectrum with a specific Potsdam Wolf-Rayet (PoWR) model. The magnitude in\nV-band varies between 13.95 and 14.14 mag, while no period is found. Based on\nthe optical spectra, the time domain data, and the indices of the astrometric\nsolution of the Gaia data, WR 121-16 is likely a transitional WN/C single star\nrather than a WN+WC binary.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The RR Lyrae Variable Population in the Phoenix Dwarf Galaxy: We present the first detailed study of the RR Lyrae variable population in\nthe Local Group dSph/dIrr transition galaxy, Phoenix, using previously obtained\nHST/WFPC2 observations of the galaxy. We utilize template light curve fitting\nroutines to obtain best fit light curves for RR Lyrae variables in Phoenix. Our\ntechnique has identified 78 highly probable RR Lyrae stars (54 ab-type; 24\nc-type) with about 40 additional candidates. We find mean periods for the two\npopulations of $\\langle P_{ab}\\rangle = 0.60 \\pm 0.03$ days and $\\langle\nP_{c}\\rangle = 0.353 \\pm 0.002$ days. We use the properties of these light\ncurves to extract, among other things, a metallicity distribution function for\nab-type RR Lyrae. Our analysis yields a mean metallicity of $\\langle\n[Fe/H]\\rangle = -1.68 \\pm 0.06$ dex for the RRab stars. From the mean period\nand metallicity calculated from the ab-type RR Lyrae, we conclude that Phoenix\nis more likely of intermediate Oosterhoff type; however the morphology of the\nBailey diagram for Phoenix RR Lyraes appears similar to that of an Oosterhoff\ntype I system. Using the RRab stars, we also study the chemical enrichment law\nfor Phoenix. We find that our metallicity distribution is reasonably well\nfitted by a closed-box model. The parameters of this model are compatible with\nthe findings of Hidalgo et al. (2009) further supporting the idea that Phoenix\nappears to have been chemically enriched as a closed-box-like system during the\nearly stage of its formation and evolution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Constraining general massive-star physics by exploring the unique\n  properties of magnetic O-stars: Rotation, macroturbulence, and sub-surface\n  convection: A quite remarkable aspect of non-interacting O-stars with detected surface\nmagnetic fields is that they all are very slow rotators. This paper uses this\nunique property to first demonstrate that the projected rotational speeds of\nmassive, hot stars, as derived using current standard spectroscopic techniques,\ncan be severely overestimated when significant \"macroturbulent\" line-broadening\nis present. This may, for example, have consequences for deriving the\nstatistical distribution of rotation rates in massive-star populations, and for\nthe use of these rates in stellar evolution models. It is next shown how such\nmacroturbulence (seemingly a universal feature of hot, massive stars) is\npresent in all but one of the magnetic O-stars, namely NGC 1624-2. Assuming\nthen a simple model in which NGC 1624-2's exceptionally strong, large-scale\nmagnetic field suppresses atmospheric motions down to layers where the magnetic\nand gas pressures are comparable, first empirical constraints on the formation\ndepth of this enigmatic hot-star macroturbulence are derived. The results\nsuggest an origin in the thin sub-surface convection zone of massive stars,\nconsistent with a physical origin due to, e.g., stellar pulsations excited by\nthe convective motions.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Coadded Spectroscopic Stellar Parameters and Abundances from the LAMOST\n  Low Resolution Survey: I combine duplicate spectroscopic stellar parameter estimates in the Large\nSky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data Release 6 Low\nResolution Spectral Survey A, F, G, and K Type stellar parameter catalog.\nCombining repeat measurements results in a factor of two improvement in the\nprecision of the spectroscopic stellar parameter estimates. Moreover, this\ntrivializes the process of performing coordinate-based cross-matching with\nother catalogs. Similarly, I combine duplicate stellar abundance estimates for\nthe Xiang et al. catalog which was produced using LAMOST Data Release 5 Low\nResolution Spectral Survey data. These data have numerous applications in\nstellar, galactic, and exoplanet astronomy. The catalogs I produce are\navailable as machine-readable tables at https://doi.org/10.7281/T1/QISGRU .",
        "category": "astro-ph_SR"
    },
    {
        "text": "Correlation of the sunspot number and the waiting time distribution of\n  solar flares, coronal mass ejections, and solar wind switchback events\n  observed with the Parker Solar Probe: Waiting time distributions of solar flares and {\\sl coronal mass ejections\n(CMEs)} exhibit power law-like distribution functions with slopes in the range\nof $\\alpha_{\\tau} \\approx 1.4-3.2$, as observed in annual data sets during 4\nsolar cycles (1974-2012). We find a close correlation between the waiting time\npower law slope $\\alpha_\\tau$ and the {\\sl sunspot number (SN)}, i.e.,\n$\\alpha_\\tau$ = 1.38 + 0.01 $\\times$ SN. The waiting time distribution can be\nfitted with a Pareto-type function of the form $N(\\tau) = N_0$ $(\\tau_0 +\n\\tau)^{-\\alpha_{\\tau}}$, where the offset $\\tau_0$ depends on the instrumental\nsensitivity, the detection threshold of events, and pulse pile-up effects. The\ntime-dependent power law slope $\\alpha_{\\tau}(t)$ of waiting time distributions\ndepends only on the global solar magnetic flux (quantified by the sunspot\nnumber) or flaring rate, independent of other physical parameters of {\\sl\nself-organized criticality (SOC)} or {\\sl magneto-hydrodynamic (MHD)}\nturbulence models. Power law slopes of $\\alpha_{\\tau}\\approx 1.2-1.6$ were also\nfound in solar wind switchback events, as observed with the {\\sl Parker Solar\nProbe (PSP)}. We conclude that the annual variability of switchback events in\nthe heliospheric solar wind is modulated by flare and CME rates originating in\nthe photosphere and lower corona.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Near Infrared studies of the carbon-monoxide and dust forming nova V5668\n  Sgr: We present near-infrared (NIR) observations of Nova V5668 Sgr, discovered in\noutburst on 2015 March 15.634 UT, between 2d to 107d after outburst. NIR\nspectral features are used to classify it as a FeII class of nova. The spectra\nfollow the evolution of the spectral lines from a P Cygni stage to a pure\nemission phase where the shape of the profiles suggests the presence of a\nbipolar flow. A notable feature is the presence of carbon monoxide first\novertone bands which are seen in emission. The CO emission is modeled to make\nestimates of the mass, temperature and column density to be (0.5--2.0)$\\times$\n10$^{-8}$ M$_\\odot$, 4000 $\\pm$ 300K and (0.36--1.94)$\\times$ 10$^{19}$\ncm$^{-2}$ respectively. The $^{12}$C/$^{13}$C ratio is estimated to be $\\sim$\n1.5. V5668 Sgr was a strong dust producer exhibiting the classical deep dip in\nits optical light curve during dust formation. Analysis of the dust SED yields\na dust mass of 2.7 $\\times$ 10${^{\\rm -7}}$ $M_\\odot $, a blackbody angular\ndiameter of the dust shell of 42 mas and a distance estimate to the nova of\n1.54 kpc which agrees with estimates made from MMRD relations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Investigating 2MASS J06593158-0405277: a FUor burst in a triple system?: FUor outbursts in young stellar objects (YSOs) are the most dramatic events\namong episodic accretion phenomena. The origin of these bursts is not clear:\ndisk instabilities and/or disk perturbations by an external body being the most\nviable hypotheses. Here, we report our VLT/SINFONI high angular resolution\nAO-assisted observations of 2MASS J06593158-0405277, which is undergoing a\nrecently discovered FUor outburst. Our observations reveal the presence of an\nextended disc-like structure around the FUor, a very low-mass companion (2MASS\nJ06593158-0405277B) at ~100 au in projection, and, possibly, a third closer\ncompanion at ~11 au. These sources appear to be young, displaying accretion\nsignatures. Assuming the components are physically linked, 2MASS\nJ06593158-0405277 would then be one of the very few triple systems observed in\nFUors.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Ultrashort-period MS eclipsing systems. New observations and light curve\n  solutions of six NSVS binaries: We carried out photometric and low-resolution spectral observations of six\neclipsing ultrashort-period binaries with MS components. The light curve\nsolutions of the Rozhen observations show that all targets are overcontact\nsystems. We found well-defined empirical relation \"period -- semi-major axis\"\nfor the short-period binaries and used it for estimation of the global\nparameters of the targets. Our results revealed that NSVS 925605 is quite\ninteresting target: (a) it is one of a few contact binaries with M components;\n(b) it exhibits high activity (emission in H$\\alpha$ line, X-ray emission,\nlarge cool spots, non-Planck energy distribution); (c) its components differ in\ntemperature by 700 K. All appearances of high magnetic activity and huge\nfillout factor (0.7) of NSVS 925605 might be assumed as a precursor of the\npredicted merging of close magnetic binaries. Another unusual binary is NSVS\n2700153 which reveals considerable long-term variability.",
        "category": "astro-ph_SR"
    },
    {
        "text": "First Observation of Chromospheric Waves in a Sunspot by DKIST/ViSP: The\n  Anatomy of an Umbral Flash: The Visible Spectro-Polarimeter (ViSP) of the NSF Daniel K. Inouye Solar\nTelescope (DKIST) collected its Science Verification data on May 7-8, 2021. The\ninstrument observed multiple layers of a sunspot atmosphere simultaneously, in\npassbands of Ca-II 397 nm (H-line), Fe-I 630 nm, and Ca-II 854 nm, scanning the\nregion with a spatial sampling of 0.041\" and average temporal cadence of 7.76\nseconds, for a 38.8 minute duration. The slit moves southward across the\nplane-of-the-sky at 3.83 km/s. The spectropolarimetric scans exhibit prominent\noscillatory 'ridge' structures which lie nearly perpendicular to the direction\nof slit motion (north to south). These ridges are visible in maps of line\nintensity, central wavelength, line width, and both linear and circular\npolarizations. Contemporaneous Atmospheric Imaging Assembly observations\nindicate these ridges are purely temporal in character and likely attributed to\nthe familiar chromospheric 3-minute umbral oscillations. We observe in detail a\nsteady umbral flash near the center of the sunspot umbra. Although bad seeing\nlimited the spatial resolution, the unique high signal-to-noise enable us to\nestimate the shock Mach numbers (= 2), propagation speeds (= 9 km/s), and their\nimpact on longitudinal magnetic field (delta B = 50 G), gas pressure, and\ntemperature (delta T/T = 0.1) of the subshocks over 30 seconds. We also find\nevidence for rarefaction waves situated between neighboring wave-train shocks.\nThe Ca-II 854 nm line width is steady throughout the umbral flash except for a\nsharp 1.5 km/s dip immediately before, and comparable spike immediately after,\nthe passage of the shock front. This zig-zag in line width is centered on the\nsubshock and extends over 0.4\".",
        "category": "astro-ph_SR"
    },
    {
        "text": "Validating Forward Modeling and Inversions of Helioseismic Holography\n  Measurements: Here we use synthetic data to explore the performance of forward models and\ninverse methods for helioseismic holography. Specifically, this work presents\nthe first comprehensive test of inverse modeling for flows using\nlateral-vantage (deep-focus) holography. We derive sensitivity functions in the\nBorn approximation. We then use these sensitivity functions in a series of\nforward models and inversions of flows from a publicly available\nmagnetohydrodynamic quiet-Sun simulation. The forward travel times computed\nusing the kernels generally compare favorably with measurements obtained by\napplying holography, in a lateral-vantage configuration, on a 15-hour time\nseries of artificial Dopplergrams extracted from the simulation. Inversions for\nthe horizontal flow components are able to reproduce the flows in the upper 3Mm\nof the domain, but are compromised by noise at greater depths.",
        "category": "astro-ph_SR"
    },
    {
        "text": "New Parallaxes for the Upper Scorpius OB Association: Upper Scorpius is a subgroup of the nearest OB association,\nScorpius--Centaurus. Its young age makes it an important association to study\nstar and planet formation. We present parallaxes to 52 low mass stars in Upper\nScorpius, 28 of which have full kinematics. We measure ages of the individual\nstars by combining our measured parallaxes with pre-main sequence evolutionary\ntracks. We find there is a significant difference in the ages of stars with and\nwithout circumstellar disks. The stars without disks have a mean age of\n4.9+/-0.8 Myr and those with disks have an older mean age of 8.2+/-0.9 Myr.\nThis somewhat counterintuitive result suggests that evolutionary effects in\nyoung stars can dominate their apparent ages. We also attempt to use the 28\nstars with full kinematics (i.e.\\ proper motion, radial velocity, and parallax)\nto trace the stars back in time to their original birthplace to obtain a\ntrackback age. We find, as expected given large measurement uncertainties on\navailable radial velocity measurements, that measurement uncertainties alone\ncause the group to diverge after a few Myr.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Distances of CVs and related objects derived from Gaia Data Release 1: We consider the parallaxes of sixteen cataclysmic variables and related\nobjects that are included in the TGAS catalogue, which is part of the Gaia\nfirst data release, and compare these with previous parallax measurements. The\nparallax of the dwarf nova SS Cyg is consistent with the parallax determination\nmade using the VLBI, but with only one of the analyses of the HST Fine Guidance\nSensor (FGS) observations of this system. In contrast, the Gaia parallaxes of\nV603 Aql and RR Pic are broadly consistent, but less precise than the HST/FGS\nmeasurements. The Gaia parallaxes of IX Vel, V3885 Sgr, and AE Aqr are\nconsistent with, but much more accurate than the Hipparcos measurements. We\ntake the derived Gaia distances and find that absolute magnitudes of\noutbursting systems show a weak correlation with orbital period. For systems\nwith measured X-ray fluxes we find that the X-ray luminosity is a clear\nindicator of whether the accretion disc is in the hot and ionised or cool and\nneutral state. We also find evidence for the X-ray emission of both low and\nhigh state discs correlating with orbital period, and hence the long-term\naverage accretion rate. The inferred mass accretion rates for the nova-like\nvariables and dwarf novae are compared with the critical mass accretion rate\npredicted by the Disk Instability Model. While we find agreement to be good for\nmost systems there appears to be some uncertainty in the system parameters of\nSS Cyg. Our results illustrate how future Gaia data releases will be an\nextremely valuable resource in mapping the evolution of cataclysmic variables.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Photometric Investigation of V921 Her using the Lunar-based\n  Ultraviolet Telescope of Chang'e-3 mission: The light curve of V921 Her in ultraviolet band observed by the Lunar-based\nUltraviolet Telescope (LUT) is analyzed by the Wilson-Devinney code. Our\nsolutions conclude that V921 Her is an early type marginal contact binary\nsystem with an additional close-in component. The binary system is under poor\nthermal contact with a temperature difference of nearly $700K$ between the two\ncomponents. The close-in component contributes about $19\\,\\%$ of the total\nluminosity in the triple system. Combining the radial velocity study together\nwith our photometric solutions, the mass of the primary star and secondary one\nare calculated to be $M_1 = 1.784(\\pm0.055)M_\\odot$, $M_2 =\n0.403(\\pm0.012)M_\\odot$. The evolutionary scenario of V921 Her is discussed.\nAll times of light minimum of V921 Her available in the bibliography are taken\ninto account and the $O - C$ curve is analyzed for the first time. The most\nprobable fitting results are discussed in the paper, which also confirm the\nexistence of a third component ($P_3=10.2$ year) around the binary system. The\nperiod of V921 Her is also undergoing a continuously rapid increase at a rate\nof $dP/dt=+2.79\\times{10^{-7}}day\\cdot year^{-1}$, which may due to mass\ntransfer from the less massive component to the more massive one.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The circumbinary rings of GG Carinae: indications of disc eccentricity\n  growth in the B[e] supergiant's atomic emission lines: B[e] supergiants have unusual circumstellar environments which may include\nthin, concentric rings displaying atomic line emission. GG Carinae is a B[e]\nsupergiant binary which exhibits such a geometry in its circumbinary\nenvironment. We study atomic emission lines arising from GG Carinae's\ncircumbinary disc in FEROS spectra collected between 1998 and 2015. We find\nthat semi-forbidden Fe\\,II] and permitted Ca\\,II emission are formed in the\nsame thin circumbinary ring previously reported to have forbidden [O\\,I] and\n[Ca\\,II] emission. We find that there are two circumbinary rings orbiting with\nprojected velocities of $84.6\\pm1.0$\\,km\\,s$^{-1}$ and\n$27.3\\pm0.6$\\,km\\,s$^{-1}$. Deprojecting these velocities from the\nline-of-sight, and using updated binary masses presented by\n\\cite{Porter2021GGPhotometry}, we find that the radii of the circumbinary rings\nare $2.8^{+0.9}_{-1.1}$\\,AU and $27^{+9}_{-10}$\\,AU for the inner ring and\nouter ring respectively. We find evidence of subtle dynamical change in the\ninner circumbinary ring over the 17 years spanned by the data, manifesting in\nvariability in the ratio of the intensity of the blueshifted peak to the\nredshifted peak of its emission lines and the central velocity becoming more\nblueshifted. We perform smoothed-particle hydrodynamic simulations of the\nsystem which suggest that these observed changes are consistent with pumping of\nthe eccentricity of a radially thin circumbinary ring by the inner binary. We\nfind a systemic velocity of the GG Carinae system of $-23.2 \\pm\n0.4$\\,km\\,s$^{-1}$.",
        "category": "astro-ph_SR"
    },
    {
        "text": "ASAS Light Curves of Intermediate Mass Eclipsing Binaries and the\n  Parameters of HI Mon: We present a catalog of 56 candidate intermediate mass eclipsing binary\nsystems extracted from the 3rd data release of the All Sky Automated Survey. We\ngather pertinent observational data and derive orbital properties, including\nephemerides, for these systems as a prelude to anticipated spectroscopic\nobservations. We find that 37 of the 56, or ~66% of the systems are not\nidentified in the Simbad Astronomical Database as known binaries. As a specific\nexample, we show spectroscopic data obtained for the system HI Mon (B0 V + B0.5\nV) observed at key orbital phases based on the computed ephemeris, and we\npresent a combined spectroscopic and photometric solution for the system and\ngive stellar parameters for each component.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Particle acceleration by circularly and elliptically polarised\n  dispersive Alfven waves in a transversely inhomogeneous plasma in the\n  inertial and kinetic regimes: Dispersive Alfven waves (DAWs) offer, an alternative to magnetic\nreconnection, opportunity to accelerate solar flare particles. We study the\neffect of DAW polarisation, L-, R-, circular and elliptical, in different\nregimes inertial and kinetic on the efficiency of particle acceleration. We use\n2.5D PIC simulations to study how particles are accelerated when DAW, triggered\nby a solar flare, propagates in transversely inhomogeneous plasma that mimics\nsolar coronal loop. (i) In inertial regime, fraction of accelerated electrons\n(along the magnetic field), in density gradient regions is ~20% by the time\nwhen DAW develops 3 wavelengths and is increasing to ~30% by the time DAW\ndevelops 13 wavelengths. In all considered cases ions are heated in transverse\nto the magnetic field direction and fraction of the heated particles is ~35%.\n(ii) The case of R-circular, L- and R- elliptical polarisation DAWs, with the\nelectric field in the non-ignorable transverse direction exceeding several\ntimes that of in the ignorable direction, produce more pronounced parallel\nelectron beams and transverse ion beams in the ignorable direction. In the\ninertial regime such polarisations yield the fraction of accelerated electrons\n~20%. In the kinetic regime this increases to ~35%. (iii) The parallel electric\nfield that is generated in the density inhomogeneity regions is independent of\nm_i/m_e and exceeds the Dreicer value by 8 orders of magnitude. (iv) Electron\nbeam velocity has the phase velocity of the DAW. Thus electron acceleration is\nvia Landau damping of DAWs. For the Alfven speeds of 0.3c the considered\nmechanism can accelerate electrons to energies circa 20 keV. (v) The increase\nof mass ratio from m_i/m_e=16 to 73.44 increases the fraction of accelerated\nelectrons from 20% to 30-35% (depending on DAW polarisation). For the mass\nratio m_i/m_e=1836 the fraction of accelerated electrons would be >35%.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A CCD Search for Variable Stars of Spectral Type B in the Northern\n  Hemisphere Open Clusters. VII. NGC 1502: We present results of variability search in the field of the young open\ncluster NGC 1502. Eight variable stars were discovered. Of six other stars in\nthe observed field that were suspected for variability, we confirm variability\nof two, including one beta Cep star, NGC 1502-26. The remaining four suspects\nwere found to be constant in our photometry. In addition, UBVI photometry of\nthe well-known massive eclipsing binary SZ Cam was obtained. The new variable\nstars include: two eclipsing binaries of which one is a relatively bright\ndetached system with an EA-type light curve, an alpha2 CVn-type variable, an\nSPB candidate, a field RR Lyrae star and three other variables showing\nvariability of unknown origin. The variability of two of them is probably\nrelated to their emission in Halpha, which has been measured by means of the\nalpha index obtained for 57 stars brighter than V=16 mag in the central part of\nthe observed field. Four other non-variable stars with emission in Halpha were\nalso found. Additionally, we provide VI photometry for stars down to V=17 mag\nand UB photometry for about 50 brightest stars in the observed field. We also\nshow that the 10-Myr isochrone fits very well the observed color-magnitude\ndiagram if a distance of 1 kpc and mean reddening, E(V-I)=0.9 mag, are adopted.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Long-Term Measurements of Sunspot Magnetic Tilt Angles: Tilt angles of close to 30,600 sunspots are determined using Mount Wilson\ndaily averaged magnetograms taken from 1974 to 2012, and MDI/SoHO magnetograms\ntaken from 1996 to 2010. Within a cycle, more than 90% of sunspots have a\nnormal polarity alignment along the east-west direction following Hale's law.\nThe median tilts increase with increasing latitude (Joy's law) at a rate of\n~0.5 degree per degree of latitude. Tilt angles of spots appear largely\ninvariant with respect to time at a given latitude, but they decrease by\n~0.9degree per year on average, a trend which largely reflects Joy's law\nfollowing the butterfly diagram. We find an asymmetry between the hemispheres\nin the mean tilt angles. On average, the tilts are greater in the southern than\nin the northern hemisphere for all latitude zones, and the differences increase\nwith increasing latitude.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Iron abundance in the prototype PG1159 star, GW Vir pulsator PG1159-035,\n  and related objects: We performed an iron abundance determination of the hot, hydrogen deficient\npost-AGB star PG1159-035, which is the prototype of the PG1159 spectral class\nand the GW Vir pulsators, and of two related objects (PG1520+525, PG1144+005),\nbased on the first detection of Fe VIII lines in stellar photospheres. In\nanother PG1159 star, PG1424+535, we detect Fe VII lines. In all four stars,\neach within Teff = 110,000 - 150,000 K, we find a solar iron abundance. This\nresult agrees with our recent abundance analysis of the hottest PG1159 stars\n(Teff = 150,000 - 200,000 K) that exhibit Fe X lines. On the whole, we find\nthat the PG1159 stars are not significantly iron deficient, in contrast to\nprevious notions.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Torsional oscillations of nonbare strange stars: Strange stars are one of the possible compact stellar objects that can be\nformed after a supernova collapse. We consider a model of strange star having\nan inner core in the color-flavor locked phase surmounted by a crystalline\ncolor superconducting layer. These two phases constitute the {\\it quarksphere},\nwhich we assume to be the largest and heaviest part of the strange star. The\nnext layer consists of standard nuclear matter forming a ionic crust, hovering\non the top of the quarksphere and prevented from falling by a strong dipolar\nelectric field. The dipolar electric field arises because quark matter is\nconfined in the quarksphere by the strong interaction, but electrons can leak\noutside forming a few hundreds Fermi thick electron layer separating the ionic\ncrust from the underlying quark matter. The ionic matter and the crystalline\ncolor superconducting matter constitute two electromagnetically coupled crust\nlayers. We study the torsional oscillations of these two layers. Remarkably, we\nfind that if a fraction larger than $10^{-4}$ of the energy of a Vela-like\nglitch is conveyed to a torsional oscillation, the ionic crust will likely\nbreak. The reason is that the very rigid and heavy crystalline color\nsuperconducting crust layer will absorb only a small fraction of the glitch\nenergy, leading to a large amplitude torsional oscillation of the ionic crust.",
        "category": "astro-ph_SR"
    },
    {
        "text": "ASASSN-18di: discovery of a $\u0394V \\sim 10$ flare on a mid-M dwarf: We report and characterize a white-light superflare on a previously\nundiscovered M dwarf detected by the ASAS-SN survey. Employing various\ncolor-magnitude and color-spectral type relationships, we estimate several\nstellar parameters, including the quiescent V-band magnitude, from which we\nderive a flare amplitude of $\\Delta V \\sim 10$. We determine an r-band absolute\nmagnitude of $M_{r} = 11.4$, consistent with a mid-M dwarf, and an approximate\ndistance to the source of $2.2$ kpc. Using classical-flare models, we infer a\nflare energy of $E_{V} \\simeq (4.1\\pm 2.2)\\times 10^{36}$ ergs, making this one\nof the strongest flares documented on an M dwarf.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Vector Magnetic Fields and Current Helicities in Coronal Holes and Quiet\n  Regions: In the solar photosphere, many properties of coronal holes (CHs) are not\nknown, especially vector magnetic fields. Using observations from\n\\emph{Hinode}, we investigate vector magnetic fields, current densities and\ncurrent helicities in two CHs and compare them with two normal quiet regions\n(QRs) for the first time. We find that, in the CHs and QRs, the areas where\nlarge current helicities are located are mainly co-spatial with strong vertical\nand horizontal field elements both in shape and location. In the CHs,\nhorizontal magnetic fields, inclination angles, current densities and current\nhelicities are larger than those in the QRs. The mean vertical current density\nand current helicity, averaged over all the observed areas including the CHs\nand QRs, are approximately 0.008 A m$^{-2}$ and 0.005 G$^{2}$ m$^{-1}$,\nrespectively. The mean current density in magnetic flux concentrations where\nthe vertical fields are stronger than 100 G is as large as 0.012 $\\pm$ 0.001 A\nm$^{-2}$, consistent with that in the flare productive active regions. Our\nresults imply that the magnetic fields, especially the strong fields, both in\nthe CHs and QRs are nonpotential.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The First TESS Self-Lensing Pulses: Revisiting KIC 12254688: We report the observations of two self-lensing pulses from KIC 12254688 in\nTransiting Exoplanet Survey Satellite (TESS) light curves. This system,\ncontaining a F2V star and white-dwarf companion, was amongst the first\nself-lensing binary systems discovered by the Kepler Space Telescope over the\npast decade. Each observed pulse occurs when the white dwarf transits in front\nof its companion star, gravitationally lensing the star's surface, thus making\nit appear brighter to a distant observer. These two pulses are the very first\nself-lensing events discovered in TESS observations. We describe the methods by\nwhich the data were acquired and detrended, as well as the best-fit binary\nparameters deduced from our self-lensing+radial velocity model. We highlight\nthe difficulties of finding new self-lensing systems with TESS, and we discuss\nthe types of self-lensing systems that TESS may be more likely to discover in\nthe future.",
        "category": "astro-ph_SR"
    },
    {
        "text": "From Predicting Solar Activity to Forecasting Space Weather: Practical\n  Examples of Research-to-Operations and Operations-to-Research: The successful transition of research to operations (R2O) and operations to\nresearch (O2R) requires, above all, interaction between the two communities. We\nexplore the role that close interaction and ongoing communication played in the\nsuccessful fielding of three separate developments: an observation platform, a\nnumerical model, and a visualization and specification tool. Additionally, we\nwill examine how these three pieces came together to revolutionize\ninterplanetary coronal mass ejection (ICME) arrival forecasts. A discussion of\nthe importance of education and training in ensuring a positive outcome from\nR2O activity follows. We describe efforts by the meteorological community to\nmake research results more accessible to forecasters and the applicability of\nthese efforts to the transfer of space-weather research.We end with a\nforecaster \"wish list\" for R2O transitions. Ongoing, two-way communication\nbetween the research and operations communities is the thread connecting it\nall.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Solar Electron Beam -- Langmuir Wave Interactions and How They Modify\n  Solar Electron Beam Spectra: Solar Orbiter Observations of a Match Made in\n  the Heliosphere: Solar Orbiter's four in-situ instruments have recorded numerous energetic\nelectron events at heliocentric distances between 0.5 and 1au. We analyse\nenergetic electron fluxes, spectra, pitch angle distributions, associated\nLangmuir waves, and type III solar radio bursts for 3 events to understand what\ncauses modifications in the electron flux and identify the origin and\ncharacteristics of features observed in the electron spectrum. We investigate\nwhat electron beam properties and solar wind conditions are associated with\nLangmuir wave growth and spectral breaks in the electron peak flux as a\nfunction of energy. We observe velocity dispersion and quasilinear relaxation\nin the electron flux caused by the resonant wave-particle interactions in the\ndeca-keV range, at the energies at which we observe breaks in the electron\nspectrum, co-temporal with the local generation of Langmuir waves. We show, via\nthe evolution of the electron flux at the time of the event, that these\ninteractions are responsible for the spectral signatures observed around 10 and\n50keV, confirming the results of simulations by Kontar & Reid (2009). These\nsignatures are independent of pitch angle scattering. Our findings highlight\nthe importance of using overlapping FOVs when working with data from different\nsensors. In this work, we exploit observations from all in-situ instruments to\naddress, for the first time, how the energetic electron flux is modified by the\nbeam-plasma interactions, and results into specific features to appear in the\nlocal spectrum. Our results, corroborated with numerical simulations, can be\nextended to a wider range of heliocentric distances.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Wobbling jets in common envelope evolution: We find that the convective motion in the envelopes of red supergiant (RSG)\nstars supplies a non-negligible stochastic angular momentum to the mass that a\nsecondary star accretes in a common envelope evolution (CEE), such that jets\nthat the secondary star launches wobble. The orbital motion of the secondary\nstar in a CEE and the density gradient in the envelope impose a non-zero\nangular momentum to the accreted mass with a constant direction parallel to the\norbital angular momentum. From one-dimensional stellar evolution simulations\nwith the numerical code \\textsc{mesa} we find that the stochastic convection\nmotion in the envelope of RSG stars adds a stochastic angular momentum\ncomponent with an amplitude that is about 0.1-1 times that of the constant\ncomponent due to the orbital motion. We mimic a CEE of the RSG star by removing\nenvelope mass at a high rate and by depositing energy into its envelope. The\nstochastic angular momentum implies that the accretion disk around the\nsecondary star (which we do not simulate), and therefore the jets that it\nlaunches, wobble with angles of up to tens of degrees with respect to the\norbital angular momentum axis. This wobbling makes it harder for jets to break\nout from the envelope and can shape small bubbles in the ejecta that compress\nfilaments that appear as arcs in the ejected nebula, i.e., in planetary nebulae\nwhen the giant is an asymptotic giant branch star.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Wolf-Rayet binaries of the nitrogen sequence in the Large Magellanic\n  Cloud: spectroscopy, orbital analysis, formation, and evolution: Massive Wolf-Rayet (WR) stars dominate the radiative and mechanical energy\nbudget of galaxies and probe a critical phase in the evolution of massive stars\nprior to core-collapse. It is not known whether core He-burning WR stars\n(classical WR, cWR) form predominantly through wind-stripping (w-WR) or binary\nstripping (b-WR). With spectroscopy of WR binaries so-far largely avoided due\nto its complexity, our study focuses on the 44 WR binaries / binary candidates\nof the Large Magellanic Cloud (LMC, metallicity Z~0.5 Zsun), identified on the\nbasis of radial velocity variations, composite spectra, or high X-ray\nluminosities. Relying on a diverse spectroscopic database, we aim to derive the\nphysical and orbital parameters of our targets, confronting evolution models of\nevolved massive stars at sub-solar metallicity, and constraining the impact of\nbinary interaction in forming them. Spectroscopy is performed using the Potsdam\nWolf-Rayet (PoWR) code and cross-correlation techniques. Disentanglement is\nperformed using the code Spectangular or the shift-and-add algorithm.\nEvolutionary status is interpreted using the Binary Population and Spectral\nSynthesis (BPASS) code, exploring binary interaction and chemically-homogeneous\nevolution.\n  No obvious dichotomy in the locations of apparently-single and binary WN\nstars on the Hertzsprung-Russell diagram is apparent. According to commonly\nused stellar evolution models (BPASS, Geneva), most apparently-single WN stars\ncould not have formed as single stars, implying that they were stripped by an\nundetected companion. Otherwise, it must follow that pre-WR mass-loss/mixing\n(e.g., during the red supergiant phase) are strongly underestimated in standard\nstellar evolution models.",
        "category": "astro-ph_SR"
    },
    {
        "text": "First Results from the CHEPS: Exoplanets and the Discovery of an\n  Eccentric Brown Dwarf in the Desert: We report the discovery of a brown dwarf on an eccentric orbit and with a\nsemimajor axis that places it in the brown dwarf desert region around the star\nHD191760. The star has a spectral type of G3IV/V and a metallicity ([Fe/H]) of\n0.29 dex. HD191760 adds to the small number of metal-rich stars with brown\ndwarf companions. The brown dwarf (HD191760b) is found to have an orbital\nperiod of 505.57+/-0.40 days and semimajor axis of 1.35+/-0.01 AU, placing it\nfirmly in the brown dwarf desert. The eccentricity of HD191760b is found to be\n0.63+/-0.01, meaning it reaches as close as 0.5 AU from the host star.\nDynamical simulations indicate that no inner planets could reside at\nseparations beyond ~0.17 AU due to the disastrous gravity imposed by HD191760b.\nIn addition to these first results we also refine the orbits found for the\nexoplanets around the stars HD48265, HD143361 and HD154672. All 1-planet\nsolutions are in agreement with those previously published by the Magellan\nPlanet Search.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Surface convection: from the Sun to red giant stars: We check how the change in surface conditions between the Sun and red giant\nbranch stars changes the characteristic surface convection length scale to be\nused in models. We investigate the question in the case of the mixing length\ntheory and of the phenomenology of full spectrum of turbulence. For the\nobservational part, we rely on independent measurements of effective\ntemperatures and interferometric radii of nearby red giants. We find that the\nlocal red giant branch cannot be explained taking into account the solar\ncalibrated convective length scale.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Tracing the ISM magnetic field morphology: The potential of\n  multi-wavelength polarization measurements: $\\textit{Aims.}$ We present a case study to demonstrate the potential of\nmulti-wavelength polarization measurements. The aim is to investigate the\neffects that dichroic polarization and thermal re-emission have on tracing the\nmagnetic field in the interstellar medium (ISM). Furthermore, we analyze the\ncrucial influence of imperfectly aligned compact dust grains on the resulting\nsynthetic continuum polarization maps.$\\\\ \\textit{Methods.}$ We developed an\nextended version of the well-known 3D Monte-Carlo radiation transport code MC3D\nfor multi-wavelength polarization simulations running on an adaptive grid.We\ninvestigated the interplay between radiation, magnetic fields and dust grains.\nOur results were produced by post-processing both ideal density distributions\nand sophisticated magnetohydrodynamic (MHD) collapse simulations with radiative\ntransfer simulations. We derived spatially resolved maps of intensity, optical\ndepth, and linear and circular polarization at various inclination angles and\nscales in a wavelength range from 7 $\\mu m$ to 1 $mm$.$\\\\ \\textit{Results.}$ We\npredict unique patterns in linear and circular polarization maps for different\ntypes of density distributions and magnetic field morphologies for test setups\nand sophisticated MHD collapse simulations. We show that alignment processes of\ninterstellar dust grains can significantly influence the resulting synthetic\npolarization maps. Multi-wavelength polarization measurements allow one to\npredict the morphology of the magnetic field inside the ISM. The interpretation\nof polarization measurements of complex structures still remains ambiguous\nbecause of the large variety of the predominant parameters in the ISM.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Likely Members of the Beta Pictoris and AB Doradus Moving Groups in the\n  North: We present first results from follow-up of targets in the northern hemisphere\nBeta Pictoris and AB Doradus moving group candidate list of Schlieder, Lepine,\nand Simon (2012). We obtained high-resolution, near-infrared spectra of 27\ncandidate members to measure their radial velocities and confirm consistent\ngroup kinematics. We identify 15 candidates with consistent predicted and\nmeasured radial velocities, perform analyses of their 6-dimensional\n(U,V,W,X,Y,Z) Galactic kinematics, and compare to known group member\ndistributions. Based on these analyses, we propose that 7 Beta Pic and 8 AB Dor\ncandidates are likely new group members. Four of the likely new Beta Pic stars\nare binaries; one a double lined spectroscopic system. Three of the proposed AB\nDor stars are binaries. Counting all binary components, we propose 22 likely\nmembers of these young, moving groups. The majority of the proposed members are\nM2 to M5 dwarfs, the earliest being of type K2. We also present preliminary\nparameters for the two new spectroscopic binaries identified in the data, the\nproposed Beta Pic member and a rejected Beta Pic candidate. Our candidate\nselection and follow-up has thus far identified more than 40 low-mass, likely\nmembers of these two moving groups. These stars provide a new sample of nearby,\nyoung targets for studies of local star formation, disks and exoplanets via\ndirect imaging, and astrophysics in the low-mass regime.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Gravitoturbulent dynamo in global simulations of gaseous disks: The turbulence driven by gravitational instabilities (GIs) can amplify\nmagnetic fields in massive gaseous disks. This GI dynamo may appear in young\ncircumstellar disks, whose weak ionization challenges other amplification\nroutes, as well as in active galactic nuclei. Although regarded as a\nlarge-scale dynamo, only local simulations have so far described its kinematic\nregime. We study the GI dynamo in global magnetohydrodynamic (MHD) models of\naccretion disks, focusing on its kinematic phase. We perform resistive MHD\nsimulations with the Pluto code for different radiative cooling times and\nelectrical resistivities. A weak magnetic field seeds the dynamo, and we adopt\nmean-field and heuristic models to capture its essence. We recover the same\ninduction process leading to magnetic field amplification as previously\nidentified in local simulations. The dynamo is, however, global in nature,\nconnecting distant annuli of the disk via a large-scale dynamo mode of a fixed\ngrowth rate. This large-scale amplification can be described by a mean-field\nmodel that does not rely on conventional alpha-Omega effects. When varying the\ndisk parameters we find an optimal resistivity that facilitates magnetic\namplification, whose magnetic Reynolds number, Rm < 10, is substantially\nsmaller than in local simulations. Unlike local simulations, we find an optimal\ncooling rate and the existence of global oscillating dynamo modes. The\nnonlinear saturation of the dynamo puts the disk in a strongly magnetized\nturbulent state on the margins of the effective range of GI. In our\nsimulations, the accretion power eventually exceeds the threshold required by\nlocal thermal balance against cooling, leaving the long-term nonlinear outcome\nof the GI dynamo uncertain.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The rotation of field stars from CoRoT data: We present period measurements of a large sample of field stars in the solar\nneighbourhood, observed by CoRoT in two different directions of the Galaxy. The\npresence of a period was detected using the Scargle Lomb Normalized Periodogram\ntechnique and the autocorrelation analysis. The assessment of the results has\nbeen performed through a consistency verification supported by the folded light\ncurve analysis. The data analysis procedure has discarded a non-negligible\nfraction of light curves due to instrumental artifacts, however it has allowed\nus to identify pulsators and binaries among a large number of field stars. We\nmeasure a wide range of periods, from 0.25 to 100 days, most of which are\nrotation periods. The final catalogue includes 1978 periods, with 1727 of them\nidentified as rotational periods, 169 are classified as pulsations and 82 as\norbital periods of binary systems. Our sample suffers from selection biases not\neasily corrected for, thus we do not use the distribution of rotation periods\nto derive the age distribution of the main-sequence population. Nevertheless,\nusing rotation as a proxy for age, we can identify a sample of young stars (<\n600 Myr), that will constitute a valuable sample, supported by further\nspectroscopic observations, to study the recent star formation history in the\nsolar neighborhood.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Rapid Enhancement of Sheared Evershed Flow Along the Neutral Line\n  Associated with an X6.5 Flare Observed by Hinode: We present G-band and Ca II H observations of NOAA AR 10930 obtained by\nHinode/SOT on 2006 December 6 covering an X6.5 flare. Local Correlation\nTracking (LCT) technique was applied to the foreshortening-corrected G-band\nimage series to acquire horizontal proper motions in this complex\nbeta-gamma-delta active region. With the continuous high quality, spatial and\ntemporal resolution G-band data, we not only confirm the rapid decay of outer\npenumbrae and darkening of the central structure near the flaring neutral line,\nbut also unambiguously detect for the first time the enhancement of the sheared\nEvershed flow (average horizontal flow speed increased from 330+-3.1 to\n403+-4.6 m/s) along the neutral line right after the eruptive white-light\nflare. Post-flare Ca II H images indicate that the originally fanning out field\nlines at the two sides of the neutral line get connected. Since penumbral\nstructure and Evershed flow are closely related to photospheric magnetic\ninclination or horizontal field strength, we interpret the rapid changes of\nsunspot structure and surface flow as the result of flare-induced magnetic\nrestructuring down to the photosphere. The magnetic fields turn from fanning\nout to inward connection causing outer penumbrae decay, meanwhile those near\nthe flaring neutral line become more horizontal leading to stronger Evershed\nflow there. The inferred enhancement of horizontal magnetic field near the\nneutral line is consistent with recent magnetic observations and theoretical\npredictions of flare-invoked photospheric magnetic field change.",
        "category": "astro-ph_SR"
    },
    {
        "text": "High-frequency wave power observed in the solar chromosphere with IBIS\n  and ALMA: We present observational constraints on the solar chromospheric heating\ncontribution from acoustic waves with frequencies between 5 and 50 mHz. We\nutilize observations from the Dunn Solar Telescope in New Mexico complemented\nwith observations from the Atacama Large Millimeter Array collected on 2017\nApril 23. The properties of the power spectra of the various quantities are\nderived from the spectral lines of Ca II 854.2 nm, H I 656.3 nm, and the\nmillimeter continuum at 1.25 mm and 3 mm. At the observed frequencies the\ndiagnostics almost all show a power law behavior, whose particulars (slope,\npeak and white noise floors) are correlated with the type of solar feature\n(internetwork, network, plage). In order to disentangle the vertical versus\ntransverse plasma motions we examine two different fields of view; one near\ndisk center and the other close to the limb. To infer the acoustic flux in the\nmiddle chromosphere, we compare our observations with synthetic observables\nfrom the time-dependent radiative hydrodynamic RADYN code. Our findings show\nthat acoustic waves carry up to about 1 kW m$^{-2}$ of energy flux in the\nmiddle chromosphere, which is not enough to maintain the quiet chromosphere,\ncontrary to previous publications.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The ISO Long Wavelength Spectrometer line spectrum of VY Canis Majoris\n  and other oxygen-rich evolved stars: The far-infrared spectra of circumstellar envelopes around various\noxygen-rich stars were observed using the ISO Long Wavelength Spectrometer\n(LWS). We have examined high signal-to-noise ISO LWS observations of the\nluminous supergiant star, VY CMa, with the aim of identifying all of the\nspectral lines. By paying particular attention to water lines, we aim to\nseparate the lines due to other species, in particular, to prepare for\nforthcoming observations that will cover the same spectral range using Herschel\nPACS and at higher spectral resolution using Herschel HIFI and SOFIA. We have\ndeveloped a fitting method to account for blended water lines using a simple\nweighting scheme to distribute the flux. We have applied this approach to\nseveral other stars which we compare with VY CMa. We present line fluxes for\nthe unblended H2O and CO lines, and present detections of several possible\nnu_2=1 vibrationally excited water lines. We also identify blended lines of OH,\none unblended and several blended lines of NH3, and one possible detection of\nH3O+.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Inversion of physical parameters in solar atmospheric seismology: Magnetohydrodynamic (MHD) wave activity is ubiquitous in the solar\natmosphere. MHD seismology aims to determine difficult to measure physical\nparameters in solar atmospheric magnetic and plasma structures by a combination\nof observed and theoretical properties of MHD waves and oscillations. This\ntechnique, similar to seismology or helio-seismology, demands the solution of\ntwo problems. The direct problem involves the computation of wave properties of\ngiven theoretical models. The inverse problem implies the calculation of\nunknown physical parameters, by means of a comparison of observed and\ntheoretical wave properties. Solar atmospheric seismology has been successfully\napplied to different structures such as coronal loops, prominence fine\nstructures, spicules, or jets. However, it is still in its infancy. Far more is\nthere to come. We present an overview of recent results, with particular\nemphasis in the inversion procedure.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Formation pathway for lonely stripped-envelope supernova progenitors:\n  implications for Cassiopeia A: We explore a new scenario for producing stripped-envelope supernova\nprogenitors. In our scenario, the stripped-envelope supernova is the second\nsupernova of the binary, in which the envelope of the secondary was removed\nduring its red supergiant phase by the impact of the first supernova. Through\n2D hydrodynamical simulations, we find that $\\sim$50-90$\\%$ of the envelope can\nbe unbound as long as the pre-supernova orbital separation is $\\lesssim5$ times\nthe stellar radius. Recombination energy plays a significant role in the\nunbinding, especially for relatively high mass systems ($\\gtrsim18M_\\odot$). We\npredict that more than half of the unbound mass should be distributed as a\none-sided shell at about $\\sim$10-100pc away from the second supernova site. We\ndiscuss possible applications to known supernova remnants such as Cassiopeia A,\nRX J1713.7-3946, G11.2-0.3, and find promising agreements. The predicted rate\nis $\\sim$0.35-1$\\%$ of the core-collapse population. This new scenario could be\na major channel for the subclass of stripped-envelope or type IIL supernovae\nthat lack companion detections like Cassiopeia A.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Grids of stellar models with rotation - I. Models from 0.8 to 120 Msun\n  at solar metallicity (Z = 0.014): [abridged] Many topical astrophysical research areas, such as the properties\nof planet host stars, the nature of the progenitors of different types of\nsupernovae and gamma ray bursts, and the evolution of galaxies, require\ncomplete and homogeneous sets of stellar models at different metallicities in\norder to be studied during the whole of cosmic history. We present here a first\nset of models for solar metallicity, where the effects of rotation are\naccounted for in a homogeneous way.\n  We computed a grid of 48 different stellar evolutionary tracks, both rotating\nand non-rotating, at Z=0.014, spanning a wide mass range from 0.8 to 120 Msun.\nFor each of the stellar masses considered, electronic tables provide data for\n400 stages along the evolutionary track and at each stage, a set of 43 physical\ndata are given. These grids thus provide an extensive and detailed data basis\nfor comparisons with the observations. The rotating models start on the ZAMS\nwith a rotation rate Vini/Vcrit=0.4. The evolution is computed until the end of\nthe central carbon-burning phase, the early AGB phase, or the core helium-flash\nfor, respectively, the massive, intermediate, and both low and very low mass\nstars. The initial abundances are those deduced by Asplund and collaborators,\nwhich best fit the observed abundances of massive stars in the solar\nneighbourhood. We update both the opacities and nuclear reaction rates, and\nintroduce new prescriptions for the mass-loss rates as stars approach the\nEddington and/or the critical velocity. We account for both atomic diffusion\nand magnetic braking in our low-mass star models. [...]",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the identification of time interval threshold in the twin-CME\n  scenario: Recently it has been suggested that the \"twin-CME\" scenario Li.etal2012 may\nbe a very effective mechanism in causing extreme Solar Energetic Particle (SEP)\nevents and in particular Ground Level Enhancement (GLE) events. Ding.etal2013\nperformed a statistical examination of the twin-CME scenario with a total of\n$126$ fast and wide western Coronal Mass Ejections (CMEs). They found that CMEs\nhaving a preceding CME with a speed $>$ 300 $km/s$ within $9$ hours from the\nsame active region have larger probability of leading to large SEP events than\nCMEs that do not have preceding CMEs. The choice of $9$ hours being the time\nlag $\\tau$ between the preceding CME and the main CME was based on some crude\nestimates of the decay time of the turbulence downstream of the shock driven by\nthe preceding CME. In this work, we examine this choice. For the $126$ fast\nwide CMEs examined in Ding.etal2013, we vary the time lag $\\tau$ from $1$ hour\nto $24$ hours with an increment of $1$ hour. By considering three quantities\nwhose values depend on the choice of this time lag $\\tau$, we show that the\nchoice of $13$ hours for $\\tau$ is more appropriate. Our study confirms our\nearlier result that twin CMEs are more likely to lead to large SEP events than\nsingle fast CMEs. The results shown here are of great relevance to space\nweather studies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Radial velocity photon limits for the dwarf stars of spectral classes\n  F--M: The determination of extrasolar planet masses with the radial velocity (RV)\ntechnique requires spectroscopic Doppler information from the planet's host\nstar, which varies with stellar brightness and temperature. We analyze Doppler\ninformation in spectra of F--M dwarfs utilizing empirical information from\nHARPS and CARMENES, and from model spectra. We come to the conclusions that an\noptical setup ($BVR$-bands) is more efficient that a near-infrared one ($YJHK$)\nin dwarf stars hotter than 3200\\,K. We publish a catalogue of 46,480\nwell-studied F--M dwarfs in the solar neighborhood and compare their\ndistribution to more than one million stars from Gaia DR2. For all stars, we\nestimate the RV photon noise achievable in typical observations assuming no\nactivity jitter and slow rotation. We find that with an ESPRESSO-like\ninstrument at an 8m-telescope, a photon noise limit of 10\\,cm\\,s$^{-1}$ or\nlower can be reached in more than 280 stars in a 5\\,min observation. At\n4m-telescopes, a photon noise limit of 1\\,m\\,s$^{-1}$ can be reached in a\n10\\,min exposure in approx.\\ 10,000 predominantly sun-like stars with a\nHARPS-like (optical) instrument. The same applies to $\\sim$3000 stars for a\nred-optical setup covering the $RIz$-bands, and to $\\sim$700 stars for a\nnear-infrared instrument. For the latter two, many of the targets are nearby M\ndwarfs. Finally, we identify targets in which Earth-mass planets within the\nliquid water habitable zone can cause RV amplitudes comparable to the RV photon\nnoise. Assuming the same exposure times, we find that an ESPRESSO-like\ninstrument can reach this limit for 1\\,M$_\\Earth$ planets in more than 1000\nstars. The optical, red-optical, and near-infrared configurations reach the\nlimit for 2\\,M$_\\Earth$ planets in approximately 500, 700, and 200 stars,\nrespectively.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the accretion process in a high-mass star forming region - A\n  multitransitional THz Herschel-HIFI study of ammonia toward G34.26+0.15: [Abridged] Our aim is to explore the gas dynamics and the accretion process\nin the early phase of high-mass star formation. The inward motion of molecular\ngas in the massive star forming region G34.26+0.15 is investigated by using\nhigh-resolution profiles of seven transitions of ammonia at THz frequencies\nobserved with Herschel-HIFI. The shapes and intensities of these lines are\ninterpreted in terms of radiative transfer models of a spherical, collapsing\nmolecular envelope. An accelerated Lambda Iteration (ALI) method is used to\ncompute the models. The seven ammonia lines show mixed absorption and emission\nwith inverse P-Cygni-type profiles that suggest infall onto the central source.\nA trend toward absorption at increasingly higher velocities for higher\nexcitation transitions is clearly seen in the line profiles. The $J =\n3\\leftarrow2$ lines show only very weak emission, so these absorption profiles\ncan be used directly to analyze the inward motion of the gas. This is the first\ntime a multitransitional study of spectrally resolved rotational ammonia lines\nhas been used for this purpose. Broad emission is, in addition, mixed with the\nabsorption in the $1_0-0_0$ ortho-NH$_3$ line, possibly tracing a molecular\noutflow from the star forming region. The best-fitting ALI model reproduces the\ncontinuum fluxes and line profiles, but slightly underpredicts the emission and\nabsorption depth in the ground-state ortho line $1_0-0_0$. The derived\northo-to-para ratio is approximately 0.5 throughout the infalling cloud core\nsimilar to recent findings for translucent clouds in sight lines toward W31C\nand W49N. We find evidence of two gas components moving inwards toward the\ncentral region with constant velocities: 2.7 and 5.3 km$\\,$s$^{-1}$, relative\nto the source systemic velocity. The inferred mass accretion rates derived are\nsufficient to overcome the expected radiation pressure from G34.26+0.15.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Shapes of AGB Envelopes as Probes of Binary Companions: We describe how the large scale geometry of the circumstellar envelopes of\nasymptotic giant branch stars can be used to probe the presence of unseen\nstellar companions. A nearby companion modifies the mass loss by\ngravitationally focusing the wind towards the orbital plane, and thereby\ndetermines the shape of the envelope at large distances from the star. Using\navailable simulations, we develop a prescription for the observed shapes of\nenvelopes in terms of the binary parameters, envelope orientation, and type of\nobservation. The prescription provides a tool for the analysis of envelope\nimages at optical, infrared, and millimetre wavelengths, which can be used to\nconstrain the presence of companions in well observed cases. We illustrate this\napproach by examining the possible role of binary companions in triggering the\nonset of axi-symmetry in planetary nebula formation. If interaction with the\nprimary leads to axi-symmetry, the spherical halos widely seen around newly\nformed nebulae set limits on the companion mass. Only low mass objects may\norbit close to the primary without observable shaping effects: they remain\ninvisible until the interaction causes a sudden change in the mass loss\ngeometry.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Survey for H-alpha Emission from Late L dwarfs and T dwarfs: Recently, studies of brown dwarfs have demonstrated that they possess strong\nmagnetic fields and have the potential to produce radio and optical auroral\nemissions powered by magnetospheric currents. This emission provides the only\nwindow on magnetic fields in the coolest brown dwarfs and identifying\nadditional benchmark objects is key to constraining dynamo theory in this\nregime. To this end, we conducted a new red optical (6300 - 9700 Angstrom)\nsurvey with the Keck telescopes looking for H-alpha emission from a sample of\nlate L dwarfs and T dwarfs. Our survey gathered optical spectra for 29 targets,\n18 of which did not have previous optical spectra in the literature, greatly\nexpanding the number of moderate resolution (R~2000) spectra available at these\nspectral types. Combining our sample with previous surveys, we confirm an\nH-alpha detection rate of 9.2 (+3.5/-2.1) % for L and T dwarfs in the optical\nspectral range of L4 - T8. This detection rate is consistent with the recently\nmeasured detection rate for auroral radio emission from Kao et al. (2016),\nsuggesting that geometrical selection effects due to the beaming of the radio\nemission are small or absent. We also provide the first detection of H-alpha\nemission from 2MASS 0036+1821, previously notable as the only electron\ncyclotron maser radio source without a confirmed detection of H-alpha emission.\nFinally, we also establish optical standards for spectral types T3 and T4,\nfilling in the previous gap between T2 and T5.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Differential rotation in main-sequence solar-like stars: Qualitative\n  inference from asteroseismic data: Understanding differential rotation of Sun-like stars is of great importance\nfor insight into the angular momentum transport in these stars. One means of\ngaining such information is that of asteroseismology. By a forward modeling\napproach we analyze in a qualitative manner the impact of different\ndifferential rotation profiles on the splittings of p-mode oscillation\nfrequencies. The optimum modes for inference on differential rotation are\nidentified along with the best value of the stellar inclination angle. We find\nthat in general it is not likely that asteroseismology can be used to make an\nunambiguous distinction between a rotation profile such as, e.g., a conical\nSun-like profile and a cylindrical profile. In addition, it seems unlikely that\nasteroseismology of Sun-like stars will result in inferences on the radial\nprofile of the differential rotation, such as can be done for, e.g., red\ngiants. At best one could possibly obtain the sign of the radial differential\nrotation gradient. Measurements of the extent of the latitudinal differential\nfrom frequency splitting are, however, more promising. One very interesting\naspect that could likely be tested from frequency splittings is whether the\ndifferential rotation is solar-like or anti-solar-like in nature, in the sense\nthat a solar-like profile has an equator rotating faster than the poles.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Reconstruction of a Large-scale Pre-flare Coronal Current Sheet\n  Associated with an Homologous X-shaped Flare: As a fundamental magnetic structure in the solar corona, electric current\nsheets (CSs) can form either prior to or during solar flare, and they are\nessential for magnetic energy dissipation in the solar corona by enabling\nmagnetic reconnection. However static reconstruction of CS is rare, possibly\ndue to limitation inherent in available coronal field extrapolation codes. Here\nwe present the reconstruction of a large-scale pre-flare CS in solar active\nregion 11967 using an MHD-relaxation model constrained by SDO/HMI vector\nmagnetogram. The CS is found to be associated with a set of peculiar homologous\nflares that exhibit unique X-shaped ribbons and loops occurring in a\nquadrupolar magnetic configuration. This is evidenced by that the field lines\ntraced from the CS to the photosphere form an X shape which nearly precisely\nreproduces the shape of the observed flare ribbons, suggesting that the flare\nis a product of the dissipation of the CS through reconnection. The CS forms in\na hyperbolic flux tube, which is an intersection of two quasi-separatrix\nlayers. The recurrence of the X-shaped flares might be attributed to the\nrepetitive formation and dissipation of the CS, as driven by the photospheric\nfootpoint motions. These results demonstrate the power of data-constrained MHD\nmodel in reproducing CS in the corona as well as providing insight into the\nmagnetic mechanism of solar flares.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Masses of the Hyades white dwarfs: A gravitational redshift measurement: Context. It is possible to accurately measure the masses of the white dwarfs\n(WDs) in the Hyades cluster using gravitational redshift, because the radial\nvelocity of the stars can be obtained independently of spectroscopy from\nastrometry and the cluster has a low velocity dispersion. Aims. We aim to\nobtain an accurate measurement of the Hyades WD masses by determining the\nmass-to-radius ratio (M/R) from the observed gravitational redshift, and to\ncompare them with masses derived from other methods. Methods. We analyse\narchive high-resolution UVES-VLT spectra of six WDs belonging to the Hyades to\nmeasure their Doppler shift, from which M/R is determined after subtracting the\nastrometric radial velocity. We estimate the radii using Gaia photometry as\nwell as literature data. Results. The M/R error associated to the gravitational\nredshift measurement is about 5%. The radii estimates, evaluated with different\nmethods, are in very good agreement, though they can differ by up to 4%\ndepending on the quality of the data. The masses based on gravitational\nredshift are systematically smaller than those derived from other methods, by a\nminimum of $\\sim 0.02$ up to 0.05 solar masses. While this difference is within\nour measurement uncertainty, the fact that it is systematic indicates a likely\nreal discrepancy between the different methods. Conclusions. We show that the\nM/R derived from gravitational redshift measurements is a powerful tool to\ndetermine the masses of the Hyades WDs and could reveal interesting properties\nof their atmospheres. The technique can be improved by using dedicated\nspectrographs, and can be extended to other clusters, making it unique in its\nability to accurately and empirically determine the masses of WDs in open\nclusters. At the same time we prove that gravitational redshift in WDs agrees\nwith the predictions of stellar evolution models to within a few percent.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Celestial Sign in the Anglo-Saxon Chronicle in the 770s: Insights on\n  Contemporary Solar Activity: The anomalous concentration of radiocarbon in 774/775 attracted intense\ndiscussion on its origin, including the possible extreme solar event(s)\nexceeding any events in observational history. Anticipating such extreme solar\nevents, auroral records were also surveyed in historical documents and those\nincluding the red celestial sign after sunset in the Anglo-Saxon Chronicle\n(ASC) were subjected to consideration. Usoskin et al. (2013: U13) interpreted\nthis record as an aurora and suggested enhanced solar activity around 774/775.\nConversely, Neuhauser and Neuhauser (2015a, 2015b: N15a and N15b) interpreted\n\"after sunset\" as during sunset or twilight; they considered this sign as a\nhalo display and suggested a solar minimum around 774. However, so far these\nrecords have not been discussed in comparison with eyewitness auroral records\nduring the known extreme space-weather events, although they were discussed in\nrelationship with potential extreme events in 774/775. Therefore, we\nreconstruct the observational details based on the original records in the ASC\nand philological references, compare them with eyewitness auroral observations\nduring known extreme space-weather events, and consider contemporary solar\nactivity. We clarify the observation was indeed \"after sunset\", reject the\nsolar halo hypothesis, define the observational time span between 25 Mar. 775\nand 25 Dec. 777, and note the parallel halo drawing in 806 in the ASC shown in\nN15b was not based on the original observation in England. We show examples of\neyewitness auroral observations during twilight in known space-weather events,\nand this celestial sign does not contradict the observational evidence.\nAccordingly, we consider this event happened after the onset of the event in\n774/775, but shows relatively enhanced solar activity, with other historical\nauroral records in the mid-770s, as also confirmed by the Be data from ice\ncores.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Stellar mid-life crisis: subcritical magnetic dynamos of solar-like\n  stars and the breakdown of gyrochronology: Recent observations reveal the surprising breakdown of stellar gyrochronology\nrelations at about the Sun's age hinting that middle-aged, solar-like stars\ntransition to a magnetically inactive future. We provide a theoretical basis\nfor these intriguing observations inspired by simulations with a mathematical\ndynamo model that can explore long-term solar cycle fluctuations. We reproduce\nthe observed bimodal distribution of sunspot numbers, but only for subcritical\ndynamos. Based on a bifurcation analysis, we argue that ageing of solar-like\nstars makes the magnetically-weak dynamo regime readily accessible. Weak\nmagnetic field production in this regime compromises wind-driven angular\nmomentum losses thus disrupting the hegemony of magnetic braking on stellar\nrotational spin-down. This hypothesis of {\\emph{subcritical magnetic dynamos}}\nof solar-like stars provides a self-consistent, unifying physical basis for a\ndiversity of solar-stellar phenomena such as why stars beyond their mid-life do\nnot spin-down as fast as in their youth, the break-down of stellar\ngyrochronology relations, the observed bimodal distribution of long-term\nsunspot observations and recent findings suggesting that the Sun may be\ntransitioning to a magnetically inactive future.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the energy dissipation rate at the inner edge of circumbinary discs: We study, by means of numerical simulations and analysis, the details of the\naccretion process from a disc onto a binary system. We show that energy is\ndissipated at the edge of a circumbinary disc and this is associated with the\ntidal torque that maintains the cavity: angular momentum is transferred from\nthe binary to the disc through the action of compressional shocks and viscous\nfriction. These shocks can be viewed as being produced by fluid elements which\ndrift into the cavity and, before being accreted, are accelerated onto\ntrajectories that send them back to impact the disc. The rate of energy\ndissipation is approximately equal to the product of potential energy per unit\nmass at the disc's inner edge and the accretion rate, estimated from the disc\nparameters just beyond the cavity edge, that would occur without the binary.\nFor very thin discs, the actual accretion rate onto the binary may be\nsignificantly less. We calculate the energy emitted by a circumbinary disc\ntaking into account energy dissipation at the inner edge and also irradiation\narising there from reprocessing of light from the stars. We find that, for\ntight PMS binaries, the SED is dominated by emission from the inner edge at\nwavelengths between 1-4 and 10 $\\mu$m. This may apply to systems like CoRoT\n223992193 and V1481 Ori.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The First Extensive Exploration of UV-bright Stars in the Globular\n  Cluster NGC 2808: In this study, we identified and characterized the hot and luminous UV-bright\nstars in the globular cluster NGC 2808. We combined data from the Ultra Violet\nImaging Telescope (UVIT) on-board the Indian space satellite, AstroSat, with\nthe Hubble Space Telescope UV Globular Cluster Survey (HUGS) data for the\ncentral region (within $\\sim$ $\\ang[angle-symbol-over-decimal]{;2.7;} \\times\n\\ang[angle-symbol-over-decimal]{;2.7;}$) and Gaia and ground-based optical\nphotometry for the outer parts of the cluster. We constructed the UV and\nUV-optical color-magnitude diagrams, compared the horizontal branch (HB)\nmembers with the theoretical zero-age HB and terminal-age HB models and\nidentified 34 UV-bright stars. The spectral energy distributions of the\nUV-bright stars were fitted with theoretical models to estimate their effective\ntemperatures (12500 K - 100,000 K), radii (0.13 to 2.2 $R_{\\odot}$), and\nluminosities ($\\sim 40$ to $3000$ $L_{\\odot}$) for the first time. These stars\nwere then placed on the H-R diagram, along with theoretical post-HB (pHB)\nevolutionary tracks to assess their evolutionary status. The models suggest\nthat most of these stars are in the AGB-manqu\\'e phase and all, except three,\nhave evolutionary masses $<$ 0.53 $M_{\\odot}$. We also calculated the\ntheoretically expected number of hot post-(early)-AGB (p(e)AGB) stars in this\ncluster and found the range to match our observations. Seven UV-bright stars\nlocated in the outer region of the cluster, identified from the AstroSat/UVIT\nimages, are ideal candidates for detailed follow-up spectroscopic studies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Testing Angular Momentum Transport and Wind Loss in Intermediate Mass\n  Core Helium Burning Stars: Stars between two and three solar masses rotate rapidly on the main sequence,\nand their rotation rates in the core helium burning (secondary clump) phase can\ntherefore be used to test models of angular momentum loss used for\ngyrochronology in a new regime. Because both their core and surface rotation\nrates can be measured, these stars can also be used to set strong constraints\non angular momentum transport inside stars. We find that they are rotating\nslower than angular momentum conservation and rigid rotation would predict. Our\nresults are insensitive to the degree of core-envelope coupling because of the\nsmall moment of inertia of the radiative core. We discuss two possible\nmechanisms for slowing down the surfaces of these stars: (1) substantial\nangular momentum loss, and (2) radial differential rotation in the surface\nconvection zone. Modern angular momentum loss prescriptions used for solar-type\nstars predict secondary clump surface rotation rates in much better agreement\nwith the data than prior variants used in the literature, and we argue that\nsuch enhanced loss is required to understand the combination of core and\nsurface rotation rates. However, we find that the assumed radial differential\nrotation profile in convective regions has a strong impact on the predicted\nsurface rotation rates, and that a combination of enhanced loss and radial\ndifferential rotation in the surface convection zone is also consistent with\nthe data. We discuss future tests that can quantify the impact of both\nphenomena. Current data tentatively suggests that some combination of the two\nprocesses fits the data better than either one alone.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Binary stars as the key to understanding planetary nebulae: Planetary nebulae are traditionally considered to represent the final\nevolutionary stage of all intermediate-mass stars ($\\sim$0.7-8Msol). Recent\nevidence seems to contradict this picture. In particular, since the launch of\nthe Hubble Space Telescope it has become clear that planetary nebulae display a\nwide range of striking morphologies which cannot be understood in a single star\nscenario, instead pointing towards a binary evolution in a majority of systems.\nHere, we summarise our current understanding of the importance of binarity in\nthe formation and shaping of planetary nebulae, as well as the surprises that\nrecent observational studies have revealed with respect to our understanding of\nbinary evolution in general. These advances have critical implications,\nincluding for the understanding of mass transfer processes in binary stars -\nparticularly the all-important but ever-so poorly understood `common envelope\nphase' - as well as the formation of cosmologically important type Ia\nsupernovae.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A detailed study of the barium central star of the planetary nebula\n  Abell 70: We present a detailed study of the barium star at the heart of the planetary\nnebula Abell 70. Time-series photometry obtained over a period of more than ten\nyears demonstrates that the barium-contaminated companion is a rapid rotator\nwith temporal variability due to spots. The amplitude and phasing of the\nphotometric variability changes abruptly, however there is no evidence for a\nchange in the rotation period (P = 2.06~d) over the course of the observations.\nThe co-addition of 17 high-resolution spectra obtained with VLT-UVES allow us\nto measure the physical and chemical properties of the companion, confirming it\nto be a chromospherically-active, late G-type sub-giant with more than +1~dex\nof barium enhancement. We find no evidence of radial velocity variability in\nthe spectra, obtained over the course of approximately 130~d with a single\nadditional point some 8 years later, with the radial velocities of all epochs\napproximately $-$10 \\kms{} from the previously measured systemic velocity of\nthe nebula. This is perhaps indicative that the binary has a relatively long\nperiod (P $\\gtrsim$ 2~yr) and high eccentricity ($e\\gtrsim$ 0.3), and that all\nthe observations were taken around radial velocity minimum. However, unless the\nbinary orbital plane is not aligned with the waist of the nebula or the\nsystemic velocity of the binary is not equal to the literature value for the\nnebula, this would imply an unfeasibly large mass for the nebular progenitor.",
        "category": "astro-ph_SR"
    },
    {
        "text": "GALEX J184559.8-413827: a new extreme helium star identified using SALT: A high-resolution spectrum of the helium-rich `hot subdwarf' GALEX\nJ184559.8-413827 (J1845-4138) obtained with SALT HRS demonstrates it to be the\nfirst extreme helium (EHe) star to be discovered in nearly 40 years. A\nquantitative analysis demonstrates it to have an atmosphere described by Teff =\n26 170 +/- 750 K, log g /(cm s^-2) = 4.22 +/- 0.10, and a surface chemistry\ncharacterised by CNO-processed helium, a 1% contamination of hydrogen (by\nnumber), and a metallicity 0.4 dex subsolar. Its distance and position are\nconsistent with membership of the Galactic bulge. Its sharp absorption lines\nplace strong constraints on both the rotation and microturbulent velocities.\nSpectroscopically, J1845-4138 closely resembles the pulsating EHe star V652\nHer, generally considered to be the product of a double helium white dwarf\nmerger evolving to become a helium-rich sdO star.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Driving Turbulence and Triggering Star Formation by Ionizing Radiation: We present high resolution simulations on the impact of ionizing radiation of\nmassive O-stars on the surrounding turbulent interstellar medium (ISM). The\nsimulations are performed with the newly developed software iVINE which\ncombines ionization with smoothed particle hydrodynamics (SPH) and\ngravitational forces. We show that radiation from hot stars penetrates the ISM,\nefficiently heats cold low density gas and amplifies over-densities seeded by\nthe initial turbulence. The formation of observed pillar-like structures in\nstar forming regions (e.g. in M16) can be explained by this scenario. At the\ntip of the pillars gravitational collapse can be induced, eventually leading to\nthe formation of low mass stars. Detailed analysis of the evolution of the\nturbulent spectra shows that UV-radiation of O-stars indeed provides an\nexcellent mechanism to sustain and even drive turbulence in the parental\nmolecular cloud.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Active red giants: close binaries versus single rapid rotators: The objective of this work is to determine what fraction of red-giant (RG)\nstars shows photometric rotational modulation, and understand its origin. One\nof the underlying questions is the role of close binarity in this population,\nstanding upon the fact that RGs in short-period binary systems (<150 days or\nso) have been observed to display strong rotational modulation. We select a\nsample of about 4500 relatively bright RGs observed by Kepler, and show that\n370 of them (8%) display rotational modulation. Almost all have oscillation\namplitudes below the median of the sample, while 30 of them are not oscillating\nat all. Of the 85 of these RGs with rotational modulation chosen for follow-up\nradial-velocity observation and analysis, 34 show clear evidence of\nspectroscopic binarity. Surprisingly, 26 of the 30 non-oscillators are in this\ngroup of binaries. To the contrary, about 85% of the active RGs with detectable\noscillations are not part of close binaries. With the help of stellar masses\nand evolutionary states computed from the oscillation properties, it appears\nthat low-mass red-giant branch stars tend to be magnetically inactive, while\nintermediate-mass ones tend to be highly active. The opposite trends are true\nfor helium-core burning (red clump) stars, whereby the lower-mass clump stars\nare comparatively more active and the higher-mass ones less so. In other words,\nwe find that low-mass red-giant branch stars gain angular momentum as they\nevolve to clump stars, while higher-mass ones lose angular momentum. The trend\nobserved with low-mass stars leads to possible scenarios of planet engulfment\nor other merging events during the shell-burning phase. Regarding\nintermediate-mass stars, the rotation periods are long with respect to\ntheoretical expectations reported in the literature, which reinforces the\nexistence of an unidentified sink of angular momentum after the main sequence.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Eccentricity and Spin-Orbit Misalignment in Short-Period Stellar\n  Binaries as a Signpost of Hidden Tertiary Companions: Eclipsing binaries are observed to have a range of eccentricities and\nspin-orbit misalignments (stellar obliquities). Whether such properties are\nprimordial, or arise from post-formation dynamical interactions remains\nuncertain. This paper considers the scenario in which the binary is the inner\ncomponent of a hierarchical triple stellar system, and derives the requirements\nthat the tertiary companion must satisfy in order to raise the eccentricity and\nobliquity of the inner binary. Through numerical integrations of the secular\noctupole-order equations of motion of stellar triples, coupled with the spin\nprecession of the oblate primary star due to the torque from the secondary, we\nobtain a simple, robust condition for producing spin-orbit misalignment in the\ninner binary: In order to excite appreciable obliquity, the precession rate of\nthe stellar spin axis must be smaller than the orbital precession rate due to\nthe tertiary companion. This yields quantitative requirements on the mass and\norbit of the tertiary. We also present new analytic expressions for the maximum\neccentricity and range of inclinations allowing eccentricity excitation\n(Lidov-Kozai window), for stellar triples with arbitrary masses and including\nthe non-Keplerian potentials introduced by general relativity, stellar tides\nand rotational bulges. The results of this paper can be used to place\nconstraints on unobserved tertiary companions in binaries that exhibit high\neccentricity and/or spin-orbit misalignment, and will be helpful in guiding\nefforts to detect external companions around stellar binaries. As an\napplication, we consider the eclipsing binary DI Herculis, and identify the\nrequirements that a tertiary companion must satisfy to produce the observed\nspin-orbit misalignment.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The winds of young Solar-type stars in Coma Berenices and Hercules-Lyra: We present wind models of ten young Solar-type stars in the Hercules-Lyra\nassociation and the Coma Berenices cluster aged around 0.26 Gyr and 0.58 Gyr\nrespectively. Combined with five previously modelled stars in the Hyades\ncluster, aged 0.63 Gyr, we obtain a large atlas of fifteen observationally\nbased wind models. We find varied geometries, multi-armed structures in the\nequatorial plane, and a greater spread in quantities such as the angular\nmomentum loss. In our models we infer variation of a factor of ~6 in wind\nangular momentum loss $\\dot J$ and a factor of ~2 in wind mass loss $\\dot M$\nbased on magnetic field geometry differences when adjusting for the unsigned\nsurface magnetic flux. We observe a large variation factor of ~4 in wind\npressure for an Earth-like planet; we attribute this to variations in the\n'magnetic inclination' of the magnetic dipole axis with respect to the stellar\naxis of rotation. Within our models, we observe a tight correlation between\nunsigned open magnetic flux and angular momentum loss. To account for possible\nunderreporting of the observed magnetic field strength we investigate a second\nseries of wind models where the magnetic field has been scaled by a factor of\n5. This gives $\\dot M \\propto B^{0.4}$ and $\\dot J \\propto B^{1.0}$ as a result\nof pure magnetic scaling.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Episodic Accretion on to Strongly Magnetic Stars: Some accreting neutron stars and young stars show unexplained episodic flares\nin the form of quasi-periodic oscillations or recurrent outbursts. In a series\nof two papers we present new work on an instability that can lead to episodic\noutbursts when the accretion disc is truncated by the star's strong magnetic\nfield close to the corotation radius (where the Keplerian frequency matches the\nstar's rotational frequency). In this paper we outline the physics of the\ninstability and use a simple parameterization of the disc-field interaction to\nexplore the instability numerically, which we show can lead to repeated bursts\nof accretion as well as steady-state solutions, as first suggested by Sunyaev\nand Shakura. The cycle time of these bursts increases with decreasing accretion\nrate. These solutions show that the usually assumed `propeller' state, in which\nmass is ejected from the system, does not need to occur even at very low\naccretion rates.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Two-dimensional simulations of internal gravity waves in a 5 $M_{\\odot}$\n  Zero-Age-Main-Sequence model: Main-sequence intermediate-mass stars present a radiative envelope that\nsupports internal gravity waves (IGWs). Excited at the boundary with the\nconvective core, IGWs propagate towards the stellar surface and are suspected\nto impact physical processes such as rotation and chemical mixing. Using the\nfully compressible time-implicit code MUSIC, we study IGWs in two-dimensional\nsimulations of a zero-age-main-sequence 5 solar mass star model up to 91\\% of\nthe stellar radius with different luminosity and radiative diffusivity\nenhancements. Our results show that low frequency waves excited by core\nconvection are strongly impacted by radiative effects as they propagate. This\nimpact depends on the radial profile of radiative diffusivity which increases\nby almost 5 orders of magnitude between the centre of the star and the top of\nthe simulation domain. In the upper layers of the simulation domain, we observe\nan increase of the temperature. Our study suggests that this is due to heat\nadded in these layers by IGWs damped by radiative diffusion. We show that\nnon-linear effects linked to large amplitude IGWs may be relevant just above\nthe convective core. Both these effects are intensified by the artificial\nenhancement of the luminosity and radiative diffusivity, with enhancement\nfactors up to $10^4$ times the realistic values. Our results also highlight\nthat direct comparison between numerical simulations with enhanced luminosity\nand observations must be made with caution. Finally, our work suggests that\nthermal effects linked to the damping of IGWs could have a non-negligible\nimpact on stellar structure.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Mode identification in three pulsating hot subdwarfs observed with TESS\n  satellite: We report on the detection of pulsations of three pulsating subdwarf B stars\nobserved by the TESS satellite and our results of mode identification in these\nstars based on an asymptotic period relation. SB 459 (TIC 067584818), SB 815\n(TIC 169285097) and PG 0342+026 (TIC 457168745) have been monitored during\nsingle sectors resulting in 27 days coverage. These datasets allowed for\ndetecting, in each star, a few tens of frequencies, which we interpreted as\nstellar oscillations. We found no multiplets, though we partially constrained\nmode geometry by means of period spacing, which recently became a key tool in\nanalyses of pulsating subdwarf B stars. Standard routine that we have used\nallowed us to select candidates for trapped modes that surely bear signatures\nof non-uniform chemical profile inside the stars. We have also done statistical\nanalysis using collected spectroscopic and asteroseismic data of previously\nknown subdwarf B stars along with our three stars. Making use of high precision\ntrigonometric parallaxes from the Gaia mission and spectral energy\ndistributions we converted atmospheric parameters to stellar ones. Radii,\nmasses and luminosities are close to their canonical values for extreme\nhorizontal branch stars. In particular, the stellar masses are close to the\ncanonical one of 0.47 M$_\\odot$ for all three stars but uncertainties on the\nmass are large. The results of the analyses presented here will provide\nimportant constrains for asteroseismic modelling.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Vertical Current Approximation Nonlinear Force-Free Field Code -\n  Description, Performance Tests, and Measurements of Magnetic Energies\n  Dissipated in Solar Flares: In this work we provide an updated description of the Vertical Current\nApproximation Nonlinear Force-Free Field (VCA-NLFFF) code, which is designed to\nmeasure the evolution of the potential, nonpotential, free energies, and the\ndissipated magnetic energies during solar flares. This code provides a\ncomplementary and alternative method to existing traditional NLFFF codes. The\nchief advantages of the VCA-NLFFF code over traditional NLFFF codes are the\ncircumvention of the unrealistic assumption of a force-free photosphere in the\nmagnetic field extrapolation method, the capability to minimize the\nmisalignment angles between observed coronal loops (or chromospheric fibril\nstructures) and theoretical model field lines, as well as computational speed.\nIn performance tests of the VCA-NLFFF code, by comparing with the NLFFF code of\nWiegelmann (2004), we find agreement in the potential, nonpotential, and free\nenergy within a factor of about 1.3, but the Wiegelmann code yields in the\naverage a factor of 2 lower flare energies. The VCA-NLFFF code is found to\ndetect decreases in flare energies in most X, M, and C-class flares. The\nsuccessful detection of energy decreases during a variety of flares with the\nVCA-NLFFF code indicates that current-driven twisting and untwisting of the\nmagnetic field is an adequate model to quantify the storage of magnetic\nenergies in active regions and their dissipation during flares. - The VCA-NLFFF\ncode is also publicly available in the Solar SoftWare (SSW).",
        "category": "astro-ph_SR"
    },
    {
        "text": "Precise mass measurements for the double neutron star system J1829+2456: PSR J1829+2456 is a radio pulsar in a relativistic binary system with another\nneutron star. It has a rotational period of 41 ms and a mildly eccentric ($e =\n0.14$) 28-hr orbit. We have continued its observations with the Arecibo radio\ntelescope and have now measured the individual neutron star masses of this\nsystem. The pulsar and companion masses are $1.306\\,\\pm\\,0.007\\,M_{\\odot}$ and\n$1.299\\,\\pm\\,0.007\\,M_{\\odot}$ (2$\\sigma$ - 95% confidence, unless stated\notherwise), respectively. We have also measured the proper motion for this\nsystem and used it to estimate a space velocity of 49$^{+77}_{-30}$ km s$^{-1}$\nwith respect to the local standard of rest. The relatively low values for\ncompanion mass, space velocity and orbital eccentricity in this system make it\nsimilar to other double neutron star systems in which the second-formed neutron\nstar is thought to have formed in a low-kick, low mass-loss, symmetric\nsupernova.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Imaging Discovery of the Debris Disk Around HIP 79977: We present Subaru/HiCIAO H-band high-contrast images of the debris disk\naround HIP 79977, whose pres- ence was recently inferred from an infrared\nexcess. Our images resolve the disk for the first time, allowing\ncharacterization of its shape, size, and dust grain properties. We use angular\ndifferential imaging (ADI) to reveal the disk geometry in unpolarized light out\nto a radius of ~2\", as well as polarized differential imaging (PDI) to measure\nthe degree of scattering polarization out to ~1.5\". In order to strike a\nfavorable balance between suppression of the stellar halo and conservation of\ndisk flux, we explore the application of principal component analysis (PCA) to\nboth ADI and reference star subtraction. This allows accurate forward modeling\nof the effects of data reduction on simulated disk images, and thus direct\ncomparison with the imaged disk. The resulting best-fit values and well-fitting\nintervals for the model parameters are a surface brightness power-law slope of\nS_out = -3.2 [-3.6,-2.9], an inclination of i = 84{\\deg} [81{\\deg},86{\\deg}], a\nhigh Henyey-Greenstein forward-scattering parameter of g = 0.45 [0.35, 0.60],\nand a non-significant disk-star offset of u = 3.0 [-1.5, 7.5] AU = 24 [-13, 61]\nmas along the line of nodes. Furthermore, the tangential linear polarization\nalong the disk rises from ~10% at 0.5\" to ~45% at 1.5\". These measurements\npaint a consistent picture of a disk of dust grains produced by collisional\ncascades and blown out to larger radii by stellar radiation pressure.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Mass-Loss Induced Eccentric Kozai Mechanism: A New Channel for the\n  Production of Close Compact Object-Stellar Binaries: Over a broad range of initial inclinations and eccentricities an appreciable\nfraction of hierarchical triple star systems with similar masses are\nessentially unaffected by the Kozai-Lidov mechanism (KM) until the primary in\nthe central binary evolves into a compact object. Once it does, it may be much\nless massive than the other components in the ternary, enabling the \"eccentric\nKozai mechanism (EKM):\" the mutual inclination between the inner and outer\nbinary can flip signs driving the inner binary to very high eccentricity,\nleading to a close binary or collision. We demonstrate this \"Mass-loss Induced\nEccentric Kozai\" (MIEK) mechanism by considering an example system and defining\nan ad-hoc minimal separation between the inner two members at which tidal\naffects become important. For fixed initial masses and semi-major axes, but\nuniform distributions of eccentricity and cosine of the mutual inclination,\n~10% of systems interact tidally or collide while the primary is on the MS due\nto the KM or EKM. Those affected by the EKM are not captured by earlier\nquadrupole-order secular calculations. We show that fully ~30% of systems\ninteract tidally or collide for the first time as the primary swells to AU\nscales, mostly as a result of the KM. Finally, ~2% of systems interact tidally\nor collide for the first time after the primary sheds most of its mass and\nbecomes a WD, mostly as a result of the MIEK mechanism. These findings motivate\na more detailed study of mass-loss in triple systems and the formation of close\nNS/WD-MS and NS/WD-NS/WD binaries without an initial common envelope phase.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Using Machine Learning Methods to Forecast If Solar Flares Will Be\n  Associated with CMEs and SEPs: Among the eruptive activity phenomena observed on the Sun, the most\ntechnology threatening ones are flares with associated coronal mass ejections\n(CMEs) and solar energetic particles (SEPs). Flares with associated CMEs and\nSEPs are produced by magnetohydrodynamical processes in magnetically active\nregions (ARs) on the Sun. However, these ARs do not only produce flares with\nassociated CMEs and SEPs, they also lead to flares and CMEs, which are not\nassociated with any other event. In an attempt to distinguish flares with\nassociated CMEs and SEPs from flares and CMEs, which are unassociated with any\nother event, we investigate the performances of two machine learning\nalgorithms. To achieve this objective, we employ support vector machines (SVMs)\nand multilayer perceptrons (MLPs) using data from the Space Weather Database of\nNotification, Knowledge, Information (DONKI) of NASA Space Weather Center, {\\it\nthe Geostationary Operational Environmental Satellite} ({\\it GOES}), and the\nSpace-Weather Heliospheric and Magnetic Imager Active Region Patches (SHARPs).\nWe show that True Skill Statistics (TSS) and Heidke Skill Scores (HSS)\ncalculated for SVMs are slightly better than those from the MLPs. We also show\nthat the forecasting time frame of 96 hours provides the best results in\npredicting if a flare will be associated with CMEs and SEPs (TSS=0.92$\\pm$0.09\nand HSS=0.92$\\pm$0.08). Additionally, we obtain the maximum TSS and HSS values\nof 0.91$\\pm$0.06 for predicting that a flare will not be associated with CMEs\nand SEPs for the 36 hour forecast window, while the 108 hour forecast window\ngive the maximum TSS and HSS values for predicting CMEs will not be\naccompanying any events (TSS=HSS=0.98$\\pm$0.02).",
        "category": "astro-ph_SR"
    },
    {
        "text": "Magnetospherically driven optical and radio aurorae at the end of the\n  stellar main sequence: Aurorae are detected from all the magnetized planets in our Solar System,\nincluding Earth. They are powered by magnetospheric current systems that lead\nto the precipitation of energetic electrons into the high-latitude regions of\nthe upper atmosphere. In the case of the gas-giant planets, these aurorae\ninclude highly polarized radio emission at kilohertz and megahertz frequencies\nproduced by the precipitating electrons, as well as continuum and line emission\nin the infrared, optical, ultraviolet and X-ray parts of the spectrum,\nassociated with the collisional excitation and heating of the\nhydrogen-dominated atmosphere. Here we report simultaneous radio and optical\nspectroscopic observations of an object at the end of the stellar main\nsequence, located right at the boundary between stars and brown dwarfs, from\nwhich we have detected radio and optical auroral emissions both powered by\nmagnetospheric currents. Whereas the magnetic activity of stars like our Sun is\npowered by processes that occur in their lower atmospheres, these aurorae are\npowered by processes originating much further out in the magnetosphere of the\ndwarf star that couple energy into the lower atmosphere. The dissipated power\nis at least four orders of magnitude larger than what is produced in the Jovian\nmagnetosphere, revealing aurorae to be a potentially ubiquitous signature of\nlarge-scale magnetospheres that can scale to luminosities far greater than\nthose observed in our Solar System. These magnetospheric current systems may\nalso play a part in powering some of the weather phenomena reported on brown\ndwarfs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Detailed computation of hot-plasma atomic spectra: We present recent evolutions of the detailed opacity code SCO-RCG which\ncombines statistical modelings of levels and lines with fine-structure\ncalculations. The code now includes the Partially-Resolved-Transition-Array\nmodel, which allows one to replace a complex transition array by a small-scale\ndetailed calculation preserving energy and variance of the genuine transition\narray and yielding improved high-order moments. An approximate method for\nstudying the impact of strong magnetic field on opacity and emissivity was also\nrecently implemented. The Zeeman line profile is modeled by fourth-order\nGram-Charlier expansion series, which is a Gaussian multiplied by a linear\ncombination of Hermite polynomials. Electron collisional line broadening is\noften modeled by a Lorentzian function and one has to calculate the convolution\nof a Lorentzian with Gram-Charlier distribution for a huge number of spectral\nlines. Since the numerical cost of the direct convolution would be prohibitive,\nwe propose, in order to obtain the resulting profile, a fast and precise\nalgorithm, relying on a representation of the Gaussian by cubic splines.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Observations of magnetic reconnection and particle acceleration\n  locations in solar coronal jets: We present a multi-wavelength analysis of two flare-related jets on November\n13, 2014, using data from SDO/AIA, RHESSI, Hinode/XRT, and IRIS. Unlike most\ncoronal jets where hard X-ray (HXR) emissions are usually observed near the jet\nbase, in these events HXR emissions are found at several locations, including\nin the corona. We carry out the first differential emission measure (DEM)\nanalysis that combines both AIA (and XRT when available) bandpass filter data\nand RHESSI HXR measurements for coronal jets, and obtain self-consistent\nresults across a wide temperature range and into non-thermal energies. In both\nevents, hot plasma first appeared at the jet base, but as the base plasma\ngradually cooled, hot plasma also appeared near the jet top. Moreover,\nnon-thermal electrons, while only mildly energetic, are found in multiple HXR\nlocations and contain a large amount of total energy. Particularly, the\nenergetic electrons that produced the HXR sources at the jet top were\naccelerated near the top location, rather than traveling from a reconnection\nsite at the jet base. This means that there was more than one particle\nacceleration site in each event. Jet velocities are consistent with previous\nstudies, including upward and downward velocities around ~200 km/s and ~100\nkm/s respectively, and fast outflows of 400-700 km/s. We also examine the\nenergy partition in the later event, and find that the non-thermal energy in\naccelerated electrons is most significant compared to other energy forms\nconsidered. We discuss the interpretations and provide constraints on\nmechanisms for coronal jet formation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Hubble Space Telescope Snapshot Survey for Resolved Companions of\n  Galactic Cepheids: Final Results: Cepheids in multiple systems provide information on the outcome of the\nformation of massive stars. They can also lead to exotic end-stage objects.\nThis study concludes our survey of 70 galactic Cepheids using the {\\it Hubble\nSpace Telescope\\} (\\HST) Wide Field Camera~3 (WFC3) with images at two\nwavelengths to identify companions closer than $5\\arcsec$. In the entire WFC3\nsurvey we identify 16 probable companions for 13 Cepheids. The seven Cepheids\nhaving resolved candidate companions within $2\"$ all have the surprising\nproperty of themselves being spectroscopic binaries (as compared with a 29\\%\nincidence of spectroscopic binaries in the general Cepheid population). That is\na strong suggestion that an inner binary is linked to the scenario of a third\ncompanion within a few hundred~AU\\null. This characteristic is continued for\nmore widely separated companions. Under a model where the outer companion is\nformed first, it is unlikely that it can anticipate a subsequent inner binary.\nRather it is more likely that a triple system has undergone dynamical\ninteraction, resulting in one star moving outward to its current location. {\\it\nChandra\\} and {\\it Gaia\\} data as well as radial velocities and \\HSTSTIS and\n{\\it IUE\\} spectra are used to derive properties of the components of the\nCepheid systems.\n  The colors of the companion candidates show a change in distribution at\napproximately 2000~AU separations, from a range including both hot and cool\ncolors for closer companions, to only low-mass companions for wider\nseparations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Probing X-ray photoevaporative winds through their interaction with\n  ionising radiation in cluster environments: the case for X-ray proplyds: We show that if young low mass stars undergo vigorous X-ray driven disc\nwinds, these may be detected in clusters through their interaction with\nionising radiation from massive stars. We argue that in the ONC (Orion Nebula\nCluster) one should see $\\sim$ 10s of `X-ray proplyds' ( objects with optically\nimaged offset ionisation fronts) in the range $0.3-0.6$pc from $\\theta_1$C Ori\n(the dominant O star in the ONC). Such objects lie outside the central `FUV\nzone' in the ONC where proplyds are instead well explained by neutral winds\ndriven by external Far Ultraviolet (FUV) emission from $\\theta_1$C. We show\nthat the predicted numbers and sizes of X-ray proplyds are compatible with\nthose observed and that this may also explain at least some of the far flung\nproplyds seen in the Carina nebula. We compare the sizes of observed proplyds\noutside the FUV region of the ONC with model predictions based on the current\nobserved X-ray luminosities of these sources ( bearing in mind that the current\nsize is actually set by the X-ray luminosity a few hundred years ago, $\\sim$\nthe flow time to the ionisation front). We discuss whether variability on this\ntimescale can plausibly explain proplyd size data on a case by case basis. We\nalso show that the predicted radio free-free emission signature of X-ray\nproplyds is readily detectable. Monitoring is however required to distinguish\nsuch emission from non-thermal emission from active coronae. We also predict\nthat it is only at $> 1$ pc from $\\theta_1$C that the free-free emission from\nsuch offset ionised structures would be clearly distinguishable from an\nexternally driven ionised disc wind. We argue that the fortuitous proximity of\nmassive stars in the ONC can be used as a beacon to light up internally driven\nX-ray winds and that this is a promising avenue for observationally testing\nX-ray photoevaporation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spicule Dynamics over Plage Region: We studied spicular jets over a plage area and derived their dynamic\ncharacteristics using Hinode Solar Optical Telescope (SOT) high-resolution\nimages. The target plage region was near the west limb of the solar disk. This\nlocation permitted us to study the dynamics of spicular jets without the\noverlapping effect of spicular structures along the line of sight.\n  In this work, to increase the ease with which we can identify spicules on the\ndisk, we applied the image processing method `MadMax' developed by Koutchmy et\nal. (1989). It enhances fine, slender structures (like jets), over a diffuse\nbackground. We identified 169 spicules over the target plage. This sample\npermits us to derive statistically reliable results regarding spicular\ndynamics.\n  The properties of plage spicules can be summarized as follows: (1) In a plage\narea, we clearly identified spicular jet features. (2) They were shorter in\nlength than the quiet region limb spicules, and followed ballistic motion under\nconstant deceleration. (3) The majority (80%) of the plage spicules showed the\ncycle of rise and retreat, while 10% of them faded out without a complete\nretreat phase. (4) The deceleration of the spicule was proportional to the\nvelocity of ejection (i.e. the initial velocity).",
        "category": "astro-ph_SR"
    },
    {
        "text": "Resistive magnetohydrodynamic simulations of the ideal tearing mode: We study the linear and nonlinear evolution of the tearing instability on\nthin current sheets by means of two-dimensional numerical simulations, within\nthe framework of compressible, resistive magnetohydrodynamics. In particular we\nanalyze the behavior of current sheets whose inverse aspect ratio scales with\nthe Lundquist number $S$ as $S^{-1/3}$. This scaling has been recently\nrecognized to yield the threshold separating fast, ideal reconnection, with an\nevolution and growth which are independent of $S$ provided this is high enough,\nas it should be natural having the ideal case as a limit for $S\\to\\infty$. Our\nsimulations confirm that the tearing instability growth rate can be as fast as\n$\\gamma\\approx 0.6\\,{\\tau_A}^{-1}$, where $\\tau_A$ is the ideal Alfv\\'enic time\nset by the macroscopic scales, for our least diffusive case with $S=10^7$. The\nexpected instability dispersion relation and eigenmodes are also retrieved in\nthe linear regime, for the values of $S$ explored here. Moreover, in the\nnonlinear stage of the simulations we observe secondary events obeying the same\ncritical scaling with $S$, here calculated on the \\emph{local}, much smaller\nlengths, leading to increasingly faster reconnection. These findings strongly\nsupport the idea that in a fully dynamic regime, as soon as current sheets\ndevelop, thin and reach this critical threshold in their aspect ratio, the\ntearing mode is able to trigger plasmoid formation and reconnection on the\nlocal (ideal) Alfv\\'enic timescales, as required to explain the explosive\nflaring activity often observed in solar and astrophysical plasmas.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Signature of mass supply to quiet coronal loops: Aims. The physical implication of large blue shift of Ne viii in the quiet\nSun region is investigated in this paper. Methods. We compare the significant\nNe viii blue shifts, which are visible as large blue patches on the\nDoppler-shift map of a middlelatitude quiet-Sun region observed by SUMER, with\nthe coronal magnetic-field structures as reconstructed from a simultaneous\nphotospheric magnetogram by means of a force-free-field extrapolation. Results.\nWe show for the first time that coronal funnels also exist in the quiet Sun.\nThe region studied contains several small funnels that originate from network\nlanes, expand with height and finally merge into a single wide open-field\nregion. However, the large blue shifts of the Ne viii line are not generally\nassociated with funnels. A comparison between the projections of coronal loops\nonto the solar x-y-plane and the Ne viii dopplergram indicates that there are\nsome loops that reveal large Ne viii blue shifts in both legs, and some loops\nwith upflow in one and downflow in the other leg. Conclusions. Our results\nsuggest that strong plasma outflow, which can be traced by large Ne viii blue\nshift, is not necessarily associated with the solar wind originating in coronal\nfunnels but appears to be a signature of mass supply to coronal loops. Under\nthe assumption that the measured Doppler shift of the Ne viii line represents\nthe real outflow velocity of the neon ions being markers of the proton flow, we\nestimate the mass supply rate to coronal loops to be about 10\\^{34} s\\^{-1}.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The first photometric investigation of the contact binary OQ Cam: The first charge-coupled device photometric light curves in B, V, R${_c}$,\nand I${_c}$ bands of the short-period contact binary star OQ Cam are presented.\nThrough analyzing the light curves with the Wilson-Devinney code, it is found\nthat OQ Cam is a W-type shallow contact binary star with a contact degree of f\n=20.2%. Based on the O-C analyzing, the orbit period displays a long term\nincreasing with a rate of $dP/dt=4.40\\times10{^{-7}} day \\cdot year{^{-1}}$.\nThe increasing in orbit period can be interpreted by mass transfer from the\nless massive component to the more massive one. As the orbit period increasing,\nOQ Cam would evolve from the present shallow contact configuration to a none\ncontact stage. So it may be a potential candidate to confirm the thermal\nrelaxation oscillation theory.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Low-Mass Pre-Main-Sequence Eclipsing Binary in Lower Centaurus Crux\n  Discovered with TESS: We report the discovery of 2M1222-57 as a low-mass, pre-main-sequence (PMS)\neclipsing binary (EB) in the Lower Centaurus Crux (LCC) association for which,\nusing Gaia parallaxes and proper motions with a neural-net age estimator, we\ndetermine an age of 16.2$\\pm$2.2 Myr. The broadband spectral energy\ndistribution (SED) shows clear excess at ~10 um indicative of a circumbinary\ndisk, and new speckle-imaging observations reveal a faint, tertiary companion\nseparated by ~100 AU. H-alpha emission is modulated on the orbital period,\nconsistent with theoretical models of orbitally pulsed accretion streams\nreaching from the inner disk edge to the central stars. From a joint analysis\nof spectroscopically determined radial velocities and TESS light curves,\ntogether with additional tight constraints provided by the SED and the Gaia\nparallax, we measure masses for the eclipsing stars of 0.74 Msun and 0.67 Msun;\nradii of 0.98 Rsun and 0.94 Rsun; and effective temperatures of 3750 K and 3645\nK. The masses and radii of both stars are measured to an accuracy of ~1%. The\nmeasured radii are inflated, and the temperatures suppressed, relative to\npredictions of standard PMS evolutionary models at the age of LCC; also, the Li\nabundances are ~2 dex less depleted than predicted by those models. However,\nmodels that account for the global and internal effects of surface magnetic\nfields are able to simultaneously reproduce the measured radii, temperatures,\nand Li abundances at an age of 17.0$\\pm$0.5 Myr. Altogether, the 2M1222-57\nsystem presents very strong evidence that magnetic activity in young stars\nalters both their global properties and the physics of their interiors.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Lagrangian coherent structures in nonlinear dynamos: Turbulence and chaos play a fundamental role in stellar convective zones\nthrough the transportof particles, energy and momentum, and in fast dynamos,\nthrough the stretching, twisting and folding of magnetic flux tubes. A\nparticularly revealing way to describe turbulent motions is through the\nanalysis of Lagrangian coherent structures (LCS), which are material lines or\nsurfaces that act as transport barriers in the fluid. We report the detection\nof Lagrangian coherent structures in helical MHD dynamo simulations with scale\nseparation. In an ABC--flow, two dynamo regimes, a propagating coherent\nmean--field regime and an intermittent regime, are identified as the magnetic\ndiffusivity is varied. The sharp contrast between the chaotic tangle of\nattracting and repelling LCS in both regimes permits a unique analysis of the\nimpact of the magnetic field on the velocity field. In a second example, LCS\nreveal the link between the level of chaotic mixing of the velocity field and\nthe saturation of a large--scale dynamo when the magnetic field exceeds the\nequipartition value.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Discovery of a new photometric sub-class of faint and fast classical\n  novae: We present photometric and spectroscopic follow-up of a sample of\nextragalactic novae discovered by the Palomar 60-inch telescope during a search\nfor \"Fast Transients In Nearest Galaxies\" (P60-FasTING). Designed as a fast\ncadence (1-day) and deep (g < 21 mag) survey, P60-FasTING was particularly\nsensitive to short-lived and faint optical transients. The P60-FasTING nova\nsample includes 10 novae in M31, 6 in M81, 3 in M82, 1 in NGC2403 and 1 in\nNGC891. This significantly expands the known sample of extragalactic novae\nbeyond the Local Group, including the first discoveries in a starburst\nenvironment. Surprisingly, our photometry shows that this sample is quite\ninconsistent with the canonical Maximum Magnitude Rate of Decline (MMRD)\nrelation for classical novae. Furthermore, the spectra of the P60-FasTING\nsample are indistinguishable from classical novae. We suggest that we have\nuncovered a sub-class of faint and fast classical novae in a new phase space in\nluminosity-timescale of optical transients. Thus, novae span two orders of\nmagnitude in both luminosity and time. Perhaps, the MMRD, which is\ncharacterized only by the white dwarf mass, was an over-simplification. Nova\nphysics appears to be characterized by quite a rich four-dimensional parameter\nspace in white dwarf mass, temperature, composition and accretion rate.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Sunspot umbra atmosphere from full Stokes inversion: Sunspots are prominent manifestations of the solar cycle and provide key\nconstraints for understanding its operation. Also, knowing internal structure\nof sunspots allows us to gain insights on the energy transport in strong\nmagnetic fields and, thus, on the processes inside the convection zone, where\nsolar magnetic fields are generated and amplified before emerging at the\nsurface on various scales, even during solar minima. In this paper, we present\nresults of a spectropolarimetric analysis of a sunspot observed during the\ndeclining phase of the solar cycle 23. By inversion of full Stokes spectra\nobserved in several spectral regions in the optical at the THEMIS facility we\ninfer the height dependence of physical quantities such as the temperature and\nthe magnetic field strength for different sunspot regions. The simultaneous use\nof atomic (Fe{\\sc i} 5250.2 and 5250.6 \\AA) and highly temperature sensitive\nmolecular (TiO 7055 \\AA and MgH 5200 \\AA) lines allow us to improve a model of\nthe sunspot umbra.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A JWST project on 47 Tucanae. Overview, photometry and early\n  spectroscopic results of M dwarfs, and observation of brown dwarfs: The James Webb Space Telescope (JWST) observations have been demonstrated to\nbe efficient in detecting globular clusters' (GCs) multiple stellar populations\nin the low mass regime of M dwarfs. We present an overview, and first results,\nof different projects that can be explored by using the JWST observations\ngathered under the GO2560 for 47 Tucanae, a first program entirely devoted to\nthe investigation of multiple populations in very low mass stars, which\nincludes spectroscopic data for the faintest GC stars for which spectra are\navailable. Our color-magnitude diagram (CMD) shows some substructures for\nultracool stars, including gaps and breaks in slope. In particular, we observe\nboth a gap and a minimum in the F322W2 luminosity function less than one\nmagnitude apart, and discuss which one could be associated with the H-burning\nlimit. We detect stars fainter than this minimum, very likely the brown dwarfs.\nWe corroborate the ubiquity of the multiple populations across different\nmasses, from ~0.1 solar masses up to red giants (~0.8 solar masses). The oxygen\nrange inferred from the M dwarfs, both from the CMD and from the spectra of two\nM dwarfs associated with different populations, is similar to that observed in\ngiants. We have not detected any difference between the fractions of stars in\ndistinct populations across stellar masses >~0.1 solar masses. This work\ndemonstrates the JWST's capability in uncovering multiple populations within M\ndwarfs and illustrates the possibility to analyse very low-mass stars in GCs\napproaching the H-burning limit and the brown-dwarf sequence.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Using ForeCAT Deflections and Rotations to Constrain the Early Evolution\n  of CMEs: To accurately predict the space weather effects of coronal mass ejection\n(CME) impacts at Earth one must know if and when a CME will impact Earth, and\nthe CME parameters upon impact. Kay et al. (2015b) presents Forecasting a CME's\nAltered Trajectory (ForeCAT), a model for CME deflections based on the magnetic\nforces from the background solar magnetic field. Knowing the deflection and\nrotation of a CME enables prediction of Earth impacts, and the CME orientation\nupon impact. We first reconstruct the positions of the 2008 April 10 and the\n2012 July 12 CMEs from the observations. The first of these CMEs exhibits\nsignificant deflection and rotation (34 degrees deflection and 58 degrees\nrotation), while the second shows almost no deflection or rotation (<3 degrees\neach). Using ForeCAT, we explore a range of initial parameters, such as the CME\nlocation and size, and find parameters that can successfully reproduce the\nbehavior for each CME. Additionally, since the deflection depends strongly on\nthe behavior of a CME in the low corona (Kay et al. (2015a, 2015b)), we are\nable to constrain the expansion and propagation of these CMEs in the low\ncorona.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Solar cycle variations of large frequency separations of acoustic modes:\n  Implications for asteroseismology: We have studied solar cycle changes in the large frequency separations that\ncan be observed in Birmingham Solar Oscillations Network (BiSON) data. The\nlarge frequency separation is often one of the first outputs from asteroseismic\nstudies because it can help constrain stellar properties like mass and radius.\nWe have used three methods for estimating the large separations: use of\nindividual p-mode frequencies, computation of the autocorrelation of\nfrequency-power spectra, and computation of the power spectrum of the power\nspectrum. The values of the large separations obtained by the different methods\nare offset from each other and have differing sensitivities to the realization\nnoise. A simple model was used to predict solar cycle variations in the large\nseparations, indicating that the variations are due to the well-known solar\ncycle changes to mode frequency. However, this model is only valid over a\nrestricted frequency range. We discuss the implications of these results for\nasteroseismology.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Suzaku observation of the transient X-ray pulsar GRO J1008-57: We report the timing and broad-band spectral properties of the Be transient\nhigh mass X-ray binary pulsar GRO J1008-57 using a Suzaku observation in the\ndeclining phase of its 2007 November-December outburst. Pulsations with a\nperiod of 93.737 s were clearly detected in the light curves of the pulsar up\nto the 80-100 keV energy band. The pulse profile was found to be strongly\nenergy dependent, a double peaked profile at soft X-ray energy bands (< 8 keV)\nand a single peaked smooth profile at hard X-rays. The broad-band energy\nspectrum of the pulsar, reported for the first instance in this paper, is well\ndescribed with three different continuum models viz. (i) a high energy cut-off\npower-law, (ii) a Negative and Positive power-law with EXponential cut-off\n(NPEX), and (iii) a partial covering power-law with high energy cut-off.\nInspite of large value of absorption column density in the direction of the\npulsar, a blackbody component of temperature ~0.17 keV for the soft excess was\nrequired for the first two continuum models. A narrow iron K_\\alpha emission\nline was detected in the pulsar spectrum. The partial covering model, however,\nis found to explain the phase averaged and phase resolved spectra well. The dip\nlike feature in the pulse profile can be explained by the presence of an\nadditional absorption component with high column density and covering fraction\nat the same pulse phase. The details of the results are described in the paper.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Twelve years of spectroscopic monitoring in the Galactic Center: the\n  closest look at S-stars near the black hole: We study the young S-stars within a distance of 0.04 pc from the supermassive\nblack hole in the center of our Galaxy. Given how inhospitable the region is\nfor star formation, their presence is more puzzling the younger we estimate\ntheir ages. In this study, we analyse the result of 12 years of high resolution\nspectroscopy within the central arcsecond of the Galactic Center (GC). By\nco-adding between 55 and 105 hours of spectra we have obtained high signal to\nnoise H- and K-band spectra of eight stars orbiting the central supermassive\nblack hole. Using deep H-band spectra, we show that these stars must be high\nsurface gravity (dwarf) stars. We compare these deep spectra to detailed model\natmospheres and stellar evolution models to infer the stellar parameters. Our\nanalysis reveals an effective temperature of 21000-28500 K, a rotational\nvelocity of 60-170 km/s, and a surface gravity of 4.1-4.2. These parameters\nimply a spectral type of B0-B3V for these stars. The inferred masses lie within\n8-14 Msun. We derive an age of 6.6^{+3.4}{-4.7} Myr for the star S2, which is\ncompatible with the age of the clockwise rotating young stellar disk in the GC.\nWe estimate the age of all other studied S-stars to be less than 15 Myr, which\nare compatible with the age of S2 within the uncertainties. The relatively low\nages for these S-stars favor a scenario in which the stars formed in a local\ndisk rather than the field-binary-disruption scenario throughout a longer\nperiod of time.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Chemical Abundances in a Sample of Red Giants in the Open Cluster NGC\n  2420 from APOGEE: NGC 2420 is a $\\sim$2 Gyr-old well-populated open cluster that lies about 2\nkpc beyond the solar circle, in the general direction of the Galactic\nanti-center. Most previous abundance studies have found this cluster to be\nmildly metal-poor, but with a large scatter in the obtained metallicities for\nthis open cluster. Detailed chemical abundance distributions are derived for 12\nred-giant members of NGC 2420 via a manual abundance analysis of\nhigh-resolution (R = 22,500) near-infrared ($\\lambda$1.5 - 1.7$\\mu$m) spectra\nobtained from the Apache Point Observatory Galactic Evolution Experiment\n(APOGEE) survey. The sample analyzed contains 6 stars that are identified as\nmembers of the first-ascent red giant branch (RGB), as well as 6 members of the\nred clump (RC). We find small scatter in the star-to-star abundances in NGC\n2420, with a mean cluster abundance of [Fe/H] = -0.16 $\\pm$ 0.04 for the 12 red\ngiants. The internal abundance dispersion for all elements (C, N, O, Na, Mg,\nAl, Si, K, Ca, Ti, V, Cr, Mn, Co and Ni) is also very small ($\\sim$0.03 - 0.06\ndex), indicating a uniform cluster abundance distribution within the\nuncertainties. NGC 2420 is one of the clusters used to calibrate the APOGEE\nStellar Parameter and Chemical Abundance Pipeline (ASPCAP). The results from\nthis manual analysis compare well with ASPCAP abundances for most of the\nelements studied, although for Na, Al and V there are more significant offsets.\nNo evidence of extra-mixing at the RGB luminosity bump is found in the $^{12}$C\nand $^{14}$N abundances from the pre-luminosity-bump RGB stars in comparison to\nthe post-He core-flash RC stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The 26 December 2001 Solar Eruptive Event Responsible for GLE63. II.\n  Multi-Loop Structure of Microwave Sources in a Major Long-Duration flare: Analysis of the observations of the SOL2001-12-26 event related to\nground-level-event GLE63, including microwave spectra and images from NoRH at\n17 and 34 GHz, SSRT at 5.7 GHz, and TRACE in 1600 \\AA\\ has led to the following\nresults. A flare ribbon overlapped with the sunspot umbra, which is typical of\nlarge particle events. Atypical were: i) long duration of the flare of more\nthan one hour; ii) moderate intensity of a microwave burst, about $10^4$ sfu;\niii) low peak frequency of the gyrosynchrotron spectrum, around 6 GHz; and its\ninsensitivity to the flux increase by more than one order of magnitude. This\nwas accompanied by a nearly constant ratio of the flux emitted by the volume in\nthe high-frequency part of the spectrum to its elevated low-frequency part\ndetermined by the area of the source. With the self-similarity of the spectrum,\na similarity was observed between the moving microwave sources and the\nbrightest parts of the flare ribbons in 1600 \\AA. Comparison of the 17 GHz and\n1600 \\AA\\ images has confirmed that the microwave sources were associated with\nmultiple flare loops, whose footpoints appeared in ultraviolet as intermittent\nbright kernels. To understand the properties of the event, we simulated its\nmicrowave emission using a system of several homogeneous gyrosynchrotron\nsources above the ribbons. The scatter between the spectra and sizes of the\nindividual sources is determined by the inhomogeneity of the magnetic field\nwithin the ribbons. The microwave flux is mainly governed by the magnetic flux\npassing through the ribbons and the sources. An apparent simplicity of\nmicrowave structures is caused by a poorer spatial resolution and dynamic range\nof the microwave imaging. The results indicate that microwave manifestations of\naccelerated electrons correspond to the structures observed in thermal\nemissions, as well-known models predict.",
        "category": "astro-ph_SR"
    },
    {
        "text": "NSE abundance data: Novel method of calculating Nuclear Statistical Equilibrium is presented.\nBasic equations are carefully solved using arbitrary precision arithmetic.\nSpecial interpolation procedure is then used to retrieve all abundances using\ntabulated results for neutrons and protons, together with basic nuclear data.\nProton and neutron abundance tables, basic nuclear data and partition functions\nfor nuclides used in calculations are provided. Simple interpolation algorithm\nusing pre-calculated p and n abundances tabulated as a functions of kT, rho and\nYe is outlined. Unique properties of this method are: (1) ability to pick-up\nout of NSE selected nuclei only (2) computational time scaling linearly with\nnumber of re-calculated abundances (3) relatively small amount of stored data:\nonly two large tables (4) slightly faster than solving NSE equations using\ntraditional Newton-Raphson methods for small networks (few tens of species);\nsuperior for huge (800-3000) networks (5) do not require initial guess; works\nwell on random input (6) can tailored to specific application (7) ability to\nuse third-party NSE solvers to obtain fully compatible tables (8) encapsulation\nof the NSE code for bug-free calculations.\n  Range of applications for this approach is possible: coverage test of\ntraditional NSE Newton-Raphson codes, generating starting values, code-to-code\nverification and possible replacement of the old legacy procedures in supernova\nsimulations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Identification of red supergiants in nearby galaxies with mid-IR\n  photometry: The role of episodic mass loss in massive star evolution is one of the most\nimportant open questions of current stellar evolution theory. Episodic mass\nloss produces dust and therefore causes evolved massive stars to be very\nluminous in the mid-infrared and dim at optical wavelengths. We aim to increase\nthe number of investigated luminous mid-IR sources to shed light on the late\nstages of these objects. To achieve this we employed mid-IR selection criteria\nto identity dusty evolved massive stars in two nearby galaxies. The method is\nbased on mid-IR colors, using 3.6 {\\mu}m and 4.5 {\\mu}m photometry from\narchival Spitzer Space Telescope images of nearby galaxies and J-band\nphotometry from 2MASS. We applied our criteria to two nearby star-forming dwarf\nirregular galaxies, Sextans A and IC 1613, selecting eight targets, which we\nfollowed up with spectroscopy. Our spectral classification and analysis yielded\nthe discovery of two M-type supergiants in IC 1613, three K-type supergiants\nand one candidate F-type giant in Sextans A, and two foreground M giants. We\nshow that the proposed criteria provide an independent way for identifying\ndusty evolved massive stars, that can be extended to all nearby galaxies with\navailable Spitzer/IRAC images at 3.6 {\\mu}m and 4.5 {\\mu}m.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Fine structures of radio bursts from flare star AD Leo with FAST\n  observations: Radio bursts from nearby active M-dwarfs have been frequently reported and\nextensively studied in solar or planetary paradigms. Whereas, their\nsub-structures or fine structures remain rarely explored despite their\npotential significance in diagnosing the plasma and magnetic field properties\nof the star. Such studies in the past have been limited by the sensitivity of\nradio telescopes. Here we report the inspiring results from the high\ntime-resolution observations of a known flare star AD Leo with the\nFive-hundred-meter Aperture Spherical radio Telescope (FAST). We detected many\nradio bursts in the two days of observations with fine structures in the form\nof numerous millisecond-scale sub-bursts. Sub-bursts on the first day display\nstripe-like shapes with nearly uniform frequency drift rates, which are\npossibly stellar analogs to Jovian S-bursts. Sub-bursts on the second day,\nhowever, reveal a different blob-like shape with random occurrence patterns and\nare akin to solar radio spikes. The new observational results suggest that the\nintense emission from AD Leo is driven by electron cyclotron maser instability\nwhich may be related to stellar flares or interactions with a planetary\ncompanion.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Alfv\u00e9n wave experiments with liquid rubidium in a pulsed magnetic\n  field: Magnetic fields are key ingredients for heating the solar corona to\ntemperatures of several million Kelvin. A particularly important region with\nrespect to this is the so-called magnetic canopy below the corona, where sound\nand Alfv\\'en waves have roughly the same speed and can, therefore, easily\ntransform into each other. We present the results of an Alfv\\'en-wave\nexperiment with liquid rubidium carried out in a pulsed field of up to 63 T. At\nthe critical point of 54 T, where the speeds of Alfv\\'en waves and sound\ncoincide, a new 4 kHz signal appears in addition to the externally excited 8\nkHz torsional wave. This emergence of an Alfv\\'en wave with a doubled period is\nin agreement with the theoretical predictions of a parametric resonance between\nthe two wave types. We also present preliminary results from numerical\nsimulations of Alfv\\'en and magneto-sonic waves using a compressible MHD code.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Empirical 2MASS-WFC3/IR filter transformations from synthetic photometry: Near-infrared bandpasses on spaceborne observatories diverge from their\nground-based counterparts as they are free of atmospheric telluric absorption.\nAvailable transformations between respective filter systems in the literature\nrely on theoretical stellar atmospheres, which are known to have difficulties\nreproducing observed spectral energy distributions of cool giants. We present\nnew transformations between the 2MASS $JHK_S$ and HST WFC3/IR F110W, F125W, &\nF160W photometric systems based on synthetic photometry of empirical stellar\nspectra from four spectral libraries. This sample comprises over 1000\nindividual stars, which together span nearly the full HR diagram and sample\nstellar populations from the solar neighborhood out to the Magellanic Clouds,\ncovering a broad range of ages, metallicities, and other relevant stellar\nproperties. In addition to global color-dependent transformations, we examine\nband-to-band differences for cool, luminous giant stars in particular,\nincluding multiple types of primary distance indicators.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Jeans Instability in a viscoelastic fluid: The well known Jeans instability is studied for a viscoelastic, gravitational\nfluid using generalized hydrodynamic equations of motions. It is found that the\nthreshold for the onset of instability appears at higher wavelengths in a\nviscoelastic medium. Elastic effects playing a role similar to thermal pressure\nare found to lower the growth rate of the gravitational instability. Such\nfeatures may manifest themselves in matter constituting dense astrophysical\nobjects.",
        "category": "astro-ph_SR"
    },
    {
        "text": "HD 51106 and HD 50747: an ellipsoidal binary and a triple system\n  observed with CoRoT: We present an analysis of the observations of HD 51106 and HD 50747 by the\nsatellite CoRoT, obtained during its initial run, and of the spectroscopic\npreparatory observations.\n  AIMS: We complete an analysis of the light curve, extract the main\nfrequencies observed, and discuss some preliminary interpretations about the\nstars.\n  Methods: We used standard Fourier transform and pre-whitening methods to\nextract information about the periodicities of the stars.\n  Results: HD 51106 is an ellipsoidal binary, the light curve of which can be\ncompletely explained by the tidal deformation of the star and smaller secondary\neffects. HD 50747 is a triple system containing a variable star, which exhibits\nmany modes of oscillation with periods in the range of a few hours. On the\nbasis of this period range and the analysis of the physical parameters of the\nstar, we conclude that HD 50747 is a Gamma-Doradus star.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Luminosities of Carbon-rich Asymptotic Giant Branch stars in the Milky\n  Way: Stars evolving along the Asymptotic Giant Branch can become Carbon-rich in\nthe final part of their evolution. They replenish the inter-stellar medium with\nnuclear processed material via strong radiative stellar winds. The\ndetermination of the luminosity function of these stars, even if far from being\nconclusive, is extremely important to test the reliability of theoretical\nmodels. In particular, strong constraints on the mixing treatment and the\nmass-loss rate can be derived.\n  We present an updated Luminosity Function of Galactic Carbon Stars obtained\nfrom a re-analysis of available data already published in previous papers.\nStarting from available near- and mid-infrared photometric data, we\nre-determine the selection criteria. Moreover, we take advantage from updated\ndistance estimates and Period-Luminosity relations and we adopt a new\nformulation for the computation of Bolometric Corrections. This leads us to\ncollect an improved sample of carbon-rich sources from which we construct an\nupdated Luminosity Function.\n  The Luminosity Function of Galactic Carbon Stars peaks at magnitudes around\n-4.9, confirming the results obtained in a previous work. Nevertheless, the\nLuminosity Function presents two symmetrical tails instead of the larger high\nluminosity tail characterizing the former Luminosity Function. The derived\nLuminosity Function of Galactic Carbon Stars matches the indications coming\nfrom recent theoretical evolutionary Asymptotic Giant Branch models, thus\nconfirming the validity of the choices of mixing treatment and mass-loss\nhistory. Moreover, we compare our new Luminosity Function with its counterpart\nin the Large Magellanic Cloud finding that the two distributions are very\nsimilar for dust-enshrouded sources, as expected from stellar evolutionary\nmodels. Finally, we derive a new fitting formula aimed to better determine\nBolometric Corrections for C-stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "First axion bounds from a pulsating helium-rich white dwarf star: The Peccei-Quinn mechanism proposed to solve the CP problem of Quantum\nChromodynamics has as consequence the existence of axions, hypothetical weakly\ninteracting particles whose mass is constrained to be on the sub-eV range. If\nthese particles exist and interact with electrons, they would be emitted from\nthe dense interior of white dwarfs, becoming an important energy sink for the\nstar. Due to their well known physics, white dwarfs are good laboratories to\nstudy the properties of fundamental particles such as the axions. We study the\ngeneral effect of axion emission on the evolution of helium-rich white dwarfs\nand on their pulsational properties. To this aim, we calculate evolutionary\nhelium-rich white dwarf models with axion emission, and asses the pulsational\nproperties of this models. Our results indicate that the rates of change of\npulsation periods are significantly affected by the existence of axions. We are\nable for the first time to independently constrain the mass of the axion from\nthe study of pulsating helium-rich white dwarfs. To do this, we use an\nestimation of the rate of change of period of the pulsating white dwarf PG\n1351+489 corresponding to the dominant pulsation period. From an\nasteroseismological model of PG 1351+489 we obtain $g_{ae}<3.3\\times10^{-13}$\nfor the axion-electron coupling constant, or $m_a\\cos^2{\\beta}\\lesssim$ 11.5\nmeV for the axion mass. This constraint is relaxed to\n$g_{ae}<5.5\\times10^{-13}$ ($m_a\\cos^2{\\beta}\\lesssim$ 19.5 meV), when no\ndetailed asteroseismological model is adopted for the comparison with\nobservations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On modeling ICME cross-sections as static MHD columns: Solar coronal mass ejections are well known to expand as they propagate\nthrough the heliosphere. Despite this, their cross-sections are usually modeled\nas static plasma columns within the magnetohydrodynamics (MHD) framework. We\ntest the validity of this approach using in-situ plasma data from 151 magnetic\nclouds (MCs) observed by the WIND spacecraft and 45 observed by the Helios\nspacecrafts. We find that the most probable cross-section expansion speeds for\nthe WIND events are only $\\approx 0.06$ times the Alfv\\'en speed inside the MCs\nwhile the most probable cross-section expansion speeds for the Helios events is\n$\\approx 0.03$. MC cross-sections can thus be considered to be nearly static\nover an Alfv\\'en crossing timescale. Using estimates of electrical conductivity\narising from Coulomb collisions, we find that the Lundquist number inside MCs\nis high ($\\approx 10^{13}$), suggesting that the MHD description is well\njustified. The Joule heating rates using our conductivity estimates are several\norders of magnitude lower than the requirement for plasma heating inside MCs\nnear the Earth. While the (low) heating rates we compute are consistent with\nthe MHD description, the discrepancy with the heating requirement points to\npossible departures from MHD and the need for a better understanding of plasma\nheating in MCs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "An abundance analysis from the STIS-HST UV spectrum of the non-magnetic\n  Bp star HR 6000: The sharp-line spectrum of the Bp star HR 6000 has peculiarities that\ndistinguish it from those of the HgMn stars with which it is sometimes\nassociated. The position of the star close to the center of the Lupus 3\nmolecular cloud, whose estimated age is on the order of 9.1 +/- 2.1 Myr, has\nlead to the hypothesis that the anomalous peculiarities of HR 6000 can be\nexplained by the young age of the star. Observational material from HST\nprovides the opportunity to extend the abundance analysis previously performed\nfor the optical region and clarify the properties of this remarkable peculiar\nstar. Our aim was to obtain the atmospheric abundances for all the elements\nobserved in a broad region from 1250 to 10000 A. An LTE synthetic spectrum was\ncompared with a high-resolution spectrum observed with STIS equipment in the\n1250-3040 A interval. The adopted model is an ATLAS12 model already used for\nthe abundance analysis of a high-resolution optical spectrum observed at ESO\nwith UVES. The stellar parameters are Teff=13450K, logg=4.3, and zero\nmicroturbulent velocity. Abundances for 28 elements and 7 upper limits were\nderived from the ultraviolet spectrum. Adding results from previous work, we\nhave now quantitative results for 37 elements, some of which show striking\ncontrasts with those of a broad sample of HgMn stars. The analysis has pointed\nout ionization anomalies and line-to-line variation in the derived abundances,\nin particular for silicon. The inferred discrepancies could be explained by\nnon-LTE effects and with the occurrence of diffusion and vertical abundance\nstratification. In the framework of the last hypothesis, we obtained, by means\nof trial and error, empirical step functions of abundance versus optical depth\nlog(tau_5000) for carbon, silicon, manganese, and gold, while we failed to find\nsuch a function for phosphorous.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Observations of Reconnection Flows in a Flare on the Solar Disk: Magnetic reconnection is a well-accepted part of the theory of solar eruptive\nevents, though the evidence is still circumstantial. Intrinsic to the\nreconnection picture of a solar eruptive event, particularly in the standard\nmodel for two-ribbon flares (\"CSHKP\" model), are an advective flow of\nmagnetized plasma into the reconnection region, expansion of field above the\nreconnection region as a flux rope erupts, retraction of heated\npost-reconnection loops, and downflows of cooling plasma along those loops. We\nreport on a unique set of SDO/AIA imaging and Hinode/EIS spectroscopic\nobservations of the disk flare SOL2016-03-23T03:54 in which all four flows are\npresent simultaneously. This includes spectroscopic evidence for a plasma\nupflow in association with large-scale expanding closed inflow field. The\nreconnection inflows are symmetric, and consistent with fast reconnection, and\nthe post-reconnection loops show a clear cooling and deceleration as they\nretract. Observations of coronal reconnection flows are still rare, and most\nevents are observed at the solar limb, obscured by complex foregrounds, making\ntheir relationship to the flare ribbons, cusp field and arcades formed in the\nlower atmosphere difficult to interpret. The disk location and favorable\nperspective of this event have removed these ambiguities giving a clear picture\nof the reconnection dynamics.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evidence of a Mira-like tail and bow shock about the semi-regular\n  variable V CVn from four decades of polarization measurements: Polarization is a powerful tool for understanding stellar atmospheres and\ncircumstellar environments. Mira and semi-regular variable stars have been\nobserved for decades and some are known to be polarimetrically variable,\nhowever, the semi-regular variable V Canes Venatici displays an unusually\nlarge, unexplained amount of polarization. We present ten years of optical\npolarization observations obtained with the HPOL instrument, supplemented by\npublished observations spanning a total interval of about forty years for V\nCVn. We find that V CVn shows large polarization variations ranging from 1 -\n6%. We also find that for the past forty years the position angle measured for\nV CVn has been virtually constant suggesting a long-term, stable, asymmetric\nstructure about the star. We suggest that this asymmetry is caused by the\npresence of a stellar wind bow shock and tail, consistent with the star's large\nspace velocity.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Far Ultraviolet Spectroscopy of Three Long Period Nova-Like Variables: We have selected three nova-like variables at the long period extreme of\nnova-like orbital periods: V363 Aur, RZ Gru and AC Cnc, all with IUE archival\nfar ultraviolet spectra. All are UX UMa type nova-like variables and all have\n$P_{orb} > 7$h. V363 Aur is a bona fide SW Sex star, and AC Cnc is a probable\none, while RZ Gru has not proven to be a member of the SW Sex subclass. We have\ncarried out the first synthetic spectral analysis of far ultraviolet spectra of\nthe three systems using state-of-the-art models both of accretion disks and\nwhite dwarf photospheres. We find that the FUV spectral energy distribution of\nboth V363 Aur and RZ Gru are in agreement with optically thick steady state\naccretion disk models in which the luminous disk accounts for 100% of the FUV\nlight. We present accretion rates and model-derived distances for V363 Aur and\nRZ Gru. For AC Cnc, we find that a hot accreting white dwarf accounts for\n$\\sim$ 60% of the FUV light with an accretion disk providing the rest. We\ncompare our accretion rates and model-derived distances with estimates in the\nliterature.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Chemical Composition of the Atmospheres of Red Giants with High Space\n  Velocities: The results of a comparative analysis of the elemental abundances in the\natmospheres of 14 red giants with high Galactic space velocities are presented.\nFor almost all of the chemical elements considered, the their abundance trends\nwith metallicity correspond to those constructed for thick-disk dwarfs. In the\ncase of sodium, the main factor affecting the [Na/Fe] abundance in the stellar\natmosphere for red giants is the surface gravity that characterizes the degree\nof development of the convective envelope. The difference between the [Na/Fe]\nabundances in the atmospheres of thin-and thick-disk red giants has been\nconfirmed.",
        "category": "astro-ph_SR"
    },
    {
        "text": "MESA Models of the Evolutionary State of the Interacting Binary epsilon\n  Aurigae: Using MESA code (Modules for Experiments in Stellar Astrophysics, version\n9575), an evaluation was made of the evolutionary state of the epsilon Aurigae\nbinary system (HD 31964, F0Iap + disk). We sought to satisfy several\nobservational constraints: 1) requiring evolutionary tracks to pass close to\nthe current temperature and luminosity of the primary star; 2) obtaining a\nperiod near the observed value of 27.1 years; 3) matching a mass function of\n3.0; 4) concurrent Roche lobe overflow and mass transfer; 5) an isotopic ratio\n12 C/ 13 C = 5 and, (6) matching the interferometrically determined angular\ndiameter. A MESA model starting with binary masses of 9.85 + 4.5 Msun , with a\n100 day initial period, produces a 1.2 + 10.6 Msun result having a 547 day\nperiod, and a single digit 12 C/ 13 C ratio. These values were reached near an\nage of 20 Myr, when the donor star comes close to the observed luminosity and\ntemperature for epsilon Aurigae A, as a post-RGB/pre-AGB star.\nContemporaneously, the accretor then appears as an upper main sequence, early\nB-type star. This benchmark model can provide a basis for further exploration\nof this interacting binary, and other long period binary stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "IRTF Observations of White Dwarfs with Possible Near-Infrared Excess: Near-infrared photometry and spectroscopy is obtained for a heterogeneous\nsample of nearby white dwarfs with possible excess flux as identified primarily\nin the Two Micron All Sky Survey. Among the sample of 43 stars are a number of\nwhite dwarfs that are either metal-rich, magnetic, or binary suspects. With a\nfew notable exceptions in four (or possibly five) distinct categories, the\nnewly obtained JHK photometric data fail to corroborate the putative excesses,\nwith K_IRTF - K_2MASS = +0.31 mag. Where available, GALEX photometric data are\nused to better constrain the overall spectral energy distribution of the white\ndwarfs, enabling any excess near-infrared flux to stand out more readily\nagainst the expected stellar photosphere.\n  With superior data, a near-infrared photometric excess is confirmed at three\nmetal-rich white dwarfs and ruled out at nine others. Several new binaries are\nconfirmed or suggested; five white dwarf - red dwarf pairs and five double\ndegenerates. Four apparently single magnetic white dwarfs -- two DA and two DQp\n-- display modest to strong near-infrared excess (relative to non-magnetic\nmodels), which may be better described as two effective temperatures owing to a\nredistribution of energy in highly magnetic or peculiar atmospheres.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Astro2020 Science White Paper: gravity-wave asteroseismology of\n  intermediate- and high-mass stars: The evolution of a star is driven by the physical processes in its interior\nmaking the theory of stellar structure and evolution the most crucial\ningredient for not only stellar evolution studies, but any field of astronomy\nwhich relies on the yields along stellar evolution. High-precision time-series\nphotometric data assembled by recent space missions revealed that current\nmodels of stellar structure and evolution show major shortcomings already in\nthe two earliest nuclear burning phases, impacting all subsequent phases prior\nto the formation of the end-of-life remnant. This white paper focuses\nspecifically on the transport of chemical elements and of angular momentum in\nthe stellar structure and evolution models of stars born with convective core,\nas revealed by their gravity-mode oscillations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Magnetoclinicity Instability: In strongly compressible magnetohydrodynamic turbulence, obliqueness between\nthe large-scale density gradient and magnetic field gives an electromotive\nforce mediated by density variance (intensity of density fluctuation). This\neffect is named ``magnetoclinicity'', and is expected to play an important role\nin large-scale magnetic-field generation in astrophysical compressible\nturbulent flows. Analysis of large-scale instability due to the\nmagnetoclinicity effect shows that the mean magnetic-field perturbation is\ndestabilised at large scales in the vicinity of strong mean density gradient in\nthe presence of density variance.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Early Time Light Curves of Type Ia Supernovae Observed with TESS: We present early time light curves of Type Ia supernovae observed in the\nfirst six sectors of TESS data. Ten of these supernovae were discovered by\nASAS-SN, seven by ATLAS, six by ZTF, and one by \\textit{Gaia}. For nine SNe\nwith sufficient dynamic range ($>$3.0 mag from detection to peak), we fit power\nlaw models and search for signatures of companion stars. We find a diversity of\nearly time light curve shapes, although most of our sources are consistent with\nfireball models where the flux increases $\\propto t^2$. Three SN display a\nflatter rise with flux $\\propto t$. We do not find any evidence for additional\nstructure such as multiple power law components in the early rising light\ncurves. For assumptions about the SN properties and the observer viewing angle,\nand further assuming that companion stars would be in Roche-lobe overflow, we\nplace limits on the radii of companions for six SNe with complete coverage of\nthe early time light curves. The upper limits are $\\lesssim$\\,32 R$_\\odot$ for\nthese six supernovae, $\\lesssim$\\,20 R$_\\odot$ for five of these six, and\n$\\lesssim$\\,4 R$_\\odot$ for two of these six. The small sample size does not\nconstrain occurrence rates of single degenerate Type Ia SN progenitors, but we\nexpect that TESS observed enough SNe in its primary mission (26 sectors) to\ninform this measurement. We also show that TESS is capable of detecting\nemission from a 1 \\rsun\\ companion for a Type Ia SN within 50 Mpc, and may do\nso after about six years.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Keplerian frequency of uniformly rotating neutron stars and quark stars: We calculate Keplerian (mass shedding) configurations of rigidly rotating\nneutron stars and quark stars with crusts. We check the validity of empirical\nformula for Keplerian frequency, f_K, proposed by Lattimer & Prakash, f_K(M)=C\n(M/M_sun)^1/2 (R/10km)^-3/2, where M is the (gravitational) mass of Keplerian\nconfiguration, R is the (circumferential) radius of the non-rotating\nconfiguration of the same gravitational mass, and C = 1.04 kHz. Numerical\ncalculations are performed using precise 2-D codes based on the multi-domain\nspectral methods. We use a representative set of equations of state (EOSs) of\nneutron stars and quark stars. We show that the empirical formula for f_K(M)\nholds within a few percent for neutron stars with realistic EOSs, provided 0.5\nM_sun < M < 0.9 M_max,stat, where M_max,stat is the maximum allowable mass of\nnon-rotating neutron stars for an EOS, and C=C_NS=1.08 kHz. Similar precision\nis obtained for quark stars with 0.5 M_sun < M < 0.9 M_max,stat. For maximal\ncrust masses we obtain C_QS = 1.15 kHz, and the value of C_QS is not very\nsensitive to the crust mass. All our C's are significantly larger than the\nanalytic value from the relativistic Roche model, C_Roche = 1.00 kHz. For 0.5\nM_sun < M < 0.9 M_max,stat, the equatorial radius of Keplerian configuration of\nmass M, R_K(M), is, to a very good approximation, proportional to the radius of\nthe non-rotating star of the same mass, R_K(M) = aR(M), with a_NS \\approx a_QS\n\\approx 1.44. The value of a_QS is very weakly dependent on the mass of the\ncrust of the quark star. Both a's are smaller than the analytic value a_Roche =\n1.5 from the relativistic Roche model.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Predicting the Magnetic Fields of a Stealth CME Detected by Parker Solar\n  Probe at 0.5 AU: Stealth coronal mass ejection (CMEs) are eruptions from the Sun that are not\nassociated with appreciable low-coronal signatures. Because they often cannot\nbe linked to a well-defined source region on the Sun, analysis of their initial\nmagnetic configuration and eruption dynamics is particularly problematic. In\nthis manuscript, we address this issue by undertaking the first attempt at\npredicting the magnetic fields of a stealth CME that erupted in 2020 June from\nthe Earth-facing Sun. We estimate its source region with the aid of off-limb\nobservations from a secondary viewpoint and photospheric magnetic field\nextrapolations. We then employ the Open Solar Physics Rapid Ensemble\nInformation (OSPREI) modelling suite to evaluate its early evolution and\nforward-model its magnetic fields up to Parker Solar Probe, which detected the\nCME in situ at a heliocentric distance of 0.5 AU. We compare our hindcast\nprediction with in-situ measurements and a set of flux rope reconstructions,\nobtaining encouraging agreement on arrival time, spacecraft crossing location,\nand magnetic field profiles. This work represents a first step towards reliable\nunderstanding and forecasting of the magnetic configuration of stealth CMEs and\nslow, streamer-blowout events.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Rich Circumstellar Chemistry of SMP LMC 11: Carbon-rich evolved stars from the asymptotic giant branch to the planetary\nnebula phase are characterized by a rich and complex carbon chemistry in their\ncircumstellar envelopes. A peculiar object is the preplanetary nebula SMP LMC\n11, whose Spitzer-IRS spectrum shows remarkable and diverse molecular\nabsorption bands. To study how the molecular composition in this object\ncompares to our current understanding of circumstellar carbon chemistry, we\nmodeled this molecular absorption. We find high abundances for a number of\nmolecules, perhaps most notably benzene. We also confirm the presence of\npropyne (CH3C2H) in this spectrum. Of all the cyanopolyynes, only HC3N is\nevident; we can detect at best a marginal presence of HCN. From comparisons to\nvarious chemical models, we can conclude that SMP LMC 11 must have an unusual\ncircumstellar environment (a torus rather than an outflow).",
        "category": "astro-ph_SR"
    },
    {
        "text": "Photometric and spectroscopic study of the intermediate-age open cluster\n  NGC 2355: In this paper we analyse the evolutionary status and properties of the old\nopen cluster NGC 2355, located in the Galactic anticentre direction, as a part\nof the long term programme BOCCE. NGC 2355 was observed with LBC@LBT using the\nBessel $B$, $V$, and $I_c$ filters. The cluster parameters have been obtained\nusing the synthetic colour-magnitude diagram (CMD) method, as done in other\npapers of this series. Additional spectroscopic observations with FIES@NOT of\nthree giant stars were used to determine the chemical properties of the\ncluster. Our analysis shows that NGC 2355 has metallicity slightly less than\nsolar, with [Fe/H]$=-0.06$ dex, age between 0.8 and 1 Gyr, reddening $E(B-V)$\nin the range 0.14 and 0.19 mag, and distance modulus $(m-M)_0$ of about 11 mag.\nWe also investigated the abundances of O, Na, Al, $\\alpha$, iron-peak, and\nneutron capture elements, showing that NGC 2355 falls within the abundance\ndistribution of similar clusters (same age and metallicity). The Galactocentric\ndistance of NGC~2355 places it at the border between two regimes of metallicity\ndistribution; this makes it an important cluster for the study of the chemical\nproperties and evolution of the disc.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spectroscopic Studies of 30 Short-period Cataclysmic Variable Stars, and\n  Remarks on the Evolution and Population of Similar Objects: We present spectroscopy and orbital periods Porb for 30 apparently\nnon-magnetic cataclysmic binaries with periods below about 3 hours, nearly all\nof which are dwarf novae, mostly of the SU Ursae Majoris subclass. We then turn\nto the evidence supporting the prediction that short-period dwarf novae evolve\ntoward longer periods after passing through a minimum period -- the \"period\nbounce\" phenomenon. Plotting data from the literature reveals that for\nsuperhump period excess $\\epsilon = (P_{\\rm sh} - P_{\\rm orb} )/P_{\\rm orb}$\nbelow about 0.015, the period appears to increase with decreasing epsilon,\nagreeing at least qualitatively with the predicted behavior. Next, motivated by\nthe long (decadal) outburst intervals of the WZ Sagittae subclass of\nshort-period dwarf novae, we ask whether there could be a sizable population of\n\"lurkers\" -- systems that resemble dwarf novae at minimum light, but do not\noutburst over accessible timescales (or at all), and therefore do not draw\nattention to themselves. By examining the outburst history of the Sloan Digital\nSky Survey sample of CVs, which were selected by color and not by outburst, we\nfind that a large majority of the color-selected dwarf-nova-like objects have\nbeen observed to outburst, and conclude that \"lurkers\", if they exist, are a\nrelatively minor part of the CV population.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The formation of IRIS diagnostics VIII. IRIS observations in the C II\n  133.5 nm multiplet: The C II 133.5 nm multiplet has been observed by NASA's Interface Region\nImaging Spectrograph (IRIS) in unprecedented spatial resolution. The aims of\nthis work are to characterize these new observations of the C II lines, place\nthem in context with previous work, and to identify any additional value the C\nII lines bring when compared with other spectral lines. We make use of wide,\nlong exposure IRIS rasters covering the quiet Sun and an active region. Line\nproperties such as velocity shift and width are extracted from individual\nspectra and analyzed. The lines have a variety of shapes (mostly single-peak or\ndouble-peak), are strongest in active regions and weaker in the quiet Sun. The\nratio between the 133.4 nm and 133.5 nm components is always less than 1.8,\nindicating that their radiation is optically thick in all locations. Maps of\nthe C II line widths are a powerful new diagnostic of chromospheric structures,\nand their line shifts are a robust velocity diagnostic. Compared with earlier\nquiet Sun observations, we find similar absolute intensities and mean line\nwidths, but smaller red shifts; this difference can perhaps be attributed to\ndifferences in spectral resolution and spatial coverage. The C II intensity\nmaps are somewhat similar to those of transition region lines, but also share\nsome features with chromospheric maps such as those from the Mg II k line,\nindicating that they are formed between the upper chromosphere and transition\nregion. C II intensity, width, and velocity maps can therefore be used to\ngather additional information about the upper chromosphere.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The orbital and superhump periods of the SU UMa-type dwarf nova V1212\n  Tauri: We report CCD photometry of the superoutburst of the dwarf nova V1212 Tau\nobtained during 2011 January and February. The outburst amplitude was at least\n6 magnitudes and it lasted at least 12 days. Three distinct superhump regimes\nwere observed. Initially low amplitude superhumps (0.03 to 0.05 magnitude\npeak-to-peak) with Psh = 0.0782(52) d were present. The superhumps reached a\nmaximum amplitude of 0.31 magnitudes at the beginning of the plateau phase,\nwith Psh = 0.07031(96) d. Subsequently the star began to fade slowly. During\nthe first part of the decline, the period increased with dPsh/dt = +1.62(9) x\n10-3 and the amplitude of the superhumps also declined. Mid way through the\nslow decline, the superhumps partially regrew and this point coincided with a\nchange to a new superhump regime during which the period decreased with dPsh/dt\n= -1.50(39) x 10-3. We determined the orbital period as Porb = 0.06818(64) d\nand the superhump period excess as epsilon = 0.034(15)",
        "category": "astro-ph_SR"
    },
    {
        "text": "3D Stellar Reddening Map from 2MASS Photometry: An Improved Version: An improved version of the 3D stellar reddening map in a space with a radius\nof 1200 pc around the Sun and within 600 pc of the Galactic midplane is\npresented. As in the previous 2010 and 2012 versions of the map, photometry\nwith an accuracy better than $0.05^m$ in the $J$ and $K_s$ bands for more than\n70 million stars from the 2MASS catalogue is used in the new version. However,\nthe data reduction technique is considerably more complicated. As before, an\nanalysis of the distribution of stars near the main-sequence turnoff on the\n$(J-K_s)$ - $K_s$ diagram, where they form a distribution maximum, provides a\nbasis for the method. The shift of this maximum, i.e., the $mode(J-K_s)$, along\n$(J-K_s)$ and $K_s$, given the spatial variations of the mean de-reddened color\n$(J-K_s)_0$ of these stars, is interpreted as a growth of the reddening with\nincreasing distance. The main distinction of the new method is that instead of\nthe fixed mean absolute magnitude, de-reddened color, distance, and reddening\nfor each cell, the individual values of these quantities are calculated for\neach star by iterations when solving the system of equations relating them.\nThis has allowed one to increase the random accuracy of the map to $0.01^m$ and\nits spatial resolution to 20 pc in coordinates and distance and to $1^{\\circ}$\nin longitude and latitude. Comparison with other reddening estimates for the\nsame spatial cells and Gaia DR1 TGAS stars shows that the constructed map is\none of the best maps for the space under consideration. Its systematic errors\nhave been estimated to be $\\sigma(E(J-K_s))=0.025^m$, or\n$\\sigma(E(B-V))=0.04^m$. The main purpose of the map is to analyze the\ncharacteristics of Galactic structures, clouds, and cloud complexes. For this\npurpose, the reddening map within each spatial cell has also been computed by\nanalyzing the reddening along each line of sight.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Pulsating Components in Binary and Multiple Stellar Systems --- A\n  Catalog of Oscillating Binaries: We present an up-to-date catalog of pulsating binaries, i.e. the binary and\nmultiple stellar systems containing pulsating components, along with a\nstatistics on them. Compared to the earlier compilation by Soydugan et\nal.(2006a) of 25 delta Scuti-type `oscillating Algol-type eclipsing binaries'\n(oEA), the recent collection of 74 oEA by Liakos et al.(2012), and the\ncollection of Cepheids in binaries by Szabados (2003a), the numbers and types\nof pulsating variables in binaries are now extended. The total numbers of\npulsating binary/multiple stellar systems have increased to be 515 as of 2014\nOctober 26, among which 262+ are oscillating eclipsing binaries and the oEA\ncontaining delta Scuti components are updated to be 96. The catalog is intended\nto be a collection of various pulsating binary stars across the\nHertzsprung-Russell diagram. We reviewed the open questions, advances and\nprospects connecting pulsation/oscillation and binarity. The observational\nimplication of binary systems with pulsating components, to stellar evolution\ntheories is also addressed. In addition, we have searched the Simbad database\nfor candidate pulsating binaries. As a result, 322 candidates were extracted.\nFurthermore, a brief statistics on Algol-type eclipsing binaries (EA) based on\nthe existing catalogs is given. We got 5315 EA, of which there are 904 EA with\nspectral types A and F. The present catalog has a sortable web version allowing\neasy updating and maintenance at http://www.chjaa.org/COB/ .",
        "category": "astro-ph_SR"
    },
    {
        "text": "The G-O Rule and Waldmeier Effect in the Variations of the Numbers of\n  Large and Small Sunspot Groups: We have analysed the combined Greenwich and Solar Optical Observing Network\n(SOON) sunspot group data during the period of 1874-2011 and determined\nvariations in the annual numbers (counts) of the small, large and big sunspot\ngroups (these classifications are made on the basis of the maximum areas of the\nsunspot groups). We found that the amplitude of an even-numbered cycle of the\nnumber of large groups is smaller than that of its immediately following\nodd-numbered cycle. This is consistent with the well known Gnevyshev and Ohl\nrule or G-O rule of solar cycles, generally described by using the Zurich\nsunspot number (Rz). During cycles 12-21 the G-O rule holds good for the\nvariation in the number of small groups also, but it is violated by cycle pair\n(22, 23) as in the case of Rz. This behaviour of the variations in the small\ngroups is largely responsible for the anomalous behaviour of Rz in cycle pair\n(22, 23). It is also found that the amplitude of an odd-numbered cycle of the\nnumber of small groups is larger than that of its immediately following\neven-numbered cycle. This can be called as `reverse G-O rule'. In the case of\nthe number of the big groups, both cycle pairs (12, 13) and (22, 23) violated\nthe G-O rule. In many cycles the positions of the peaks of the small, large,\nand big groups are different and considerably differ with respect to the\ncorresponding positions of the Rz peaks. In the case of cycle 23, the\ncorresponding cycles of the small and large groups are largely symmetric/less\nasymmetric (Waldmeier effect is weak/absent) with their maxima taking place two\nyears later than that of Rz. The corresponding cycle of the big groups is more\nasymmetric (strong Waldmeier effect) with its maximum epoch taking place at the\nsame time as that of Rz.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On effects of surface bipolar magnetic regions on the convection zone\n  dynamo: We investigate the effect of the surface bipolar magnetic regions (BMR) on\nthe large-scale dynamo distributed in the bulk of the convection zone. The\nstudy employs the nonlinear 3D mean-field dynamo model. We model the emergence\nof the BMR on the surface through the nonaxisymmetric magnetic buoyancy effect,\nwhich acts on the large-scale toroidal magnetic field in the upper half of the\nconvection zone. The nonaxisymmetric magnetic field which results from this\nmechanism is shallow. On the surface, the effect of the BMR on the magnetic\nfield generation is dominant. {However, because of the shallow BMR\ndistribution, its effect on the global dynamo is less compared to the\nconvective zone dynamo.} We find that the mean-field $\\alpha$ effect, which\nacts on the nonaxisymmetric magnetic field of the BMRs, provides the greater\ncontribution to the dynamo process than the BMR's tilt does. Even so, the\nfluctuations of the BMR's tilt lead to the parity braking in the global dynamo.\nAt the surface the nonaxisymmetric magnetic field, which are generated because\nof the BMR's activity, shows a tendency for the bihelical spectrum with the\npositive sign for the low $\\ell$ modes during the maximum of the magnetic\nactivity cycle.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Discovery of Radial Spectral Hardening in the Hot Bubble of Planetary\n  Nebula BD+30 3639 with Median Energy Imaging: We introduce a new imaging analysis technique to study the spatial\ndistribution of the X-ray emission from the hot bubble of planetary nebula\nBD+30 3639. Hot bubble emission is typically photon-starved, thus limiting the\nmethods for spatial-spectral analysis, however, this new technique uses the\nstatistics of photon energies across the nebula to identify spatial variations.\nUsing the median energy value of the X-ray photons, we identified a rise in\nmedian energy values towards the projected edge of the nebula, which we refer\nto as radial spectral hardening. We explored the origin of this radial spectral\nhardening with X-ray spectral analysis of distinct regions of high- and\nlow-median energy values. Given that the hot bubble is embedded within a young,\ndense, planetary nebula, we argue that the radial spectral hardening is due to\nan increased column density at the projected nebular edge. Median energy\nimaging provides a promising new methodology for exploring the spatial\nvariations in faint extended X-ray sources.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The data center for the Spectrometer and Telescope for Imaging X-rays\n  (STIX) onboard Solar Orbiter: The Spectrometer and Telescope for Imaging X-rays (STIX) on board Solar\nOrbiter observes solar X-ray emission in the range of 4 &ndash; 150 keV and\nproduces spectra and images of solar flares over a wide range of flare\nmagnitudes. During nominal operation, STIX continuously generates data. A\nconstant data flow requires fully automated data-processing pipelines to\nprocess and analyze the data, and a data platform to manage, visualize, and\ndistribute the data products to the scientific community. The STIX Data Center\nhas been built to fulfill these needs. In this paper, we outline its main\ncomponents to help the community better understand the tools and data it\nprovides. The STIX Data Center is operated at the University of Applied\nSciences and Arts Northwestern Switzerland (FHNW) and consists of automated\nprocessing pipelines and a data platform. The pipelines process STIX telemetry\ndata, perform common analysis tasks, and generate data products at different\nprocessing levels. They have been designed to operate fully automatically with\nminimal human intervention. The data platform provides web-based user\ninterfaces and application programmable interfaces for searching and\ndownloading STIX data products. The STIX Data Center has been operating\nsuccessfully for more than two years. The platform facilitates instrument\noperations and provides vital support to STIX data users.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Population of post-nova supersoft X-ray sources: Novae undergo a supersoft X-ray phase of varying duration after the optical\noutburst. Such transient post-nova supersoft X-ray sources (SSSs) are the\nmajority of the observed SSSs in M31. In this paper, we use the post-nova\nevolutionary models of Wolf et al. to compute the expected population of\npost-nova SSSs in M31. We predict that depending on the assumptions about the\nWD mass distribution in novae, at any instant there are about 250-600 post-nova\nSSSs in M31 with (unabsorbed) 0.2-1.0 keV luminosity L_x>10^36 erg/s. Their\ncombined unabsorbed luminosity is of the order of ~10^39 erg/s. Their\nluminosity distribution shows significant steepening around log(L_x)~37.7-38\nand becomes zero at L_x~2x10^38 erg/s, the maximum L_x achieved in the\npost-nova evolutionary tracks. Their effective temperature distribution has a\nroughly power law shape with differential slope of ~4-6 up to the maximum\ntemperature of T_eff~1.5x10^6 K.\n  We compare our predictions with the results of the XMM-Newton monitoring of\nthe central field of M31 between 2006 and 2009. The predicted number of\npost-nova SSSs exceed the observed number by a factor of ~2-5, depending on the\nassumed WD mass distribution in novae. This is good agreement, considering the\nnumber and magnitude of uncertainties involved in calculations of the post-nova\nevolutionary models and their X-ray output. Furthermore, only a moderate\ncircumstellar absorption, with hydrogen column density of the order of ~10^21\ncm^-2, will remove the discrepancy.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The planetary nebula population of M33 and its metallicity gradient: A\n  look into the galaxy's distant past: The Planetary Nebula (PN) population of M33 is studied via multi-fiber\nspectroscopy with Hectospec at the MMT. In this paper we present the spectra of\n102 PNe, whereas plasma diagnostic and chemical abundances were performed on\nthe 93 PNe where the necessary diagnostic lines were measured. About 20% of the\nPNe are compatible with being Type I; the rest of the sample is the progeny of\nan old disk stellar population, with main sequence masses M<3M${_\\odot}$ and\nages t$>$0.3 Gyr.\n  By studying the elemental abundances of the PNe in the M33 disk we were able\nto infer that: (1) there is a tight correlation between O/H and Ne/H, broadly\nexcluding the evolution of oxygen; (2) the average abundances of the\n$\\alpha$-elements are consistent with those of \\hii regions, indicating a\nnegligible global enrichment in the disk of M33 from the epoch of the formation\nof the PN progenitors to the present time; (3) the radial oxygen gradient\nacross the M33 disk has a slope of -0.031$\\pm$0.013 dex kpc$^{-1}$, in\nagreement, within the errors, with the corresponding gradient derived from HII\nregions. Our observations do not seem to imply that the metallicity gradient\nacross the M33 disk has flattened considerably with time. We report also the\ndiscovery of a PN with Wolf-Rayet features, PN039, belonging the class of late\n[WC] stars",
        "category": "astro-ph_SR"
    },
    {
        "text": "SN 2008gz - most likely a normal type IIP event: We present BV RI photometric and low-resolution spectroscopic investigation\nof a type II core-collapse supernova (SN) 2008gz, which occurred in a star\nforming arm and within a half-light radius (solar metallicity region) of a\nnearby spiral galaxy NGC 3672. The SN event was detected late and a detailed\ninvestigation of its light curves and spectra spanning 200 days suggest that it\nis an event of type IIP similar to archetypal SNe 2004et and 1999em. However,\nin contrast to other events of its class, the SN 2008gz exhibits rarely\nobserved V magnitude drop of 1.5 over the period of a month during plateau to\nnebular phase. Using 0.21 mag of Av as a lower limit and a distance of 25.5\nMpc, we estimate synthesized $^{56}$Ni mass of 0.05 \\pm 0.01 M* and a\nmid-plateau Mv of -16.6 \\pm 0.2 mag. The photospheric velocity is observed to\nbe higher than that was observed for SN 2004et at similar epochs, indicating\nexplosion energy was comparable to or higher than SN 2004et. Similar trend was\nalso seen for the expansion velocity of H-envelopes. By comparing its\nproperties with other well studied events as well as by using a recent\nsimulation of pre-SN models of Dessart, Livne & Waldman (2010), we infer an\nexplosion energy range of 2 - 3 x 10$^{51}$ erg and this coupled with the\nobserved width of the forbidden [O I] 6300-6364 {\\AA} line at 275 days after\nthe explosion gives an upper limit for the main-sequence (non-rotating, solar\nmetallicity) progenitor mass of 17 M*. Our narrow-band H{\\alpha} observation,\ntaken nearly 560 days after the explosion and the presence of an emission kink\nat zero velocity in the Doppler corrected spectra of SN indicate that the event\ntook place in a low luminosity star forming H II region.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Distances to Recent Near-Earth Supernovae From Geological and Lunar 60Fe: Near-Earth supernova blasts which engulf the solar system have left traces of\ntheir ejecta in the geological and lunar records. There is now a wealth of data\non live radioactive ${}^{60}$Fe pointing to a supernova at 3 Myr ago, as well\nas the recent discovery of an event at 7 Myr ago. We use the available\nmeasurements to evaluate the distances to these events. For the better analyzed\nsupernova at 3 Myr, samples include deep-sea sediments, ferromanganese crusts,\nand lunar regolith; we explore the consistency among and across these\nmeasurements, which depends sensitively on the uptake of iron in the samples as\nwell as possible anisotropies in the ${}^{60}$Fe fallout. There is also\nsignificant uncertainty in the astronomical parameters needed for these\ncalculations. We take the opportunity to perform a parameter study on the\neffects that the ejected ${}^{60}$Fe mass from a core-collapse supernova and\nthe fraction of dust that survives the remnant have on the resulting distance.\nWe find that with an ejected ${}^{60}$Fe mass of $3\\times10^{-5} M_\\odot$ and a\ndust fraction of 10%, the distance range for the supernova 3 Myr ago is $D \\sim\n20 - 140$ pc, with the most likely range between $50 - 65$ pc. Using the same\nastrophysical parameters, the distance for the supernova at 7 Myr ago is $D\n\\sim 110$ pc. We close with a brief discussion of geological and astronomical\nmeasurements that can improve these results.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Power-law statistics and stellar rotational velocities in the Pleiades: In this paper we will show that, the non-gaussian statistics framework based\non the Kaniadakis statistics is more appropriate to fit the observed\ndistributions of projected rotational velocity measurements of stars in the\nPleiades open cluster. To this end, we compare the results from the $\\kappa$\nand $q$-distributions with the Maxwellian.",
        "category": "astro-ph_SR"
    },
    {
        "text": "GYRE: A New Open-Source Stellar Oscillation Code: We introduce GYRE, a new open-source stellar oscillation code which solves\nthe adiabatic/non-adiabatic pulsation equations using a novel Magnus Multiple\nShooting (MMS) numerical scheme. The code has a global error scaling of up to\n6th order in the grid spacing, and can therefore achieve high accuracy with few\ngrid points. It is moreover robust and efficiently makes use of multiple\nprocessor cores and/or nodes. We present an example calculation using GYRE, and\ndiscuss recent work to integrate GYRE into the asteroseismic optimization\nmodule of the MESA stellar evolution code.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Local Helioseismology of Sunspots: Current Status and Perspectives\n  (Invited Review): Mechanisms of the formation and stability of sunspots are among the\nlongest-standing and intriguing puzzles of solar physics and astrophysics.\nSunspots are controlled by subsurface dynamics hidden from direct observations.\nRecently, substantial progress in our understanding of the physics of the\nturbulent magnetized plasma in strong-field regions has been made by using\nnumerical simulations and local helioseismology. Both the simulations and\nhelioseismic measurements are extremely challenging, but it becomes clear that\nthe key to understanding the enigma of sunspots is a synergy between models and\nobservations. Recent observations and radiative MHD numerical models have\nprovided a convincing explanation to the Evershed flows in sunspot penumbrae.\nAlso, they lead to the understanding of sunspots as self-organized magnetic\nstructures in the turbulent plasma of the upper convection zone, which are\nmaintained by a large-scale dynamics. Local helioseismic diagnostics of\nsunspots still have many uncertainties, some of which are discussed in this\nreview. However, there have been significant achievements in resolving these\nuncertainties, verifying the basic results by new high-resolution observations,\ntesting the helioseismic techniques by numerical simulations, and comparing\nresults obtained by different methods. For instance, a recent analysis of\nhelioseismology data from the Hinode space mission has successfully resolved\nseveral uncertainties and concerns (such as the inclined-field and phase-speed\nfiltering effects) that might affect the inferences of the subsurface\nwave-speed structure of sunspots and the flow pattern. It becomes clear that\nfor the understanding of the phenomenon of sunspots it is important to further\nimprove the helioseismology methods and investigate the whole life cycle of\nactive regions, from magnetic-flux emergence to dissipation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Magnetic imprints of eruptive and non-eruptive Solar flares as observed\n  by Solar Dynamics Observatory: The abrupt and permanent changes of photospheric magnetic field in the\nlocalized regions of active regions during solar flares called magnetic\nimprints (MIs), have been observed for the past nearly three decades. The well\nknown \"coronal implosion\" model is assumed to explain such flare associated\nchanges but the complete physical understanding is still missing and debatable.\nIn this study, we made a systematic analysis of flare-related changes of\nphotospheric magnetic field during 21 flares (14 eruptive and 7 non-eruptive)\nusing the high-cadence (\\texttt{135s}) vector-magnetogram data obtained from\nHelioseismic and Magnetic Imager. The MI regions for eruptive flares are found\nto be strongly localised, whereas the majority of non-eruptive events\n($>70~\\%$) have scattered imprint regions. To quantify the strength of the MIs,\nwe derived the integrated change of horizontal field and total change of\nLorentz force over an area. These quantities correlate well with the flare\nstrength, irrespective of whether flares being eruptive or not, short or long\nduration. Further, the free-energy (FE), determined from virial-theorem\nestimates, exhibits statistically significant downward trend which starts\naround the flare time is observed in majority of flares. The change of FE\nduring flares do not depend on eruptivity but have a strong positive\ncorrelation ($\\approx 0.8$) with the Lorentz force change, indicating that the\npart of FE released would penetrate into the photosphere. While these results\nstrongly favor the idea of significant feedback from corona on the photospheric\nmagnetic field, the characteristics of MIs are quite indistinguishable for\nflares being eruptive or not.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Characterisation of the turbulent electromotive force and its\n  magnetically-mediated quenching in a global EULAG-MHD simulation of solar\n  convection: We perform a mean-field analysis of the EULAG-MHD millenium simulation of\nglobal magnetohydrodynamical convection presented in Passos et al. 2014. The\nturbulent electromotive force operating in the simulation is assumed to be\nlinearly related to the cyclic axisymmetric mean magnetic field and its first\nspatial derivatives. At every grid point in the simulation's meridional plane,\nthis assumed relationship involves 27 independent tensorial coefficients.\nExpanding on Racine et al. 2011, we extract these coefficients from the\nsimulation data through a least-squares minimization procedure based on\nsingular value decomposition. The reconstructed alpha-tensor shows good\nagreement with that obtained by Racine et al. 2011, who did not include\nderivatives of the mean-field in their fit, as well as with the alpha-tensor\nextracted by Augustson et al. 2015 from a distinct ASH MHD simulation. The\nisotropic part of the turbulent magnetic diffusivity tensor beta is positive\ndefinite and reaches values of 5.0x10^7 m2s-1 in the middle of the convecting\nfluid layers. The spatial variations of both alpha_phiphi and beta_phiphi\ncomponent are well reproduced by expressions obtained under the SOCA, with a\ngood matching of amplitude requiring a turbulent correlation time about five\ntimes smaller than the estimated turnover time of the small-scale turbulent\nflow. We find the magnetic quenching of the alpha-effect to be driven primarily\nby a reduction of the small-scale flow's kinetic helicity, with variations of\nthe current helicity playing a lesser role in most locations in the simulation\ndomain. Our measurements of turbulent diffusivity quenching are restricted to\nthe beta_phiphi component, but indicate a weaker quenching, by a factor of\n1.36, than of the alpha effect, which in our simulation drops by a factor of\nthree between the minimum and maximum phases of the magnetic cycle.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Heavy element abundances in planetary nebulae: A theorist's perspective: The determination of heavy element abundances from planetary nebula (PN)\nspectra provides an exciting opportunity to study the nucleosynthesis occurring\nin the progenitor asymptotic giant branch (AGB) star. We perform\npost-processing calculations on AGB models of a large range of mass and\nmetallicity to obtain predictions for the production of neutron-capture\nelements up to the first s-process peak at strontium. We find that solar\nmetallicity intermediate-mass AGB models provide a reasonable match to the\nheavy element composition of Type I PNe. Likewise, many of the Se and Kr\nenriched PNe are well fitted by lower mass models with solar or close-to-solar\nmetallicities. However the most Kr-enriched objects, and the PN with sub-solar\nSe/O ratios are difficult to explain with AGB nucleosynthesis models.\nFurthermore, we compute s-process abundance predictions for low-mass AGB models\nof very low metallicity ([Fe/H] =-2.3) using both scaled solar and an\nalpha-enhanced initial composition. For these models, O is dredged to the\nsurface, which means that abundance ratios measured relative to this element\n(e.g., [X/O]) do not provide a reliable measure of initial abundance ratios, or\nof production within the star owing to internal nucleosynthesis.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Numerical methods for solution of the stochastic differential equations\n  equivalent to the non-stationary Parker's transport equation: We derive the numerical schemes for the strong order integration of the set\nof the stochastic differential equations (SDEs) corresponding to the\nnon-stationary Parker transport equation (PTE). PTE is 5-dimensional (3 spatial\ncoordinates, particles energy and time) Fokker- Planck type equation describing\nthe non-stationary the galactic cosmic ray (GCR) particles transport in the\nheliosphere. We present the formulas for the numerical solution of the obtained\nset of SDEs driven by a Wiener process in the case of the full\nthree-dimensional diffusion tensor. We introduce the solution applying the\nstrong order Euler-Maruyama, Milstein and stochastic Runge-Kutta methods. We\ndiscuss the advantages and disadvantages of the presented numerical methods in\nthe context of increasing the accuracy of the solution of the PTE.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Fundamental parameters of Ap-star HD 108662: We present the results of a self-consistent spectroscopic analysis of the\natmosphere of Ap-star HD 108662 based on high resolution spectrum and low\nresolution spectrophotometric observations. Magnetic field effects, such as\nZeeman broadening and polarized line formation, were taking into account in the\nanalysis of spectral line profiles. We derived abundances of 24 chemical\nelements as well as the stratification of Fe -- an element which is the main\ncontributor to the line absorption in the visible wavelengths. Another abundant\nchemical element -- Chromium -- was found to be distributed homogeneously in\nthe atmosphere of the star. From our analysis we determined the following\nfundamental parameters of HD~108662: $T_{eff}$ = 10212 K, log$g$ = 4.0,\n$R/R_{\\odot}$ = 2.09 and log$(L/L_{\\odot})$ = 1.63. The estimated rotational\nvelocity of the star is $v\\,sini$ = 20.4 km/s and the strength of the surface\nmagnetic field is $B_s$ = 3300 G.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Disk-Driven Rotating Bipolar Outflow in Orion Source I: One of the outstanding problems in star-formation theory concerns the\ntransfer of angular momentum such that mass can accrete onto a newly born young\nstellar object (YSO). From a theoretical standpoint, outflows and jets are\npredicted to play an essential role in angular momentum transfer and their\nrotation motions have been reported for both low- and high-mass YSOs. However,\nlittle quantitative discussion on outflow launching mechanisms have been\npresented for high-mass YSOs due to a lack of observational data. Here we\npresent a clear signature of rotation in the bipolar outflow driven by Orion\nSource I, a high-mass YSO candidate, using the Atacama Large\nMillimeter/Submillimeter Array (ALMA). A rotational transition of silicon\nmonoxide (Si18O) reveals a velocity gradient perpendicular to the outflow axis\nwhich is consistent with that of the circumstellar disk traced by a\nhigh-excitation water (H2O) line. The launching radii and outward velocity of\nthe outflow are estimated to be >10 au and 10 km s-1, respectively. These\nparameters rule out a possibility that the observed outflow is produced by\nentrainment of a high-velocity jet, and that contribution from stellar-wind or\nX-wind which have smaller launching radii are significant in the case of Source\nI. Thus, present results provide a convincing evidence of a rotating outflow\ndirectly driven by the magneto-centrifugal disk wind launched by a high-mass\nYSO candidate.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Constraining the amplitude of turbulence in solar corona using\n  observations of angular broadening of radio sources: The angular broadening of compact radio sources observed through a medium\nhaving turbulent density irregularities is usually estimated using the phase\nstructure function. We employ an exact formulation for the phase structure\nfunction that helps in obtaining an accurate estimate of angular broadening\nwhen the baseline lengths are comparable to the inner scale of the turbulent\nspectrum.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Flare Rates, Rotation Periods. and Spectroscopic Activity Indicators of\n  a Volume-Complete Sample of Mid-to-Late M dwarfs within 15 Parsecs: We present a study of flare rates, rotation periods, and spectroscopic\nactivity indicators of 125 single stars within 15 parsecs and with masses\nbetween 0.1$-$0.3 $M_\\odot$ observed during the first year of the TESS mission,\nwith the goal of elucidating the relationship between these various\nmagnetically connected phenomena. We gathered multi-epoch high resolution\nspectra of each target and we measured equivalent widths of the activity\nindicators Helium I D$_3$, $H\\alpha$, and the Calcium infrared triplet line at\n8542.09 angstroms. We present 18 new rotation periods from MEarth photometry\nand 19 new rotation periods from TESS photometry. We present a catalog of 1392\nflares. After correcting for sensitivity, we find the slope of the flare\nfrequency distribution for all stars to have a standard value of $\\alpha$ =\n1.98 $\\pm$ 0.02. We determine R$_{31.5}$, the rate of flares per day with\nenergies above E = 3.16$\\times$10$^{31}$ ergs in the TESS bandpass. We find\nthat below a critical value of $H\\alpha$ EW = -0.71 angstroms, log R$_{31.5}$\nincreases linearly with increasing $H\\alpha$ emission; above this value, log\nR$_{31.5}$ declines rapidly. The stars divide into two groups: 26% have\n$H\\alpha$ in emission, high flare rates with typical values of log R$_{31.5}$ =\n-1.30 $\\pm$ 0.08, and have Rossby numbers $<$ 0.50. The remaining 74% show\nlittle to no $H\\alpha$ in emission and exhibit log R$_{31.5}$ $<$ -3.86, with\nthe majority of these stars not showing a single flare during the TESS\nobservations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Monotonicity of the cores of massive stars: Massive stars are linked with diverse astronomical processes and objects\nincluding star formation, supernovae and their remnants, cosmic rays,\ninterstellar media, and galaxy evolution. Understanding their properties is of\nprimary importance for modern astronomy, and finding simple rules that\ncharacterize them is especially useful. However, theoretical simulations have\nnot yet realized such relations, instead finding that the late evolutionary\nphases are significantly affected by a complicated interplay between nuclear\nreactions, chemical mixing, and neutrino radiation, leading to non-monotonic\ninitial mass dependencies of the iron core mass and the compactness parameter.\nWe conduct a set of stellar evolution simulations, in which evolutions of He\nstar models are followed until their central densities uniformly reach\n10$^{10}$ g cm$^{-3}$, and analyze their final structures as well as their\nevolutionary properties including the lifetime, surface radius change, and\npresumable fates after core collapse. Based on the homogeneous data set, we\nhave found that monotonicity is inherent in the cores of massive stars. We show\nthat not only the density, entropy, and chemical distributions, but also their\nlifetimes and explosion properties such as the proto-neutron-star mass and the\nexplosion energy can be simultaneously ordered into a monotonic sequence. This\nmonotonicity can be regarded as an empirical principle that characterizes the\ncores of massive stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Shaken and stirred: the effects of turbulence and rotation on disc and\n  outflow formation during the collapse of magnetised molecular cloud cores: We present the results of eighteen magnetohydrodynamical calculations of the\ncollapse of a molecular cloud core to form a protostar. Some calculations\ninclude radiative transfer in the flux-limited diffusion approximation while\nothers employ a barotropic equation of state. We cover a wide parameter space,\nwith mass-to-flux ratios ranging from $\\mu = 5$ to $20$; initial turbulent\namplitudes ranging from a laminar calculation (i.e. where the Mach number,\n$\\mathscr{M} = 0$) to transonic $\\mathscr{M} = 1$; and initial rotation rates\nfrom $\\beta_\\mathrm{rot} = 0.005$ to $0.02$. We first show that using a\nradiative transfer scheme produces warmer pseudo-discs than the barotropic\nequation of state, making them more stable. We then `shake' the core by\nincreasing the initial turbulent velocity field, and find that at all three\nmass-to-flux ratios transonic cores are weakly bound and do not produce\npseudo-discs; $\\mathscr{M} = 0.3$ cores produce very disrupted discs; and\n$\\mathscr{M} = 0.1$ cores produce discs broadly comparable to a laminar core.\nIn our previous paper (arXiv:1701.08741), we showed that a pseudo-disc coupled\nwith sufficient magnetic field is necessary to form a bipolar outflow. Here we\nshow that only weakly turbulent cores exhibit collimated jets. We finally take\nthe $\\mathscr{M} = 1.0$, $\\mu = 5$ core and `stir' it by increasing the initial\nangular momentum, finding that once the degree of rotational energy exceeds the\nturbulent energy in the core the disc returns, with a corresponding (though\nslower), outflow. These conclusions place constraints on the initial mixtures\nof rotation and turbulence in molecular cloud cores which are conducive to the\nformation of bipolar outflows early in the star formation process.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Closer Look at the Alpha Persei Coronal Conundrum: A ROSAT survey of the Alpha Per open cluster in 1993 detected its brightest\nstar, mid-F supergiant Alpha Persei: the X-ray luminosity and spectral hardness\nwere similar to coronally active late-type dwarf members. Later, in 2010, a\nHubble Cosmic Origins Spectrograph SNAPshot of Alpha Persei found\nfar-ultraviolet coronal proxy SiIV unexpectedly weak. This, and a suspicious\noffset of the ROSAT source, suggested that a late-type companion might be\nresponsible for the X-rays. Recently, a multi-faceted program tested that\npremise. Groundbased optical coronography, and near-UV imaging with HST Wide\nField Camera 3, searched for any close-in faint candidate coronal objects, but\nwithout success. Then, a Chandra pointing found the X-ray source single and\ncoincident with the bright star. Significantly, the SiIV emissions of Alpha\nPersei, in a deeper FUV spectrum collected by HST COS as part of the joint\nprogram, aligned well with chromospheric atomic oxygen (which must be intrinsic\nto the luminous star), within the context of cooler late-F and early-G\nsupergiants, including Cepheid variables. This pointed to the X-rays as the\nfundamental anomaly. The over-luminous X-rays still support the case for a\nhyperactive dwarf secondary, albeit now spatially unresolved. However, an\nalternative is that Alpha Persei represents a novel class of coronal source.\nResolving the first possibility now has become more difficult, because the easy\nsolution -- a well separated companion -- has been eliminated. Testing the\nother possibility will require a broader high-energy census of the early-F\nsupergiants.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A comprehensive study of young B stars in NGC 2264: I. Space photometry\n  and asteroseismology: Space photometric time series of the most massive members of the young open\ncluster NGC 2264 allow us to study their different sources of variability down\nto the millimagnitude level and permits a search for Slowly Pulsating B (SPB)\ntype pulsation among objects that are only a few million years old. Our goal is\nto conduct a homogeneous study of young B type stars in the cluster NGC 2264\nusing photometric time series from space in combination with high-resolution\nspectroscopy and spectropolarimetry obtained from the ground. The latter will\nbe presented in a separate follow-up article. We performed frequency analyses\nfor eleven B stars in the field of the young cluster NGC 2264 using photometric\ntime series from the MOST, CoRoT and Spitzer space telescopes and the routines\nPeriod04 and SigSpec. We employ the MESA stellar evolution code in combination\nwith the oscillation code GYRE to identify the pulsation modes for two SPB\nstars which exhibit short period spacing series. From our analysis we identify\nfour objects that show SPB pulsations, five stars that show rotational\nmodulation of their light curves caused by spots, one star that is identified\nto be a binary, and one object in the field of the cluster that is found to be\na non-member Be star. In two SPB stars we detect a number of regularly spaced\npulsation modes that are compatible with being members of a g mode period\nseries. Despite NGC 2264's young age, our analysis illustrates that its B type\nmembers have already arrived on the zero-age main sequence (ZAMS). Our\nasteroseismic analysis yields masses between 4 and 6 Msun and ages between 1\nand 6 million years, which agree well to the overall cluster age.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Diagnostics of non-thermal distributions in solar flare spectra observed\n  by RESIK and RHESSI: We focus on the non-thermal components of the electron distribution in the\nkeV range and analyse high-energy resolution X-ray spectra detected by RESIK\nand RHESSI for three solar flares.In the 2-4 keV range we assume that the\nelectron distribution can be modelled by an n-distribution. Using a method of\nline-intensity ratios, we analyse allowed and satellite lines of Si observed by\nRESIK and estimate the parameters of this n-distribution. At higher energies we\nexplore RHESSI bremsstrahlung spectra. Adopting a forward-fitting approach and\nthick-target approximation, we determine the characteristics of injected\nelectron beams. RHESSI non-thermal component associated with the electron beam\nis correlated well with presence of the non-thermal n-distribution obtained\nfrom the RESIK spectra. In addition, such an n-distribution occurs during radio\nbursts observed in the 0.61-15.4 GHz range. Furthermore, we show that the\nn-distribution could also explain RHESSI emission below ~5 keV. Therefore, two\nindependent diagnostics methods indicate the flare plasma being affected by the\nelectron beam can have a non-thermal component in the ~2-5 keV range, which is\ndescribed by the n-distribution well. Finally, spectral line analysis reveals\nthat the n-distribution does not occupy the same location as the thermal\ncomponent detected by RHESSI at ~10 keV.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The floor in the interplanetary magnetic field: Estimation on the basis\n  of relative duration of ICME observations in solar wind during 1976-2000: To measure the floor in interplanetary magnetic field and estimate the time-\ninvariant open magnetic flux of Sun, it is necessary to know a part of magnetic\nfield of Sun carried away by CMEs. In contrast with previous papers, we did not\nuse global solar parameters: we identified different large-scale types of solar\nwind for 1976-2000 interval, obtained a fraction of interplanetary CMEs (ICMEs)\nand calculated magnitude of interplanetary magnetic field B averaged over 2\nCarrington rotations. The floor of magnetic field is estimated as B value at\nsolar cycle minimum when the ICMEs were not observed and it was calculated to\nbe 4,65 \\pm 6,0 nT. Obtained value is in a good agreement with previous\nresults.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Determination of the inclination of the multi-planet hosting star HR8799\n  using asteroseismology: Direct imaging of the HR8799 system was a major achievement in the study of\nexoplanets. HR8799 is a $\\gamma$\\,Doradus variable and asteroseismology can\nprovide an independent constraint on the inclination. Using 650 high\nsignal-to-noise, high resolution, full visual wavelength spectroscopic\nobservations obtained over two weeks at Observatoire de Haute Provence (OHP)\nwith the SOPHIE spectrograph we find that the main frequency in the radial\nvelocity data is 1.9875 d$^{-1}$. This frequency corresponds to the main\nfrequency as found in previous photometric observations. Using the FAMIAS\nsoftware to identify the pulsation modes, we find this frequency is a prograde\n$\\ell$=1 sectoral mode and obtain the constraint that inclination\n$i\\gtrsim$40$^{\\circ}$.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Two-step evolution of a rising flux rope resulting in a confined solar\n  flare: Combining the Solar Dynamics Observatory and the New Vacuum Solar Telescope\nobservations, we study a confined flare triggered by a rising flux rope within\nthe trailing sunspots of active region 12733. The flux rope lying above the\nsheared polarity inversion line can be constructed through magnetic\nextrapolation but could not be detected in multi-wavelength images at the\npre-flare stage. The conspicuous shearing motions between the opposite-polarity\nfields in the photosphere are considered to be responsible for the flux rope\nformation. The maximum twist of the flux rope is as high as -1.76, and then the\nflux rope rises due to the kink instability. Only when the flare starts can the\nflux rope be observed in high-temperature wavelengths. The differential\nemission measure results confirm that this flux rope is a high-temperature\nstructure. Associated with the rising flux rope, there appear many post-flare\nloops and a pair of flare ribbons. When the rising flux rope meets and\nreconnects with the large-scale overlying field lines, a set of large-scale\ntwisted loops are formed, and two flare ribbons propagating in opposite\ndirections appear on the outskirts of the former ribbons, indicating that the\ntwist of the flux rope is transferred to a much larger system. These results\nimply that the external reconnection between the rising flux rope and the\nlarge-scale overlying loops plays an important role in the confined flare\nformation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Radiation-pressure-driven dust waves inside bursting interstellar\n  bubbles: Massive stars drive the evolution of the interstellar medium through their\nradiative and mechanical energy input. After their birth, they form bubbles of\nhot gas surrounded by a dense shell. Traditionally, the formation of bubbles is\nexplained through the input of a powerful stellar wind, even though direct\nevidence supporting this scenario is lacking. Here we explore the possibility\nthat interstellar bubbles seen by the Spitzer- and Herschel space telescopes,\nblown by stars with log(L/L_sun) < 5.2, form and expand due to the thermal\npressure accompanying ionization of the surrounding gas. We show that density\ngradients in the natal cloud or a puncture in the swept up shell lead to an\nionized gas flow through the bubble into the general interstellar medium, which\nis traced by a dust wave near the star, demonstrating the importance of\nradiation pressure during this phase. Dust waves provide a natural explanation\nfor the presence of dust inside H II bubbles, offer a novel method to study\ndust in H II regions and provide direct evidence that bubbles are relieving\ntheir pressure into the ISM through a champagne flow, acting as a probe of the\nradiative interaction of a massive star with its surroundings. We create a\nparameter space connecting the ambient density, the ionizing source luminosity,\nand the position of the dust wave, while using the well-studied H II bubbles\nRCW 120 and RCW 82 as benchmarks of our model. Finally, we briefly examine the\nimplications of our study for the environments of super star clusters formed in\nUltraLuminous InfraRed Galaxies (ULIRGs), merging galaxies and the early\nUniverse, which occur in very luminous and dense environments and where\nradiation pressure is expected to dominate the dynamical evolution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The physics of neutron stars: Topical problems in the physics of and basic facts about neutron stars are\nreviewed. The observational manifestations of neutron stars, their core and\nenvelope structure, magnetic fields, thermal evolution, and masses and radii\nare briefly discussed, along with the underlying microphysics.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Statistics of the two-point cross-covariance function of solar\n  oscillations: Context: The cross-covariance of solar oscillations observed at pairs of\npoints on the solar surface is a fundamental ingredient in time-distance\nhelioseismology. Wave travel times are extracted from the cross-covariance\nfunction and are used to infer the physical conditions in the solar interior.\nAims: Understanding the statistics of the two-point cross-covariance function\nis a necessary step towards optimizing the measurement of travel times.\nMethods: By modeling stochastic solar oscillations, we evaluate the variance of\nthe cross-covariance function as function of time-lag and distance between the\ntwo points. Results: We show that the variance of the cross-covariance is\nindependent of both time-lag and distance in the far field, i.e., when they are\nlarge compared to the coherence scales of the solar oscillations. Conclusions:\nThe constant noise level for the cross-covariance means that the\nsignal-to-noise ratio for the cross-covariance is proportional to the amplitude\nof the expectation value of the cross-covariance. This observation is important\nfor planning data analysis efforts.",
        "category": "astro-ph_SR"
    },
    {
        "text": "An interesting candidate for isolated massive star formation in the\n  Small Magellanic Cloud: The region of the Small Magellanic Cloud (SMC) with which this paper is\nconcerned contains the highest concentration of IRAS/Spitzer sources, H I\nemission, and molecular clouds in this neighboring galaxy. However very few\nstudies have been devoted to it, despite these signs of star formation. We\npresent the first detailed study of the compact H II region N33 in the SMC by\nplacing it in a wider context of massive star formation. Moreover, we show that\nN33 is a particularly interesting candidate for isolated massive star\nformation. This analysis is based mainly on optical ESO NTT observations, both\nimaging and spectroscopy, coupled with other archive data, notably Spitzer\nimages (IRAC 3.6, 4.5, 5.8, and 8.0 mic) and 2MASS observations. We derive a\nnumber of physical characteristics of the compact H II region N33 for the first\ntime. This gas and dust formation of 7\".4 (2.2 pc) in diameter is powered by a\nmassive star of spectral type O6.5-O7 V. The compact H II region belongs to a\nrare class of H II regions in the Magellanic Clouds, called high-excitation\nblobs (HEBs). We show that this H II region is not related to any star cluster.\nSpecifically, we do not find any traces of clustering around N33 on scales\nlarger than 10\" (~ 3 pc). On smaller scales, there is a marginal stellar\nconcentration, the low density of which, below the 3 sigma level, does not\nclassify it as a real cluster. We also verify that N33 is not a member of any\nlarge stellar association. Under these circumstances, N33 is also therefore\nattractive because it represents a remarkable case of isolated massive-star\nformation in the SMC. Various aspects of the relevance of N33 to the topic of\nmassive-star formation in isolation are discussed.",
        "category": "astro-ph_SR"
    },
    {
        "text": "AstroSat study of the globular cluster NGC 2298: probable evolutionary\n  scenarios of hot HB stars: We present the far-UV (FUV) photometry of images acquired with UVIT on\nAstroSat to probe the horizontal branch (HB) population of the Galactic\nglobular cluster NGC 2298. UV-optical color-magnitude diagrams (CMDs) are\nconstructed for member stars in combination with HST UV Globular Cluster Survey\n(HUGS) data for the central region and Gaia and ground-based photometric data\nfor the outer region. Blue HB (BHB) sequence with a spread and four hot HB\nstars are detected in all FUV-optical CMDs and are compared with theoretical\nupdated BaSTI isochrones and synthetic HB models with a range in helium\nabundance, suggesting that the hot HB stars are helium enhanced when compared\nto the BHB. The estimated effective temperature, radius, and luminosity of HB\nstars, using best SED fits, were compared with various HB models. BHB stars\nspan a temperature range from 7,500-12,250 K. The three hot HB stars have\n35,000-40,000 K, whereas one star has around 100,000K. We suggest the following\nevolutionary scenarios: two stars are likely to be the progeny of extreme HB\n(EHB) stars formed through an early hot-flasher scenario; one is likely to be\nan EHB star with probable helium enrichment, the hottest HB star is about to\nenter the WD cooling phase, could have evolved from BHB phase. Nevertheless,\nthese are interesting spectroscopic targets to understand the late stages of\nevolution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The dispersion of period spacing for DAV stars: Three groups of DAV star models are evolved with time-dependent element\ndiffusion by \\texttt{WDEC}. The core compositions of these models are directly\nfrom white dwarf models evolved by \\texttt{MESA}, which are results of\nthermonuclear burning. Based on these DAV star models, we try to study the\ndispersion of period spacing. The thickness of hydrogen atmosphere can\nseriously affect the deviation degree of minimal period spacings. The minimal\nperiod spacings dominate the dispersion of period spacing. The thinner the\nhydrogen atmosphere, basically, the more dispersive the period spacing.\nStandard deviations are used to analyze the dispersion of period spacing.\nStudying the dispersion of period spacing on a DAV star KUV03442+0719\npreliminarily, we suggest that log($M_{\\rm H}/M_{*}$) is from -8.5 to -5.5. In\naddition, modes partly trapped in C/O core are found based on those DAV star\nmodels. The identified modes and average period spacings indicate that\nKUV03442+0719 may be the first star to 'observe' modes partly trapped in C/O\ncore.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Surrogate forward models for population inference on compact binary\n  mergers: Rapidly growing catalogs of compact binary mergers from advanced\ngravitational-wave detectors allow us to explore the astrophysics of massive\nstellar binaries. Merger observations can constrain the uncertain parameters\nthat describe the underlying processes in the evolution of stars and binary\nsystems in population models. In this paper, we demonstrate that binary black\nhole populations - namely, detection rates, chirp masses, and redshifts - can\nbe used to measure cosmological parameters describing the redshift-dependent\nstar formation rate and metallicity distribution. We present a method that uses\nartificial neural networks to emulate binary population synthesis computer\nmodels, and construct a fast, flexible, parallelisable surrogate model that we\nuse for inference.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The disappearance of the helium-giant progenitor of the type Ib\n  supernova iPTF13bvn and constraints on its companion: We report and discuss post-explosion observations of supernova iPTF13bvn. We\nfind that the brightness of the SN at +740 days is below the level of the\npre-explosion source and thus confirm that the progenitor star has gone. We\nestimate that the late-time brightness is still dominated by the supernova,\nwhich constrains the magnitude and thus mass of a possible companion star to\nbelow approximately 10Msun. In turn this implies that the progenitor's initial\nmass is constrained to a narrow range of between 10 to 12Msun. The progenitor\nof iPTF13bvn would have been a helium giant rather than a Wolf-Rayet star. In\naddition, we suggest that sufficiently deep observations acquired in 2016 would\nnow stand a chance to directly observe the companion star.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A characterization of ASAS-SN core-collapse supernova environments with\n  VLT+MUSE: I. Sample selection, analysis of local environments, and\n  correlations with light curve properties: The analysis of core-collapse supernova (CCSN) environments can provide\nimportant information on the life cycle of massive stars and constrain the\nprogenitor properties of these powerful explosions. The MUSE instrument at the\nVLT enables detailed local environment constraints of the progenitors of large\nsamples of CCSNe. Using a homogeneous SN sample from the ASAS-SN survey has\nenabled us to perform a minimally biased statistical analysis of CCSN\nenvironments. We analyze 111 galaxies observed by MUSE that hosted 112 CCSNe\ndetected or discovered by the ASAS-SN survey between 2014 and 2018. The\nmajority of the galaxies were observed by the the AMUSING survey. Here we\nanalyze the immediate environment around the SN locations and compare the\nproperties between the different CCSN types and their light curves. We used\nstellar population synthesis and spectral fitting techniques to derive physical\nparameters for all HII regions detected within each galaxy, including the star\nformation rate (SFR), H$\\alpha$ equivalent width (EW), oxygen abundance, and\nextinction. We found that stripped-envelope (SE) SNe occur in environments with\na higher median SFR, H$\\alpha$ EW, and oxygen abundances than SNe II and SNe\nIIn/Ibn. The distributions of SNe II and IIn are very similar, indicating that\nthese events explode in similar environments. For the SESNe, SNe Ic have higher\nmedian SFRs, H$\\alpha$ EWs, and oxygen abundances than SNe Ib. SNe IIb have\nenvironments with similar SFRs and H$\\alpha$ EWs to SNe Ib, and similar oxygen\nabundances to SNe Ic. We also show that the postmaximum decline rate, $s$, of\nSNe II correlates with the H$\\alpha$ EW, and that the luminosity and the\n$\\Delta m_{15}$ parameter of SESNe correlate with the oxygen abundance,\nH$\\alpha$ EW, and SFR at their environments. This suggests a connection between\nthe explosion mechanisms of these events to their environment properties.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The sunspot observations by Toaldo and Comparetti at November 1779: There is a low frequency of sunspot observations in some years of the 17th\nand 18th century. In particular, the interval 1777-1795 is one of the periods\nwith scarce observations in the databases. In this note, we report a sunspot\nobservation carried out the 3rd November 1779 by Giuseppe Toaldo and Andrea\nComparetti.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Plasma sloshing in pulse-heated solar and stellar coronal loops: There is evidence that coronal heating is highly intermittent, and flares are\nthe high energy extreme. The properties of the heat pulses are difficult to\nconstrain. Here hydrodynamic loop modeling shows that several large amplitude\noscillations (~ 20% in density) are triggered in flare light curves if the\nduration of the heat pulse is shorter that the sound crossing time of the\nflaring loop. The reason is that the plasma has not enough time to reach\npressure equilibrium during the heating and traveling pressure fronts develop.\nThe period is a few minutes for typical solar coronal loops, dictated by the\nsound crossing time in the decay phase. The long period and large amplitude\nmake these oscillations different from typical MHD waves. This diagnostic can\nbe applied both to observations of solar and stellar flares and to future\nobservations of non-flaring loops at high resolution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Measuring mean densities of delta Scuti stars with asteroseismology.\n  Theoretical properties of large separations using TOUCAN: We aim at studying the theoretical properties of the regular spacings found\nin the oscillation spectra of delta Scuti stars. We performed a multi-variable\nanalysis covering a wide range of stellar structure and seismic properties and\nmodel parameters representative of intermediate-mass, main sequence stars. The\nwork-flow is entirely done using a new Virtual Observatory tool: TOUCAN (the VO\ngateway for asteroseismic models), which is presented in this paper. A linear\nrelation between the large separation and the mean density is predicted to be\nfound in the low frequency frequency domain (i.e. radial orders spanning from 1\nto 8, approximately) of the main-sequence, delta Scuti stars' oscillation\nspectrum. We found that such a linear behavior stands whatever the mass,\nmetallicity, mixing length, and overshooting parameters considered in this\nwork. The intrinsic error of the method is discussed. This includes the\nuncertainty in the large separation determination and the role of rotation. The\nvalidity of the relation found is only guaranteed for stars rotating up to 40\npercent of their break-up velocity. Finally, we applied the diagnostic method\npresented in this work to five stars for which regular patterns have been\nfound. Our estimates for the mean density and the frequency of the fundamental\nradial mode match with those given in the literature within a 20 percent of\ndeviation. Asteroseismology has thus revealed an independent direct measure of\nthe average density of delta Scuti stars, analogous to that of the Sun. This\nplaces tight constraints on the mode identification and hence on the stellar\ninternal structure and dynamics, and allows a determination the radius of\nplanets orbiting around delta Scuti stars with unprecedented precision. This\nopens the way for studying the evolution of regular patterns in pulsating\nstars, and its relation with stellar structure and evolution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Non-Linear Growth of the Magnetic Rayleigh-Taylor Instability: This work examines the effect of the embedded magnetic field strength on the\nnon-linear development of the magnetic Rayleigh-Taylor Instability (RTI) (with\na field-aligned interface) in an ideal gas close to the incompressible limit in\nthree dimensions. Numerical experiments are conducted in a domain sufficiently\nlarge so as to allow the predicted critical modes to develop in a physically\nrealistic manner. The ratio between gravity, which drives the instability in\nthis case (as well as in several of the corresponding observations), and\nmagnetic field strength is taken up to a ratio which accurately reflects that\nof observed astrophysical plasma, in order to allow comparison between the\nresults of the simulations and the observational data which served as\ninspiration for this work. This study finds reduced non-linear growth of the\nrising bubbles of the RTI for stronger magnetic fields, and that this is\ndirectly due to the change in magnetic field strength, rather than the indirect\neffect of altering characteristic length scales with respect to domain size. By\nexamining the growth of the falling spikes, the growth rate appears to be\nenhanced for the strongest magnetic field strengths, suggesting that rather\nthan affecting the development of the system as a whole, increased magnetic\nfield strengths in fact introduce an asymmetry to the system. Further\ninvestigation of this effect also revealed that the greater this asymmetry, the\nless efficiently the gravitational energy is released. By better understanding\nthe under-studied regime of such a major phenomenon in astrophysics, deeper\nexplanations for observations may be sought, and this work illustrates that the\nstrength of magnetic fields in astrophysical plasmas influences observed RTI in\nsubtle and complex ways.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A multi-wavelength study of the young star V1118 Orionis in outburst: Abriged version for astroph: The young late-type star V1118 Orionis was in\noutburst from 2005 to 2006. We followed the outburst with optical and\nnear-infrared photometry; the X-ray emission was further probed with\nobservations taken with XMM-Newton and Chandra during and after the outburst.\nIn addition, we obtained mid-infrared photometry and spectroscopy with Spitzer\nat the peak of the outburst and in the post-outburst phase. The spectral energy\ndistribution of V1118 Ori varied significantly over the course of the outburst.\nThe optical flux showed the largest variations, most likely due to enhanced\nemission by a hot spot. The latter dominated the optical and near-infrared\nemission at the peak of the outburst, while the disk emission dominated in the\nmid-infrared. The X-ray flux correlated with the optical and infrared fluxes,\nindicating that accretion affected the magnetically active corona and the\nstellar magnetosphere. The thermal structure of the corona was variable with\nsome indication of a cooling of the coronal temperature in the early phase of\nthe outburst with a gradual return to normal values. Color-color diagrams in\nthe optical and infrared showed variations during the outburst, with no obvious\nsignature of reddening due to circumstellar matter. Using MC realizations of\nstar+disk+hotspot models to fit the SED in ``quiescence'' and at the peak of\nthe outburst, we determined that the mass accretion rate varied from about\n2.5E-7 Msun/yr to 1E-6 Msun/yr; in addition the fractional area of the hotspot\nincreased significantly as well. The multi-wavelength study of the V1118 Ori\noutburst helped us to understand the variations in spectral energy\ndistributions and demonstrated the interplay between the disk and the stellar\nmagnetosphere in a young, strongly accreting star.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Binarity and Accretion in AGB Stars: HST/STIS Observations of UV\n  Flickering in Y Gem: Binarity is believed to dramatically affect the history and geometry of mass\nloss in AGB and post-AGB stars, but observational evidence of binarity is\nsorely lacking. As part of a project to search for hot binary companions to\ncool AGB stars using the GALEX archive, we discovered a late-M star, Y Gem, to\nbe a source of strong and variable UV and X-ray emission. Here we report UV\nspectroscopic observations of Y Gem obtained with the Hubble Space Telescope\nthat show strong flickering in the UV continuum on time-scales of <~20 s,\ncharacteristic of an active accretion disk. Several UV lines with P-Cygni-type\nprofiles from species such as Si IV and C IV are also observed, with emission\nand absorption features that are red- and blue- shifted by velocities of ~500\nkm/s from the systemic velocity. Our model for these (and previous)\nobservations is that material from the primary star is gravitationally captured\nby a companion, producing a hot accretion disk. The latter powers a fast\noutflow that produces blue-shifted features due to absorption of UV continuum\nemitted by the disk, whereas the red-shifted emission features arise in heated\ninfalling material from the primary. The outflow velocities support a previous\ninference by Sahai et al. (2015) that Y Gem's companion is a low-mass\nmain-sequence star. Blackbody fitting of the UV continuum implies an accretion\nluminosity of about 13 Lsun and thus a mass-accretion rate >5e-7 Msun/yr; we\ninfer that Roche lobe overflow is the most likely binary accretion mode for Y\nGem.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spectroscopic factor of the 1+, 25Al(p,g)26Si resonance at Ex=5.68 MeV: Nuclear shell model predictions for the proton spectroscopic factor of the\n1+, Ex = 5.68 MeV level in 26Si are about fifty times smaller than the value\nsuggested by the measured (a,3He) cross section for the Ex = 5.69 MeV mirror\nlevel in 26Mg, assuming purely single-particle transfer. Given that the 5.69\nMeV level has been very weakly, if it all, populated in previous studies of the\nsimpler 25Mg(d,p) reaction, it is unclear if the (a,3He) result is a true\nsingle-particle spectroscopic factor. If we assume the (a,3He) result, the\nthermonuclear rate of the 25Al(p,g)26Si reaction would increase by factors of 6\n- 50 over stellar temperatures of T = 0.05 - 0.2 GK. We examine the\nimplications of this enhanced rate for model predictions of nucleosynthesis in\nclassical nova explosions.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Multicolour photometry of pulsating stars in the Galactic Bulge fields: We present a study of photometric properties of very crowded stellar fields\ntoward the Galactic Bulge. We performed a search for pulsating stars among\nthousands of variable stars from the OGLE-II survey supplementing the\nvariability study with photometric measurements in four Johnson-Cousins\n$UBVI_{\\rm C}$ passbands. Using these data, we analysed the properties of\nobjects located at different distances and, whenever possible, classified them.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Parallax of the L4.5 dwarf 2M1821$+$14 from high-precision astrometry\n  with OSIRIS at GTC: We used the OSIRIS camera at the 10.4 m Gran Telescopio Canarias (GTC) to\nmonitor the astrometric motion of the L4.5 dwarf 2M1821$+$14 over 17 months.\nThe astrometric residuals of eleven epochs have a r.m.s. dispersion of 0.4 mas,\nwhich is larger than the average precision of 0.23 mas per epoch and hints\ntowards an additional signal or excess noise. Comparison of the\npoint-spread-functions in OSIRIS and FORS2/VLT images reveals no differences\ncritical for high-precision astrometry, despite the GTC's segmented primary\nmirror. We attribute the excess noise to an unknown effect that may be\nuncovered with additional data. For 2M1821$+$14, we measured a relative\nparallax of $106.15 \\pm 0.18$ mas and determined a correction of $0.50\\pm0.05$\nmas to absolute parallax, leading to a distance of $9.38 \\pm0.03$ pc. We\nexcluded at 3-$\\sigma$ confidence the presence of a companion to 2M1821$+$14\ndown to a mass ratio of 0.1 ($\\approx 5\\, M_\\mathrm{Jupiter}$) with a period of\n50--1000 days and a separation of 0.1--0.7 au. The accurate parallax allowed us\nto estimate the age and mass of 2M1821$+$14 of 120--700 Myr and\n0.049$^{+0.014}_{-0.024}$ M$_\\odot$, thus confirming its intermediate age and\nsubstellar mass. We complement our study with a parallax and proper motion\ncatalogue of 587 stars ($i'\\simeq15.5-22$) close to 2M1821$+$14, used as\nastrometric references. This study demonstrates sub-mas astrometry with the\nGTC, a capability applicable for a variety of science cases including the\nsearch for extrasolar planets and relevant for future astrometric observations\nwith E-ELT and TMT.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Rotational periods and evolutionary models for subgiant stars observed\n  by CoRoT: We present rotation period measurements for subgiants observed by CoRoT.\nInterpreting the modulation of stellar light that is caused by star-spots on\nthe time scale of the rotational period depends on knowing the fundamental\nstellar parameters. Constraints on the angular momentum distribution can be\nextracted from the true stellar rotational period. By using models with an\ninternal angular momentum distribution and comparing these with measurements of\nrotation periods of subgiant stars we investigate the agreement between\ntheoretical predictions and observational results. With this comparison we can\nalso reduce the global stellar parameter space compatible with the rotational\nperiod measurements from subgiant light curves. We can prove that an evolution\nassuming solid body rotation is incompatible with the direct measurement of the\nrotational periods of subgiant stars. Measuring the rotation periods relies on\ntwo different periodogram procedures, the Lomb-Scargle algorithm and the\nPlavchan periodogram. Angular momentum evolution models were computed to give\nus the expected rotation periods for subgiants, which we compared with measured\nrotational periods. We find evidence of a sinusoidal signal that is compatible\nin terms of both phase and amplitude with rotational modulation. Rotation\nperiods were directly measured from light curves for 30 subgiant stars and\nindicate a range of 30 to 100 d for their rotational periods. Our models\nreproduce the rotational periods obtained from CoRoT light curves. These new\nmeasurements of rotation periods and stellar models probe the non-rigid\nrotation of subgiant stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the nature of Nova 1670 (CK Vulpeculae): a merger of a red giant with\n  a helium white dwarf: Nova 1670 is a historical transient bearing strong similarities to a\nrecently-recognized type of stellar eruptions known as red novae, which are\nthought to be powered by stellar mergers. The remnant of the transient, CK Vul,\nis observable today mainly through cool circumstellar gas and dust, and\nrecombining plasma, but we have no direct view on the stellar object. Within\nthe merger hypothesis, we aim to infer the most likely makeup of the progenitor\nsystem that resulted in Nova 1670. We collect and summarize the literature data\non the physical properties of the outburst and the remnant, and on the chemical\ncomposition of the circumstellar material which resulted from optical and\nsubmillimeter observations of the circumstellar gas of CK Vul. Simple\nsimulations yield the form and level of mixing of material associated with the\nmerger. Products of nuclear burning are identified, among them ashes of H\nburning in the CNO cycles and the MgAl chain, as well as of partial He burning.\nBased on the luminosity and chemical composition of the remnant, we find that\nthe progenitor primary had to be an evolutionarily advanced red-giant branch\nstar of a mass of 1-2 M$_{\\odot}$. The secondary was either a very similar\ngiant, or a He white dwarf. While the eruption event was mainly powered by\naccretion, we estimate that about 12% of total energy might have come from He\nburning activated during the merger. The coalescence of a first-ascent giant\nwith a He white dwarf created a star with a rather unique internal structure\nand composition, which resemble these of early R-type carbon stars. Nova 1670\nwas the result of a merger between a He white dwarf and a first-ascent red\ngiant and is likely now evolving to become an early R-type carbon star.",
        "category": "astro-ph_SR"
    },
    {
        "text": "An Infrared Census Of Dust In Nearby Galaxies With Spitzer (DUSTiNGS):\n  V. The Period-luminosity Relation For Dusty Metal-Poor AGB Stars: The survey for DUST In Nearby Galaxies with Spitzer (DUSTiNGS) has identified\nhundreds of candidate dust-producing Asymptotic Giant Branch (AGB) stars in\nseveral nearby metal-poor galaxies. We have obtained multi-epoch follow-up\nobservations for these candidates with the Spitzer Space Telescope and measured\ntheir infrared (IR) lightcurves. This has allowed us to confirm their AGB\nnature and investigate pulsation behavior at very low metallicity. We have\nobtained high-confidence pulsation periods for 88 sources in seven galaxies. We\nhave confirmed DUSTiNGS variable star candidates with a 20% success rate, and\ndetermined the pulsation properties of 19 sources already identified as\nThermally-Pulsing AGB (TP-AGB) stars. We find that the AGB pulsation properties\nare similar in all galaxies surveyed here, with no discernible difference\nbetween the DUSTiNGS galaxies (down to 1.4% solar metallicity; [Fe/H]=-1.85)\nand the far more metal-rich Magellanic Clouds (up to 50% solar metallicity;\n[Fe/H]=-0.38). These results strengthen the link between dust production and\npulsation in AGB stars and establish the IR Period-Luminosity (P - L) relation\nas a reliable tool (+/- 4%) for determining distances to galaxies, regardless\nof metallicity.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Large ion-neutral drift velocities and plasma heating in partially\n  ionized coronal rain blobs: In this paper we present a numerical study of the dynamics of partially\nionized coronal rain blobs. We use a two-fluid model to perform a\nhigh-resolution 2D simulation that takes into account the collisional\ninteraction between the charged and neutral particles contained in the plasma.\nWe follow the evolution of a cold plasma condensation as it falls through an\nisothermal vertically stratified atmosphere that represents the much hotter and\nlighter solar corona. We study the consequences of the different degrees of\ncollisional coupling that are present in the system. On the one hand, we find\nthat at the dense core of the blob there is a very strong coupling and the\ncharged and neutral components of the plasma behave as a single fluid, with\nnegligible drift velocities (of a few cm s^-1). On the other hand, at the edges\nof the blob the coupling is much weaker and larger drift velocities (of the\norder of 1 km s^-1) appear. In addition, frictional heating causes large\nincreases of temperature at the transition layers between the blob and the\ncorona. For the first time we show that such large drift velocities and\ntemperature enhancements can develop as a consequence of ion-neutral decoupling\nassociated to coronal rain dynamics. This can lead to enhanced emission coming\nfrom the plasma at the coronal rain-corona boundary, which possesses transition\nregion temperature.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Mid-infrared observations of O-type stars: spectral morphology: We present mid-infrared observations for a sample of 16 O-type stars. The\ndata were acquired with the NASA Spitzer Space Telescope, using the IRS\ninstrument at moderate resolution (R $\\sim$ 600), covering the range $\\sim\n10-37$ microns. Our sample includes early, mid and late O supergiants and\ndwarfs. We explore for the first time their mid-IR spectral morphology in a\nquantitative way. We use NLTE expanding atmosphere models to help with line\nidentifications, analyze profile contributions and line-formation regions. The\nO supergiants present a rich emission line spectra. The most intense features\nare from hydrogen - $6\\alpha$, 7$\\alpha$, and $8\\alpha$ - which have\nnon-negligible contributions of HeI or HeII lines, depending on the spectral\ntype. The spectrum of early O supergiants is a composite of HI and HeII lines,\nHeI lines being absent. On the other hand, late O supergiants present features\ncomposed mainly by HI and HeI lines. All emission lines are formed throughout\nthe stellar wind. We found that O dwarfs exhibit a featureless mid-IR spectrum.\nTwo stars of our sample exhibit very similar mid-IR features, despite having a\nvery different optical spectral classification. The analysis of O-type stars\nbased on mid-IR spectra alone to infer spectral classes or to estimate physical\nparameters may thus be prone to substantial errors. Our results may therefore\ninform spectroscopic observations of massive stars located in heavily obscured\nregions and help establish an initial framework for observations of massive\nstars using the Mid-Infrared Instrument on the James Webb Space Telescope.",
        "category": "astro-ph_SR"
    },
    {
        "text": "An Incipient Debris Disk in the Chamaeleon I Cloud: The point at which a protoplanetary disk becomes a debris disk is difficult\nto identify. To better understand this, here we study the $\\sim$40~AU\nseparation binary T~54 in the Chamaeleon I cloud. We derive a K5 spectral type\nfor T~54~A (which dominates the emission of the system) and an age of\n$\\sim$2~Myr. However, the dust disk properties of T~54 are consistent with\nthose of debris disks seen around older and earlier-type stars. At the same\ntime, T~54 has evidence of gas remaining in the disk as indicated by [Ne II],\n[Ne III], and [O I] line detections. We model the spectral energy distribution\nof T~54 and estimate that $\\sim$3$\\times$10$^{-3}$ Earth-masses of small dust\ngrains ($<$0.25~$\\mu$m) are present in an optically thin circumbinary disk\nalong with at least $\\sim$3$\\times$10$^{-7}$ Earth-masses of larger\n($>$10~$\\mu$m) grains within a circumprimary disk. Assuming a solar-like\nmixture, we use Ne line luminosities to place a minimum limit on the gas mass\nof the disk ($\\sim$3$\\times$10$^{-4}$ Earth-masses) and derive a gas-to-dust\nmass ratio of $\\sim$0.1. We do not detect substantial accretion, but we do see\nH$\\alpha$ in emission in one epoch, suggestive that there may be intermittent\ndumping of small amounts of matter onto the star. Considering the low dust\nmass, the presence of gas, and young age of T~54, we conclude that this system\nis on the bridge between the protoplanetary and debris disk stages.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The LAMOST spectroscopic survey of star clusters in M31. II.\n  Metallicities, ages and masses: We select from Paper I a sample of 306 massive star clusters observed with\nthe Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) in the\nvicinity fields of M31 and M33 and determine their metallicities, ages and\nmasses. Metallicities and ages are estimated by fitting the observed integrated\nspectra with stellar synthesis population (SSP) models with a pixel-to-pixel\nspectral fitting technique. Ages for most young clusters are also derived by\nfitting the multi-band photometric measurements with model spectral energy\ndistributions (SEDs). The estimated cluster ages span a wide range, from\nseveral million years to the age of the universe. The numbers of clusters\nyounger and older than 1 Gyr are respectively 46 and 260. With ages and\nmetallicities determined, cluster masses are then estimated by comparing the\nmulti-band photometric measurements with SSP model SEDs. The derived masses\nrange from $\\sim 10^{3}$ to $\\sim 10^7$ $M_{\\odot}$, peaking at $\\sim 10^{4.3}$\nand $\\sim 10^{5.7}$ $M_{\\odot}$ for young ($< 1$ Gyr) and old ($>1$ Gyr)\nclusters, respectively. Our estimated metallicities, ages and masses are in\ngood agreement with available literature values. Old clusters richer than\n[Fe/H] $\\sim -0.7$ dex have a wide range of ages. Those poorer than [Fe/H]\n$\\sim -0.7$ dex seem to be composed of two groups, as previously found for\nGalactic GCs -- one of the oldest ages with all values of metallicity down to\n$\\sim -2$ dex and another with metallicity increasing with decreasing age. The\nold clusters in the inner disk of M\\,31 (0 -- 30 kpc) show a clear metallicity\ngradient measured at $-0.038\\pm0.023$ dex/kpc.",
        "category": "astro-ph_SR"
    },
    {
        "text": "HD 156324: a tidally locked magnetic triple spectroscopic binary with a\n  disrupted magnetosphere: HD 156324 is an SB3 (B2V/B5V/B5V) system in the Sco OB4 association. The\nHe-strong primary possesses both a strong magnetic field, and H$\\alpha$\nemission believed to originate in its Centrifugal Magnetosphere (CM). We\nanalyse a large spectroscopic and high-resolution spectropolarimetric dataset.\nThe radial velocities (RVs) indicate that the system is composed of two\nsub-systems, which we designate A and B. Period analysis of the RVs of the\nthree components yields orbital periods $P_{\\rm orb} = 1.5806(1)$~d for the Aa\nand Ab components, and 6.67(2)~d for the B component, a PGa star. Period\nanalysis of the longitudinal magnetic field \\bz~and H$\\alpha$ equivalent\nwidths, which should both be sensitive to the rotational period $P_{\\rm rot}$\nof the magnetic Aa component, both yield $\\sim$1.58~d. Since $P_{\\rm orb} =\nP_{\\rm rot}$ Aa and Ab must be tidally locked. Consistent with this, the orbit\nis circularized, and the rotational and orbital inclinations are identical\nwithin uncertainty, as are the semi-major axis and the Kepler corotation\nradius. The star's H$\\alpha$ emission morphology differs markedly from both\ntheoretical and observational expectations in that there is only one, rather\nthan two, emission peaks. We propose that this unusual morphology may be a\nconsequence of modification of the gravitocentrifugal potential by the presence\nof the close stellar companion. We also obtain upper limits on the magnetic\ndipole strength $B_{\\rm d}$ for the Ab and B components, respectively finding\n$B_{\\rm d} < 2.6$~kG and $<0.7$~kG.",
        "category": "astro-ph_SR"
    },
    {
        "text": "He white dwarfs with large H contamination: Convective mixing or\n  accretion?: White dwarfs are compact objects with atmospheres containing mainly light\nelements, hydrogen or helium. Because of their surface high gravitational\nfield, heavy elements diffuse downwards in a very short timescale compared to\nthe evolutionary timescale, leaving the lightest ones on the top of the\nenvelope. This results in the main classification of white dwarfs as hydrogen\nrich or helium rich. But many helium rich white dwarfs show also the presence\nof hydrogen traces in their atmosphere, whose origin is still unsettled. Here\nwe study, by means of full evolutionary calculations, the case for a\nrepresentative model of the \"He-H-Z\" white dwarfs, a sub-group of helium rich\nwhite dwarfs showing both heavy elements and a large amount of hydrogen in\ntheir atmosphere. We find it impossible to explain its hydrogen atmospheric\ncontent by the convective mixing of a primordial hydrogen present in the star.\nWe conclude that the most likely explanation is the accretion of hydrogen rich\nmaterial, presumably water-bearing, coming from a debris disk.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Coronal Fourier power spectra: implications for coronal seismology and\n  coronal heating: The dynamics of regions of the solar corona are investigated using\nAtmospheric Imaging Assembly (AIA) 171\\AA\\ and 193\\AA\\ data. The coronal\nemission from the quiet Sun, coronal loop footprints, coronal moss, and from\nabove a sunspot is studied. It is shown that the mean Fourier power spectra in\nthese regions can be described by a power law at lower frequencies that tails\nto flat spectrum at higher frequencies, plus a Gaussian-shaped contribution\nthat varies depending on the region studied. This Fourier spectral shape is in\ncontrast to the commonly-held assumption that coronal time-series are well\ndescribed by the sum of a long time-scale background trend plus\nGaussian-distributed noise, with some specific locations also showing an\noscillatory signal. The implications of this discovery to the field of coronal\nseismology and the automated detections of oscillations are discussed. The\npower law contribution to the shape of the Fourier power spectrum is\ninterpreted as being due to the summation of a distribution of exponentially\ndecaying emission events along the line of sight. This is consistent with the\nidea that the solar atmosphere is heated everywhere by small energy deposition\nevents.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Observations of the forward scattering Hanle effect in the Ca i 4227\n  \u00c5 line: Chromospheric magnetic fields are notoriously diffcult to measure. The\nchromospheric lines are broad, while the fields are producing a minuscule\nZeeman-effect polarization. A promising diagnostic alternative is provided by\nthe forward-scattering Hanle effect, which can be recorded in chromospheric\nlines such as the He i 10830 {\\AA} and the Ca i 4227 {\\AA} lines. We present a\nset of spectropolarimetric observations of the full Stokes vector obtained near\nthe center of the solar disk in the Ca i 4227 {\\AA} line with the ZIMPOL\npolarimeter at the IRSOL observatory.We detect a number of interesting\nforward-scattering Hanle effect signatures, which we model successfully using\npolarized radiative transfer. Here we focus on the observational aspects, while\na separate companion paper deals with the theoretical modeling.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Universal Law for Solar-Wind Turbulence at Electron Scales: The interplanetary magnetic fluctuation spectrum obeys a Kolmogorovian power\nlaw at scales above the proton inertial length and gyroradius which is well\nregarded as an inertial range. Below these scales a power law index around\n$-2.5$ is often measured and associated to nonlinear dispersive processes.\nRecent observations reveal a third region at scales below the electron inertial\nlength. This region is characterized by a steeper spectrum that some refer to\nit as the dissipation range. We investigate this range of scales in the\nelectron magnetohydrodynamic approximation and derive an exact and universal\nlaw for a third-order structure function. This law can predict a magnetic\nfluctuation spectrum with an index of $-11/3$ which is in agreement with the\nobserved spectrum at the smallest scales. We conclude on the possible existence\nof a third turbulence regime in the solar wind instead of a dissipation range\nas recently postulated.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evolution towards and beyond accretion-induced collapse of massive white\n  dwarfs and formation of millisecond pulsars: Millisecond pulsars (MSPs) are generally believed to be old neutron stars\n(NSs), formed via type Ib/c core-collapse supernovae (SNe), which have been\nspun up to high rotation rates via accretion from a companion star in a\nlow-mass X-ray binary (LMXB). In an alternative formation channel, NSs are\nproduced via the accretion-induced collapse (AIC) of a massive white dwarf (WD)\nin a close binary. Here we investigate binary evolution leading to AIC and\nexamine if NSs formed in this way can subsequently be recycled to form MSPs\nand, if so, how they can observationally be distinguished from pulsars formed\nvia the standard core-collapse SN channel in terms of their masses, spins,\norbital periods and space velocities. Numerical calculations with a detailed\nstellar evolution code were used for the first time to study the combined pre-\nand post-AIC evolution of close binaries. We investigated the mass transfer\nonto a massive WD in 240 systems with three different types of non-degenerate\ndonor stars: main-sequence stars, red giants, and helium stars. When the WD is\nable to accrete sufficient mass (depending on the mass-transfer rate and the\nduration of the accretion phase) we assumed it collapses to form a NS and we\nstudied the dynamical effects of this implosion on the binary orbit.\nSubsequently, we followed the mass-transfer epoch which resumes once the donor\nstar refills its Roche lobe and calculated the continued LMXB evolution until\nthe end. We demonstrate that the final properties of these MSPs are, in\ngeneral, remarkably similar to those of MSPs formed via the standard\ncore-collapse SN channel. However, the resultant MSPs created via the AIC\nchannel preferentially form in certain orbital period intervals. Finally, we\ndiscuss the link between AIC and young NSs in globular clusters. Our\ncalculations are also applicable to progenitor binaries of SNe Ia under certain\nconditions. [Abridged]",
        "category": "astro-ph_SR"
    },
    {
        "text": "First solar butterfly diagram from Schwabe's observations in 1825-1867: The original sunspot observations by Heinrich Samuel Schwabe of 1825-1867\nwere digitized and a first subset of spots was measured. In this initial\nproject, we determined more than 14 000 sunspot positions and areas comprising\nabout 11% of the total amount of spots available from that period. The\nresulting butterfly diagram has a typical appearance, but with evident\nnorth-south asymmetries.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Is it possible to detect coronal mass ejections on solar-type stars\n  through extreme-ultraviolet spectral observations?: Stellar coronal mass ejections (CMEs) from host stars are an important factor\nthat affects the habitability of exoplanets. Although their solar counterparts\nhave been well observed for decades, it is still very difficult to find solid\nevidence of stellar CMEs. Using the spectral line profile asymmetry caused by\nthe Doppler shift of erupting plasma, several stellar CME candidates have been\nidentified from spectral lines formed at chromospheric or transition region\ntemperatures of the stars. However, a successful detection of stellar CME\nsignals based on the profile asymmetries of coronal lines is still lacking. It\nis unclear whether we can detect such signals. Here we construct an analytical\nmodel for CMEs on solar-type stars, and derive an expression of stellar\nextreme-ultraviolet (EUV) line profiles during CME eruptions. For different\ninstrumental parameters, exposure times, CME propagation directions and stellar\nactivity levels, we synthesized the corresponding line profiles of Fe IX 171.07\n\\AA\\ and Fe XV 284.16 \\AA. Further investigations provide constraints on the\ninstrumental requirements for successful detection and characterization of\nstellar CMEs. Our results show that it is possible to detect stellar CME\nsignals and infer their velocities based on spectral profile asymmetries using\nan EUV spectrometer with a moderate spectral resolution and signal-to-noise\nratio. Our work provides important references for the design of future EUV\nspectrometers for stellar CME detection and the development of observation\nstrategies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Stability of the Electron Strahl against the Oblique\n  Fast-magnetosonic/Whistler Instability in the Inner Heliosphere: We analyze the micro-kinetic stability of the electron strahl in the solar\nwind depending on heliocentric distance. The oblique fast-magnetosonic/whistler\n(FM/W) instability has emerged in the literature as a key candidate mechanism\nfor the effective scattering of the electron strahl into the electron halo\npopulation. Using data from Parker Solar Probe (PSP) and Helios, we compare the\nmeasured strahl properties with the analytical thresholds for the oblique FM/W\ninstability in the low- and high-$\\beta_{\\parallel c}$ regimes, where\n$\\beta_{\\parallel c}$ is the ratio of the core parallel thermal pressure to the\nmagnetic pressure. Our PSP and Helios data show that the electron strahl is on\naverage stable against the oblique FM/W instability in the inner heliosphere.\nOur analysis suggests that the instability, if at all, can only be excited\nsporadically and on short timescales. We discuss the caveats of our analysis\nand potential alternative explanations for the observed scattering of the\nelectron strahl in the solar wind. Furthermore, we recommend the numerical\nevaluation of the stability of individual distributions in the future to\naccount for any uncertainties in the validity of the analytical expressions for\nthe instability thresholds.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Characteristics of transverse waves in chromospheric mottles: Using data obtained by the high temporal and spatial resolution Rapid\nOscillations in the Solar Atmosphere (ROSA) instrument on the Dunn Solar\nTelescope, we investigate at an unprecedented level of detail transverse\noscillations in chromospheric fine structures near the solar disk center. The\noscillations are interpreted in terms of propagating and standing\nmagnetohydrodynamic kink waves. Wave characteristics including the maximum\ntransverse velocity amplitude and the phase speed are measured as a function of\ndistance along the structure's length. Solar magneto-seismology is applied to\nthese measured parameters to obtain diagnostic information on key plasma\nparameters (e.g., magnetic field, density, temperature, flow speed) of these\nlocalised waveguides. The magnetic field strength of the mottle along the\n$\\sim$2 Mm length is found to decrease by a factor of 12, while the local\nplasma density scale height is $\\sim280\\pm$80 km.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Physical properties and astrometry of radio-emitting brown dwarf\n  TVLM513-46546 revisited: We present multi-epoch astrometric observations of the M9 ultra-cool dwarf\nTVLM513-46546 that is placed at the brown dwarf boundary. The new observations\nhave been performed with the European VLBI Network (EVN) at 6cm band. The\ntarget has been detected at 7~epochs spanning three years, with measured\nquiescent emission flux in the range 180-300 muJy. We identified four\nshort-duration flaring events (0.5-2 mJy) with very high circular polarization\n(~75%-100%). Properties of the observed radio flares support the physical model\nof the source that is characterized by the electron cyclotron maser instability\nresponsible for outbursts of radio emission. Combined with Very Long Baseline\nArray (VLBA) earlier data, our detections make it possible to refine the\nabsolute parallax pi=93.27(+0.18,-0.17) mas. Our measurements rule out\nTVLM513-46546 companions more massive than Jupiter in orbits with periods\nlonger than ~1 yr.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The CARMENES search for exoplanets around M dwarfs: Wing asymmetries of\n  H$\u03b1$, Na I D, and He I lines: Stellar activity is ubiquitously encountered in M dwarfs and often\ncharacterised by the H$\\alpha$ line. In the most active M dwarfs, H$\\alpha$ is\nfound in emission, sometimes with a complex line profile. Previous studies have\nreported extended wings and asymmetries in the H$\\alpha$ line during flares. We\nused a total of 473 high-resolution spectra of 28 active M dwarfs obtained by\nthe CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths\nwith Near-infrared and optical Echelle Spectrographs) spectrograph to study the\noccurrence of broadened and asymmetric H$\\alpha$ line profiles and their\nassociation with flares, and examine possible physical explanations. We\ndetected a total of 41 flares and 67 broad, potentially asymmetric, wings in\nH$\\alpha$. The broadened H$\\alpha$ lines display a variety of profiles with\nsymmetric cases and both red and blue asymmetries. Although some of these line\nprofiles are found during flares, the majority are at least not obviously\nassociated with flaring. We propose a mechanism similar to coronal rain or\nchromospheric downward condensations as a cause for the observed red\nasymmetries; the symmetric cases may also be caused by Stark broadening. We\nsuggest that blue asymmetries are associated with rising material, and our\nresults are consistent with a prevalence of blue asymmetries during the flare\nonset. Besides the H$\\alpha$ asymmetries, we find some cases of additional line\nasymmetries in \\ion{He}{i} D$_{3}$, \\ion{Na}{i}~D lines, and the \\ion{He}{i}\nline at 10830\\,\\AA\\, taken all simultaneously thanks to the large wavelength\ncoverage of CARMENES. Our study shows that asymmetric H$\\alpha$ lines are a\nrather common phenomenon in M~dwarfs and need to be studied in more detail to\nobtain a better understanding of the atmospheric dynamics in these objects.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The B Fields in OB Stars (BOB) Survey: The B fields in OB stars (BOB) survey is an ESO large programme collecting\nspectropolarimetric observations for a large number of early-type stars in\norder to study the occurrence rate, properties, and ultimately the origin of\nmagnetic fields in massive stars. As of July 2014, a total of 98 objects were\nobserved over 20 nights with FORS2 and HARPSpol. Our preliminary results\nindicate that the fraction of magnetic OB stars with an organised, detectable\nfield is low. This conclusion, now independently reached by two different\nsurveys, has profound implications for any theoretical model attempting to\nexplain the field formation in these objects. We discuss in this contribution\nsome important issues addressed by our observations (e.g., the lower bound of\nthe field strength) and the discovery of some remarkable objects.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Note on the Power-2 Limb Darkening Law: Recently there has been a renewed interest in the power-2 limb darkening law\nfor modeling exoplanet transits. This law provides a better match to the\nintensities generated by spherical stellar atmosphere models than other\n2-parameter laws. To help facilitate a wider use of the power-2 law we correct\na minor error and, expanding on previous work, suggest a parametrization that\ncan improve the sampling required by some numerical methods such as MCMC.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Examining the Magnetic Field Strength and the Horizontal and Vertical\n  Motions in an Emerging Active Region: Earlier observational studies have used the time evolution of emerging\nmagnetic flux regions at the photosphere to infer their subsurface structures,\nassuming that the flux structure does not change significantly over the\nnear-surface layer.In this study, we test the validity of this assumption by\ncomparing the horizontal and vertical motions of an emerging active region. The\ntwo motions would be correlated if the emerging structure is rigid. The\nselected active region (AR) NOAA 11645 is not embedded in detectable\npreexisting magnetic field. The observed horizontal motion is quantified by the\nseparation of the two AR polarities and the extension of the region. The\nvertical motion is derived from the magnetic buoyancy theory. Our results show\nthat the separation of the polarities is fastest at the beginning with a\nvelocity of $\\approx$~4~Mm hr$^{-1}$ and decreases to $\\le$~1~Mm hr$^{-1}$\nafter the main growing phase of flux emergence. The derived thick flux-tube\nbuoyant velocity is between 1 and 3~Mm hr$^{-1}$ while the thin flux-tube\napproximation results in an unreasonably high buoyant velocity, consistent with\nthe expectation that the approximation is inappropriate at the surface layer.\nThe observed horizontal motion is not found to directly correlate with either\nthe magnetic field strength or the derived buoyant velocities. However, the\npercentage of the horizontally oriented fields and the temporal derivatives of\nthe field strength and the buoyant velocity show some positive correlations\nwith the separation velocity. The results of this study imply that the\nassumption that the emerging active region is the cross section of a rising\nflux tube whose structure can be considered rigid as it rises through the\nnear-surface layer should be taken with caution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Gray Needle: Large Grains in the HD 15115 Debris Disk from\n  LBT/PISCES/Ks and LBTI/LMIRcam/L' Adaptive Optics Imaging: We present diffraction-limited \\ks band and \\lprime adaptive optics images of\nthe edge-on debris disk around the nearby F2 star HD 15115, obtained with a\nsingle 8.4 m primary mirror at the Large Binocular Telescope. At \\ks band the\ndisk is detected at signal-to-noise per resolution element (SNRE) \\about 3-8\nfrom \\about 1-2\\fasec 5 (45-113 AU) on the western side, and from \\about\n1.2-2\\fasec 1 (63-90 AU) on the east. At \\lprime the disk is detected at SNRE\n\\about 2.5 from \\about 1-1\\fasec 45 (45-90 AU) on both sides, implying more\nsymmetric disk structure at 3.8 \\microns . At both wavelengths the disk has a\nbow-like shape and is offset from the star to the north by a few AU. A surface\nbrightness asymmetry exists between the two sides of the disk at \\ks band, but\nnot at \\lprime . The surface brightness at \\ks band declines inside 1\\asec\n(\\about 45 AU), which may be indicative of a gap in the disk near 1\\asec. The\n\\ks - \\lprime disk color, after removal of the stellar color, is mostly grey\nfor both sides of the disk. This suggests that scattered light is coming from\nlarge dust grains, with 3-10 \\microns -sized grains on the east side and 1-10\n\\microns dust grains on the west. This may suggest that the west side is\ncomposed of smaller dust grains than the east side, which would support the\ninterpretation that the disk is being dynamically affected by interactions with\nthe local interstellar medium.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Flares and rotation of M dwarfs with habitable zones accessible to TESS\n  planet detections: We characterize the magnetic activity of M dwarfs to provide the planet\ncommunity with information on the energy input from the star; in particular, in\naddition to the frequency of optical flares directly observed with TESS, we aim\nat estimating the corresponding X-ray flare frequencies, making use of the\nsmall pool of known events observed simultaneously in both wavebands. We\nidentified 112 M dwarfs with a TESS magnitude <= 11.5 for which TESS can probe\nthe full habitable zone for transits. These 112 stars have 1276 two-minute\ncadence TESS light curves from the primary mission, which we searched for\nrotational modulation and flares. We study the link between rotation and flares\nand between flare properties, for example the flare amplitude-duration relation\nand cumulative flare energy frequency distributions (FFDs). Assuming that each\noptical flare is associated with a flare in the X-ray band, and making use of\npublished simultaneous Kepler/K2 and XMM-Newton flare studies, we estimate the\nX-ray energy released by our detected TESS flare events. Our calibration also\ninvolves the relation between flare energies in the TESS and K2 bands. We\ndetected more than 2500 optical flare events on a fraction of about 32% of our\ntargets and found reliable rotation periods only for 12 stars, which is a\nfraction of about 11%. For these 12 targets, we present cumulative FFDs and FFD\npower law fits. We construct FFDs in the X-ray band by calibrating optical\nflare energies to the X-rays. In the absence of directly observed X-ray FFDs\nfor main-sequence stars, our predictions can serve for estimates of the\nhigh-energy input to the planet of a typical fast-rotating early- or mid-M\ndwarf.",
        "category": "astro-ph_SR"
    },
    {
        "text": "An Interface Region Imaging Spectrograph first view on Solar Spicules: Solar spicules have eluded modelers and observers for decades. Since the\ndiscovery of the more energetic type II, spicules have become a heated topic\nbut their contribution to the energy balance of the low solar atmosphere\nremains unknown. Here we give a first glimpse of what quiet Sun spicules look\nlike when observed with NASA's recently launched Interface Region Imaging\nSpectrograph (IRIS). Using IRIS spectra and filtergrams that sample the\nchromosphere and transition region we compare the properties and evolution of\nspicules as observed in a coordinated campaign with Hinode and the Atmospheric\nImaging Assembly. Our IRIS observations allow us to follow the thermal\nevolution of type II spicules and finally confirm that the fading of Ca II H\nspicules appears to be caused by rapid heating to higher temperatures. The IRIS\nspicules do not fade but continue evolving, reaching higher and falling back\ndown after 500-800 s. Ca II H type II spicules are thus the initial stages of\nviolent and hotter events that mostly remain invisible in Ca II H filtergrams.\nThese events have very different properties from type I spicules, which show\nlower velocities and no fading from chromospheric passbands. The IRIS spectra\nof spicules show the same signature as their proposed disk counterparts,\nreinforcing earlier work. Spectroheliograms from spectral rasters also confirm\nthat quiet Sun spicules originate in bushes from the magnetic network. Our\nresults suggest that type II spicules are indeed the site of vigorous heating\n(to at least transition region temperatures) along extensive parts of the\nupward moving spicular plasma.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Imaging from STIX visibility amplitudes: Aims: To provide the first demonstration of STIX Fourier-transform X-ray\nimaging using semi-calibrated (amplitude-only) visibility data acquired during\nthe Solar Orbiter's cruise phase. Methods: We use a parametric imaging approach\nby which STIX visibility amplitudes are fitted by means of two non-linear\noptimization methods: a fast meta-heuristic technique inspired by social\nbehavior, and a Bayesian Monte Carlo sampling method, which, although slower,\nprovides better quantification of uncertainties. Results: When applied to a set\nof solar flare visibility amplitudes recorded by STIX on November 18, 2020 the\ntwo parametric methods provide very coherent results. The analysis also\ndemonstrates the ability of STIX to reconstruct high time resolution\ninformation and, from a spectral viewpoint, shows the reliability of a\ndouble-source scenario consistent with a thermal versus nonthermal\ninterpretation. Conclusions: In this preliminary analysis of STIX imaging based\nonly on visibility amplitudes, we formulate the imaging problem as a non-linear\nparametric issue we addressed by means of two high-performance optimization\ntechniques that both showed the ability to sample the parametric space in an\neffective fashion, thus avoiding local minima.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Star-spot distributions and chromospheric activity on the RS CVn type\n  eclipsing binary SV Cam: Using a time series of high-resolution spectra and high-quality multi-colour\nphotometry, we reconstruct surface maps of the primary component of the RS CVn\ntype rapidly rotating eclipsing binary, SV Cam (F9V + K4V). We measure a mass\nratio, q, of 0.641(2) using our highest quality spectra and obtain surface\nbrightness maps of the primary component, which exhibit predominantly\nhigh-latitude spots located between 60-70-degree latitudes with a mean filling\nfactor of about 35%. This is also indicated by the R-band light curve\ninversion, subjected to rigourous numerical tests. The spectral subtraction of\nthe H-alpha line reveals strong activity of the secondary component. The excess\nH-alpha absorption detected near the secondary minimum hints to the presence of\ncool material partially obscuring the primary star. The flux ratios of Ca II\nIRT excess emission indicate that the contribution of chromospheric plage\nregions associated with star-spots is dominant, even during the passage of the\nfilament-like absorption feature.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Can Kilonova Light curves be Standardized?: Binary neutron star mergers have been recently confirmed to be the\nprogenitors of the optical transients kilonovae (KNe). KNe are powered by the\nradioactive decay of neutron-rich elements (r-process elements) which are\nbelieved to be the product of disruption of neutron stars during their merger.\nKNe exhibit interesting parallels with type Ia supernovae (SNe), whose light\ncurves show specific correlations which allow them to be used as standardizable\ncandles. In this paper, we investigate the possibility of the KN light curves\nexhibiting similar correlations. While a satisfactory answer to this question\ncan only be provided by future KN observations, employing theoretical models we\nexplore whether there is any ground for harboring such expectations. Using\nsemi-analytic models of KN light curves in conjunction with results from\nnumerical relativity simulations of binary neutron star mergers, we obtain the\nmaximum bolometric luminosity ($L_{\\mathrm{Bol}}^{\\mathrm{max}}$) and decline\nin luminosity ($\\Delta L_{\\mathrm{Bol}}$) for a simulated population of\nmergers. We find that theoretical light curves of KNe show remarkable\ncorrelations despite the complex physics governing their behavior. This\npresents a possibility of future observations to uncover such correlations in\nthe observed light curves, eventually allowing observers to standardize these\nlight curves and to use them for local distance measurements.",
        "category": "astro-ph_SR"
    },
    {
        "text": "ASASSN-15jd: WZ Sge-type star with intermediate superoutburst between\n  single and double ones: We present optical photometry of a WZ Sge-type dwarf nova (DN), ASASSN-15jd.\nIts light curve showed a small dip in the middle of the superoutburst in 2015\nfor the first time among WZ Sge-type DNe. The unusual light curve implies a\ndelay in the growth of the 3:1 resonance tidal instability. Also, the light\ncurve is similar to those of other two WZ Sge-type stars, SSS\nJ122221.7$-$311523 and OT J184228.1$+$483742, which are believed to be the best\ncandidates for a period bouncer on the basis of their small values of the mass\nratio ($q \\equiv M_{2}/M_{1}$). Additionally, the small mean superhump\namplitude ($<$ 0.1 mag) and the long duration of no ordinary superhumps at the\nearly stage of the superoutburst are common to the best candidates for a period\nbouncer. The average superhump period was $P_{\\rm sh}$ = 0.0649810(78) d and no\nearly superhumps were detected. Although we could not estimate the mass ratio\nof ASASSN-15jd with high accuracy, this object is expected to be a candidate\nfor a period bouncer, a binary accounting for the missing population of\npost-period minimum cataclysmic variables, based on the above characteristics.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Two-stage evolution of an extended C-class eruptive flaring activity\n  from sigmoid active region NOAA 12734: SDO and Udaipur-CALLISTO observations: We present a multi-wavelength investigation of a C-class flaring activity\nthat occurred in the active region NOAA 12734 on 8 March 2019. The\ninvestigation utilises data from AIA and HMI on board the SDO and the\nUdaipur-CALLISTO solar radio spectrograph of the Physical Research Laboratory.\nThis low intensity C1.3 event is characterised by typical features of a long\nduration event (LDE), viz. extended flare arcade, large-scale two-ribbon\nstructures and twin coronal dimmings. The eruptive event occurred in a coronal\nsigmoid and displayed two distinct stages of energy release, manifested in\nterms of temporal and spatial evolution. The formation of twin dimming regions\nare consistent with the eruption of a large flux rope with footpoints lying in\nthe western and eastern edges of the coronal sigmoid. The metric radio\nobservations obtained from Udaipur-CALLISTO reveals a broad-band\n($\\approx$50-180 MHz), stationary plasma emission for $\\approx$7 min during the\nsecond stage of the flaring activity that resemble a type IV radio burst. A\ntype III decametre-hectometre radio bursts with starting frequency of\n$\\approx$2.5 MHz precedes the stationary type IV burst observed by\nUdaipur-CALLISTO by $\\approx$5 min. The synthesis of multi-wavelength\nobservations and Non-Linear Force Free Field (NLFFF) coronal modelling together\nwith magnetic decay index analysis suggests that the sigmoid flux rope\nunderwent a zipping-like uprooting from its western to eastern footpoints in\nresponse to the overlying asymmetric magnetic field confinement. The\nasymmetrical eruption of the flux rope also accounts for the observed\nlarge-scale structures viz. apparent eastward shift of flare ribbons and post\nflare loops along the polarity inversion line (PIL), and provides an evidence\nfor lateral progression of magnetic reconnection site as the eruption proceeds.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A multi-wavelength analysis of BL Her stars: Models versus Observations: We present new theoretical period--luminosity (PL) and period--radius (PR)\nrelations at multiple wavelengths (Johnson--Cousins--Glass and {\\sl Gaia}\npassbands) for a fine grid of BL~Herculis models computed using {\\sc mesa-rsp}.\nThe non-linear models were computed for periods typical of BL~Her stars, i.e.\n$1\\leq P ({\\rm days}) \\leq4$, covering a wide range of input parameters:\nmetallicity ($-$2.0 dex $\\leq$ [Fe/H] $\\leq$ 0.0 dex), stellar mass (0.5--0.8\nM$_{\\odot}$), luminosity (50--300 L$_{\\odot}$) and effective temperature (full\nextent of the instability strip; in steps of 50K). We investigate the impact of\nfour sets of convection parameters on multi-wavelength properties. Most\nempirical relations match well with theoretical relations from the BL~Her\nmodels computed using the four sets of convection parameters. No significant\nmetallicity effects are seen in the PR relations. Another important result from\nour grid of BL~Her models is that it supports combining PL relations of RR\nLyrae and Type~II Cepheids together as an alternative to classical Cepheids for\nthe extragalactic distance scale calibration.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Uncovering a 260 pc wide, 35 Myr old filamentary relic of star formation: Several recent studies have shown that the Vela OB2 region hosts a complex\nconstellation of sub-populations with ages in the range 10 to 50 Myr. Such\npopulations might represent the best example of the outcome of clustered star\nformation in Giant Molecular clouds (GMC). We use Gaia DR2 data over an area of\n40 deg radius around the open cluster Collinder 135 to extend the study of the\nstellar populations of the Vela OB2 region over an area of several hundreds of\nparsecs on sky. Detailed clustering algorithms combined with the exquisite\nastrometric quality of the GAIA catalogue allow us to detect a new cluster\nnamed BBJ 1 that shows the same age as NGC 2547 (30 to 35 Myr), but located at\na distance of 260 pc from it. Deeper investigation of the region via clustering\nin 5D parameter space and in the colour-magnitude diagram allows us to detect a\nfilamentary structure of stars that bridges the two clusters. Given the extent\nin space of such structure (260 pc) and the young age (~35 Myr), we exclude\nthat such population originates by the same mechanism responsible to create\ntidal streams around older clusters. Even if we miss a complete picture of the\n3D motion of the studied stellar structure because of the lack of accurate\nradial velocity measurements, we propose that such structure represent the\ndetection of a 35 Myr old outcome of a mechanism of filamentary star formation\nin a GMC.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The timing behaviour of radio pulsars: The purpose of this review paper is to summarise the pulsar timing method, to\nprovide an overview of recent research into the spin-down of pulsars over\ndecadal timescales and to highlight the science that can be achieved using\nhigh-precision timing of millisecond pulsars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Luminous Blue Variable RMC127 as seen with ALMA and ATCA: We present ALMA and ATCA observations of the luminous blue variable \\rmc. The\nradio maps show for the first time the core of the nebula and evidence that the\nnebula is strongly asymmetric with a Z-pattern shape. Hints of this morphology\nare also visible in the archival \\emph{HST} $\\rm H\\alpha$ image, which overall\nresembles the radio emission. The emission mechanism in the outer nebula is\noptically thin free-free in the radio. At high frequencies, a component of\npoint-source emission appears at the position of the star, up to the ALMA\nfrequencies. The rising flux density distribution ($S_{\\nu}\\sim\n\\nu^{0.78\\pm0.05}$) of this object suggests thermal emission from the ionized\nstellar wind and indicates a departure from spherical symmetry with\n$n_{e}(r)\\propto r^{-2}$. We examine different scenarios to explain this excess\nof thermal emission from the wind and show that this can arise from a bipolar\noutflow, supporting the suggestion by other authors that the stellar wind of\n\\rmc is aspherical. We fit the data with two collimated ionized wind models and\nwe find that the mass-loss rate can be a factor of two or more smaller than in\nthe spherical case. We also fit the photometry obtained by IR space telescopes\nand deduce that the mid- to far-IR emission must arise from extended, cool\n($\\sim80\\,\\rm K$) dust within the outer ionized nebula. Finally we discuss two\npossible scenarios for the nebular morphology: the canonical single star\nexpanding shell geometry, and a precessing jet model assuming presence of a\ncompanion star.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Dynamic kink instability and transverse motions of solar spicules: Hydrodynamic jets are unstable to the kink instability (m=1 mode in\ncylindrical geometry) owing to the centripetal force, which increases the\ntransverse displacement of the jet. When the jet moves along a magnetic field,\nthen the Lorentz force tries to decrease the displacement and stabilises the\ninstability of sub-Alfvenic flows. The threshold of the instability depends on\nthe Alfven Mach number (the ratio of Alfven and jet speeds). We suggest that\nthe dynamic kink instability may be of importance to explain observed\ntransverse motions of type II spicules in the solar atmosphere. We show that\nthe instability may start for spicules which rise up at the peripheries of\nvertically expanding magnetic flux tubes owing to the decrease of the Alfven\nspeed in both, the vertical and the radial directions. Therefore, inclined\nspicules may be more unstable and have more higher transverse speeds. Periods\nand growth times of unstable modes in the conditions of type II spicules have\nthe values of 30 s and 25-100 s, respectively, which are comparable to the life\ntime of the structures. This may indicate to the interconnection between high\nspeed flow and rapid disappearance of type II spicules in chromospheric\nspectral lines.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Photospheric magnetic structure of coronal holes: In this study, we investigate in detail the photospheric magnetic structure\nof 98 coronal holes using line-of-sight magnetograms of SDO/HMI, and for a\nsubset of 42 coronal holes using HINODE/SOT G-band filtergrams. We divided the\nmagnetic field maps into magnetic elements and quiet coronal hole regions by\napplying a threshold at $\\pm 25$ G. We find that the number of magnetic bright\npoints in magnetic elements is well correlated with the area of the magnetic\nelements (cc=$0.83\\pm 0.01$). Further, the magnetic flux of the individual\nmagnetic elements inside coronal holes is related to their area by a power law\nwith an exponent of $1.261\\pm 0.004$ (cc=$0.984\\pm 0.001$). Relating the\nmagnetic elements to the overall structure of coronal holes, we find that on\naverage ($69\\pm 8$) % of the overall unbalanced magnetic flux of the coronal\nholes arises from long-lived magnetic elements with lifetimes > 40 hours. About\n($22\\pm 4$) % of the unbalanced magnetic flux arises from a very weak\nbackground magnetic field in the quiet coronal hole regions with a mean\nmagnetic field density of about 0.2 to 1.2 G. This background magnetic field is\ncorrelated to the flux of the magnetic elements with lifetimes of > 40 h\n(cc=$0.88\\pm 0.02$). The remaining flux arises from magnetic elements with\nlifetimes < 40 hours. By relating the properties of the magnetic elements to\nthe overall properties of the coronal holes, we find that the unbalanced\nmagnetic flux of the coronal holes is completely determined by the total area\nthat the long-lived magnetic elements cover (cc=$0.994\\pm 0.001$).",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spatially resolving the inhomogeneous structure of the dynamical\n  atmosphere of Betelgeuse with VLTI/AMBER: We present spatially resolved high-spectral resolution K-band observations of\nthe red supergiant Betelgeuse (alpha Ori) using AMBER at the Very Large\nTelescope Interferometer (VLTI). Betelgeuse was observed between 2.28 and 2.31\nmicron using baselines of 16, 32, and 48m with spectral resolutions of 4800 --\n12000. Spectrally dispersed interferograms have been obtained in the 2nd, 3rd,\nand 5th lobes, which represents the highest spatial resolution (9 mas) achieved\nfor Betelgeuse, corresponding to 5 resolution elements over its stellar disk.\nThe AMBER data in the continuum can be reasonably fitted by a uniform disk with\na diameter of 43.19+/-0.03 mas or a limb-darkening disk with 43.56+/-0.06 mas.\nThe K-band interferometric data taken at various epochs suggest that Betelgeuse\nseen in the continuum shows much smaller deviations from the above\nuniform/limb-darkened disk than predicted by 3-D convection simulations. On the\nother hand, our AMBER data in the CO lines reveal that the blue and red wings\nof the CO lines originate in spatially distinct regions over the stellar disk,\nindicating an inhomogeneous velocity field. Our AMBER data in the CO lines can\nbe roughly explained by a simple model, in which a patch of CO gas is moving\noutward or inward at velocities of 10--15 km s^-1, while the CO gas in the\nremaining region in the atmosphere is moving in the opposite direction at the\nsame velocities. The AMBER data are also consistent with the presence of warm\nmolecular layers at ~1.4--1.5 Rstar with a CO column density of ~1 x 10^20\ncm^-2. Our AMBER observations of Betelgeuse are the first spatially resolved\nstudy of the so-called macroturbulence in a stellar atmosphere other than the\nSun. The spatially resolved CO gas motion is likely to be related to convective\nmotion or intermittent mass ejections in clumps or arcs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The intermediate neutron capture process. V. The i-process in AGB stars\n  with overshoot: The intermediate neutron capture process (i-process) can develop during\nproton ingestion events (PIE), potentially during the early stages of low-mass\nlow-metallicity asymptotic giant branch (AGB) stars. We examine the impact of\novershoot mixing on the triggering and development of i-process nucleosynthesis\nin AGB stars of various initial masses and metallicities. We computed AGB\nstellar models, with initial masses of 1, 2, 3, and 4 M$_{\\odot}$ and\nmetallicities in the $-2.5 \\le $ [Fe/H] $\\le 0$ range, using the stellar\nevolution code STAREVOL with a network of 1160 nuclei coupled to the transport\nequations. We considered different overshooting profiles below and above the\nthermal pulses, and below the convective envelope. The occurrence of PIEs is\nfound to be primarily governed by the amount of overshooting at the top of\npulse ($f_{\\rm top}$) and to increase with rising $f_{\\rm top}$. For $f_{\\rm\ntop} =$ 0, 0.02, 0.04, and 0.1, we find that 0 %, 6 %, 24 %, and 86 % of our 21\nAGB models with $-2<$ [Fe/H] $<0$ experience a PIE, respectively. We also find\nthat PIEs leave a $^{13}$C-pocket at the bottom of the pulse that can give rise\nto an additional radiative s-process nucleosynthesis, and ultimately produce a\nnoticeable mixed i+s chemical signature at the surface. Finally, the chemical\nabundance patterns of 22 observed r/s-stars candidates with $-2<$ [Fe/H] $<-1$\nare found to be in reasonable agreement with our AGB model predictions. The\nbinary status of the dwarfs/giants being unclear, we suggest that these stars\nhave acquired their chemical pattern either from the mass transfer of a\nnow-extinct AGB companion or from an early generation AGB star that polluted\nthe natal cloud. Stricter constraints from multi-dimensional hydrodynamical\nmodels on overshoot coefficients could deliver new insights into the\ncontribution of AGB stars to heavy elements in the Universe.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A new class of Roche lobe-filling hot subdwarf binaries: We present the discovery of the second binary with a Roche lobe-filling hot\nsubdwarf transferring mass to a white dwarf (WD) companion. This 56 minute\nbinary was discovered using data from the Zwicky Transient Facility.\nSpectroscopic observations reveal an He-sdOB star with an effective temperature\nof $T_{\\rm eff}=33,700\\pm1000$ K and a surface gravity of $log(g)=5.54\\pm0.11$.\nThe GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the\nHe-sdOB star and shows an eclipse of the He-sdOB by an accretion disk as well\nas a weak eclipse of the WD. We infer a He-sdOB mass of $M_{\\rm\nsdOB}=0.41\\pm0.04$ M$_\\odot$ and a WD mass of $M_{\\rm WD}=0.68\\pm0.05$\nM$_\\odot$. The weak eclipses imply a WD black-body temperature of\n$63,000\\pm10,000$ K and a radius $R_{\\rm WD}=0.0148\\pm0.0020$ M$_\\odot$ as\nexpected for a WD of such high temperature.\n  The He-sdOB star is likely undergoing hydrogen shell burning and will\ncontinue transferring mass for $\\approx1$ Myrs at a rate of $10^{-9} M_\\odot\n{\\rm yr}^{-1}$ which is consistent with the high WD temperature. The hot\nsubdwarf will then turn into a WD and the system will merge in $\\approx30$\nMyrs. We suggest that Galactic reddening could bias discoveries towards\npreferentially finding Roche lobe-filling systems during the short-lived shell\nburning phase. Studies using reddening corrected samples should reveal a large\npopulation of helium core-burning hot subdwarfs with $T_{\\rm eff}\\approx25,000$\nK in binaries of 60-90 minutes with WDs. Though not yet in contact, these\nbinaries would eventually come into contact through gravitational wave emission\nand explode as a sub-luminous thermonuclear supernova or evolve into a massive\nsingle WD.",
        "category": "astro-ph_SR"
    },
    {
        "text": "IAU 2015 Resolution B2 on Recommended Zero Points for the Absolute and\n  Apparent Bolometric Magnitude Scales: The XXIXth IAU General Assembly in Honolulu adopted IAU 2015 Resolution B2 on\nrecommended zero points for the absolute and apparent bolometric magnitude\nscales. The resolution was proposed by the IAU Inter-Division A-G Working Group\non Nominal Units for Stellar and Planetary Astronomy after consulting with a\nbroad spectrum of researchers from the astronomical community. Resolution B2\nresolves the long-standing absence of an internationally-adopted zero point for\nthe absolute and apparent bolometric magnitude scales. Resolution B2 defines\nthe zero point of the absolute bolometric magnitude scale such that a radiation\nsource with $M_{\\rm Bol}$ = 0 has luminosity L$_{\\circ}$ = 3.0128e28 W. The\nzero point of the apparent bolometric magnitude scale ($m_{\\rm Bol}$ = 0)\ncorresponds to irradiance $f_{\\circ}$ = 2.518021002e-8 W/m$^2$. The zero points\nwere chosen so that the nominal solar luminosity (3.828e26 W) adopted by IAU\n2015 Resolution B3 corresponds approximately to $M_{\\rm Bol}$(Sun) = 4.74, the\nvalue most commonly adopted in recent literature. The nominal total solar\nirradiance (1361 W/m$^2$) adopted in IAU 2015 Resolution B3 corresponds\napproximately to apparent bolometric magnitude $m_{\\rm bol}$(Sun) = -26.832.\nImplicit in the IAU 2015 Resolution B2 definition of the apparent bolometric\nmagnitude scale is an exact definition for the parsec (648000/$\\pi$ au) based\non the IAU 2012 Resolution B2 definition of the astronomical unit.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Water deuterium fractionation in the high-mass star-forming region\n  G34.26+0.15 based on Herschel/HIFI data: Understanding water deuterium fractionation is important for constraining the\nmechanisms of water formation in interstellar clouds. Observations of HDO and\nH$_2^{18}$O transitions were carried out towards the high-mass star-forming\nregion G34.26+0.15 with the HIFI instrument onboard the Herschel Space\nObservatory, as well as with ground-based single-dish telescopes. Ten HDO lines\nand three H$_2^{18}$O lines covering a broad range of upper energy levels\n(22-204 K) were detected. We used a non-LTE 1D analysis to determine the\nHDO/H$_2$O ratio as a function of radius in the envelope. Models with different\nwater abundance distributions were considered in order to reproduce the\nobserved line profiles. The HDO/H$_2$O ratio is found to be lower in the hot\ncore ($\\sim$3.5 $\\times$ 10$^{-4}$ - 7.5 $\\times$ 10$^{-4}$) than in the colder\nenvelope ($\\sim$1.0 $\\times$ 10$^{-3}$ - 2.2 $\\times$ 10$^{-3}$). This is the\nfirst time that a radial variation of the HDO/H$_2$O ratio has been found to\noccur in a high-mass source. The chemical evolution of this source was modeled\nas a function of its radius and the observations are relatively well\nreproduced. The comparison between the chemical model and the observations\nleads to an age of $\\sim$10$^5$ years after the infrared dark cloud stage.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Using stellar rotation to identify tidally stripped members of the\n  Praesepe open cluster: As an open cluster orbits the Milky Way, gravitational fields distort it,\nstripping stars from the core and forming tidal tails. Recent work has\nidentified tidal tails of the Praesepe cluster; we explore rotation periods as\na way to confirm these candidate members. In open clusters, the rotation period\ndistribution evolves over time due to magnetic braking. Since tidally stripped\nstars originally formed within the cluster, they should follow the same period\ndistribution as in the cluster core. We analyze 96 candidate members observed\nby NASA's Transiting Exoplanet Survey Satellite (TESS) mission. We measure\nreliable rotation periods for 32 stars, while 64 light curves are\nnoise-dominated. The 32 newly identified rotators are consistent with the\nperiod distribution in the core, and with past membership in Praesepe. We\ntherefore suggest that for nearby open clusters, stellar rotation offers a\nquick and inexpensive method for confirming past members dispersed into tidal\ntails.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Forward Modelling of Standing Kink Modes in Coronal Loops I. Synthetic\n  Views: Kink magnetohydrodynamic (MHD) waves are frequently observed in various\nmagnetic structures of the solar atmosphere. They may contribute significantly\nto coronal heating and could be used as a tool to diagnose the solar plasma. In\nthis study, we synthesise the \\ion{Fe}{9} $\\lambda171.073$ emission of a\ncoronal loop supporting a standing kink MHD mode. The kink MHD wave solution of\na plasma cylinder is mapped into a semi-torus structure to simulate a curved\ncoronal loop. We decompose the solution into a quasi-rigid kink motion and a\nquadrupole term, which dominate the plasma inside and outside the flux tube,\nrespectively. At the loop edges, the line-of-sight integrates relatively more\nambient plasma, and the background emission becomes significant. The plasma\nmotion associated with the quadrupole term causes spectral line broadening and\nemission suppression. The periodic intensity suppression will modulate the\nintegrated intensity and the effective loop width, which both exhibit\noscillatory variations at half of the kink period. The quadrupole term can be\ndirectly observed as a pendular motion at front view.",
        "category": "astro-ph_SR"
    },
    {
        "text": "High Fidelity Imaging of the Inner AU Mic Debris Disk: Evidence of\n  Differential Wind Sculpting?: We present new high fidelity optical coronagraphic imagery of the inner\n$\\sim$50 au of AU Mic's edge-on debris disk using the BAR5 occulter of the\nHubble Space Telescope Imaging Spectrograph (HST/STIS) obtained on 26-27 July\n2018. This new imagery reveals that \"feature A\", residing at a projected\nstellocentric separation of 14.2 au on SE-side of the disk, exhibits an\napparent \"loop-like\" morphology at the time of our observations. The loop has a\nprojected width of 1.5 au and rises 2.3 au above the disk midplane. We also\nexplored TESS photometric observations of AU Mic that are consistent with\nevidence of two starspot complexes in the system. The likely co-alignment of\nthe stellar and disk rotational axes breaks degeneracies in detailed spot\nmodeling, indicating that AU Mic's projected magnetic field axis is offset from\nits rotational axis. We speculate that small grains in AU Mic's disk could be\nsculpted by a time-dependent wind that is influenced by this offset magnetic\nfield axis, analogous to co-rotating Solar interaction regions that sculpt and\ninfluence the inner and outer regions of our own Heliosphere. Alternatively, if\nthe observed spot modulation is indicative of a significant mis-alignment of\nthe stellar and disk rotational axes, we suggest the disk could still be\nsculpted by the differential equatorial versus polar wind that it sees with\nevery stellar rotation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Revealing the structure of the outer disks of Be stars: Context. The structure of the inner parts of Be star disks (20 stellar radii)\nis well explained by the viscous decretion disk (VDD) model, which is able to\nreproduce the observable properties of most of the objects studied so far. The\nouter parts, on the ther hand, are not observationally well-explored, as they\nare observable only at radio wavelengths. A steepening of the spectral slope\nsomewhere between infrared and radio wavelengths was reported for several Be\nstars that were previously detected in the radio, but a convincing physical\nexplanation for this trend has not yet been provided. Aims. We test the VDD\nmodel predictions for the extended parts of a sample of six Be disks that have\nbeen observed in the radio to address the question of whether the observed\nturndown in the spectral energy distribution (SED) can be explained in the\nframework of the VDD model, including recent theoretical development for\ntruncated Be disks in binary systems. Methods. We combine new multi-wavelength\nradio observations from the Karl. G. Jansky Very Large Array (JVLA) and Atacama\nPathfinder Experiment (APEX) with previously published radio data and archival\nSED measurements at ultraviolet, visual, and infrared wavelengths. The density\nstructure of the disks, including their outer parts, is constrained by\nradiative transfer modeling of the observed spectrum using VDD model\npredictions. In the VDD model we include the presumed effects of possible tidal\ninfluence from faint binary companions. Results. For 5 out of 6 studied stars,\nthe observed SED shows strong signs of SED turndown between far-IR and radio\nwavelengths. A VDD model that extends to large distances closely reproduces the\nobserved SEDs up to far IR wavelengths, but fails to reproduce the radio SED.\n... (abstract continues but did not fit here)",
        "category": "astro-ph_SR"
    },
    {
        "text": "Discovery of A Binary System in IRAM 04191+1522: We present high angular resolution observations of the Class 0 protostar\nIRAM04191+1522, using the Submillimeter Array (SMA). The SMA 1.3 mm continuum\nimages reveal within IRAM04191+1522 two distinct sources with an angular\nseparation of 7.8\\,$\\pm$\\,0.2$\"$. The two continuum sources are located in the\nsoutheast-northwest direction, with total gas masses of about 0.011 M_sun and\nabout 0.005 M_sun, respectively. The southeastern source, associated with an\ninfrared source seen in the Spitzer images, is the well-known Class 0 protostar\nwith a bolometric luminosity of about 0.08 L_sun. The newly-discovered\nnorthwestern continuum source is not visible in the Spitzer images at\nwavelengths from 3.6 to 70 micron, and has an extremely low bolometric\nluminosity (< 0.03 L_sun). Complementary IRAM N2H+(1-0) data that probe the\ndense gas in the common envelope suggest that the two sources were formed\nthrough the rotational fragmentation of an elongated dense core. Furthermore,\ncomparisons between IRAM04191+1522 and other protostars suggest that most cores\nwith binary systems formed therein have ratios of rotational energy to\ngravitational energy $\\beta_{\\rm rot}$ > 1%. This is consistent with\ntheoretical simulations and indicates that the level of rotational energy in a\ndense core plays an important role in the fragmentation process.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Doppler Probe of Accretion onto a T Tauri star: The YY Ori stars are T Tauri stars with prominent time-variable redshifted\nabsorption components that flank certain emission lines. One of the brightest\nin this class is S CrA, a visual double star. We have obtained a series of\nhigh-resolution spectra of the two components during four nights with the UVES\nspectrograph at the Very Large Telescope. We followed the spectral changes\noccurring in S CrA to derive the physical structure of the accreting gas.\n  We found that both stars are very similar with regard to surface temperature,\nradius, and mass. Variable redshifted absorption components are particularly\nprominent in the SE component. During one night, this star developed a spectrum\nunique among the T Tauri stars: extremely strong and broad redshifted\nabsorption components appeared in many lines of neutral and ionized metals, in\naddition to those of hydrogen and helium. The absorption depths of cooler, low\nionization lines peak at low velocities - while more highly ionized lines have\npeak absorption depths at high velocities. The different line profiles indicate\nthat the temperature and density of the accretion stream increase as material\napproaches the star. We derive the physical conditions of the flow at several\npoints along the accretion funnel directly from the spectrum of the infalling\ngas. We estimated mass accretion rates of about 10^(-7) solar masses per year,\nwhich is similar to that derived from the relation based on the strength of H\nalpha emission line.\n  This is the first time the density and temperature distributions in accretion\nflows around a T Tauri star have been inferred from observations. Compared with\npredictions from standard models of accretion in T Tauri stars, which assume a\ndipole stellar magnetic field, we obtained higher densities and a steeper\ntemperature rise toward the star.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Stability of thermal modes in cool prominence plasmas: Context: Magnetohydrodynamic thermal modes may play an important role in the\nformation, plasma condensation, and evolution of solar prominences. Unstable\nthermal modes due to unbalance between radiative losses and heating can lead to\nrapid plasma cooling and condensation. An accurate description of the radiative\nloss function is therefore crucial for this process. Aims: We study the\nstability of thermal modes in unbounded and uniform plasmas with properties\nakin to those in solar prominences. Effects due to partial ionization are taken\ninto account. Three different parametrizations of the radiative loss function\nare used. Methods: By means of a normal mode analysis, we investigate linear\nnonadiabatic perturbations superimposed on the equilibrium state. We find an\napproximate instability criterion for thermal modes, while the exact linear\ngrowth rate is obtained by numerically solving the general dispersion relation.\nThe stability of thermal disturbances is compared for the three different loss\nfunctions considered. Results: Using up-to-date computations of radiative\nlosses derived from the CHIANTI atomic database, we find that thermal modes may\nbe unstable in prominences for lower temperatures than those predicted with\npreviously existing loss functions. Thermal instability can take place for\ntemperatures as low as 15,000 K, approximately. The obtained linear growth\nrates indicate that this instability might have an important impact on the\ndynamics and evolution of cool prominence condensations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Extended atmospheres of AGB stars: modeling and measurement: Encoded in the time- and wavelength dependent properties of pulsating AGB\nstars are the underlying fundamental parameters of mass, composition and\nevolutionary state. However, the standard technique of placing stars on a HR\ndiagram, even with the aid of pulsation periods, can not be done easily for\nextended AGB stars, because of the difficulty of defining a radius or\ntemperature. The atmospheres of Mira variables are so extended that the optical\ndepth unity radius can vary by a factor of ~3 over the energetically important\nregion of the spectrum. Many important constituents in the radiative transfer\nare far from local thermodynamic equilibrium, and for the coolest stars, the\nprocess of dust formation and destruction requires a time-dependent model of\ngrain growth. I will describe the challenges and some of the solutions to\nmodeling these atmospheres, and describe the utility of different kinds of\nobservations in helping understand both fundamental parameters and chaotic\nprocesses in complex AGB atmospheres.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The binary nature of the Galactic Centre X-ray source CXOGC\n  J174536.1-285638: X-ray and near-IR surveys of the central 2x0.8 degrees of the Galactic Centre\nhave revealed a population of X-ray bright massive stars. However, the nature\nof the emission, originating in a wind collision zone or via accretion onto a\ncompact object, is uncertain. In order to address this we investigated the\nnature of the luminous X-ray source CXOGC J174536.1-285638. An analysis of the\nnear-IR spectrum with a non-LTE model atmosphere code demonstrated that it was\nan highly luminous WN9h star, for which comparison to evolutionary tracks\nsuggests an age of 2-2.5Myr and an initial mass of ~110Msun. The X-ray\nproperties of CXOGC J174536.1-285638 resemble those of 3 of the WN9h stars\nwithin the Arches cluster and in turn other very massive WNLh colliding wind\nbinaries. Simple analytical arguments demonstrate consistency between the X-ray\nemission and a putative WN9h+mid O V-III binary. However, we may not exclude a\nhigh mass X-ray binary interpretation, which, if correct, would provide a\nunique insight into the (post-SN) evolution of extremely massive stars.\nIrrespective of the nature of the secondary, CXOGC J174536.1-285638 adds to the\ngrowing list of known and candidate WNLh binaries. Of the subset of WNLh stars\nsubject to a radial velocity survey, we find a lower limit to the binary\nfraction of ~45%; of interest for studies of massive stellar formation, given\nthat they currently possess the highest dynamically determined masses of any\ntype of star. [ABRIDGED]",
        "category": "astro-ph_SR"
    },
    {
        "text": "New Double Periodic Variable Stars in the ASAS-SN Catalog: We report the discovery of 3 new Double Periodic Variables based on the\nanalysis of ASAS-SN light curves: GSD J11630570-510306, V593 Sco and TYC\n6939-678-1. These systems have orbital periods between 10 and 20 days and long\ncycles between 300 and 600 days.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar\n  Diffusion: II. Comparison with Observation, Clump Properties, and Scaling to\n  Physical Units: Ambipolar diffusion is important in redistributing magnetic flux and in\ndamping Alfven waves in molecular clouds. The importance of ambipolar diffusion\non a length scale $\\ell$ is governed by the ambipolar diffusion Reynolds\nnumber, $\\rad=\\ell/\\lad$, where $\\lad$ is the characteristic length scale for\nambipolar diffusion. The logarithmic mean of the AD Reynolds number in a sample\nof 15 molecular clumps with measured magnetic fields (Crutcher 1999) is 17,\ncomparable to the theoretically expected value. We identify several regimes of\nambipolar diffusion in a turbulent medium, depending on the ratio of the flow\ntime to collision times between ions and neutrals; the clumps observed by\nCrutcher (1999) are all in the standard regime of ambipolar diffusion, in which\nthe neutrals and ions are coupled over a flow time. We have carried out\ntwo-fluid simulations of ambipolar diffusion in isothermal, turbulent boxes for\na range of values of $\\rad$. The mean Mach numbers were fixed at $\\calm=3$ and\n$\\ma=0.67$; self-gravity was not included. We study the properties of\noverdensities--i.e., clumps--in the simulation and show that the slope of the\nhigher-mass portion of the clump mass spectrum increases as $\\rad$ decreases,\nwhich is qualitatively consistent with Padoan et al. (2007)'s finding that the\nmass spectrum in hydrodynamic turbulence is significantly steeper than in ideal\nMHD turbulence. For a value of $\\rad$ similar to the observed value, we find a\nslope that is consistent with that of the high-mass end of the Initial Mass\nFunction for stars. However, the value we find for the spectral index in our\nideal MHD simulation differs from theirs, presumably because our simulations\nhave different initial conditions. This suggests that the mass spectrum of the\nclumps in the Padoan et al. (2007) turbulent fragmentation model for the IMF\ndepends on the environment, which would conflict with evidence ...",
        "category": "astro-ph_SR"
    },
    {
        "text": "Annihilation of Magnetic Islands at the Top of Solar Flare Loops: The dynamics of magnetic reconnection in the solar current sheet (CS) is\nstudied by high-resolution 2.5-dimensional MHD simulation. With the commence of\nmagnetic reconnection, a number of magnetic islands are formed intermittently\nand move quickly upward and downward along the CS. When colliding with the\nsemi-closed flux of flare loops, the downflow islands cause a second\nreconnection with a rate even comparable with that in the main CS. Though the\ntime-integrated magnetic energy release is still dominated by the reconnection\nin main CS, the second reconnection can release substantial magnetic energy,\nannihilating the main islands and generating secondary islands with various\nscales at the flare loop top. The distribution function of the flux of the\nsecond islands is found to follow a power-law varying from\n$f\\left(\\psi\\right)\\sim\\psi^{-1}$ (small scale) to $\\psi^{-2}$ (large scale),\nwhich seems to be independent with background plasma $\\beta$ and if including\nthermal conduction. However, the spatial scale and the strength of the\ntermination shocks driven by main reconnection outflows or islands decrease if\n$\\beta$ increases or thermal conduction is included. We suggest that the\nannihilation of magnetic islands at the flare loop top, which is not included\nin the standard flare model, plays a non-negligible role in releasing magnetic\nenergy to heat flare plasma and accelerate particles.",
        "category": "astro-ph_SR"
    },
    {
        "text": "First evidence of non-Gaussian solar flare EUV spectral line profiles\n  and accelerated non-thermal ion motion: The properties of solar flare plasma can be determined from the observation\nof optically thin lines. The emitting ion distribution determines the shape of\nthe line, with an isothermal Maxwellian ion distribution producing a Gaussian\nprofile. Non-Gaussian lines may indicate more complex ion distributions. We\ninvestigate the possibility of determining flare-accelerated non-thermal ion or\nplasma velocity distributions. We study EUV spectral lines during a flare\nSOL2013-05-15T01:45 using the Hinode EUV Imaging Spectrometer (EIS). The flare\nis located close to the eastern solar limb with an extended loop structure,\nallowing the different flare features: ribbons, hard X-ray (HXR) footpoints and\nthe loop-top source to be clearly observed in UV, EUV and X-rays. EUV line\nspectroscopy is performed in seven different regions. We study the line\nprofiles of isolated and unblended Fe XVI lines (262.976 A) formed at\ntemperatures of 2-4 MK. Fe XVI line profiles at one time close to the peak soft\nX-ray emission and free of directed mass motions are examined using: 1. a\nhigher moments analysis, 2. Gaussian fitting, and 3. by fitting a kappa\ndistribution profile convolved with a Gaussian to account for the EIS\ninstrumental profile. Fe XVI line profiles in the loop-top, HXR footpoint and\nribbon regions can be confidently fitted with a kappa line profile, giving low,\nnon-thermal kappa values between 2-3.3. A higher moments analysis also finds\nthat many of the line kurtosis values are higher than the Gaussian value of 3,\neven with the presence of a broad Gaussian instrumental profile. A\nflare-accelerated non-thermal ion population could account for both the\nobserved non-Gaussian line profiles, and for the Fe XVI excess broadening found\nfrom Gaussian fitting, if the emitting ions are interacting with a thermalised\n4 MK electron population, and the instrumental profile is well-approximated by\na Gaussian.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Three-dimensional kinetic-MHD model of the global heliosphere with the\n  heliopause-surface fitting: This paper provides a detailed description of the latest version of our model\nof the solar wind (SW) interaction with the local interstellar medium (LISM).\nThis model has already been applied to the analysis of Lyman-alpha absorption\nspectra toward nearby stars and for analyses of Solar and Heliospheric\nObservatory/SWAN data. Katushkina et al. (this issue) used the model results to\nanalyze IBEX-Lo data. At the same time, the details of this model have not yet\nbeen published. This is a three-dimensional (3D) kinetic-magnetohydrodynamical\n(MHD) model that takes into account SW and interstellar plasmas (including\n$\\alpha$ particles in SW and helium ions in LISM), the solar and interstellar\nmagnetic fields, and the interstellar hydrogen atoms. The latitudinal\ndependence of SW and the actual flow direction of the interstellar gas with\nrespect to the Sun are also taken into account in the model. It was very\nessential that our numerical code had been developed in such a way that any\nnumerical diffusion or reconnection across the heliopause had not been allowed\nin the model. The heliospheric current sheet is a rotational discontinuity in\nthe ideal MHD and can be treated kinematically. In the paper, we focus in\nparticular on the effects of the heliospheric magnetic field and on the\nheliolatitudinal dependence of SW.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Correlation between the solar magnetic field strength and the millimeter\n  brightness temperature: Images of the Sun at millimeter wavelengths obtained by ALMA show a\nsignificant correspondence with the magnetograms. In this paper, we investigate\nthis correspondence by comparing ALMA full-disk solar image taken at 1.2 mm\nwith a SDO/HMI magnetogram and analyze their correlation. It is found that\nchromospheric network and active regions show a positive correlation where\nbrightness temperature is increasing with the line-of-sight magnetic field\nstrength, while sunspots have a negative correlation. Quiet Sun regions do not\nshow any dependence of the brightness temperature with the magnetic field.\nThermal bremsstrahlung is given as the best explanation for the observed\ncorrelations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Exploring wind-driving dust species in cool luminous giants III. Wind\n  models for M-type AGB stars: dynamic and photometric properties: Stellar winds observed in asymptotic giant branch (AGB) stars are usually\nattributed to a combination of stellar pulsations and radiation pressure on\ndust. Shock waves triggered by pulsations propagate through the atmosphere,\ncompressing the gas and lifting it to cooler regions, which create favourable\nconditions for grain growth. If sufficient radiative acceleration is exerted on\nthe newly formed grains through absorption or scattering of stellar photons, an\noutflow can be triggered. Strong candidates for wind-driving dust species in\nM-type AGB stars are magnesium silicates (Mg$_2$SiO$_4$ and MgSiO$_3$). Such\ngrains can form close to the stellar surface, they consist of abundant\nmaterials and, if they grow to sizes comparable to the wavelength of the\nstellar flux maximum, they experience strong acceleration by photon scattering.\n  We use a frequency-dependent radiation-hydrodynamics code with a detailed\ndescription for the growth of Mg$_2$SiO$_4$ grains to calculate the first\nextensive set of time-dependent wind models for M-type AGB stars. The resulting\nwind properties, visual and near-IR photometry and mid-IR spectra are compared\nwith observations.We show that the models can produce outflows for a wide range\nof stellar parameters. We also demonstrate that they reproduce observed\nmass-loss rates and wind velocities, as well as visual and near-IR photometry.\nHowever, the current models do not show the characteristic silicate features at\n10 and 18 $\\mu$m as a result of the cool temperature of Mg$_2$SiO$_4$ grains in\nthe wind. Including a small amount of Fe in the grains further out in the\ncircumstellar envelope will increase the grain temperature and result in\npronounced silicate features, without significantly affecting the photometry in\nthe visual and near-IR wavelength regions.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Mass and Angular Momentum Transport in a Gravitationally Unstable\n  Protoplanetary Disk with Improved 3D Radiative Hydrodynamics: During early phases of a protoplanetary disks's life, gravitational\ninstabilities can produce significant mass transport, can dramatically alter\ndisk structure, can mix and shock-process gas and solids, and may be\ninstrumental in planet formation. We present a 3D grid-based radiative\nhydrodynamics study with varied resolutions of a 0.07 M$_\\odot$ disk orbiting a\n0.5 M$_\\odot$ star as it settles over most of its radial extent into a\nquasi-steady asymptotic state that maintains approximate balance between\nheating produced by GIs and radiative cooling governed by realistic dust\nopacities. We assess disk stability criteria, thermodynamic properties,\nstrengths of GIs, characteristics of density waves and torques produced by GIs,\nradial mass transport arising from these torques, and the level to which\ntransport can be represented as local or nonlocal processes. Physical and\nthermal processes display distinct differences between inner optically thick\nand outer optically thin regions of the disk. In the inner region,\ngravitational torques are dominated by low-order Fourier components of the\nazimuthal mass distribution. These torques are strongly variable on the local\ndynamical time and are subject to rapid flaring presumably driven by recurrent\nswing amplification. In the outer region, m=1 torques dominate. Ring-like\nstructures exhibiting strong noncircular motions, and vortices develop near the\ninner edge between 8 and 14 au. We find that GI-induced spiral modes erupt in a\nchaotic manner over the whole low-Q part of the disk, with many spiral modes\nappearing and disappearing, producing gravitoturbulence, but dominated by\nfluctuating large-scale modes, very different from a simple $\\alpha$-disk.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Origin of Quasi-Periodic Pulsation at the Base of Kink Unstable Jet: We study a blowout jet that occurs at the west limb of the Sun on August\n29$^{th}$, 2014 using high-resolution imaging/spectroscopic observations\nprovided by SDO/AIA and IRIS. An inverse $\\gamma$-shape flux-rope appears\nbefore the jet{--} morphological indication of the onset of kink instability.\nThe twisted field lines of kink-unstable flux-rope reconnect at its bright knot\nand launch the blowout jet at $\\approx$06:30:43 UT with an average speed of 234\nkm s$^{-1}$. Just after the launch, the northern leg of the flux rope erupts\ncompletely. The time-distance diagrams show multiple spikes or bright dots,\nwhich is the result of periodic fluctuations, i.e., quasi-periodic fluctuations\n(QPPs). The wavelet analysis confirms that QPPs have a dominant period of\n$\\approx$ 03 minutes. IRIS spectra (Si~{\\sc iv}, C~{\\sc ii}, and Mg~{\\sc ii})\nmay also indicate the occurrence of magnetic reconnection through existence of\nbroad $\\&$ complex profiles and bi-directional flows in the jet. Further, we\nhave found that line broadening is periodic with a period of $\\approx$ 03\nminutes, and plasma upflow is always occurs when the line width is high, i.e.,\nmultiple reconnection may produce periodic line broadening. The EM curves also\nshow the same period of $\\approx$ 03 minutes in different temperature bins. The\nimages and EM show that this jets spire is mainly cool\n(chromospheric/transition region) rather than hot (coronal) material. Further,\nline broadening, intensity, and EM curves have a period of $\\approx$03 minutes,\nwhich strongly supports that multiple magnetic reconnection triggers QPPs in\nthe blowout jet.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evolution and Consequences of Interacting CMEs of 2012 November 9-10\n  using STEREO/SECCHI and In Situ Observations: Understanding of the kinematic evolution of Coronal Mass Ejections (CMEs) in\nthe heliosphere is important to estimate their arrival time at the Earth. It is\nfound that kinematics of CMEs can change when they interact or collide with\neach other as they propagate in the heliosphere. In this paper, we analyze the\ncollision and post-interaction characteristics of two Earth-directed CMEs,\nlaunched successively on 2012 November 9 and 10, using white light imaging\nobservations from STEREO/SECCHI and in situ observations taken from WIND\nspacecraft. We tracked two density enhanced features associated with leading\nand trailing edge of November 9 CME and one density enhanced feature associated\nwith leading edge of November 10 CME by constructing J-maps. We found that the\nleading edge of November 10 CME interacted with the trailing edge of November 9\nCME. We also estimated the kinematics of these features of the CMEs and found a\nsignificant change in their dynamics after interaction. In in situ\nobservations, we identified distinct structures associated with interacted CMEs\nand also noticed their heating and compression as signatures of CME-CME\ninteraction. Our analysis shows an improvement in arrival time prediction of\nCMEs using their post-collision dynamics than using pre-collision dynamics.\nEstimating the true masses and speeds of these colliding CMEs, we investigated\nthe nature of observed collision which is found to be close to perfectly\ninelastic. The investigation also places in perspective the geomagnetic\nconsequences of the two CMEs and their interaction in terms of occurrence of\ngeomagnetic storm and triggering of magnetospheric substorms.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Mass Budget of Planet Forming Discs: Isolating the Epoch of\n  Planetesimal Formation: The high rate of planet detection among solar-type stars argues that planet\nformation is common. It is also generally assumed that planets form in\nprotoplanetary discs like those observed in nearby star forming regions. On\nwhat timescale does the transformation from discs to planets occur? Here we\nshow that current inventories of planets and protoplanetary discs are sensitive\nenough to place basic constraints on the timescale and efficiency of the planet\nformation process. A comparison of planet detection statistics and the measured\nsolid reservoirs in T Tauri discs suggests that planet formation is likely\nalready underway at the few Myr age of the discs in Taurus-Auriga, with a large\nfraction of solids having been converted into large objects with low millimeter\nopacity and/or sequestered at small disc radii where they are difficult to\ndetect at millimeter wavelengths.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Horizontal flow fields observed in Hinode G-band images III. The decay\n  of a satellite sunspot and the role of magnetic flux removal in flaring: The flare-prolific active region NOAA 10930 offered both a developing\ndelta-spot and a decaying satellite sunspot of opposite polarity. The objective\nof this study is to characterize the photometric decay of the satellite sunspot\nand the evolution of photospheric and chromospheric horizontal proper motions\nin its surroundings. We apply the local correlation tracking technique to a\n16-hour time-series of Hinode G-band and CaIIH images and study the horizontal\nproper motions in the vicinity of the satellite sunspot on 2006 December 7.\nDecorrelation times were computed to measure the lifetime of solar features in\nintensity and flow maps. We observed shear flows in the dominant umbral cores\nof the satellite sunspot. These flows vanished once the penumbra had\ndisappeared. This slow penumbral decay had an average rate of 152Mm2/day over\nan 11-hour period. Typical lifetimes of intensity features derived from an\nautocorrelation analysis are 3-5min for granulation, 25-35min for G-band bright\npoints, and up to 200-235min for penumbrae, umbrae, and pores. Long-lived\nintensity features (i.e., the dominant umbral cores) are not related to\nlong-lived flow features in the northern part of the sunspot, where flux\nremoval, slowly decaying penumbrae, and persistent horizontal flows of up to 1\nkm/s contribute to the erosion of the sunspot. Finally, the restructuring of\nmagnetic field topology was responsible for a homologous M2.0 flare, which\nshared many characteristics with an X6.5 flare on the previous day.\nNotwithstanding the prominent role of delta-spots in flaring, we conclude based\non the decomposition of the satellite sunspot, the evolution of the surrounding\nflow fields, and the timing of the M2.0 flare that the vanishing magnetic flux\nin the decaying satellite sunspot played an instrumental role in triggering the\nhomologous M2.0 flare and the eruption of a small Halpha filament.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Asteroseismic sensitivity to internal rotation along the red-giant\n  branch: Transport of angular momentum in stellar interiors is currently not well\nunderstood. Asteroseismology can provide us with estimates of internal rotation\nof stars and thereby advances our understanding of angular momentum transport.\nWe can measure core-rotation rates in red-giant stars and we can place upper\nbounds on surface-rotation rates using measurements of dipole ($l=1$) modes.\nHere, we aim to determine the theoretical sensitivity of modes of different\nspherical degree towards the surface rotation. Additionally, we aim to identify\nmodes that can potentially add sensitivity at intermediate radii. We used\nasteroseismic rotational inversions to probe the internal stellar rotation\nprofiles in red-giant models from the base of the red-giant branch up to the\nluminosity bump. We used the inversion method of multiplicative optimally\nlocalised averages (MOLA) to assess how well internal and surface rotation\nrates can be recovered from different mode sets and different synthetic\nrotation profiles. We confirm that dipole mixed modes are sufficient to set\nconstraints on the average core-rotation rates in red giants. However,\nsurface-rotation rates estimated with only dipole mixed modes are contaminated\nby the core rotation. We show that the sensitivity to the surface rotation\ndecreases from the base of the red-giant branch until it reaches a minimum at\n0.6-0.8$L_\\text{bump}$ due to a glitch in the buoyancy frequency. Thereafter a\nnarrow range of increased surface sensitivity just below the bump luminosity\nexists. Quadrupole and octopole modes have more sensitivity in the outer parts\nof the star. If observed, quadrupole and octopole modes enable us to\ndistinguish between differential and solid body rotation in the convection\nzone. To obtain accurate estimates of rotation rates at intermediate radii,\nacoustic oscillation modes with a spherical degree of $l\\approx10$ are needed.",
        "category": "astro-ph_SR"
    },
    {
        "text": "New Horizons Solar Wind Around Pluto (SWAP) Observations of the Solar\n  Wind From 11-33 AU: The Solar Wind Around Pluto (SWAP) instrument on NASA's New Horizon Pluto\nmission has collected solar wind observations en route from Earth to Pluto, and\nthese observations continue beyond Pluto. Few missions have explored the solar\nwind in the outer heliosphere making this dataset a critical addition to the\nfield. We created a forward model of SWAP count rates, which includes a\ncomprehensive instrument response function based on laboratory and flight\ncalibrations. By fitting the count rates with this model, the proton density\n(n), speed (V), and temperature (T) parameters are determined. Comparisons\nbetween SWAP parameters and both propagated 1 AU observations and prior Voyager\n2 observations indicate consistency in both the range and mean wind values.\nThese comparisons as well as our additional findings confirm that small and\nmidsized solar wind structures are worn down with increasing distance due to\ndynamic interaction of parcels of wind with different speed. For instance, the\nT-V relationship steepens, as the range in V is limited more than the range in\nT with distance. At times the T-V correlation clearly breaks down beyond 20 AU,\nwhich may indicate wind currently expanding and cooling may have an elevated T\nreflecting prior heating and compression in the inner heliosphere. The power of\nwind parameters at shorter periodicities decreases with distance as the longer\nperiodicities strengthen. The solar rotation periodicity is present in\ntemperature beyond 20 AU indicating the observed parcel temperature may reflect\nnot only current heating or cooling, but also heating occurring closer to the\nSun.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Ionization: a possible explanation for the difference of mean disk sizes\n  in star-forming regions: Surveys of protoplanetary disks in star-forming regions of similar age\nrevealed significant variations in average disk mass between some regions. For\ninstance, disks in the Orion Nebular Cluster (ONC) and Corona Australis (CrA)\nare on average smaller than disks observed in Lupus, Taurus, Chamaeleon I or\nOphiuchus. In contrast to previous models that study truncation of disks at a\nlate stage of their evolution, we investigate whether disks may already be born\nwith systematically smaller disk sizes in more massive star-forming regions as\na consequence of enhanced ionization rates. Assuming various cosmic-ray\nionization rates, we compute the resistivities for ambipolar diffusion and\nOhmic dissipation with a chemical network, and perform 2D non-ideal\nmagnetohydrodynamical protostellar collapse simulations. A higher ionization\nrate leads to stronger magnetic braking, and hence to the formation of smaller\ndisks. Accounting for recent findings that protostars act as forges of cosmic\nrays and considering only mild attenuation during the collapse phase, we show\nthat a high average cosmic-ray ionization rate in star-forming regions like the\nONC or CrA can explain the detection of smaller disks in these regions. Our\nresults show that on average a higher ionization rate leads to the formation of\nsmaller disks. Therefore, smaller disks in regions of similar age can be the\nconsequence of different levels of ionization, and may not exclusively be\ncaused by disk truncation via external photoevaporation. We strongly encourage\nobservations that allow measuring the cosmic-ray ionization degrees in\ndifferent star-forming regions to test our hypothesis.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Stellar disk destruction by dynamical interactions in the Orion\n  Trapezium star cluster: We compare the observed size distribution of circum stellar disks in the\nOrion Trapezium cluster with the results of $N$-body simulations in which we\nincorporated an heuristic prescription for the evolution of these disks. In our\nsimulations, the sizes of stellar disks are affected by close encounters with\nother stars (with disks). We find that the observed distribution of disk sizes\nin the Orion Trapezium cluster is excellently reproduced by truncation due to\ndynamical encounters alone. The observed distribution appears to be a sensitive\nmeasure of the past dynamical history of the cluster, and therewith on the\nconditions of the cluster at birth. The best comparison between the observed\ndisk size distribution and the simulated distribution is realized with a\ncluster of $N = 2500\\pm500$ stars with a half-mass radius of about 0.5\\,pc in\nvirial equilibrium (with a virial ratio of $Q = 0.5$, or somewhat colder $Q\n\\simeq 0.3$), and with a density structure according to a fractal dimension of\n$F \\simeq 1.6$. Simulations with these parameters reproduce the observed\ndistribution of circum stellar disks in about 0.2--0.5\\,Myr.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Direct diagnostics of forming massive stars: stellar pulsation and\n  periodic variability of maser sources: The 6.7 GHz methanol maser emission, a tracer of forming massive stars,\nsometimes shows enigmatic periodic flux variations over several 10-100 days. In\nthis Letter, we propose that this periodic variations could be explained by the\npulsation of massive protostars growing under rapid mass accretion with rates\nof Mdot > 10^-3 Msun/yr. Our stellar evolution calculations predict that the\nmassive protostars have very large radius exceeding 100 Rsun at maximum, and we\nhere study the pulsational stability of such the bloated protostars by way of\nthe linear stability analysis. We show that the protostar becomes pulsationally\nunstable with various periods of several 10-100 days, depending on different\naccretion rates. With the fact that the stellar luminosity when the star is\npulsationally unstable also depends on the accretion rate, we derive the\nperiod-luminosity relation log (L/Lsun) = 4.62 + 0.98log(P/100 day), which is\ntestable with future observations. Our models further show that the radius and\nmass of the pulsating massive protostar should also depend on the period. It\nwould be possible to infer such protostellar properties and the accretion rate\nwith the observed period. Measuring the maser periods enables a direct\ndiagnosis of the structure of accreting massive protostars, which are deeply\nembedded in dense gas and inaccessible with other observations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Modelling depletion by re-accretion of gas from a dusty disc in post-AGB\n  stars: Many disc-type post-asymptotic giant branch (post-AGB) stars are chemically\npeculiar, showing underabundances of refractory elements in their photospheres\nthat correlate with condensation temperature. The aim of this paper is to\ninvestigate how accretion from a circumbinary disc can cause this phenomenon of\ndepletion and how this impacts the evolution of post-AGB stars. We used the\n\\texttt{MESA} code to evolve stars in the post-AGB phase, while including\naccretion of metal-poor gas. We compared the models to a sample of 58 observed\ndisc-type post-AGB stars with chemical abundance data. For each of these stars,\nwe estimated the luminosity and the mass using the Gaia distance. We modelled\nthe accretion rate onto the binary from a viscously evolving disc for a range\nof initial accretion rates and disc masses. We find that large initial\naccretion rates ($\\gtrsim 3\\times10^{-7}$ $M_\\odot$/yr) and large initial disc\nmasses ($\\sim10^{-2}$ $M_\\odot$) are needed to reproduce the observed depleted\npost-AGB stars. Based on these high accretion rates, the evolution timescale of\npost-AGB stars can be significantly extended by a factor between two and five.\nWe distinguish depletion patterns that are unsaturated (plateau profile) from\nthose that are saturated, and we expect that post-red giant branch (post-RGB)\nstars are much more likely to show an unsaturated abundance pattern compared to\npost-AGB stars. Finally, because of the slower evolution of the low-mass\npost-RGB stars, we find that these systems can become depleted at lower\neffective temperatures ($< 5000$ K). We conclude that accretion from a\ncircumbinary disc successfully accounts for the chemical peculiarity of\npost-AGB stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A solar blowout jet caused by the eruption of a magnetic flux rope: We investigate the three-dimensional (3D) magnetic structure of a blowout jet\noriginated in the west edge of NOAA Active Region (AR) 11513 on 02 July 2012 by\nmeans of recently developed forced field extrapolation (FFE) model. The results\nshow that the blowout jet was caused by the eruption of the magnetic flux rope\n(MFR) consisting of twisted field lines. We further calculate the twist number\n$\\mathcal{T}_{w}$ and squashing factor Q of the reconstructed magnetic field\nand find that (1) the MFR corresponds well to the high $\\mathcal{T}_{w}$ region\n(2) the MFR outer boundary corresponds well to the high Q region, probably\ninterpreting the bright structure at the base of the jet. The twist number of\nthe MFR is estimated to be $\\mathcal{T}_{w}=-1.54\\pm 0.67$. Thus, the kink\ninstability is regarded as the initiation mechanism of the blowout jet as\n$\\mathcal{T}_{w}$ reaching or even exceeding the threshold value of the kink\ninstability. Our results also indicate that the bright point at the decaying\nphase is actually comprised of some small loops that are heated by the\nreconnection occurred above. In summary, the blowout jet is mostly consistent\nwith the scenario proposed by \\citet{mcs10} except that the kink instability is\nfound to be a possible trigger.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Investigating the IBEX Ribbon Structure a Solar Cycle Apart: A Ribbon of enhanced energetic neutral atom (ENA) emissions was discovered by\nthe Interstellar Boundary Explorer (IBEX) in 2009, redefining our understanding\nof the heliosphere boundaries and the physical processes occurring at the\ninterstellar interface. The Ribbon signal is intertwined with that of a\nglobally distributed flux (GDF) that spans the entire sky. To a certain extent,\nRibbon separation methods enabled examining its evolution independent of the\nunderlying GDF. Observations over a full solar cycle revealed the Ribbon's\nevolving nature, with intensity variations closely tracking those of the solar\nwind (SW) structure after a few years delay accounting for the SW-ENA recycling\nprocess. In this work, we examine the Ribbon structure, namely, its ENA fluxes,\nangular extent, width, and circularity properties for two years, 2009 and 2019,\nrepresentative of the declining phases of two adjacent solar cycles. We find\nthat, (i) the Ribbon ENA fluxes have recovered in the nose direction and south\nof it down to ~ 25{\\deg} (for energies below 1.7 keV) and not at mid and high\necliptic latitudes; (ii) The Ribbon width exhibits significant variability as a\nfunction of azimuthal angle; (iii) Circularity analysis suggests that the 2019\nRibbon exhibits a statistically consistent radius with that in 2009. The\nRibbon's partial recovery is aligned with the consensus of a heliosphere with\nits closest point being southward of the nose region. The large variability of\nthe Ribbon width as a function of Azimuth in 2019 compared to 2009 is likely\nindicative of small-scale processes within the Ribbon.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Photometric search for variable stars in young open cluster Berkeley 59: We present time-series photometry of stars located in the extremely young\nopen cluster Berkeley 59. Using the 1.04 m telescope at ARIES, Nainital, we\nhave identified 42 variables in a field of 13'x13' around the cluster. The\nprobable members of the cluster are identified using (V, V-I) colour-magnitude\ndiagram and (J-H, H-K) colour-colour diagram. Thirty one variables are found to\nbe pre-main sequence stars associated with the cluster. The ages and masses of\npre-main sequence stars are derived from colour-magnitude diagram by fitting\ntheoretical models to the observed data points. The ages of the majority of the\nprobable pre-main sequence variable candidates range from 1 to 5 Myrs. The\nmasses of these pre-main sequence variable stars are found to be in the range\nof ~0.3 to ~3.5 Msun and these could be T Tauri stars. The present statistics\nreveal that about 90% T Tauri stars have periods < 15 days. The classical T\nTauri stars are found to have larger amplitude in comparison to the weak line T\nTauri stars. There is an indication that the amplitude decreases with increase\nof the mass, which could be due to the dispersal of disk of relatively massive\nstars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "An estimate of the magnetic field strength associated with a solar\n  coronal mass ejection from low frequency radio observations: We report ground based, low frequency heliograph (80 MHz), spectral (85-35\nMHz) and polarimeter (80 and 40 MHz) observations of drifting, non-thermal\nradio continuum associated with the `halo' coronal mass ejection (CME) that\noccurred in the solar atmosphere on 2013 March 15. The magnetic field strengths\n($B$) near the radio source were estimated to be $B \\approx 2.2 \\pm 0.4$ G at\n80 MHz and $B \\approx 1.4 \\pm 0.2$ G at 40 MHz. The corresponding radial\ndistances ($r$) are $r \\approx 1.9~R_{\\odot}$ (80 MHz) and $r \\approx\n2.2~R_{\\odot}$ (40 MHz).",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the incidence of episodic accretion in Class I YSOs from VVV: Episodic accretion is one of the competing models to explain the observed\nluminosity spread in young stellar clusters. These short-lived high accretion\nevents could also have a strong impact on planet formation. Observations of\nhigh-amplitude variability in young stellar objects (YSOs) due to large changes\nin the accretion rate provide direct observational evidence for episodic\naccretion. However, there are still uncertainties in the frequency of these\nevents and if episodic accretion is universal among YSOs. To determine the\nfrequency of outbursts in Class I YSOs, we built a large and robust sample of\nobjects at this evolutionary stage, and searched for high-amplitude\nnear-infrared ($\\Delta K_{\\rm S}>2$~mag) variability in the VIRAC2 database of\nthe Vista Variables in the Via Lactea (VVV) survey. By complementing with\nnear-IR (2MASS and DENIS) and mid-IR (WISE/Neo-WISE) data, we find that from\n$\\sim$ 7000 Class I YSOs, 97 objects can be classified as eruptive variable\nYSOs. The duration of the outbursts vary from a few months to longer than 9\nyears, and cover a similar range of amplitudes. Values of $\\Delta K_{\\rm\nS}>5$~mag, however, are only observed in outbursts with duration longer than 9\nyears. When considering different effects of completeness and contamination we\nestimate that the incidence of episodic accretion in Class I YSOs is between\n2\\% and 3\\%. Finally, we determine a recurrence timescale of long-term\noutbursts (a.k.a FUors) of $\\tau=1.75^{+1.12}_{-0.87}$~kyr. The latter value\nagrees with previous estimates and is in line with the expectations of higher\nfrequency of FUor outbursts during younger stages of evolution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Amplitude of solar gravity modes generated by penetrative plumes: The detection of gravity modes is expected to give us unprecedented insights\ninto the inner dynamics of the Sun. Within this framework, predicting their\namplitudes is essential to guide future observational strategies and seismic\nstudies. In this work, we predict the amplitude of low-frequency asymptotic\ngravity modes generated by penetrative convection at the top of the radiative\nzone. The result is found to depend critically on the time evolution of the\nplumes inside the generation region. Using a solar model, we compute the GOLF\napparent surface radial velocity of low-degree gravity modes in the frequency\nrange $10~\\mu H_z\\le \\nu \\le 100~\\mu H_z$. In case of a Gaussian plume time\nevolution, gravity modes turn out to be undetectable because of too small\nsurface amplitudes. This holds true despite a wide range of values considered\nfor the parameters of the model. In the other limiting case of an exponential\ntime evolution, plumes are expected to drive gravity modes in a much more\nefficient way because of a much higher temporal coupling between the plumes and\nthe modes than in the Gaussian case. Using reasonable values for the plume\nparameters based on semi-analytical models, the apparent surface velocities in\nthis case turn out to be one order of magnitude smaller than the 22-years GOLF\ndetection threshold and than the previous estimates considering turbulent\npressure as the driving mechanism, with a maximum value of $0.05$ cm s${}^{-1}$\nfor $\\ell =1$ and $\\nu\\approx 100~\\mu H_z$. When accounting for uncertainties\non the plume parameters, the apparent surface velocities in the most favorable\nplausible case become comparable to those predicted with turbulent pressure,\nand the GOLF observation time required for a detection at $ \\nu \\approx100~\\mu\nH_z$ and $\\ell=1$ is reduced to about 50 yrs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Low-Resolution Spectroscopy of the Recurrent Nova T Pyxidis at its Early\n  Stage of 2011 Outburst: We present our observational results of the recurrent nova T Pyxidis at its\nearly stage of 2011 outburst, using a low-resolution spectrograph\n($R\\approx400$) attached to a 28cm telescope. Total nights of our observation\nare 11, among which 9 nights are during the pre-maximum stage. As a result we\nhave obtained a detailed evolutional feature of this recurrent nova on the way\nto its maximum light. At first, on the earliest three nights ($-25 \\sim -21$\ndays before maximum), broad and prominent emission lines such as Balmer series,\nHe I, He II, N II, N III and O I together with P Cygni profile are seen on the\nspectra. The blueshifted absorption minima of H$\\alpha$ yields a maximum\nexpansion velocity of approximately 2200 km s$^{-1}$, and the velocity\ngradually decreases. Then, Helium and Nitrogen lines are weakened day by day.\nAfter that (18 days before maximum light), Fe II (multiplets) lines emerge on\nthe spectra. These lines are then strengthened day by day, and the P Cygni\nprofiles also become more prominent. Accordingly, the expansion velocities\nturns to be gradual increase. In addition, during the pre-maximum stage, nova\nspectral type of T Pyx is thought to evolve from He/N type to Fe II one.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Solar Minimum Eclipse of 2019 July 2. III. Inferring the Coronal\n  $T_e$ with a Radiative Differential Emission Measure Inversion: Differential Emission Measure (DEM) inversion methods use the brightness of a\nset of emission lines to infer the line-of-sight (LOS) distribution of the\nelectron temperature ($T_e$) in the corona. DEM inversions have been\ntraditionally performed with collisionally excited lines at wavelengths in the\nExtreme Ultraviolet (EUV) and X-ray. However, such emission is difficult to\nobserve beyond the inner corona (1.5 R$_\\odot$), particularly in coronal holes.\nGiven the importance of the $T_e$ distribution in the corona for exploring the\nviability of different heating processes, we introduce an analog of the DEM\nspecifically for radiatively excited coronal emission lines, such as those\nobserved during total solar eclipses (TSEs) and with coronagraphs. This\nRadiative DEM (R-DEM) inversion utilizes visible and infrared emission lines\nwhich are excited by photospheric radiation out to at least 3 R$_\\odot$.\nSpecifically, we use the Fe X (637 nm), Fe XI (789 nm), and Fe XIV (530 nm)\ncoronal emission lines observed during the 2019 July 2 TSE near solar minimum.\nWe find that despite a large $T_e$ spread in the inner corona, the distribution\nconverges to an almost isothermal yet bimodal distribution beyond 1.4\nR$_\\odot$, with $T_e$ ranging from 1.1 to 1.4 in coronal holes, and from 1.4 to\n1.65 MK in quiescent streamers. Application of the R-DEM inversion to the\nPredictive Science Inc. magnetohydrodynamic (MHD) simulation for the 2019\neclipse validates the R-DEM method and yields a similar LOS Te distribution to\nthe eclipse data.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Gas phase SiO in the circumstellar environment of the recurrent nova T\n  Coronae Borealis: We report the discovery of the diatomic molecule SiO in the gas phase in the\nenvironment of the recurrent nova T Coronae Borealis. While some of the SiO is\nphotospheric, a substantial portion must arise in the wind from the red giant\ncomponent of T CrB. A simple fit to the SiO feature, assuming local\nthermodynamic equilibrium, suggests a SiO column density of 2.8x10^17 /cm2 and\ntemperature ~1000K; the SiO column density is similar to that present in the\nwinds of field red giants. A search for SiO maser emission is encouraged both\nbefore and after the next anticipated eruption. We find that the 12C/13C ratio\nin the red giant is <9, with a best fit value of ~5, a factor ~18 times lower\nthan the solar value of 89. We find no convincing evidence for the presence of\ndust in the environment of T CrB, which we attribute to the destructive effects\non nucleation sites of hard X-ray emission. When the next eruption of T CrB\noccurs, the ejected material will shock the wind, producing X-ray and coronal\nline emission, as is the case for the recurrent nova RS Oph. T CrB is also a\ngood candidate for very high energy gamma-ray emission, as first observed\nduring the 2010 outburst of V407 Cyg. We include in the paper a wide variety of\ninfrared spectroscopic and photometric data.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Cycling Dynamo in a Young Sun: Grand Minima and Equatorward Propagation: We assess the global-scale dynamo action achieved in a simulation of a\nsun-like star rotating at three times the solar rate. The 3-D MHD Anelastic\nSpherical Harmonic code, using slope-limited diffusion, is employed to capture\nconvection and dynamo processes in such a young sun. The simulation is carried\nout in a spherical shell that encompasses 3.8 density scale heights of the\nsolar convection zone. We find that dynamo action with a high degree of time\nvariation occurs, with many periodic polarity reversals every 6.2 years. The\nmagnetic energy also rises and falls with a regular period, with two magnetic\nenergy cycles required to complete a polarity cycle. These magnetic energy\ncycles arise from a Lorentz-force feedback on the differential rotation,\nwhereas the polarity reversals are present due to the spatial separation of the\nequatorial and polar dynamos. Moreover, an equatorial migration of toroidal\nfield is found, which is linked to the changing differential rotation and to a\nnear-surface shear layer. This simulation also enters a grand minimum lasting\nroughly 20 years, after which the dynamo recovers its regular polarity cycles.",
        "category": "astro-ph_SR"
    },
    {
        "text": "HIFISTARS Herschel/HIFI observations of VY Canis Majoris. Molecular-line\n  inventory of the envelope around the largest known star: To gain insight into the physical conditions and kinematics of the warm\n(100-1000 K) gas around the red hyper-giant VY CMa, we performed sensitive high\nspectral resolution observations of molecular lines in the sub-mm/FIR using the\nHIFI instrument of the Herschel Space Observatory. We observed CO, H2O, and\nother molecular species, sampling excitation energies from a few tens to a few\nthousand K. These observations are part of the Herschel Guaranteed Time Key\nProgram HIFISTARS. We detected the J=6-5, J=10-9, and J=16-15 lines of 12CO and\n13CO at about 100, 300, and 750K above the ground state (and the 13CO J=9-8\nline). These lines are crucial for improving the modelling of the internal\nlayers of the envelope around VY CMa. We also detected 27 lines of H2O and its\nisotopomers, and 96 lines of species such as NH3, SiO, SO, SO2 HCN, OH and\nothers, some of them originating from vibrationally excited levels. Three lines\nwere not unambiguously assigned. Our observations confirm that VY CMa's\nenvelope must consist of two or more detached components. The molecular\nexcitation in the outer layers is significantly lower than in the inner ones,\nresulting in strong self-absorbed profiles in molecular lines that are\noptically thick in this outer envelope, for instance, low-lying lines of H2O.\nExcept for the most abundant species, CO and H2O, most of the molecular\nemission detected at these sub-mm/FIR wavelengths arise from the central parts\nof the envelope. The spectrum of VY CMa is very prominent in vibrationally\nexcited lines, which are caused by the strong IR pumping present in the central\nregions. Compared with envelopes of other massive evolved stars, VY CMa's\nemission is particularly strong in these vibrationally excited lines, as well\nas in the emission from less abundant species such as H13CN, SO, and NH3.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Photometric light curve solutions of three ultra-short period eclipsing\n  binaries: We present the results of our study of the eclipsing binary systems CSS\nJ112237.1+395219, LINEAR 1286561 and LINEAR 2602707 based on new CCD $B$, $V$,\n$R_c$ and $I_c$ complete light curves. The ultra-short period nature of the\nstars \\citep{Drake2014} is confirmed and the system's periods are revised. The\nlight curves were modelled using the 2005 version of the Wilson-Devinney code.\nWhen necessary, cool spots on the surface of the primary component were\nintroduced to account for asymmetries in the light curves. As a result, we\nfound that CSS J112237.1+395219 is a W UMa type contact binary system belonging\nto W subclass with a mass ratio of $q=1.61$ and a shallow degree of contact of\n14.8\\% where the primary component is hotter than the secondary one by $500K$.\nLINEAR 1286561 and LINEAR 2602707 are detached binary systems with mass ratios\n$q=3.467$ and $q=0.987$ respectively. These detached systems are low-mass\nM-type eclipsing binaries of similar temperatures. The marginal contact, the\nfill-out factor and the temperature difference between components of CSS\nJ112237.1+395219 suggest that this system may be at a key evolutionary state\npredicted by the Thermal Relaxation Oscillation theory (TRO). From the\nestimated absolute parameters we conclude that our systems share common\nproperties with others ultra-short period binaries.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Discovery of deep eclipses in the cataclysmic variable ZTF17aaaeefu\n  (2MASS J00594349+6454419): I performed photometric observations of the cataclysmic variable candidate\nZTF17aaaeefu and discovered very deep eclipses. The observations were obtained\nover 8 nights covering 7 months. The eclipse profile is similar to the eclipse\nprofiles observed in other cataclysmic variables. During the observations,\nZTF17aaaeefu showed brightness changes resembling dwarf nova outbursts and\nquiescent states. When ZTF17aaaeefu was bright (16.0-16.2 mag), the average\neclipse depth was 2.50+/-0.18 mag. When ZTF17aaaeefu was faint (17.2-17.4 mag),\nthe average eclipse depth was 1.45+/-0.06 mag. However, these differences in\nthe eclipse depth may be mainly caused by the contaminating effect of three\nfaint stars around ZTF17aaaeefu. In both cases, the average width of the\nprominent parts of the eclipses was the same and was about 30 min. Due to the\nlarge coverage of observations, I measured the orbital period with high\nprecision, Porb=0.18821155+/-0.00000014 d. I derived the eclipse ephemeris, the\nvalidity time of which is 700 years in accordance with the precision of the\norbital period. This ephemeris can be used for future studies of the orbital\nperiod changes. Because ZTF17aaaeefu has a long orbital period, this\ncataclysmic variable is of interest for determining the masses of its stellar\ncomponents. In future radial velocity measurements, my precise eclipse\nephemeris may be useful for determining the orbital phases.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Chemistry in a forming protoplanetary disk: main accretion phase: We investigate the chemistry in a radiation-hydrodynamics model of\nstar-forming core which evolves from a cold ($\\sim 10$ K) prestellar core to\nthe main accretion phase in $\\sim 10^5$ yr. A rotationally-supported\ngravitationally unstable disk is formed around a protostar. We extract the\ntemporal variation of physical parameters in $\\sim 1.5 \\times 10^3$ SPH\nparticles which end up in the disk, and perform post-processing calculations of\nthe gas-grain chemistry adopting a three-phase model. Inside the disk, the SPH\nparticles migrate both inward and outward. Since a significant fraction of\nvolatiles such as CO can be trapped in the water-dominant ice in the\nthree-phase model, the ice mantle composition depends not only on the current\nposition in the disk but also on whether the dust grain has ever experienced\nhigher temperatures than the water sublimation temperature. Stable molecules\nsuch as H$_2$O, CH$_4$, NH$_3$ and CH$_3$OH are already abundant at the onset\nof gravitational collapse and simply sublimated as the fluid parcels migrate\ninside the water snow line. On the other hand, various molecules such as carbon\nchains and complex organic molecules (COMs) are formed in the disk. COMs\nabundance sensitively depends on the outcomes of photodissociation and\ndiffusion rates of photofragments in bulk ice mantle. As for S-bearing species,\nH$_2$S ice is abundant in the collapse phase. In the warm regions in the disk,\nH$_2$S is sublimated to be destroyed, while SO, H$_2$CS, OCS and SO$_2$ become\nabundant.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A $\u0394R\\sim 9.5$ mag Super Flare of An Ultracool Star Detected by\n  $\\text{SVOM/GWAC}$ System: In this paper, we report the detection and follow-ups of a super stellar\nflare GWAC\\,181229A with an amplitude of $\\Delta R\\sim$9.5 mag on a M9 type\nstar by $\\text{SVOM/GWAC}$ and the dedicated follow-up telescopes. The\nestimated bolometric energy $E_{bol}$ is $(5.56-9.25)\\times10^{34}$ ergs, which\nplaces the event to be one of the most powerful flares on ultracool stars. The\nmagnetic strength is inferred to be (3.6-4.7) kG. Thanks to the sampling with a\ncadence of 15 seconds, a new component near the peak time with a very steep\ndecay is detected in the $R$-band light curve, followed by the two-component\nflare template given by Davenport et al. (2014). An effective temperature of\n$5340\\pm40$ K is measured by a blackbody shape fitting to the spectrum at the\nshallower phase during the flare. The filling factors of the flare are\nestimated to be $\\sim$30\\% and 19\\% at the peak time and at 54 min after the\nfirst detection. The detection of the particular event with large amplitude,\nhuge-emitted energy and a new component demonstrates that a high cadence sky\nmonitoring cooperating with fast follow-up observations is very essential for\nunderstanding the violent magnetic activity.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Very Small and Super Strong Zebra Pattern Burst at the Beginning of a\n  Solar Flare: Microwave emission with spectral zebra pattern structures (ZPs) is observed\nfrequently in solar flares and the Crab pulsar. The previous observations show\nthat ZP is only a structure overlapped on the underlying broadband continuum\nwith slight increments and decrements. This work reports an extremely unusual\nstrong ZP burst occurring just at the beginning of a solar flare observed\nsimultaneously by two radio telescopes located in China and Czech Republic and\nby the extreme ultraviolet (EUV) telescope on board NASA's satellite Solar\nDynamics Observatory on 2013 April 11. It is a very short and super strong\nexplosion whose intensity exceeds several times that of the underlying flaring\nbroadband continuum emission, lasting for just 18 s. EUV images show that the\nflare starts from several small flare bursting points (FBPs). There is a sudden\nEUV flash with extra enhancement in one of these FBPs during the ZP burst.\nAnalysis indicates that the ZP burst accompanying EUV flash is an unusual\nexplosion revealing a strong coherent process with rapid particle acceleration,\nviolent energy release, and fast plasma heating simultaneously in a small\nregion with short duration just at the beginning of the flare.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Frequency and Mass-Ratio Distribution of Binaries in Clusters I:\n  Description of the method and application to M67: We present a new method for probabilistic generative modelling of stellar\ncolour-magnitude diagrams (CMDs) to infer the frequency of binary stars and\ntheir mass-ratio distribution. The method invokes a mixture model to account\nfor overlapping populations of single stars, binaries and outliers in the CMD.\nWe apply the model to Gaia observations of the old open cluster, M67, and find\na frequency $f_B(q > 0.5) = 0.258 \\pm 0.019$ for binary stars with mass ratio\ngreater than 0.5. The form of the mass-ratio distribution function rises\ntowards higher mass ratios for $q > 0.3$.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Signatures of ubiquitous magnetic reconnection in the lower solar\n  atmosphere: Ellerman Bomb-like brightenings of the hydrogen Balmer line wings in the\nquiet Sun (QSEBs) are a signature of the fundamental process of magnetic\nreconnection at the smallest observable scale in the solar lower atmosphere. We\nanalyze high spatial resolution observations (0.1 arcsec) obtained with the\nSwedish 1-m Solar Telescope to explore signatures of QSEBs in the H$\\beta$\nline. We find that QSEBs are ubiquitous and uniformly distributed throughout\nthe quiet Sun, predominantly occurring in intergranular lanes. We find up to\n120 QSEBs in the FOV for a single moment in time; this is more than an order of\nmagnitude higher than the number of QSEBs found in earlier H$\\alpha$\nobservations. This suggests that about half a million QSEBs could be present in\nthe lower solar atmosphere at any given time. The QSEB brightening found in the\nH$\\beta$ line wings also persist in the line core with a temporal delay and\nspatial offset towards the nearest solar limb. Our results suggest that QSEBs\nemanate through magnetic reconnection along vertically extended current sheets\nin the solar lower atmosphere. The apparent omnipresence of small-scale\nmagnetic reconnection may play an important role in the energy balance of the\nsolar chromosphere.",
        "category": "astro-ph_SR"
    },
    {
        "text": "SN 2011ht: Confirming a Class of Interacting Supernovae with Plateau\n  Light Curves (Type IIn-P): We present photometry and spectroscopy of the Type IIn supernova (SN) 2011ht,\nidentified previously as a SN impostor. The light curve exhibits an abrupt\ntransition from a well-defined ~120 day plateau to a steep bolometric decline.\nLeading up to peak brightness, a hot emission-line spectrum exhibits signs of\ninteraction with circumstellar material (CSM), in the form of relatively narrow\nP-Cygni features of H I and He I superimposed on broad Lorentzian wings. For\nthe remainder of the plateau phase the spectrum exhibits strengthening P-Cygni\nprofiles of Fe II, Ca II, and H-alpha. By day 147, after the plateau has ended,\nthe SN entered the nebular phase, heralded by the appearance of forbidden\ntransitions of [O I], [O II], and [Ca II] over a weak continuum. At this stage,\nthe light curve exhibits a low luminosity that is comparable to that\nsub-luminous Type II-P supernovae, and a relatively fast visual-wavelength\ndecline that is significantly steeper than the Co-56 decay rate. However, the\ntotal bolometric decline, including the IR luminosity, is consistent with Co-56\ndecay, and implies a low Ni-56 mass of ~0.01 M(Sun). We therefore characterize\nSN 2011ht as a bona-fide core-collapse SN very similar to the peculiar SNe IIn\n1994W and 2009kn. These three SNe define a subclass, which are Type IIn based\non their spectrum, but that also exhibit well-defined plateaus and produce low\nNi-56 yields. We therefore suggest Type IIn-P as a name for this subclass.\nPossible progenitors of SNe IIn-P, consistent with the available data, include\n8-10 M(Sun) stars, which undergo core collapse as a result of electron capture\nafter a brief phase of enhanced mass loss, or more massive M>25 M(Sun)\nprogenitors, which experience substantial fallback of the metal-rich\nradioactive ejecta. In either case, the energy radiated by these three SNe\nduring their plateau must be dominated by CSM interaction (abridged).",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the rotation periods of the components of the triple system\n  TYC9300-0891-1AB/TYC9300-0529-1 in the Octans Association: Stellar rotation depends on different parameters. The range of values of\nthese parameters causes the dispersion in the rotation period distributions\nobserved in young stellar clusters/associations. We focus our investigation on\nthe effects of different circumstellar environments on stellar rotation. More\nspecifically, we are searching in stellar Associations for visual triple\nsystems where all stellar parameters are similar, with the only exceptions of\nthe unknown initial rotation period, and of the circum-stellar environment, in\nthe sense that one of the two about equal-mass components has a close-by third\n'perturber' component. In the present study we analyse the 35-Myr old visual\ntriple system TYC 9300-0891-1AB + TYC 9300-0529-1 in the young Octans stellar\nassociation consisting of three equal-mass K0V components. We collected from\nthe literature all information that allowed us to infer that the three\ncomponents are actually physically bound forming a triple system and are\nmembers of the Octans Association. We collected broad-band photometric\ntimeseries in two observation seasons. We discovered that all the components\nare variable, magnetically active, and from periodogram analysis we found the\nunresolved components TYC 9300-0891-1AB to have a rotation period P = 1.383d\nand TYC 9300-0529-1 a rotation period P = 1.634d. TYC 9300-0891-1A, TYC\n9300-0891-1B, and TYC 9300-0529-1 have same masses, ages, and initial chemical\ncompositions. The relatively small 16% rotation period difference measured by\nus indicates that all components had similar initial rotation periods and disc\nlifetimes, and the separation of 157AU between the component A and the\n'perturber' component B (or vice-versa) has been sufficiently large to prevent\nany significant perturbation/shortening of the accretion-disc lifetime.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Unveiling the nature of red novae cool explosions using archive plate\n  photometry: Based on archive photographic photometry and recent CCD photometric data for\nred novae V4332 Sgr and V838 Mon, we established their stellar composition,\nexploded components, and the nature of explosions. Low temperature in the\noutburst maximum is due to quasi-adiabatic expansion of a massive stellar\nenvelope after the central energy surge preceded the outburst.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Period-Colour and Amplitude-Colour relations for OGLE-$\u03b4$ Scuti\n  stars in the Galactic Bulge and LMC: We present an analysis on the behaviour of the Galactic bulge and the Large\nMagellanic Cloud (LMC) $\\delta$ Scuti stars in terms of period-colour and\namplitude-colour (PCAC) relations at maximum, mean and minimum light. The\npublicly available Optical Gravitational Lensing Experiment-IV (OGLE-IV) light\ncurves for Galactic bulge and OGLE-III light curves for LMC $\\delta$ Scuti\nstars are exploited for the analysis. It has been found that the Galactic bulge\n$\\delta$ Scuti stars obey flat PC relations at maximum/mean/minimum light while\nthe LMC $\\delta$ Scutis have sloped/sloped/flat PC relations at\nmaximum/mean/minimum light. Both the Galactic bulge and the LMC $\\delta$ Scutis\nhave sloped/flat/sloped AC relations at maximum/mean/minimum. These relations\nalso show that Galactic $\\delta$ Scutis are hotter as compared to their LMC\ncounterparts. The period-amplitude (PA) relations for $\\delta$ Scutis exhibit\ndifferent behaviour in the Galactic bulge and the LMC. The LMC variables are\nfound to have higher amplitudes at a given period. The amplitude of the\nGalactic bulge $\\delta$ Scuti shows a bimodal distribution which can be\nmodelled using a two-component Gaussian Mixture Model: one component with a\nlower amplitude and another with a higher amplitude. The observed behaviour of\nthe $\\delta$ Scuti PCAC relations can be explained using the theory of the\ninteraction of hydrogen ionization front (HIF) and stellar photosphere as well\nas the PA diagram. We use MESA-RSP to calculate theoretical non-linear\nhydrodynamical pulsation models for $\\delta$ Scuti stars with input\nmetallicities of $Z=0.02$ and $Z=0.008$ appropriate for the Galactic bulge and\nLMC, respectively. The observed PCAC relations and theoretical calculations\nsupport the HIF-photosphere interaction theory.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Brightness Oscillations in Models of Young Binary Systems with Low-Mass\n  Secondary Components: We consider a model for the cyclic brightness variations of a young star with\na low-mass companion that accretes matter from the remnants of a protostellar\ncloud. At small inclinations of the binary orbit to the line of sight, the\nstreams of matter and the density waves excited in the circumbinary disk can\nscreen the primary component of the binary from the observer. To study these\nphenomena, we have computed grids of hydrodynamic models for binary systems by\nthe SPH method based on which we have constructed the phase light curves as a\nfunction of the rotation angle of the apsidal line relative to the observer.\nThe model parameters were varied within the following ranges: the component\nmass ratio q = 0.01-0.1 and the eccentricity e = 0-0.5. We adopted optical\ngrain characteristics typical of circumstellar dust. Our computations have\nshown that the brightness oscillations with orbital phase can have a complex\nstructure. The amplitudes and shapes of the light curves depend strongly on the\ninclination of the binary orbit and its orientation relative to the observer\nand on the accretion rate. The results of our computations are used to analyze\nthe cyclic activity of UX Ori stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The $\u03ba$-cookbook: a novel generalizing approach to unify\n  $\u03ba$-like distributions for plasma particle modeling: In the literature different so-called $\\kappa$-distribution functions are\ndiscussed to fit and model the velocity (or energy) distributions of solar wind\nspecies, pickup ions or magnetospheric particles. Here we introduce a\ngeneralized (isotropic) $\\kappa$-distribution as a \"cookbook\", which admits as\nspecial cases, or \"recipes\", all the other known versions of $\\kappa$-models. A\ndetailed analysis of the generalized distribution function is performed,\nproviding general analytical expressions for the velocity moments, Debye\nlength, and entropy, and pointing out a series of general requirements that\nplasma distribution functions should satisfy. From a contrasting analysis of\nthe recipes found in the literature, we show that all of them lead to almost\nthe same macroscopic parameters with a small standard deviation between them.\nHowever, one of these recipes called the regularized $\\kappa$-distribution\nprovides a functional alternative for macroscopic parameterization without any\nconstraint for the power-law exponent $\\kappa$.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Two-sided-loop jets associated with magnetic reconnection between\n  emerging loops and twisted filament threads: Coronal jets are always produced by magnetic reconnection between emerging\nflux and pre-existing overlying magnetic fields. When the overlying field is\nvertical/obilique or horizontal, the coronal jet will appear as anemone type or\ntwo-sided-loop type. Most of observational jets are of the anemone type, and\nonly a few of two-sided-loop jets have been reported. Using the high-quality\ndata from New Vacuum Solar Telescope, Interface Region Imaging Spectrograph,\nand Solar Dynamics Observatory, we present an example of two-sided-loop jets\nsimultaneously observed in the chromosphere, transition region, and corona. The\ncontinuous emergence of magnetic flux brought in successively emerging of\ncoronal loops and the slowly rising of an overlying horizontal filament\nthreads. Sequentially, there appeared the deformation of the loops, the\nplasmoids ejection from the loop top, and pairs of loop brightenings and jet\nmoving along the untwisting filament threads. All the observational results\nindicate there exist magnetic reconnection between the emerging loops and\noverlying horizontal filament threads, and it is the first example of\ntwo-sided-loop jets associated with ejected plasmoids and twisted overlying\nfields.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Survey of multiple populations in globular clusters among very low-mass\n  stars: Recent work has shown that NIR Hubble Space Telescope (HST) photometry allows\nus to disentangle multiple populations (MPs) among M dwarfs of globular\nclusters (GCs) and investigate this phenomenon in very low-mass (VLM) stars.\nHere, we present the color-magnitude diagrams (CMDs) of nine GCs and the open\ncluster NGC 6791 in the F110W and F160W bands of HST, showing that the main\nsequences (MSs) below the knee are either broadened or split thus providing\nevidence of MPs among VLM stars. In contrast, the MS of NGC 6791 is consistent\nwith a single population. The color distribution of M-dwarfs dramatically\nchanges between different GCs and the color width correlates with the cluster\nmass. We conclude that the MP ubiquity, variety, and dependence on GC mass are\nproperties common to VLM and more-massive stars. We combined UV, optical, and\nNIR observations of NGC 2808 and NGC 6121 (M 4) to identify MPs along with a\nwide range of stellar masses (~ 0.2 - 0.8M ), from the MS turn off to the VLM\nregime, and measured, for the first time, their mass functions (MFs). We find\nthat the fraction of MPs does not depend on the stellar mass and that their MFs\nhave similar slopes. These findings indicate that the properties of MPs do not\ndepend on stellar mass. In a scenario where the second generations formed in\nhigher-density environments than the first generations, the possibility that\nthe MPs formed with the same initial MF would suggest that it does not depend\non the environment.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Data processing of Visible Emission Line Coronagraph Onboard ADITYA L1: ADITYA-L1 is India's first dedicated mission to observe the sun and its\natmosphere from a halo orbit around L1 point. Visible emission line coronagraph\n(VELC) is the prime payload on board at Aditya-L1 to observe the sun's corona.\nVELC is designed as an internally occulted reflective coronagraph to meet the\nobservational requirements of wide wavelength band and close to the solar limb\n(1.05 Ro). Images of the solar corona in continuum and spectra in three\nemission lines 5303{\\AA} [Fe xiv], 7892{\\AA} [Fe xi] and 10747 [Fe xiii]\nobtained with high cadence to be analyzed using software algorithms\nautomatically. A reasonable part of observations will be made in synoptic mode,\nthose, need to be analyzed and results made available for public use. The\nprocedure involves the calibration of instrument and detectors, converting the\nimages into fits format, correcting the images and spectra for the instrumental\neffects, align the images etc. Then, develop image processing algorithms to\ndetect the occurrence of energetic events using continuum images. Also derive\nphysical parameters, such as temperature and velocity structure of solar corona\nusing emission line observations. Here, we describe the calibration of\ndetectors and the development of software algorithms to detect the occurrence\nof CMEs and analyze the spectroscopic data.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A large stellar evolution database for population synthesis studies. V.\n  Stellar models and isochrones with CNONa abundance anticorrelations: We present a new grid of stellar models and isochrones for old stellar\npopulations, covering a large range of [Fe/H] values, for an heavy element\nmixture characterized by CNONa abundance anticorrelations as observed in\nGalactic globular cluster stars. The effect of this metal abundance pattern on\nthe evolutionary properties of low mass stars, from the main sequence to the\nhorizontal branch phase is analyzed. We perform comparisons between these new\nmodels, and our reference alpha-enhanced calculations, and discuss briefly\nimplications for CMDs showing multiple main sequence or subgiant branches. A\nbrief qualitative discussion of the effect of CN abundances on color-T_{eff}\ntransformations is also presented, highlighting the need to determine\ntheoretical color transformations for the appropriate metal mixture, if one\nwants to interpret observations in the Stroemgren system, or broadband filters\nblueward of the Johnson V-band.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Chromospheric Activity in 55 Cancri: II. Theoretical Wave Studies versus\n  Observations: In this study, we consider chromospheric heating models for 55 Cancri in\nconjunction with observations. The theoretical models, previously discussed in\nPaper I, are self-consistent, nonlinear and time-dependent ab-initio\ncomputations encompassing the generation, propagation, and dissipation of\nwaves. Our focus is the consideration of both acoustic waves and longitudinal\nflux tube waves amounting to two-component chromosphere models. 55 Cancri, a\nK-type orange dwarf, is a star of low activity, as expected by its age, which\nalso implies a relatively small magnetic filling factor. The Ca II K fluxes are\ncomputed (multi-ray treatment) assuming partial redistribution and\ntime-dependent ionization. The theoretical Ca II H+K fluxes are subsequently\ncompared with observations. It is found that for stages of lowest chromospheric\nactivity the observed Ca II fluxes are akin, though not identical, to those\nobtained by acoustic heating, but agreement can be obtained if low levels of\nmagnetic heating - consistent with the assumed photospheric magnetic filling\nfactor - are considered as an additional component; this idea is in alignment\nwith previous proposals conveyed in the literature.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Exploring B4: A Pulsating sdB star, in a Binary, in the Open Cluster NGC\n  6791: We report on Kepler photometry of the hot sdB star B4 in the open cluster NGC\n6791. We confirm that B4 is a reflection effect binary with an sdB component\nand a low-mass main sequence companion with a circular 0.3985 d orbit. The sdB\nstar is a g-mode pulsator (a V1093 Her star) with periods ranging from 2384 s\nto 7643 s. Several of the pulsation modes show symmetric splitting by 0.62\nmicroHz. Attributing this to rotational splitting, we conclude that the sdB\ncomponent has a rotation period of approximately 9.63 d, indicating that tidal\nsynchronization has not been achieved in this system. Comparison with\ntheoretical synchronization time provides a discriminant between various\ntheoretical models.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Metallicities from high resolution spectra of 49 RR Lyrae Variables: Accurate metallicities of RR Lyrae are extremely important in constraining\nperiod-luminosity-metallicity relationships (PLZ), particularly in the\nnear-infrared. We analyse 69 high-resolution spectra of Galactic RR Lyrae stars\nfrom the Southern African Large Telescope (SALT). We measure metallicities of\n58 of these RR Lyrae stars with typical uncertainties of 0.13 dex. All but one\nRR Lyrae in this sample has accurate ({\\sigma}_parallax ~ 10%) parallax from\nGaia. Combining these new high resolution spectroscopic abundances with similar\ndeterminations from the literature for 93 stars, we present new PLZ\nrelationships in WISE W1 and W2 magnitudes, and the Wesenheit magnitudes\nW(W1,V-W1) and W(W2,V-W2).",
        "category": "astro-ph_SR"
    },
    {
        "text": "Observation of \"Topological\" Microflares in the Solar Atmosphere: We report on observation of the unusual kind of solar microflares, presumably\nassociated with the so-called \"topological trigger\" of magnetic reconnection,\nwhich was theoretically suggested long time ago by Gorbachev et al. (Sov. Ast.\n1988, v.32, p.308) but has not been clearly identified so far by observations.\nAs can be seen in pictures by Hinode SOT in CaII line, there may be a bright\nloop connecting two sunspots, which looks at the first sight just as a magnetic\nfield line connecting the opposite poles. However, a closer inspection of SDO\nHMI magnetograms shows that the respective arc is anchored in the regions of\nthe same polarity near the sunspot boundaries. Yet another peculiar feature is\nthat the arc flashes almost instantly as a thin strip and then begins to expand\nand decay, while the typical chromospheric flares in CaII line are much wider\nand propagate progressively in space. A qualitative explanation of the unusual\nflare can be given by the above-mentioned model of topological trigger. Namely,\nthere are such configurations of the magnetic sources on the surface of\nphotosphere that their tiny displacements result in the formation and fast\nmotion of a 3D null point along the arc located well above the plane of the\nsources. So, such a null point can quickly ignite a magnetic reconnection along\nthe entire its trajectory. Pictorially, this can be presented as flipping the\nso-called two-dome magnetic-field structure (which is just the reason why such\nmechanism was called topological). The most important prerequisite for the\ndevelopment of topological instability in the two-dome structure is a cruciform\narrangement of the magnetic sources in its base, and this condition is really\nsatisfied in the case under consideration.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Investigation of the Progenitors of the Type Ia Supernovae Associated\n  With the LMC Supernova Remnants 0505-67.9 and 0509-68.7: Although Type Ia supernovae have been heavily scrutinized due to their use in\nmaking cosmological distance estimates, we are still unable to definitively\nidentify the progenitors for the entire population. While answers have been\npresented for certain specific systems, a complete solution remains elusive. We\npresent observations of two supernova remnants (SNRs) in the Large Magellanic\nCloud, SNR 0505-67.9 and SNR 0509-68.7, for which we have identified the center\nof the remnant and the 99.73% containment central region in which any companion\nstar left over after the supernova must be located. Both remnants have a number\nof potential ex-companion stars near their centers; all possible single and\ndouble degenerate progenitor models remain viable for these two supernovae.\nFuture observations may be able to identify the true ex-companions for both\nremnants.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Models for Metal-Poor Stars with Different Initial Abundances of C, N,\n  O, Mg, and Si. III. Grids of Isochrones for $-2.5 \\le$ [Fe/H] $\\le -0.5$ and\n  Helium Abundances $Y = 0.25$ and $0.29$ at Each Metallicity: Stellar evolutionary tracks for $0.12 \\le M/M_\\odot \\le 1.0$ have been\ncomputed for each of several variations in the abundances of C, N, and O,\nassuming mass-fraction helium abundances $Y = 0.25$ and $0.29$, and 11\nmetallicities in the range $-1.5 \\le$ [Fe/H] $\\le -0.5$, in 0.2 dex increments.\nSuch computations are provided for mixtures with [O/Fe] between $+0.4$ and\n$+0.8$, for different C:N:O ratios at a fixed value of [CNO/Fe], and for\nenhanced C. Computer codes are provided to interpolate within these grids to\nproduce isochrones for ages $> 7$ Gyr and to generate magnitudes and colours\nfor many broad-band filters using bolometric corrections based on MARCS model\natmospheres and synthetic spectra. The models are compared with (i) similar\ncomputations produced by other workers, (ii) observed UV, optical, and IR\ncolour-magnitude diagrams (CMDs), (iii) the effective temperatures, $(V-I_C)_0$\nand $(V-K_S)_0$ colours of Pop. II stars in the solar neighbourhood, and (iv)\nempirical data for the absolute magnitude of the tip of the giant branch\n(TRGB). The isochrones are especially successful in reproducing the observed\nmorphologies of observed CMDs and in satisfying the TRGB constraints. They also\nfare quite well in explaining the IR colours of low mass stars in globular\nclusters, indicating that they have [O/Fe] $\\approx +0.6$, though some\nchallenges remain.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Towards solar measurements of nuclear reaction rates: Nuclear reaction rates are a fundamental yet uncertain ingredient in stellar\nevolution models. The astrophysical S-factor pertaining to the initial reaction\nin the proton-proton chain is uncertain at the 1% level, which contributes a\nsystematic but generally unpropagated error of similar order in the theoretical\nages of stars. In this work, we study the prospect of improving the measurement\nof this and other reaction rates in the pp chain and CNO cycle using\nhelioseismology and solar neutrinos. We show that when other aspects of the\nsolar model are improved, then it shall be possible using current solar data to\nimprove the precision of this measurement by nearly an order of magnitude, and\nhence the corresponding uncertainty on the ages of low-mass stars by a similar\namount.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Revisiting the Cygnus OB associations: OB associations play an important role in Galactic evolution, though their\norigins and dynamics remain poorly studied, with only a small number of systems\nanalysed in detail. In this paper we revisit the existence and membership of\nthe Cygnus OB associations. We find that of the historical OB associations only\nCyg OB2 and OB3 stand out as real groups. We search for new OB stars using a\ncombination of photometry, astrometry, evolutionary models and an SED fitting\nprocess, identifying 4680 probable OB stars with a reliability of $>$90\\%. From\nthis sample we search for OB associations using a new and flexible clustering\ntechnique, identifying 6 new OB associations. Two of these are similar to the\nassociations Cyg OB2 and OB3, though the others bear no relationship to any\nexisting systems. We characterize the properties of the new associations,\nincluding their velocity dispersions and total stellar masses, all of which are\nconsistent with typical values for OB associations. We search for evidence of\nexpansion and find that all are expanding, albeit anistropically, with stronger\nand more significant expansion in the direction of Galactic longitude. We also\nidentify two large-scale (160 pc and 25 km s$^{-1}$) kinematic expansion\npatterns across the Cygnus region, each including three of our new\nassociations, and attribute this to the effects of feedback from a previous\ngeneration of stars. This work highlights the need to revisit the existence and\nmembership of the historical OB associations, if they are to be used to study\ntheir properties and dynamics.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A stellar relic filament in the Orion star forming region: We report the discovery of the oldest stellar substructure in the Orion star\nforming region (OSFR), the Orion relic filament. The relic filament is\nphysically associated with the OSFR as demonstrated by Gaia DR2 photometry and\nastrometry, as well as targeted radial velocity follow-up observations of a\nbright sub-sample of proper-motion selected candidate members. Gaia DR2\nparallaxes place the Orion relic filament in the more distant part of the OSFR,\napprox. 430pc from the Sun. Given its age, velocity dispersion, spatial extent,\nand shape, it is not possible for the Orion relic filament to have formed as a\nsingle stellar cluster, even taking into account residual gas expulsion. The\nrelic filament is also too young to be a tidal stream, since Galactic tides act\non much longer time scales of order 100 Myr. It therefore appears likely that\nthe structure formed from a molecular cloud filament similar to Orion A in the\nOSFR and retained its morphology despite decoupling from its natal gas. Hence,\nthe Orion relic filament bears witness to the short-lived evolutionary phase\nbetween gas removal and dispersion due to shears and tides, and provides\ncrucial new insights into how stars are formed in molecular clouds.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Detailed opacity calculations for astrophysical applications: Nowadays, several opacity codes are able to provide data for stellar\nstructure models, but the computed opacities may show significant differences.\nIn this work, we present state-of-the-art precise spectral opacity\ncalculations, illustrated by stellar applications. The essential role of\nlaboratory experiments to check the quality of the computed data is underlined.\nWe review some X-ray and XUV laser and Z-pinch photo-absorption measurements as\nwell as X-ray emission spectroscopy experiments involving hot dense plasmas\nproduced by ultra-high-intensity laser irradiation. The measured spectra are\nsystematically compared with the fine-structure opacity code SCO-RCG. Focus is\nput on iron, due to its crucial role in understanding asteroseismic\nobservations of $\\beta$ Cephei-type and Slowly Pulsating B stars, as well as of\nthe Sun. For instance, in $\\beta$ Cephei-type stars, the iron-group opacity\npeak excites acoustic modes through the \"kappa-mechanism\". A particular\nattention is paid to the higher-than-predicted iron opacity measured at the\nSandia Z-machine at solar interior conditions. We discuss some theoretical\naspects such as density effects, photo-ionization, autoionization or the\n\"filling-the-gap\" effect of highly excited states.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A $\u0394R\\sim 9.5$ mag Super Flare of An Ultracool Star Detected by\n  $\\text{SVOM/GWAC}$ System: In this paper, we report the detection and follow-ups of a super stellar\nflare GWAC\\,181229A with an amplitude of $\\Delta R\\sim$9.5 mag on a M9 type\nstar by $\\text{SVOM/GWAC}$ and the dedicated follow-up telescopes. The\nestimated bolometric energy $E_{bol}$ is $(5.56-9.25)\\times10^{34}$ ergs, which\nplaces the event to be one of the most powerful flares on ultracool stars. The\nmagnetic strength is inferred to be (3.6-4.7) kG. Thanks to the sampling with a\ncadence of 15 seconds, a new component near the peak time with a very steep\ndecay is detected in the $R$-band light curve, followed by the two-component\nflare template given by Davenport et al. (2014). An effective temperature of\n$5340\\pm40$ K is measured by a blackbody shape fitting to the spectrum at the\nshallower phase during the flare. The filling factors of the flare are\nestimated to be $\\sim$30\\% and 19\\% at the peak time and at 54 min after the\nfirst detection. The detection of the particular event with large amplitude,\nhuge-emitted energy and a new component demonstrates that a high cadence sky\nmonitoring cooperating with fast follow-up observations is very essential for\nunderstanding the violent magnetic activity.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Extended radio emission associated with a breakout eruption from the\n  back side of the Sun: Context. Coronal mass ejections (CMEs) on the Sun are the largest explosions\nin the Solar System that can drive powerful plasma shocks. The eruptions,\nshocks, and other processes associated to CMEs are efficient particle\naccelerators and the accelerated electrons in particular can produce radio\nbursts through the plasma emission mechanism. Aims. Coronal mass ejections and\nassociated radio bursts have been well studied in cases where the CME\noriginates close to the solar limb or within the frontside disc. Here, we study\nthe radio emission associated with a CME eruption on the back side of the Sun\non 22 July 2012. Methods. Using radio imaging from the Nan\\c{c}ay\nRadioheliograph, spectroscopic data from the Nan\\c{c}ay Decametric Array, and\nextreme-ultraviolet observations from the Solar Dynamics Observatory and Solar\nTerrestrial Relations Observatory spacecraft, we determine the nature of the\nobserved radio emission as well as the location and propagation of the CME.\nResults. We show that the observed low-intensity radio emission corresponds to\na type II radio burst or a short-duration type IV radio burst associated with a\nCME eruption due to breakout reconnection on the back side of the Sun, as\nsuggested by the pre-eruptive magnetic field configuration. The radio emission\nconsists of a large, extended structure, initially located ahead of the CME,\nthat corresponds to various electron acceleration locations. Conclusions. The\nobservations presented here are consistent with the breakout model of CME\neruptions. The extended radio emission coincides with the location of the\ncurrent sheet and quasi-separatrix boundary of the CME flux and the overlying\nhelmet streamer and also with that of a large shock expected to form ahead of\nthe CME in this configuration.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spectroscopic subsystems in nearby wide binaries: Radial velocity (RV) monitoring of solar-type visual binaries has been\nconducted at the CTIO/SMARTS 1.5-m telescope to study short-period systems.\nData reduction is described, mean and individual RVs of 163 observed objects\nare given. New spectroscopic binaries are discovered or suspected in 17\nobjects, for some of them orbital periods could be determined. Subsystems are\nefficiently detected even in a single observation by double lines and/or by the\nRV difference between the components of visual binaries. The potential of this\ndetection technique is quantified by simulation and used for statistical\nassessment of 96 wide binaries within 67pc. It is found that 43 binaries\ncontain at least one subsystem and the occurrence of subsystems is equally\nprobable in either primary or secondary components. The frequency of subsystems\nand their periods match the simple prescription proposed by the author (2014,\nAJ, 147, 87). The remaining 53 simple wide binaries with a median projected\nseparation of 1300AU have the distribution of the RV difference between their\ncomponents that is not compatible with the thermal eccentricity distribution\nf(e)=2e but rather matches the uniform eccentricity distribution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A catalogue of 323 cataclysmic variables from LAMOST DR6: In this work, we present a catalog of cataclysmic variables (CVs) identified\nfrom the Sixth Data Release (DR6) of the Large Sky Area Multi-Object Fiber\nSpectroscopic Telescope (LAMOST). To single out the CV spectra, we introduce a\nnovel machine-learning algorithm called UMAP to screen out a total of 169,509\nH$\\alpha$-emission spectra, and obtain a classification accuracy of the\nalgorithm of over 99.6$\\%$ from the cross-validation set. We then apply the\ntemplate matching program PyHammer v2.0 to the LAMOST spectra to obtain the\noptimal spectral type with metallicity, which helps us identify the\nchromospherically active stars and potential binary stars from the 169,509\nspectra. After visually inspecting all the spectra, we identify 323 CV\ncandidates from the LAMOST database, among them 52 objects are new. We further\ndiscuss the new CV candidates in subtypes based on their spectral features,\nincluding five DN subtype during outbursts, five NL subtype and four magnetic\nCVs (three AM Her type and one IP type). We also find two CVs that have been\npreviously identified by photometry, and confirm their previous classification\nby the LAMOST spectra.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Forward asteroseismic modeling of stars with a convective core from\n  gravity-mode oscillations: parameter estimation and stellar model selection: We propose a methodological framework to perform forward asteroseismic\nmodeling of stars with a convective core, based on gravity-mode oscillations.\nThese probe the near-core region in the deep stellar interior. The modeling\nrelies on a set of observed high-precision oscillation frequencies of\nlow-degree coherent gravity modes with long lifetimes and their observational\nuncertainties. Identification of the mode degree and azimuthal order is assumed\nto be achieved from rotational splitting and/or from period spacing patterns.\nThis paper has two major outcomes. The first is a comprehensive list and\ndiscussion of the major uncertainties of theoretically predicted gravity-mode\noscillation frequencies based on linear pulsation theory, caused by fixing\nchoices of the input physics for evolutionary models. Guided by a hierarchy\namong these uncertainties of theoretical frequencies, we subsequently provide a\nglobal methodological scheme to achieve forward asteroseismic modeling. We\nproperly take into account correlations amongst the free parameters included in\nstellar models. Aside from the stellar mass, metalicity and age, the major\nparameters to be estimated are the near-core rotation rate, the amount of\nconvective core overshooting, and the level of chemical mixing in the radiative\nzones. This modeling scheme allows for maximum likelihood estimation of the\nstellar parameters for fixed input physics of the equilibrium models, followed\nby stellar model selection considering various choices of the input physics.\nOur approach uses the Mahalanobis distance instead of the often used $\\chi^2$\nstatistic and includes heteroscedasticity. It provides estimation of the\nunknown variance of the theoretically predicted oscillation frequencies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Gaia DR1 Mass-Radius Relation for White Dwarfs: The Gaia Data Release 1 (DR1) sample of white dwarf parallaxes is presented,\nincluding 6 directly observed degenerates and 46 white dwarfs in wide binaries.\nThis data set is combined with spectroscopic atmospheric parameters to study\nthe white dwarf mass-radius relationship (MRR). Gaia parallaxes and G\nmagnitudes are used to derive model atmosphere dependent white dwarf radii,\nwhich can then be compared to the predictions of a theoretical MRR. We find a\ngood agreement between Gaia DR1 parallaxes, published effective temperatures\n(Teff) and surface gravities (log g), and theoretical MRRs. As it was the case\nfor Hipparcos, the precision of the data does not allow for the\ncharacterisation of hydrogen envelope masses. The uncertainties on the\nspectroscopic atmospheric parameters are found to dominate the error budget and\ncurrent error estimates for well-known and bright white dwarfs may be slightly\noptimistic. With the much larger Gaia DR2 white dwarf sample it will be\npossible to explore the MRR over a much wider range of mass, Teff, and spectral\ntypes.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Sunspot splitting triggering an eruptive flare: We investigate how the splitting of the leading sunspot and associated flux\nemergence and cancellation in active region NOAA 11515 caused an eruptive M5.6\nflare on 2012 July 2. Our study employs multi-wavelength observations from HMI,\nAIA and ChroTel. Emerging flux formed a neutral line ahead of the leading\nsunspot and new satellite spots. The sunspot splitting caused a long-lasting\nflow toward this neutral line, where a filament formed. Further flux emergence,\npartly of mixed-polarity, as well as episodes of flux cancellation occurred\nrepeatedly at the neutral line. Following a nearby C-class precursor flare with\nsigns of interaction with the filament, the filament erupted nearly\nsimultaneously with the onset of the M5.6 flare and evolved into a coronal mass\nejection. The sunspot stretched without forming a light bridge, splitting\nunusually fast (within about a day, complete approximately 6 hours after the\neruption) in two nearly equal parts. The front part separated strongly from the\nactive region to approach the neighbouring active region where all its coronal\nmagnetic connections were rooted. It also rotated rapidly (by 4.9 degree/hr)\nand caused significant shear flows at its edge. The eruption resulted from a\ncomplex sequence of processes in the (sub-)photosphere and corona. The\npersistent flows toward the neutral line likely caused the formation of a flux\nrope which held the filament. These flows and their associated flux\ncancellation, the emerging flux, and the precursor flare all contributed to the\ndestabilization of the flux rope. We interpret the sunspot splitting as the\nseparation of two flux bundles differently rooted in the convection zone and\nonly temporarily joined in the spot. This explains the rotation as continued\nrise of the separating flux and implies that at least this part of the sunspot\nwas still connected to its roots deep in the convection zone.",
        "category": "astro-ph_SR"
    },
    {
        "text": "High Resolution Optical and NIR Spectra of HBC 722: We present the results of high resolution (R$\\ge$30,000) optical and near-IR\nspectroscopic monitoring observations of HBC 722, a recent FU Orionis object\nthat underwent an accretion burst in 2010. We observed HBC 722 in\noptical/near-IR with the BOES, HET-HRS, and IGRINS spectrographs, at various\npoints in the outburst. We found atomic lines with strongly blueshifted\nabsorption features or P Cygni profiles, both evidence of a wind driven by the\naccretion. Some lines show a broad double-peaked absorption feature, evidence\nof disk rotation. However, the wind-driven and disk-driven spectroscopic\nfeatures are anti-correlated in time; the disk features became strong as the\nwind features disappeared. This anti-correlation might indicate that the\nrebuilding of the inner disk was interrupted by the wind pressure during the\nfirst two years. The Half-Width at Half-Depth (HWHD) of the double-peaked\nprofiles decreases with wavelength, indicative of the Keplerian rotation; the\noptical spectra with the disk feature are fitted by a G5 template stellar\nspectrum convolved with a rotation velocity of 70 km s$^{-1}$ while the near-IR\ndisk features are fitted by a K5 template stellar spectrum convolved with a\nrotation velocity of 50 km s$^{-1}$. Therefore, the optical and near-IR spectra\nseem to trace the disk at 39 and 76 $\\textit{R}_{\\odot}$, respectively. We fit\na power-law temperature distribution in the disk, finding an index of 0.8,\ncomparable to optically thick accretion disk models.",
        "category": "astro-ph_SR"
    },
    {
        "text": "OH maser toward IRAS 06056+2131: polarization parameters and evolution\n  status: We present high angular resolution observations of OH maser emission towards\nthe high-mass star forming region IRAS 06056+2131. The observations were\ncarried out using the UK radio interferometer array, Multi-Element Radio Linked\nInterferometer Network (MERLIN) in the OH main lines at 1665- and 1667-MHz, in\naddition to the OH satellite line at 1720-MHz. The results of this study\nrevealed emission in the 1665 MHz line with an estimated total intensity of\n$\\sim 4$ Jy. We did not detect any emission from the 1667-MHz and 1720-MHz\nlines.\n  The full polarization mode of MERLIN enables us to investigate the magnetic\nfield in the OH maser region. Our results show that IRAS 06056+2131 is a highly\ncircularly polarized source. In this transition, a Zeeman pair is identified\nfrom which a magnetic strength of $\\sim -1.5$ mG is inferred. The orientation\nof the linear polarization vectors suggests that the magnetic field lines at\nthe location of the OH maser emission \\textbf{might be} in agreement with the\norientation of the outflow thought to be associated with this source. The star\nforming evolutionary status of the embedded proto-stellar object is discussed.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Mass of the Compact Object in the X-Ray Binary Her X-1/HZ Her: We have obtained the first estimates of the masses of the components of the\nHer X-1/HZ Her X-ray binary system taking into account non-LTE effects in the\nformation of the H_gamma absorption line: mx=1.8Msun and mv=2.5Msun. These mass\nestimates were made in a Roche model based on the observed radial-velocity\ncurve of the optical star, HZ Her. The masses for the X-ray pulsar and optical\nstar obtained for an LTE model lie are mx=0.85\\pm0.15Msun and\nmv=1.87\\pm0.13Msun. These mass estimates for the components of Her X-1/HZ Her\nderived from the radial-velocity curve should be considered tentative. Further\nmass estimates from high-precision observations of the orbital variability of\nthe absorption profiles in a non-LTE model for the atmosphere of the optical\ncomponent should be made.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Chemical mixing in low mass stars: I. Rotation against atomic diffusion\n  including radiative accelerations: When modelling stars with masses larger than 1.2Msun with no observed\nchemical peculiarity, atomic diffusion is often neglected because, on its own,\nit causes unrealistic surface abundances compared with those observed. The\nreality is that atomic diffusion is in competition with other transport\nprocesses. The purpose of this study is to quantify the opposite or conjugated\neffects of atomic diffusion and rotationally induced mixing in stellar models\nof low mass stars. Our second goal is to estimate the impact of neglecting both\nrotational mixing and atomic diffusion in stellar parameter inferences for\nstars with masses larger than 1.3Msun. Using the AIMS code, we infer the masses\nand ages of a set of representative artificial stars for which models were\ncomputed with the CESTAM evolution code, taking into account rotationally\ninduced mixing and atomic diffusion, including radiative accelerations. We show\nthat for masses lower than 1.3Msun, rotation dominates the transport of\nchemical elements, and strongly reduces the effect of atomic diffusion, with\nnet surface abundance modifications similar to solar ones. At larger mass,\natomic diffusion and rotation are competing equally. Above 1.44Msun, atomic\ndiffusion dominates in stellar models with initial rotation smaller than\n80km.s-1 producing a chemical peculiarity which is not observed in\nKepler-legacy stars. This indicates that a transport process of chemical\nelements is missing. Importantly, neglecting rotation and atomic diffusion\n(including radiative accelerations) in the models, when inferring the\nparameters of F-type stars, may lead to errors of 5%, 2.5% and 25% respectively\nfor stellar masses, radii and ages. Atomic diffusion (including radiative\naccelerations) and rotational mixing should be taken into account in stellar\nmodels in order to determine accurate stellar parameters.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Correlation between the optical veiling and accretion properties: A case\n  study of the classical T Tauri star DK Tau: Classical T Tauri stars (cTTs) accrete from their circumstellar disk. The\nmaterial falls onto the stellar surface, producing an accretion shock, which\ngenerates veiling in a star's spectra. In addition, the shock causes a\nlocalized accretion spot at the level of the chromosphere. Our goal is to\ninvestigate the accretion, particularly the mass accretion rates (Macc), for\nthe cTTs DK Tau, over two periods of 17 and 29 days, using two different\nprocedures for comparison purposes. The first method relies on the derivation\nof the accretion luminosity via accretion-powered emission lines. The second\ncompares the variability of the optical veiling with accretion shock models to\ndetermine mass accretion rates. We used observations taken in 2010 and 2012\nwith the ESPaDOnS spectropolarimeter at the CFHT. We find peak values of the\nveiling (at 550 nm) ranging from 0.2 to 1.3, with a steeper trend across the\nwavelength range for higher peak values. When using the accretion-powered\nemission lines, we find mass accretion rate values ranging from\nlog(Macc[Msol/yr]) = -8.20 to log(Macc[Msol/yr]) = -7.40. This agrees with the\nvalues found in the literature, as well as the values calculated using the\naccretion shock models and the veiling. In addition, we identify a power-law\ncorrelation between the values of the accretion luminosity and the optical\nveiling. For the 2010 observations, using the values of the filling factors\n(which represent the area of the star covered by an accretion spot) derived\nfrom the shock models, we infer that the accretion spot was located between +45\ndegrees and +75 degrees in latitude. We show that both methods of determining\nthe mass accretion rate yield similar results. We also present a helpful means\nof confirming the accretion luminosity values by measuring the veiling at a\nsingle wavelength in the optical.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Coronal Polarization: We present an overview of the physical mechanisms responsible for the coronal\npolarization at different wavelength regimes. We also review different\ntechniques using coronal polarization to determine various quantities necessary\nfor understanding the thermodynamic properties of the solar coronal plasma.\nThis includes the coronal magnetic field, electronic densities, temperatures,\nvelocities, etc. The future needs to acquire better information on the solar\ncorona using polarization will be outlined.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The first stars: CEMP--no stars and signatures of spinstars: (Abridged) The CEMP--no stars are \"carbon-enhanced-metal-poor\" stars that in\nprinciple show no evidence of s-- and r--elements from neutron captures. We try\nto understand the origin and nucleosynthetic site of their peculiar CNO,\nNe--Na, and Mg--Al abundances. We compare the observed abundances to the\nnucleosynthetic predictions of AGB models and of models of rotating massive\nstars with internal mixing and mass loss. We also analyze the different\nbehaviors of $\\alpha$-- and CNO--elements, as well the abundances of elements\ninvolved in the Ne--Na and Mg--Al cycles. We show that CEMP-no stars exhibit\nproducts of He--burning that have gone through partial mixing and processing by\nthe CNO cycle, producing low $^{12}$C/$^{13}$C and a broad variety of [C/N] and\n[O/N] ratios. From a $^{12}$C/$^{13}$C vs. [C/N] diagram, we conclude that\nneither the yields of AGB stars (in binaries or not) nor the yields of classic\nsupernovae can fully account for the observed CNO abundances in CEMP-no stars.\nBetter agreement is obtained once the chemical contribution by stellar winds of\nfast-rotating massive stars is taken into account, where partial mixing takes\nplace, leading to various amounts of CNO being ejected. CEMP--no stars present\na wide variety in the [C/Fe], [N/Fe], [O/Fe], [Na/Fe], [Mg/Fe], [Al/Fe], and\n[Sr/Fe] ratios. We show that back-and-forth, partial mixing between the He--\nand H--regions may account for this variety. Some s--elements, mainly of the\nfirst peak, may even be produced by these processes in a small fraction of the\nCEMP--no stars. We propose a classification scheme for the CEMP--no and low--s\nstars, based on the changes in composition produced by these successive\nback-and-forth mixing motions.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Infrared Evolution of Dust in V838 Monocerotis: Luminous Red Variables (LRVs) are most likely eruptions that are the outcome\nof stellar mergers. V838 Mon is one of the best-studied members of this class,\nrepresenting an archetype for stellar mergers resulting from B-type stars. As\nresult of the merger event, nova-like eruptions occur driving mass-loss from\nthe system. As the gas cools considerable circumstellar dust is formed. V838\nMon erupted in 2002 and is undergoing very dynamic changes in its dust\ncomposition, geometry, and infrared luminosity providing a real-time laboratory\nto validate mineralogical condensation sequences in stellar mergers and\nevolutionary scenarios. We discuss recent NASA Stratospheric Observatory for\nInfrared Astronomy SOFIA 5 to 38 micron observations combined with archival\nNASA Spitzer spectra that document the temporal evolution of the freshly formed\n(within the last 20 yrs) circumstellar material in the environs of V838 Mon.\nChanges in the 10 micron spectral region are strong evidence that we are\nwitnessing a classical dust condensation sequence expected to occur in\noxygen-rich environments where alumina formation is followed by that of\nsilicates at the temperature cools.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Candidate Coronagraphic Detections of Protoplanetary Disks around Four\n  Young Stars: We present potential detections of H-band scattered light emission around\nfour young star, selected from a total sample of 45 young stars observed with\nthe CIAO coronagraph of the Subaru telescope. Two CTTS, CI Tau and DI Cep, and\ntwo WTTS, LkCa 14 and RXJ 0338.3+1020 were detected. In all four cases, the\nextended emission is within the area of the residual PSF halo, and is revealed\nonly through careful data reduction. We compare the observed extended emission\nwith simulations of the scattered light emission, to evaluate the plausibility\nand nature of the detected emission.",
        "category": "astro-ph_SR"
    },
    {
        "text": "An independent analysis of the brown dwarf atmosphere monitoring (BAM)\n  data: large-amplitude variability is rare outside the L/T transition: Observations of variability can provide valuable information about the\nprocesses of cloud formation and dissipation in brown dwarf atmospheres. Here\nwe report the results of an independent analysis of archival data from the\nBrown dwarf Atmosphere Monitoring (BAM) program. Time series data for 14 L and\nT dwarfs reported to be significantly variable over timescales of hours were\nanalyzed. We confirm large-amplitude variability (amplitudes $>$2\\%) for 4/13\ntargets and place upper limits of 0.7\\%-1.6\\% on variability in the remaining\nsample. For two targets we find evidence of weak variability at amplitudes of\n1.3\\% and 1.6\\%. Based on our revised classification of variable objects in the\nBAM study, we find strong variability outside the L/T transition to be rare at\nnear infrared wavelengths. From a combined sample of 81 L0-T9 dwarfs from the\nrevised BAM sample and the variability survey of Radigan et al. we infer an\noverall observed frequency for large-amplitude variability outside the L/T\ntransition of 3.2$_{-1.8}^{+2.8}$\\%, in contrast to 24$^{+11}_{-9}$\\% for\nL9-T3.5 spectral types. We conclude that while strong variability is not\nlimited to the L/T transition, it occurs more frequently in this spectral type\nrange, indicative of larger or more highly contrasting cloud features at these\nspectral types.",
        "category": "astro-ph_SR"
    },
    {
        "text": "H2O Maser Polarization of the Water Fountains IRAS 15445-5449 and IRAS\n  18043-2116: We present the morphology and linear polarization of the 22-GHz H2O masers in\nthe high-velocity outflow of two post-AGB sources, d46 (IRAS 15445-5449) and\nb292 (IRAS 18043-2116). The observations were performed using The Australia\nTelescope Compact Array. Different levels of saturated maser emission have been\ndetected for both sources. We also present the mid-infrared image of d46\noverlaid with the distribution of the maser features that we have observed in\nthe red-shifted lobe of the bipolar structure. The relative position of the\nobserved masers and a previous radio continuum observation suggests that the\ncontinnum is produced along the blue-shifted lobe of the jet. It is likely due\nto synchrontron radiation, implying the presence of a strong magnetic field in\nthe jet. The fractional polarization levels measured for the maser features of\nd46 indicate that the polarization vectors are tracing the poloidal component\nof the magnetic field in the emitting region. For the H2O masers of b292 we\nhave measured low levels of fractional linear polarization. The linear\npolarization in the H2O maser region of this source likely indicates a dominant\ntoroidal or poloidal magnetic field component. Since circular polarization was\nnot detected it is not possible to determine the magnetic field strength.\nHowever, we present a 3-sigma evaluation of the upper limit intensity of the\nmagnetic field in the maser emitting regions of both observed sources.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Basal magnetic flux and the local solar dynamo: The average unsigned magnetic flux density in magnetograms of the quiet Sun\nis generally dominated by instrumental noise. Due to the entirely different\nscaling behavior of the noise and the solar magnetic pattern it has been\npossible to determine the standard deviation of the Gaussian noise distribution\nand remove the noise contribution from the average unsigned flux density for\nthe whole 15-yr SOHO/MDI data set and for a selection of SDO/HMI magnetograms.\nThere is a very close correlation between the MDI disk-averaged unsigned\nvertical flux density and the sunspot number, and regression analysis gives a\nresidual level of 2.7 G when the sunspot number is zero. The selected set of\nHMI magnetograms, which spans the most quiet phase of solar activity, has a\nlower limit of 3.0 G to the noise-corrected average flux density. These\napparently cycle-independent levels may be identified as a basal flux density,\nwhich represents an upper limit to the possible flux contribution from a local\ndynamo, but not evidence for its existence. The 3.0 G HMI level, when scaled to\nthe Hinode spatial resolution, translates to 3.5 G, which means that the much\nhigher average flux densities always found by Hinode in quiet regions do not\noriginate from a local dynamo. The contributions to the average unsigned flux\ndensity come almost exclusively from the extended wings of the probability\ndensity function (PDF), also in the case of HMI magnetograms with only\nbasal-level magnetic flux. These wings represent intermittent magnetic flux.\nWhile the global dynamo appears to dominate the magnetic energy spectrum at all\nthe resolved spatial scales, there are indications from the observed Hanle\ndepolarization in atomic lines that the local dynamo may dominate the spectrum\nat scales of order 1-10 km and below.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Modelling convection in A star atmospheres. Bisectors and lineshapes of\n  HD108642: We present a code, VeDyn, for modelling envelopes and atmospheres of A to F\nstars focusing on accurate treatment of convective processes. VeDyn implements\nthe highly sophisticated convection model of Canuto and Dubovikov (1998) but is\nfast and handy enough to be used in practical astrophysical applications. We\ndeveloped the HME envelope solver for this convection model furtheron to\nconsistently model the envelope together with the stellar atmosphere. The\nsynthesis code SynthV was extended to account for the resulting velocity\nstructure. Finally, we tested our approach on atomic line bisectors. It is\nshown that our synthetic line bisectors of HD108642 bend towards the blue and\nare of a magnitude comparable to the observed ones. Even though this approach\nof modelling convection requires the solution of a coupled system of nonlinear\ndifferential equations, it is fast enough to be applicable to many of the\ninvestigation techniques relying on model atmospheres.",
        "category": "astro-ph_SR"
    },
    {
        "text": "M dwarf luminosity, radius, and $\u03b1$-enrichment from $I$-band\n  spectral features: Despite the ubiquity of M dwarfs and their growing importance to studies of\nexoplanets, Galactic evolution, and stellar structure, methods for precisely\nmeasuring their fundamental stellar properties remain elusive. Existing\ntechniques for measuring M dwarf luminosity, mass, radius, or composition are\ncalibrated over a limited range of stellar parameters or require expensive\nobservations. We find a strong correlation between the $K_S$-band luminosity\n($M_K$), the observed strength of the $I$-band sodium doublet absorption\nfeature, and [Fe/H] in M dwarfs without strong H$\\alpha$ emission. We show that\nthe strength of this feature, coupled with [Fe/H] and spectral type, can be\nused to derive M dwarf $M_K$ and radius without requiring parallax.\nAdditionally, we find promising evidence that the strengths of the $I$-band\nsodium doublet and the nearby $I$-band calcium triplet may jointly indicate\n$\\alpha$-element enrichment. The use of these $I$-band features requires only\nmoderate-resolution near-infrared spectroscopy to provide valuable information\nabout the potential habitability of exoplanets around M dwarfs, and surface\ngravity and distance for M dwarfs throughout the Galaxy. This technique has\nimmediate applicability for both target selection and candidate planet host\nsystem characterization for exoplanet missions such as \\textit{TESS} and\n\\textit{K2}.",
        "category": "astro-ph_SR"
    },
    {
        "text": "New Low-Mass Stars in the 25 Orionis Stellar Group and Orion OB1a\n  Sub-association from SDSS-III/BOSS Spectroscopy: The Orion OB1a sub-association is a rich low mass star (LMS) region. Previous\nspectroscopic studies have confirmed 160 LMSs in the 25 Orionis stellar group\n(25 Ori), which is the most prominent overdensity of Orion OB1a. Nonetheless,\nthe current census of the 25 Ori members is estimated to be less than 50%\ncomplete, leaving a large number of members to be still confirmed. We retrieved\n172 low-resolution stellar spectra in Orion OB1a observed as ancillary science\nin the SDSS-III/BOSS survey, for which we classified their spectral types and\ndetermined physical parameters. To determine memberships, we analyzed the\nH$_\\alpha$ emission, LiI$\\lambda$6708 absorption, and NaI$\\lambda\\lambda$8183,\n8195 absorption as youth indicators in stars classified as M-type. We report 50\nnew LMSs spread across the 25 Orionis, ASCC 18, and ASCC 20 stellar groups with\nspectral types from M0 to M6, corresponding to a mass range of 0.10$\\le\nm/\\textrm{M}_\\odot \\le$0.58. This represents an increase of 50% in the number\nof known LMSs in the area and a net increase of 20% in the number of 25 Ori\nmembers in this mass range. Using parallax values from the Gaia DR1 catalog, we\nestimated the distances to these three stellar groups and found that they are\nall co-distant, at 338$\\pm$66 pc. We analyzed the spectral energy distributions\nof these LMSs and classified their disks by evolutionary classes. Using H-R\ndiagrams, we found a suggestion that 25 Ori could be slightly older that the\nother two observed groups in Orion OB1a.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Granulation \"Flicker\"-based Measure of Stellar Surface Gravity: In Bastien et al. (2013) we found that high quality light curves, such as\nthose obtained by Kepler, may be used to measure stellar surface gravity via\ngranulation-driven light curve \"flicker\". Here, we update and extend the\nrelation originally presented in Bastien et al. (2013) after calibrating\nflicker against a more robust set of asteroseismically derived surface\ngravities. We describe in detail how we extract the flicker signal from the\nlight curves, including how we treat phenomena, such as exoplanet transits and\nshot noise, that adversely affect the measurement of flicker. We examine the\nlimitations of the technique, and, as a result, we now provide an updated\ntreatment of the flicker-based logg error. We briefly highlight further\napplications of the technique, such as astrodensity profiling or its use in\nother types of stars with convective outer layers. We discuss potential uses in\ncurrent and upcoming space-based photometric missions. Finally, we supply\nflicker-based logg values, and their uncertainties, for 27 628 Kepler stars not\nidentified as transiting-planet hosts, with 4500<teff<7150 K, 2.5<logg<4.6,\nKepler magnitude <13.5, and overall photometric amplitudes <10 parts per\nthousand.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Recipe for inferring sub-surface solar magnetism via local mode-coupling\n  using Slepian basis functions: Direct seismic imaging of sub-surface flow, sound-speed and magnetic field is\ncrucial for predicting flux tube emergence on the solar surface, an important\ningredient for space weather. The sensitivity of helioseismic mode-amplitude\ncross-correlation to $p$- and $f$-mode oscillations enable formal inversion of\nsuch sub-photospheric perturbations. It is well-known that such problems are\nwritten in the form of an integral equation that connects the perturbations to\nthe observations via ``sensitivity kernels\". While the sensitivity kernels for\nflow and sound-speed have been known for decades and have been used\nextensively, formulating kernels for general magnetic perturbations had been\nelusive. A recent study proposed sensitivity kernels for Lorentz-stresses\ncorresponding to global magnetic fields of general geometry. The present study\nis devoted to proposing kernels for inferring Lorentz-stresses as well as the\nsolenoidal magnetic field in a local patch on the Sun via Cartesian\nmode-coupling. Moreover, for the first time in solar physics, Slepian functions\nare employed to parameterize perturbations in the horizontal dimension. This is\nshown to increase the number of data constraints in the inverse problem,\nimplying an increase in the precision of inferred parameters. This paves the\npath to reliably imaging sub-surface solar magnetic features in, e.g.,\nsupergranules, sunspots and (emerging) active regions.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Quiet-Sun intensity contrasts in the near ultraviolet: We present high-resolution images of the Sun in the near ultraviolet spectral\nrange between 214 nm and 397 nm as obtained from the first science flight of\nthe 1-m Sunrise balloon-borne solar telescope. The quiet-Sun rms intensity\ncontrasts found in this wavelength range are among the highest values ever\nobtained for quiet-Sun solar surface structures - up to 32.8% at a wavelength\nof 214 nm. We compare with theoretical intensity contrasts obtained from\nnumerical magneto-hydrodynamic simulations. For 388 nm and 312 nm the\nobservations agree well with the numerical simulations whereas at shorter\nwavelengths discrepancies between observed and simulated contrasts remain.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Observational and model analysis of a two-ribbon flare possibly induced\n  by a neighbouring blowout jet: In this paper, we present unique observations of a blowout coronal jet that\npossibly triggered a two-ribbon confined C1.2 flare in a bipolar solar active\nregion NOAA 12615 on 2016 December 5. The jet activity initiates at\nchromospheric/transition-region heights with a small brightening that\neventually grows in a larger volume with well developed standard morphological\njet features, viz., base and spire. The spire widens up with a collimated\neruption of cool and hot plasma components, observed in the 304 and 94 A\nchannels of AIA, respectively. The speed of the plasma ejection, which forms\nthe jet's spire, was higher for the hot component (~200 km/s) than the cooler\none (~130 km/s). The NLFF model of coronal fields at pre- and post-jet phases\nsuccessfully reveal opening of previously closed magnetic field lines with a\nrather inclined/low-lying jet structure. The peak phase of the jet emission is\nfollowed by the development of a two-ribbon flare that shows coronal loop\nemission in HXRs up to ~25 keV energy. The coronal magnetic fields rooted at\nthe location of EUV flare ribbons, derived from the NLFF model, demonstrate the\npre-flare phase to exhibit an \"X-type\" configuration while the magnetic fields\nat the post-flare phase are more or less parallel oriented. The comparisons of\nmulti-wavelength measurements with the magnetic field extrapolations suggest\nthat the jet activity likely triggered the two-ribbon flare by perturbing the\nfield in the interior of the active region.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Gaia-ESO Survey: Carbon abundance in the Galactic thin and thick\n  disks: This paper focuses on carbon that is one of the most abundant elements in the\nUniverse and is of high importance in the field of nucleosynthesis and galactic\nand stellar evolution. Even nowadays, the origin of carbon and the relative\nimportance of massive and low- to intermediate-mass stars in producing it is\nstill a matter of debate. In this paper we aim at better understanding the\norigin of carbon by studying the trends of [C/H], [C/Fe],and [C/Mg] versus\n[Fe/H], and [Mg/H] for 2133 FGK dwarf stars from the fifth Gaia-ESO Survey\ninternal data release (GES iDR5). The availability of accurate parallaxes and\nproper motions from Gaia DR2 and radial velocities from GES iDR5 allows us to\ncompute Galactic velocities, orbits and absolute magnitudes and, for 1751\nstars, ages via a Bayesian approach. Three different selection methodologies\nhave been adopted to discriminate between thin and thick disk stars. In all the\ncases, the two stellar groups show different abundance ratios, [C/H], [C/Fe],\nand [C/Mg], and span different age intervals, with the thick disk stars being,\non average, older than those in the thin disk. The behaviours of [C/H], [C/Fe],\nand [C/Mg] versus [Fe/H], [Mg/H], and age all suggest that C is primarily\nproduced in massive stars like Mg. The increase of [C/Mg] for young thin disk\nstars indicates a contribution from low-mass stars or the increased C\nproduction from massive stars at high metallicities due to the enhanced mass\nloss. The analysis of the orbital parameters Rmed and |Zmax| support an\n\"inside-out\" and \"upside-down\" formation scenario for the disks of Milky Way.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Proton and electron temperatures in the solar wind and their\n  correlations with the solar wind speed: The heating and acceleration of the solar wind remains one of the fundamental\nunsolved problems in heliophysics. It is usually observed that the proton\ntemperature $T_i$ is highly correlated with the solar wind speed $V_{SW}$,\nwhile the electron temperature $T_e$ shows anti-correlation or no clear\ncorrelation with the solar wind speed. Here we inspect both Parker Solar Probe\n(PSP) and WIND data and compare the observations with simulation results. PSP\nobservations below 30 solar radii clearly show a positive correlation between\nproton temperature and wind speed and a negative correlation between electron\ntemperature and wind speed. One year (2019) of WIND data confirm that proton\ntemperature is positively correlated with solar wind speed, but the electron\ntemperature increases with the solar wind speed for slow wind while it\ndecreases with the solar wind speed for fast wind. Using a one-dimensional\nAlfv\\'en-wave-driven solar wind model with different proton and electron\ntemperatures, we for the first time find that if most of the dissipated\nAlfv\\'en wave energy heats the ions instead of electrons, a positive\n$T_i-V_{SW}$ correlation and a negative $T_e-V_{SW}$ correlation arise\nnaturally. If the electrons gain a small but finite portion of the dissipated\nwave energy, the $T_e-V_{SW}$ correlation evolves with radial distance to the\nSun such that the negative correlation gradually turns positive. The model\nresults show that Alfv\\'en waves are one of the possible explanations of the\nobserved evolution of proton and electron temperatures in the solar wind.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Exploring B4: A Pulsating sdB star, in a Binary, in the Open Cluster NGC\n  6791: We report on Kepler photometry of the hot sdB star B4 in the open cluster NGC\n6791. We confirm that B4 is a reflection effect binary with an sdB component\nand a low-mass main sequence companion with a circular 0.3985 d orbit. The sdB\nstar is a g-mode pulsator (a V1093 Her star) with periods ranging from 2384 s\nto 7643 s. Several of the pulsation modes show symmetric splitting by 0.62\nmicroHz. Attributing this to rotational splitting, we conclude that the sdB\ncomponent has a rotation period of approximately 9.63 d, indicating that tidal\nsynchronization has not been achieved in this system. Comparison with\ntheoretical synchronization time provides a discriminant between various\ntheoretical models.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Outbursts and stellar properties of the classical Be star HD 6226: The bright and understudied classical Be star HD 6226 has exhibited multiple\noutbursts in the last several years during which the star grew a viscous\ndecretion disk. We analyze 659 optical spectra of the system collected from\n2017-2020, along with a UV spectrum from the Hubble Space Telescope and high\ncadence photometry from both TESS and the KELT survey. We find that the star\nhas a spectral type of B2.5IIIe, with a rotation rate of 74% of critical. The\nstar is nearly pole-on with an inclination of $13.4$ degree. We confirm the\nspectroscopic pulsational properties previously reported, and report on three\nphotometric oscillations from KELT photometry. The outbursting behavior is\nstudied with equivalent width measurements of H$\\alpha$ and H$\\beta$, and the\nvariations in both of these can be quantitatively explained with two\nfrequencies through a Fourier analysis. One of the frequencies for the emission\noutbursts is equal to the difference between two photometric oscillations,\nlinking these pulsation modes to the mass ejection mechanism for some\noutbursts. During the TESS observation time period of 2019 October 7 to 2019\nNovember 2, the star was building a disk. With a large dataset of H$\\alpha$ and\nH$\\beta$ spectroscopy, we are able to determine the timescales of dissipation\nin both of these lines, similar to past work on Be stars that has been done\nwith optical photometry. HD 6226 is an ideal target with which to study the Be\ndisk-evolution given its apparent periodic nature, allowing for targeted\nobservations with other facilities in the future.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Correlation between lithium abundances and ages of solar twin stars: We want to determine the lithium abundances of solar twin stars as a function\nof stellar age to provide constraints for stellar evolutions models and to\ninvestigate whether there is a connection between low Li abundance and the\noccurrence of planets. For a sample of 21 solar twins observed with the HARPS\nspectrograph at high spectral resolution (R~115.000) and very high\nsignal-to-noise ratio (600 < S/N < 2400), precise lithium abundances were\nobtained by spectral synthesis of the LiI 6707.8 A line and compared to stellar\nages, masses, and metallicities determined from a spectroscopic analysis of the\nsame set of HARPS spectra. We show that for the large majority of the solar\ntwins there is a strong correlation between lithium abundance and stellar age.\nAs the age increases from 1 to 9 Gyr, the Li abundance decreases by a factor of\n~ 50. The relation agrees fairly well with predictions from non-standard\nstellar evolution models of Li destruction at the bottom of the upper\nconvection zone. Two stars deviate from the relation by having Li abundances\nenhanced by a factor of ~ 10, which may be due to planet engulfment. On the\nother hand, we find no indication of a link between planet hosting stars and\nenhanced lithium depletion.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Simultaneous X-ray and Radio Observations of Rotating Radio Transient\n  J1819-1458: We present the results of simultaneous radio and X-ray observations of PSR\nJ1819-1458. Our 94-ks XMM-Newton observation of the high magnetic field 5*10^13\nG pulsar reveals a blackbody spectrum (kT~130 eV) with a broad absorption\nfeature, possibly composed of two lines at ~1.0 and ~1.3 keV. We performed a\ncorrelation analysis of the X-ray photons with radio pulses detected in 16.2\nhours of simultaneous observations at 1-2 GHz with the Green Bank, Effelsberg,\nand Parkes telescopes, respectively. Both the detected X-ray photons and radio\npulses appear to be randomly distributed in time. We find tentative evidence\nfor a correlation between the detected radio pulses and X-ray photons on\ntimescales of less than 10 pulsar spin periods, with the probability of this\noccurring by chance being 0.46%. This suggests that the physical process\nproducing the radio pulses may also heat the polar-cap.",
        "category": "astro-ph_SR"
    },
    {
        "text": "V405 Peg (RBS 1955): A Nearby, Low-Luminosity Cataclysmic Binary: (Abridged). The cataclysmic binary V405 Peg, originally discovered as ROSAT\nBright Source (RBS) 1955 (= 1RXS J230949.6+213523), shows a strong contribution\nfrom a late-type secondary star in its optical spectrum, which led Schwope et\nal. to suggest it to be among the nearest cataclysmic binaries. We present\nextensive optical observations of V405 Peg. Time-series spectroscopy shows the\norbital period, Porb, to be 0.1776469(7) d (= 4.2635 hr), or 5.629 cycle/d. We\nclassify the secondary as M3 - M4.5. Astrometry with the MDM 2.4m telescope\ngives a parallax 7.2 +- 1.1 milli-arcsec, and a relative proper motion of 58\nmas/yr. Our best estimate of the distance yields d = 149 (+26, -20) pc. The\nsecondary stars's radial velocity has K2 = 92 +- 3 km/s, indicating a fairly\nlow orbital inclination if the masses are typical. Extensive I-band time-series\nobservations in the show the system varying between a minimum brightness level\nof I = 14.14 and states of enhanced activity about 0.2 mag brighter. While the\nlow-state shows an ellipsoidal modulation, an additional photometric modulation\nappears in the high state, with 0.1 mag amplitude and period 220-280 min. The\nfrequency of this modulation appears to be stable for a month or so, but no\nsingle period was consistently detected from one observing season to the next.\nWe estimate the system luminosity by combining optical measurements with the\narchival X-ray spectrum. The implied mass accretion rate is orders of\nmagnitudes below the predictions for the standard angular momentum loss above\nthe period gap. The system may possibly belong to a largely undiscovered\npopulation of hibernating CVs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Warm Breeze from the starboard bow: a new population of neutral helium\n  in the heliosphere: We investigate the signals from neutral He atoms observed from Earth orbit in\n2010 by IBEX. The full He signal observed during the 2010 observation season\ncan be explained as a superposition of pristine neutral interstellar He gas and\nan additional population of neutral He that we call the Warm Breeze. The Warm\nBreeze is approximately two-fold slower and 2.5 times warmer than the primary\ninterstellar He population, and its density in front of the heliosphere is ~7%\nthat of the neutral interstellar helium. The inflow direction of the Warm\nBreeze differs by ~19deg from the inflow direction of interstellar gas. The\nWarm Breeze seems a long-term feature of the heliospheric environment. It has\nnot been detected earlier because it is strongly ionized inside the\nheliosphere, which brings it below the threshold of detection via pickup ion\nand heliospheric backscatter glow observations, as well as by the direct\nsampling of GAS/Ulysses. Possible sources for the Warm Breeze include (1) the\nsecondary population of interstellar helium, created via charge exchange and\nperhaps elastic scattering of neutral interstellar He atoms on interstellar He+\nions in the outer heliosheath, or (2) a gust of interstellar He originating\nfrom a hypothetic wave train in the Local Interstellar Cloud. A secondary\npopulation is expected from models, but the characteristics of the Warm Breeze\ndo not fully conform to modeling results. If, nevertheless, this is the\nexplanation, IBEX-Lo observations of the Warm Breeze provide key insights into\nthe physical state of plasma in the outer heliosheath. If the second hypothesis\nis true, the source is likely to be located within a few thousand of AU from\nthe Sun, which is the propagation range of possible gusts of interstellar\nneutral helium with the Warm Breeze characteristics against dissipation via\nelastic scattering in the Local Cloud.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Active region upflows: 1. Multi-instrument observations: Upflows at the edges of active regions (ARs) are studied by spatially and\ntemporally combining multi-instrument observations obtained with EIS/Hinode,\nAIA and HMI/SDO and IBIS/NSO, to derive their plasma parameters. This\ninformation is used for benchmarking data-driven modelling of the upflows\n(Galsgaard et al., 2015). The studied AR upflow displays blueshifted emission\nof 5-20 km/s in Fe XII and Fe XIII and its average electron density is 1.8x10^9\ncm^3 at 1 MK. The time variation of the density shows no significant change (in\na 3sigma error). The plasma density along a single loop drops by 50% over a\ndistance of 20000 km. We find a second velocity component in the blue wing of\nthe Fe XII and Fe XIII lines at 105 km/s. This component is persistent during\nthe whole observing period of 3.5 hours with variations of only 15 km/s. We\nalso study the evolution of the photospheric magnetic field and find that\nmagnetic flux diffusion is responsible for the formation of the upflow region.\nHigh cadence Halpha observations of the chromosphere at the footpoints of the\nupflow region show no significant jet-like (spicule/rapid blue excursion)\nactivity to account for several hours/days of plasma upflow. Using an image\nenhancement technique, we show that the coronal structures seen in the AIA 193A\nchannel is comparable to the EIS Fe XII images, while images in the AIA 171A\nchannel reveals additional loops that are a result of contribution from cooler\nemission to this channel. Our results suggest that at chromospheric heights\nthere are no signatures that support the possible contribution of spicules to\nAR upflows. We suggest that magnetic flux diffusion is responsible for the\nformation of the coronal upflows. The existence of two velocity components\npossibly indicate the presence of two different flows which are produced by two\ndifferent physical mechanisms, e.g. magnetic reconnection and pressure-driven.",
        "category": "astro-ph_SR"
    },
    {
        "text": "HD66051: the first eclipsing binary hosting an early-type magnetic star: Early-type magnetic stars are rarely found in close binary systems. No such\nobjects were known in eclipsing binaries prior to this study. Here we\ninvestigated the eclipsing, spectroscopic double-lined binary HD66051, which\nexhibits out-of-eclipse photometric variations suggestive of surface brightness\ninhomogeneities typical of early-type magnetic stars. Using a new set of\nhigh-resolution spectropolarimetric observations, we discovered a weak magnetic\nfield on the primary and found intrinsic, element-dependent variability in its\nspectral lines. The magnetic field structure of the primary is dominated by a\nnearly axisymmetric dipolar component with a polar field strength $B_{\\rm\nd}\\approx600$ G and an inclination with respect to the rotation axis of\n$\\beta_{\\rm d}=13^{\\rm o}$. A weaker quadrupolar component is also likely to be\npresent. We combined the radial velocity measurements derived from our spectra\nwith archival optical photometry to determine fundamental masses (3.16 and 1.75\n$M_\\odot$) and radii (2.78 and 1.39 $R_\\odot$) with a 1-3% precision. We also\nobtained a refined estimate of the effective temperatures (13000 and 9000 K)\nand studied chemical abundances for both components with the help of\ndisentangled spectra. We demonstrate that the primary component of HD66051 is a\ntypical late-B magnetic chemically peculiar star with a non-uniform surface\nchemical abundance distribution. It is not an HgMn-type star as suggested by\nrecent studies. The secondary is a metallic-line star showing neither a strong,\nglobal magnetic field nor intrinsic spectral variability. Fundamental\nparameters provided by our work for this interesting system open unique\npossibilities for probing interior structure, studying atomic diffusion, and\nconstraining binary star evolution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Direct MD simulation of liquid-solid phase equilibria for two-component\n  plasmas: We determine the liquid-solid phase diagram for carbon-oxygen and\noxygen-selenium plasma mixtures using two-phase MD simulations. We identified\nliquid, solid, and interface regions using a bond angle metric. To study finite\nsize effects, we perform 27648 and 55296 ion simulations. To help monitor\nnon-equilibrium effects, we calculate diffusion constants $D_i$. For the\ncarbon-oxygen system we find that $D_O$ for oxygen ions in the solid is much\nsmaller than $D_C$ for carbon ions and that both diffusion constants are 80 or\nmore times smaller than diffusion constants in the liquid phase. There is\nexcellent agreement between our carbon-oxygen phase diagram and that predicted\nby Medin and Cumming. This suggests that errors from finite size and\nnon-equilibrium effects are small and that the carbon-oxygen phase diagram is\nnow accurately known. The oxygen-selenium system is a simple two-component\nmodel for more complex rapid proton capture nucleosynthesis ash compositions\nfor an accreting neutron star. Diffusion of oxygen, in a predominately selenium\ncrystal, is remarkably fast, comparable to diffusion in the liquid phase. We\nfind a somewhat lower melting temperature for the oxygen-selenium system than\nthat predicted by Medin and Cumming. This is probably because of electron\nscreening effects.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the role of tachoclines in solar and stellar dynamos: Rotational shear layers at the boundary between radiative and convective\nzones, tachoclines, play a key role in the process of magnetic field generation\nin solar-like stars. We present two sets of global simulations of rotating\nturbulent convection and dynamo. The first set considers a stellar convective\nenvelope only; the second one, aiming at the formation of a tachocline,\nconsiders also the upper part of the radiative zone. Our results indicate that\nthe resulting mean-flows and dynamo properties like the growth rate, saturation\nenergy and mode depend on the Rossby (Ro) number. For the first set of models\neither oscillatory (with ~2 yr period) or steady dynamo solutions are obtained.\nThe models in the second set naturally develop a tachocline which, in turn,\nleads to the generation of strong mean magnetic field. Since the field is also\ndeposited into the stable deeper layer, its evolutionary time-scale is much\nlonger than in the models without a tachocline. Surprisingly, the magnetic\nfield in the upper turbulent convection zone evolves in the same time scale as\nthe deep field. These models result in either an oscillatory dynamo with ~30 yr\nperiod or in a steady dynamo depending on Ro. In terms of the mean-field dynamo\ncoefficients computed using FOSA, the field evolution in the oscillatory models\nwithout a tachocline seems to be consistent with dynamo waves propagating\naccording to the Parker-Yoshimura sign rule. In the models with tachoclines the\ndynamics is more complex involving other transport mechanisms as well as\ntachocline instabilities.",
        "category": "astro-ph_SR"
    },
    {
        "text": "No evidence of chemical anomalies in the bimodal turnoff cluster NGC\n  1806 in the LMC: We have studied the chemical composition of NGC 1806, a massive,\nintermediate-age globular cluster that shows a double main sequence turnoff. We\nanalyzed a sample of high-resolution spectra (secured with FLAMES at the Very\nLarge Telescope) for 8 giant stars, members of the cluster, finding an average\niron content of [Fe/H]=--0.60 +- 0.01 dex and no evidence of intrinsic\nstar-to-star variations in the abundances of light elements (Na, O, Mg, Al).\nAlso, the (m_(F814W); m_(F336W)-m_(F814W)) color-magnitude diagram obtained by\ncombining optical and near-UV Hubble Space Telescope photometry exhibits a\nnarrow red giant branch, thus ruling out intrinsic variations of C and N\nabundances in the cluster. These findings demonstrate that NGC 1806 does not\nharbor chemically distinct sub-populations, at variance with what was found in\nold globular clusters. In turn, this indicates that the double main sequence\nturnoff phenomenon cannot be explained in the context of the self-enrichment\nprocesses usually invoked to explain the chemical anomalies observed in old\nglobulars. Other solutions (i.e., stellar rotation, merging between clusters or\ncollisions with giant molecular clouds) should be envisaged to explain this\nclass of globulars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Rapidly varying A-type stars in the SuperWASP archive: The searches for transiting exoplanets have produced a vast amount of\ntime-resolved photometric data of many millions of stars. One of the leading\nground-based surveys is the SuperWASP project. We present the initial results\nof a survey of over 1.5 million A-type stars in the search for high frequency\npulsations using SuperWASP photometry. We are able to detect pulsations down to\nthe 0.5 mmag level in the broad-band photometry. This has enabled the discovery\nof several rapidly oscillating Ap stars and over 200 delta Scuti stars with\nfrequencies above 50 d$^{-1}$, and at least one pulsating sdB star. Such a\nlarge number of results allows us to statistically study the frequency overlap\nbetween roAp and delta Scuti stars and probe to higher frequency regimes with\nexisting data.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A census of Coronal Mass Ejections on solar-like stars: Coronal Mass Ejections (CMEs) may have major importance for planetary and\nstellar evolution. Stellar CME parameters, such as mass and velocity, have yet\nnot been determined statistically. So far only a handful of stellar CMEs has\nbeen detected mainly on dMe stars using spectroscopic observations. We\ntherefore aim for a statistical determination of CMEs of solar-like stars by\nusing spectroscopic data from the ESO phase 3 and Polarbase archives. To\nidentify stellar CMEs we use the Doppler signal in optical spectral lines being\na signature of erupting filaments which are closely correlated to CMEs. We\ninvestigate more than 3700 hours of on-source time of in total 425 dF-dK stars.\nWe find no signatures of CMEs and only few flares. To explain this low level of\nactivity we derive upper limits for the non detections of CMEs and compare\nthose with empirically modelled CME rates. To explain the low number of\ndetected flares we adapt a flare power law derived from EUV data to the\nH{\\alpha} regime, yielding more realistic results for H{\\alpha} observations.\nIn addition we examine the detectability of flares from the stars by extracting\nSun-as-a-star H{\\alpha} light curves. The extrapolated maximum numbers of\nobservable CMEs are below the observationally determined upper limits, which\nindicates that the on-source times were mostly too short to detect stellar CMEs\nin H{\\alpha}. We conclude that these non detections are related to\nobservational biases in conjunction with a low level of activity of the\ninvestigated dF-dK stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Shock-Cloud Interaction in the Solar Corona: Flare associated coronal shock waves sometimes interact with solar\nprominences leading to large amplitude prominence oscillations. Such prominence\nactivation gives us unique opportunity to track time evolution of shock-cloud\ninteraction in cosmic plasmas. Although the dynamics of interstellar\nshock-cloud interaction is extensively studied, coronal shock-solar prominence\ninteraction is rarely studied in the context of shock-cloud interaction.\nAssociated with X5.4 class solar flare occurred on 7 March, 2012, a globally\npropagated coronal shock wave interacted with a polar prominence leading to\nlarge amplitude prominence oscillation. In this paper, we studied bulk\nacceleration and excitation of internal flow of the shocked prominence using\nthree-dimensional MHD simulations. We studied eight magnetohydrodynamic (MHD)\nsimulation runs with different mass density structure of the prominence, and\none hydrodynamic simulation run, and compared the result. In order to compare\nobserved motion of activated prominence with corresponding simulation, we also\nstudied prominence activation by injection of triangular shaped coronal shock.\nWe found that magnetic tension force mainly accelerate (and then decelerate)\nthe prominence. The internal flow, on the other hand, is excited during the\nshock front sweeps through the the prominence and damps almost exponentially.\nWe construct phenomenological model of bulk momentum transfer from shock to the\nprominence, which agreed quantitatively with all the simulation results. Based\non the phenomenological prominence-activation model, we diagnosed physical\nparameters of coronal shock wave. The estimated energy of the coronal shock is\nseveral percent of total energy released during the X5.4 flare.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Pair-Instability Mass Gap for Black Holes: Stellar evolution theory predicts a \"gap\" in the black hole birth function\ncaused by the pair instability. Presupernova stars that have a core mass below\nsome limiting value, Mlo, after all pulsational activity is finished, collapse\nto black holes, whereas more massive ones, up to some limiting value, Mhi,\nexplode, promptly and completely, as pair-instability supernovae. Previous work\nhas suggested Mlo is approximately 50 solar masses and Mhi is approximately 130\nsolar masses. These calculations have been challenged by recent LIGO\nobservations that show many black holes merging with individual masses, Mlo is\nleast some 65 solar masses. Here we explore four factors affecting the\ntheoretical estimates for the boundaries of this mass gap: nuclear reaction\nrates, evolution in detached binaries, rotation, and hyper-Eddington accretion\nafter black hole birth. Current uncertainties in reaction rates by themselves\nallow Mlo to rise to 64 solar masses and Mhi as large as 161 solar masses.\nRapid rotation could further increase Mlo to about 70 solar masses, depending\non the treatment of magnetic torques. Evolution in detached binaries and\nsuper-Eddington accretion can, with great uncertainty, increase Mlo still\nfurther. Dimensionless Kerr parameters close to unity are allowed for the more\nmassive black holes produced in close binaries, though they are generally\nsmaller.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Coevality in Young Eclipsing Binaries: The ages of the components in very short period pre-main sequence (PMS)\nbinaries are essential to an understanding of their formation. We considered a\nsample of 7 PMS eclipsing binaries (EBs) with ages 1 to 6.3 MY and component\nmasses 0.2 to 1.4 Msun The very high precision with which their masses and\nradii have been measured, and the capability provided by the {\\it Modules for\nExperiments in Stellar Astrophysics (MESA)} to calculate their evolutionary\ntracks at exactly the measured masses, allows the determination of age\ndifferences of the components independent of their luminosities and effective\ntemperatures. We found that the components of 5 EBs, ASAS J052821+0338.5,\nParenago 1802, JW 380, CoRoT 223992193, and UScoCTIO 5, formed within 0.3 MY of\neach other. The parameters for the components of V1174 Ori, imply an\nimplausible large age difference of 2.7 MY and should be reconsidered. The 7th\nEB in our sample, RX J0529.4+0041 fell outside the applicability of our\nanalysis.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Identification, mass and age of primary red clump stars from spectral\n  features derived with the LAMOST DR7: Although red clump (RC) stars are easy to identify due to their stability of\nluminosity and color, about 20-50% are actually red giant branch (RGB) stars in\nthe same location on the HR diagram. In this paper, a sample of 210,504 spectra\nfor 184 318 primary RC (PRC) stars from the LAMOST DR7 is identified, which has\na purity of higher than 90 percent. The RC and the RGB stars are successfully\ndistinguished through LAMOST spectra(R~1800 and SNR>10) by adopting the XGBoost\nensemble learning algorithm, and the secondary RC stars are also removed. The\nSHapley Additive exPlanations (SHAP) value is used to explain the top features\nthat the XGBoost model selected. The features are around Fe5270, MgH & MgIb,\nFe4957, Fe4207, Cr5208, and CN, which can successfully distinguish RGB and RC\nstars. The XGBoost is also used to estimate the ages and masses of PRC stars by\ntraining their spectra with Kepler labeled asteroseismic parameters. The\nuncertainties of mass and age are 13 and 31 percent, respectively. Verifying\nthe feature attribution model, we find the age-sensitive elements XGBoost gets\nare consistent with the literature. Distances of the PRC stars are derived by\n$K_{S}$ absolute magnitude calibrated by Gaia EDR3, which has an uncertainty of\nabout 6 percent and shows the stars mainly locate at the Galactic disk. We also\ntest the XGBoost with R$\\sim$250, which is the resolution of the Chinese Space\nStation Telescope(CSST) under construction, it is still capable of finding\nsensitive features to distinguish RC and RGB.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The low-mass stellar mass functions of rich, compact clusters in the\n  Large Magellanic Cloud: Context. We use Hubble Space Telescope photometry of six rich, compact star\nclusters in the Large Magellanic Cloud (LMC), with ages ranging from 0.01 to\n1.0 Gyr, to derive the clusters' stellar mass functions (MFs) at their\nhalf-mass radii.\n  Aims. The LMC is an ideal environment to study stellar MFs, because it\ncontains a large population of compact clusters at different evolutionary\nstages. We aim to obtain constraints on the initial MFs (IMFs) of our sample\nclusters on the basis of their present-day MFs, combined with our understanding\nof their dynamical and photometric evolution.\n  Methods. We derive the clusters' present-day MFs below 1.0 Msun using deep\nobservations with the Space Telescope Imaging Spectrograph and updated stellar\npopulation synthesis models.\n  Results. Since the relaxation timescales of low-mass stars are very long,\ndynamical evolution will not have affected the MFs below 1.0 Msun\nsignificantly, so that - within the uncertainties - the derived MFs are\nconsistent with the solar-neighbourhood IMF, at least for the younger clusters.\n  Conclusions. The IMF in the low-density, low-metallicity environment of the\nLMC disk is not significantly different from that in the solar neighbourhood.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the Lira law and the nature of extinction towards Type Ia Supernovae: We have studied the relation between the color evolution of Type Ia\nSupernovae (SNe Ia) from maximum light to the Lira law regime and the presence\nof narrow absorption features. Based on a nearby sample of 89 SNe Ia we have\nfound that the rate of change of B-V colors at late phases (between 35 and 80\ndays after maximum) varies significantly among different SNe Ia. At maximum\nlight, faster Lira law B-V decliners have significantly higher equivalent\nwidths of blended Na I D1 & D2 narrow absorption lines, redder colors and lower\nR_V reddening laws. We do not find faster Lira law B-V decliners to have a\nstrong preference for younger galaxy environments, where higher interstellar\nmaterial (ISM) column densities would be expected. We interpret these results\nas evidence for the presence of circumstellar material (CSM). The differences\nin colors and reddening laws found at maximum light are also present 55 days\nafterward, but unlike the colors at maximum, they show a significant variation\namong different host galaxy morphological types. This suggests that the effect\nof ISM on the colors is more apparent at late times. Finally, we discuss how\nthe transversal expansion of the ejecta in an inhomogeneous ISM could mimic\nsome of these findings.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Stellar mid-life crisis: subcritical magnetic dynamos of solar-like\n  stars and the breakdown of gyrochronology: Recent observations reveal the surprising breakdown of stellar gyrochronology\nrelations at about the Sun's age hinting that middle-aged, solar-like stars\ntransition to a magnetically inactive future. We provide a theoretical basis\nfor these intriguing observations inspired by simulations with a mathematical\ndynamo model that can explore long-term solar cycle fluctuations. We reproduce\nthe observed bimodal distribution of sunspot numbers, but only for subcritical\ndynamos. Based on a bifurcation analysis, we argue that ageing of solar-like\nstars makes the magnetically-weak dynamo regime readily accessible. Weak\nmagnetic field production in this regime compromises wind-driven angular\nmomentum losses thus disrupting the hegemony of magnetic braking on stellar\nrotational spin-down. This hypothesis of {\\emph{subcritical magnetic dynamos}}\nof solar-like stars provides a self-consistent, unifying physical basis for a\ndiversity of solar-stellar phenomena such as why stars beyond their mid-life do\nnot spin-down as fast as in their youth, the break-down of stellar\ngyrochronology relations, the observed bimodal distribution of long-term\nsunspot observations and recent findings suggesting that the Sun may be\ntransitioning to a magnetically inactive future.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The GRAVITY Young Stellar Object survey -- I. Probing the disks of\n  Herbig Ae/Be stars in terrestrial orbits: The formation and the evolution of protoplanetary disks are important stages\nin the lifetime of stars. The processes of disk evolution and planet formation\nare intrinsically linked. We spatially resolve with GRAVITY/VLTI in the K-band\nthe sub au-scale region of 27 stars to gain statistical understanding of their\nproperties. We look for correlations with stellar parameters, such as\nluminosity, mass, temperature and age. Our sample also cover a range of various\nproperties in terms of reprocessed flux, flared or flat morphology, and gaps.\nWe developed semi-physical geometrical models to fit our interferometric data.\nOur best models correspond to smooth and wide rings, implying that wedge-shaped\nrims at the dust sublimation edge are favored, as found in the H-band. The\nclosure phases are generally non-null with a median value of ~10 deg,\nindicating spatial asymmetries of the intensity distributions. Multi-size grain\npopulations could explain the closure phase ranges below 20-25 deg but other\nscenarios should be invoked to explain the largest ones. Our measurements\nextend the Radius-Luminosity relation to ~1e4 Lsun and confirm the significant\nspread around the mean relation observed in the H-band. Gapped sources exhibit\na large N-to-K band size ratio and large values of this ratio are only observed\nfor the members of our sample that would be older than 1 Ma, less massive, and\nwith lower luminosity. In the 2 Ms mass range, we observe a correlation in the\nincrease of the relative age with the transition from group II to group I, and\nan increase of the N-to-K size ratio. However, the size of the current sample\ndoes not yet permit us to invoke a clear universal evolution mechanism across\nthe HAeBe mass range. The measured locations of the K-band emission suggest\nthat these disks might be structured by forming young planets, rather than by\ndepletion due to EUV, FUV, and X-ray photo-evaporation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evolution of the post merger remnants from the coalescence of\n  oxygen-neon and carbon-oxygen white dwarf pairs: Although multidimensional simulations have investigated the processes of\ndouble WD mergers, post-merger evolution only focused on the carbon-oxygen (CO)\nWD or helium (He) WD merger remnants. In this work, we investigate for the\nfirst time the evolution of the remnants stemmed from the merger of oxygen-neon\n(ONe) WDs with CO WDs. Our simulation results indicate that the merger remnants\ncan evolve to hydrogen- and helium-deficient giants with maximum radius of\nabout 300Rsun. Our models show evidence that merger remnants more massive than\n1.95Msun can ignite Ne before significant mass-loss ensues, and they thus would\nbecome electron-capture supernovae (ECSNe). However, remnants with initial\nmasses less than 1.90Msun will experience further core contraction and longer\nevolutionary time before reaching at the conditions for Ne-burning. Therefore\ntheir fates are more dependent on mass-loss rates due to stellar winds, and\nthus more uncertain. Relatively high mass-loss rates would cause such remnants\nto end their lives as ONe WDs. Our evolutionary models can naturally explain\nthe observational properties of the double WD merger remnant IRAS 00500+6713\n(J005311). As previously suggested in the literature, we propose and justify\nthat J005311 may be the remnant from the coalescence of an ONe WD and an CO WD.\nWe deduce that the final outcome of J005311 would be a massive ONe WD rather\nthan a supernova explosion. Our investigations may be able to provide possible\nconstraints on the wind mass-loss properties of the giants which have\nCO-dominant envelopes.",
        "category": "astro-ph_SR"
    },
    {
        "text": "FUV variability of HD 189733. Is the star accreting material from its\n  hot Jupiter?: Hot Jupiters are subject to strong irradiation from the host stars and, as a\nconsequence, they do evaporate. They can also interact with the parent stars by\nmeans of tides and magnetic fields. Both phenomena have strong implications for\nthe evolution of these systems. Here we present time resolved spectroscopy of\nHD~189733 observed with the Cosmic Origin Spectrograph (COS) on board to HST.\nThe star has been observed during five consecutive HST orbits, starting at a\nsecondary transit of the planet ($\\phi$ ~0.50-0.63). Two main episodes of\nvariability of ion lines of Si, C, N and O are detected, with an increase of\nline fluxes. Si IV lines show the highest degree of variability. The FUV\nvariability is a signature of enhanced activity in phase with the planet\nmotion, occurring after the planet egress, as already observed three times in\nX-rays. With the support of MHD simulations, we propose the following\ninterpretation: a stream of gas evaporating from the planet is actively and\nalmost steadily accreting onto the stellar surface, impacting at $70-90\\deg$\nahead of the sub-planetary point.",
        "category": "astro-ph_SR"
    },
    {
        "text": "G-band and Hard X-ray Emissions of the 2006 December 14 flare observed\n  by Hinode/SOT and RHESSI: We report on G-band emission observed by the Solar Optical Telescope onboard\nthe Hinode satellite in association with the X1.5-class flare on 2006 December\n14. The G-band enhancements originate from the footpoints of flaring coronal\nmagnetic loops, coinciding with non-thermal hard X-ray bremsstrahlung sources\nobserved by the Reuven Ramaty High Energy Solar Spectroscopic Imager. At the\navailable 2 minute cadence, the G-band and hard X-ray intensities are\nfurthermore well correlated in time. Assuming that the G-band enhancements are\ncontinuum emission from a blackbody, we derived the total radiative losses of\nthe white-light flare (white-light power). If the G-band enhancements\nadditionally have a contribution from lines, the derived values are\noverestimates. We compare the white-light power with the power in hard X-ray\nproducing electrons using the thick target assumption. Independent of the\ncutoff energy of the accelerated electron spectrum, the white-light power and\nthe power of accelerated electrons are roughly proportional. Using the observed\nupper limit of ~30 keV for the cutoff energy, the hard X-ray producing\nelectrons provide at least a factor of 2 more power than needed to produce the\nwhite-light emission. For electrons above 40 keV, the powers roughly match for\nall four of the time intervals available during the impulsive phase. Hence, the\nflare-accelerated electrons contain enough energy to produce the white-light\nflare emissions. The observed correlation in time, space, and power strongly\nsuggests that electron acceleration and white-light production in solar flares\nare closely related. However, the results also call attention to the\ninconsistency in apparent source heights of the hard X-ray (chromosphere) and\nwhite-light (upper photosphere) sources.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Magnetic feature tracking, what determines the speed?: Recent observations revealed that small magnetic elements abundant at the\nsolar surface move poleward with a velocity which seems to be lower than the\nplasma velocity $U_{\\theta}$. Guerrero et al. (2011) explained this discrepancy\nas a consequence of diffusive spreading of the magnetic elements due to a\npositive radial gradient of $|U_{\\theta}|$. As the gradient's sign (inferred by\nlocal helioseismology) is still unclear, cases with a negative gradient are\nstudied in this paper. Under this condition, the velocity of the magnetic\ntracers turns out to be larger than the plasma velocity, in disagreement with\nthe observations. Alternative mechanisms for explaining them independently are\nproposed. For the turbulent magnetic pumping it is shown that it has to be\nunrealistically strong to reconcile the model with the observations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The AstraLux Multiplicity Survey: Extension to Late M-dwarfs: The distribution of multiplicity among low-mass stars is a key issue to\nunderstanding the formation of stars and brown dwarfs, and recent surveys have\nyielded large enough samples of nearby low-mass stars to study this issue\nstatistically to good accuracy. Previously, we have presented a multiplicity\nstudy of ~700 early/mid M-type stars observed with the AstraLux high-resolution\nLucky Imaging cameras. Here, we extend the study of multiplicity in M-type\nstars through studying 286 nearby mid/late M-type stars, bridging the gap\nbetween our previous study and multiplicity studies of brown dwarfs. Most of\nthe targets have been observed more than once, allowing us to assess common\nproper motion to confirm companionship. We detect 68 confirmed or probable\ncompanions in 66 systems, of which 41 were previously undiscovered. Detections\nare made down to the resolution limit of ~100 mas of the instrument. The raw\nmultiplicity in the AstraLux sensitivity range is 17.9%, leading to a total\nmultiplicity fraction of 21-27% depending on the mass ratio distribution, which\nis consistent with being flat down to mass ratios of ~0.4, but cannot be\nstringently constrained below this value. The semi-major axis distribution is\nwell represented by a log-normal function with mu_a = 0.78 and sigma_a = 0.47,\nwhich is narrower and peaked at smaller separations than for a Sun-like sample.\nThis is consistent with a steady decrease in average semi-major axis from the\nhighest-mass binary stars to the brown dwarf binaries.",
        "category": "astro-ph_SR"
    },
    {
        "text": "J-type carbon stars: A dominant source of 14N-rich presolar SiC grains\n  of type AB: We report Mo isotopic data of 27 new presolar SiC grains, including 12\n14N-rich AB (14N/15N>440, AB2) and 15 mainstream (MS) grains, and their\ncorrelated Sr and Ba isotope ratios when available. Direct comparison of the\ndata for the MS grains, which came from low-mass asymptotic giant branch (AGB)\nstars with large s-process isotope enhancements, with the AB2 grain data\ndemonstrates that AB2 grains show near-solar isotopic compositions and lack\ns-process enhancements. The near-normal Sr, Mo, and Ba isotopic compositions of\nAB2 grains clearly exclude born-again AGB stars, where the intermediate\nneutron-capture process (i-process) takes place, as their stellar source. On\nthe other hand, low-mass CO novae, and early R- and J-type carbon stars show\n13C and 14N excesses but no s-process enhancements and are thus potential\nstellar sources of AB2 grains. Since both early R-type carbon stars and CO\nnovae are rare objects, the abundant J-type carbon stars (10-15% of all carbon\nstars) are thus likely to be a dominant source of AB2 grains.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Precision southern hemisphere VLBI pulsar astrometry II: Measurement of\n  seven parallaxes: Accurate measurement of pulsar distances via astrometry using very long\nbaseline interferometry enables the improvement of Galactic electron density\ndistribution models, improving distance estimates for the vast majority of\npulsars for which parallax measurements are unavailable. However, pulsars at\nsouthern declinations have been under-represented in previous interferometric\nastrometry campaigns. In order to redress this imbalance, we have conducted a\ntwo-year astrometric campaign targeting eight southern pulsars with the\nAustralian Long Baseline Array. The program summarized in this paper has\nresulted in the measurement of seven new pulsar parallaxes, with success on\nobjects down to a mean flux density of 0.8 mJy at 1600 MHz. Our results\nhighlight the substantial uncertainties that remain when utilizing free\nelectron density models for individual pulsar distances. Until this study, PSR\nJ0630-2834 was believed to convert 16% of its spin-down energy into x-rays, but\nour measured parallax distance of 332 (+52 -40) pc has revised this value to\n<1%. In contrast, PSR J0108-1431 was found to be almost a factor of two more\ndistant than previously thought, making its conversion of spin-down energy to\nx-rays the most efficient known (>1%). The 8.5 second radio pulsar J2144-3933\nwas found to be closer than previously predicted, making its apparent 1400 MHz\nradio luminosity the lowest of any known pulsar (20 microJy kpc^2). We have\nexamined the growing population of neutron stars with accurate parallaxes to\ndetermine the effect of distance errors on the underlying neutron star velocity\ndistribution, and find that typical distance errors may be biasing the\nestimated mean pulsar velocity upwards by 5%, and are likely to exaggerate the\ndistribution's high-velocity tail.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Triple evolution: an important channel in the formation of type Ia\n  supernovae: Type Ia supernovae (SNe Ia) are thought to be the result of thermonuclear\nexplosions in white dwarfs (WDs). Commonly considered formation pathways\ninclude two merging WDs (the double degenerate channel), and a single WD\naccreting material from a H or He donor (the single degenerate channel). Since\nthe predicted SNe Ia rates from WD in binaries are thought to be insufficient\nto explain the observed SNe Ia rate, it is important to study similar\ninteractions in higher-order multiple star systems such as triple systems. We\nuse the evolutionary population synthesis code Multiple Stellar Evolution (MSE)\nto study stellar evolution, binary interactions and gravitational dynamics of\nthe triple-star systems. Also, unlike previous studies, prescriptions are\nincluded to simultaneously take into account the single and double degenerate\nchannels, and we consider triples across the entire parameter space (including\nthose with tight inner binaries). We explore the impact of typically ignored or\nuncertain physics such as fly-bys and CE prescription parameters on our\nresults. The majority of systems undergo circular mergers to explode as SNe Ia,\nwhile eccentric collisions contribute to $0.4-4$ per cent of SNe Ia events. The\ntime-integrated SNe Ia rate from the triple channel is found to be $(3.60 \\pm\n0.04) \\times {10^{-4}}\\,\\mathrm{M_{\\odot}}^{-1}$ which is, surprisingly,\nsimilar to that of the isolated binary channel where the SNe Ia rate is $(3.2\n\\pm 0.1) \\times {10^{-4}}\\,\\mathrm{M_{\\odot}}^{-1}$. This implies that triples,\nwhen considering their entire parameter space, yield an important contribution\nto the overall SNe Ia rate.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Observation of flux tube crossings in the solar wind: Current sheets are ubiquitous in the solar wind.They are a major source of\nthe solar wind MHD turbulence intermittency. They may result from non-linear\ninteractions of the solar wind MHD turbulence or are the boundaries of flux\ntubes that originate from the solar surface. Some current sheets appear in\npairs and are the boundaries of transient structures such as magnetic holes and\nreconnection exhausts, or the edges of pulsed Alfv\\'{e}n waves. For an\nindividual current sheet, discerning whether it is a flux tube boundary or due\nto non-linear interactions, or the boundary of a transient structure is\ndifficult. In this work, using data from the {\\sl Wind} spacecraft, we identify\ntwo three-current-sheet events. Detailed examination of these two events\nsuggest that they are best explained by the flux tube crossing scenario. Our\nstudy provides a convincing evidence supporting the scenario that the solar\nwind consists of flux tubes where distinct plasmas reside.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Chromospheric swirls I. Automated detection in H$\u03b1$ observations\n  and their statistical properties: Chromospheric swirls are considered to play a significant role in the\ndynamics and heating of the upper solar atmosphere. It is important to\nautomatically detect and track them in chromospheric observations and determine\ntheir properties. We applied a recently developed automated chromospheric swirl\ndetection method to time-series observations of a quiet region of the solar\nchromosphere obtained in the H$\\alpha$-0.2 \\r{A} wavelength of the H$\\alpha$\nspectral line by the CRISP instrument at the Swedish 1-m Solar Telescope. The\nalgorithm exploits the morphological characteristics of swirling events in high\ncontrast chromospheric observations and results in the detection of these\nstructures in each frame of the time series and their tracking over time. We\nconducted a statistical analysis to determine their various properties,\nincluding a survival analysis for deriving the mean lifetime. A mean number of\n146 $\\pm$ 9 swirls was detected within the FOV at any given time. The mean\nsurface density is found equal to $\\sim$0.08 swirls$ $Mm$^{-2}$ and the\noccurrence rate is $\\sim$10$^{-2}$ swirls$ $Mm$^{-2}$ min$^{-1}$. These values\nare much higher than those previously reported from chromospheric observations.\nThe radii of the detected swirls range between 0.5 and 2.5 Mm, with a mean\nvalue equal to 1.3 $\\pm$ 0.3 Mm, which is slightly higher than previous\nreports. The lifetimes range between 1.5 min and 33.7 min with an arithmetic\nmean value of $\\sim$8.5 min. A survival analysis of the lifetimes, however,\nusing the Kaplan-Meier estimator in combination with a parametric model results\nin a mean lifetime of 10.3 $\\pm$ 0.6 min. An automated method sheds more light\non their abundance than visual inspection, while higher cadence, higher\nresolution observations will most probably result in the detection of a higher\nnumber of such features on smaller scales and with shorter lifetimes.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evolved Massive Stars at Low-metallicity II. Red Supergiant Stars in the\n  Small Magellanic Cloud: We present the most comprehensive RSG sample for the SMC up to now, including\n1,239 RSG candidates. The initial sample is derived based on a source catalog\nfor the SMC with conservative ranking. Additional spectroscopic RSGs are\nretrieved from the literature, as well as RSG candidates selected from the\ninspection of CMDs. We estimate that there are in total $\\sim$ 1,800 or more\nRSGs in the SMC. We purify the sample by studying the infrared CMDs and the\nvariability of the objects, though there is still an ambiguity between AGBs and\nRSGs. There are much less RSGs candidates ($\\sim$4\\%) showing PAH emission\nfeatures compared to the Milky Way and LMC ($\\sim$15\\%). The MIR variability of\nRSG sample increases with luminosity. We separate the RSG sample into two\nsubsamples (\"risky\" and \"safe\") and identify one M5e AGB star in the \"risky\"\nsubsample. Most of the targets with large variability are also the bright ones\nwith large MLR. Some targets show excessive dust emission, which may be related\nto previous episodic mass loss events. We also roughly estimate the total gas\nand dust budget produced by entire RSG population as $\\rm\n\\sim1.9^{+2.4}_{-1.1}\\times10^{-6}~M_{\\odot}/yr$ in the most conservative case.\nBased on the MIST models, we derive a linear relation between $T_{\\rm eff}$ and\nobserved $\\rm J-K_S$ color with reddening correction for the RSG sample. By\nusing a constant bolometric correction and this relation, the Geneva\nevolutionary model is compared with our RSG sample, showing a good agreement\nand a lower initial mass limit of $\\sim$7 $\\rm M_\\odot $ for the RSG\npopulation. Finally, we compare the RSG sample in the SMC and the LMC. Despite\nthe incompleteness of LMC sample in the faint end, the result indicates that\nthe LMC sample always shows redder color (except for the $\\rm IRAC1-IRAC2$ and\n$\\rm WISE1-WISE2$ colors due to CO absorption) and larger variability than the\nSMC sample.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Primeval very low-mass stars and brown dwarfs - VI. Population\n  properties of metal-poor degenerate brown dwarfs: We presented 15 new T dwarfs that were selected from UKIRT Infrared Deep Sky\nSurvey, Visible and Infrared Survey Telescope for Astronomy, and Wide-field\nInfrared Survey Explorer surveys, and confirmed with optical to near infrared\nspectra obtained with the Very Large Telescope and the Gran Telescopio\nCanarias. One of these new T dwarfs is mildly metal-poor with slightly\nsuppressed $K$-band flux. We presented a new X-shooter spectrum of a known\nbenchmark sdT5.5 subdwarf, HIP 73786B. To better understand observational\nproperties of brown dwarfs, we discussed transition zones (mass ranges) with\nlow-rate hydrogen, lithium, and deuterium burning in brown dwarf population.\nThe hydrogen burning transition zone is also the substellar transition zone\nthat separates very low-mass stars, transitional, and degenerate brown dwarfs.\nTransitional brown dwarfs have been discussed in previous works of the Primeval\nseries. Degenerate brown dwarfs without hydrogen fusion are the majority of\nbrown dwarfs. Metal-poor degenerate brown dwarfs of the Galactic thick disc and\nhalo have become T5+ subdwarfs. We selected 41 T5+ subdwarfs from the\nliterature by their suppressed $K$-band flux. We studied the spectral-type -\ncolour correlations, spectral-type - absolute magnitude correlations,\ncolour-colour plots, and HR diagrams of T5+ subdwarfs, in comparison to these\nof L-T dwarfs and L subdwarfs. We discussed the T5+ subdwarf discovery\ncapability of deep sky surveys in the 2020s.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Twenty-Five Sub-Arcsecond Binaries Discovered By Lunar Occultations: We report on 25 sub-arcsecond binaries, detected for the first time by means\nof lunar occultations in the near-infrared as part of a long-term program using\nthe ISAAC instrument at the ESO Very Large Telescope. The primaries have\nmagnitudes in the range K=3.8 to 10.4, and the companions in the range K=6.4 to\n12.1. The magnitude differences have a median value of 2.8, with the largest\nbeing 5.4. The projected separations are in the range 6 to 748 milliarcseconds\nand with a median of 18 milliarcseconds, or about 3 times less than the\ndiffraction limit of the telescope. Among our binary detections are a pre-main\nsequence star and an enigmatic Mira-like variable previously suspected to have\na companion. Additionally, we quote an accurate first-time near-IR detection of\na previously known wider binary.\n  We discuss our findings on an individual basis as far as made possible by the\navailable literature, and we examine them from a statistical point of view. We\nderive a typical frequency of binarity among field stars of ~10%, in the\nresolution and sensitivity range afforded by the technique (~0.003\" to ~0.5\",\nand K~12mag, respectively). This is in line with previous results by the same\ntechnique but we point out interesting differences that we can trace up to\nsensitivity, time sampling, and average distance of the targets. Finally, we\ndiscuss the prospects for further follow-up studies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Derivations and Observations of Prominence Bulk Motions and Mass: In this chapter we review observations and techniques for measuring both bulk\nflows in prominences and prominence mass. Measuring these quantities is\nessential to development and testing of models discussed throughout this book.\nProminence flows are complex and various, ranging from the relatively linear\nflows along prominence spines to the complex, turbulent patterns exhibited by\nhedgerow prominences. Techniques for measuring flows include time slice and\noptical flow techniques used for motions in the plane of the sky and the use of\nspectral line profiles to determine Doppler velocities along the line of sight.\nProminence mass measurement is chiefly done via continuum absorption\nmeasurements, but mass has also been estimated using cloud modeling and white\nlight measurements.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Saturation of large-scale dynamo in anisotropically forced turbulence: Turbulent dynamo theories have faced difficulties in obtaining evolution of\nlarge-scale magnetic fields on short dynamical time-scales due to the\nconstraint imposed by magnetic helicity balance. This has critical implications\nfor understanding the large-scale magnetic field evolution in astrophysical\nsystems like the Sun, stars and galaxies. Direct numerical simulations (DNS) in\nthe past with isotropically forced helical turbulence have shown that\nlarge-scale dynamo saturation time-scales are dependent on the magnetic\nReynolds number (Rm). In this work, we have carried out periodic box DNS of\nhelically forced turbulence leading to a large-scale dynamo with two kinds of\nforcing function, an isotropic one based on that used in PENCIL-CODE and an\nanisotropic one based on Galloway-Proctor flows. We show that when the\nturbulence is forced anisotropically, the nonlinear (saturation) behaviour of\nthe large-scale dynamo is only weakly dependent on Rm. In fact the magnetic\nhelicity evolution on small and large scales in the anisotropic case is\ndistinctly different from that in the isotropic case. This result possibly\nholds promise for the alleviation of important issues like catastrophic\nquenching.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Mapping the Local Halo: Statistical Parallax Analysis of SDSS Low-Mass\n  Subdwarfs: We present a statistical parallax study of nearly 2,000 M subdwarfs with\nphotometry and spectroscopy from the Sloan Digital Sky Survey. Statistical\nparallax analysis yields the mean absolute magnitudes, mean velocities and\nvelocity ellipsoids for homogenous samples of stars. We selected homogeneous\ngroups of subdwarfs based on their photometric colors and spectral appearance.\nWe examined the color-magnitude relations of low-mass subdwarfs and quantified\ntheir dependence on the newly-refined metallicity parameter, zeta. We also\ndeveloped a photometric metallicity parameter, delta(g-r), based on the g-r and\nr-z colors of low-mass stars and used it to select stars with similar\nmetallicities. The kinematics of low-mass subdwarfs as a function of color and\nmetallicity were also examined and compared to main sequence M dwarfs. We find\nthat the SDSS subdwarfs share similar kinematics to the inner halo and thick\ndisk. The color-magnitude relations derived in this analysis will be a powerful\ntool for identifying and characterizing low-mass metal-poor subdwarfs in future\nsurveys such as GAIA and LSST, making them important and plentiful tracers of\nthe stellar halo.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Type Ia Single Degenerate Survivors Must Be Overluminous: In the single-degenerate (SD) channel of a Type Ia supernovae (SN Ia)\nexplosion, a main-sequence (MS) donor star survives the explosion but it is\nstripped of mass and shock heated. An essentially unavoidable consequence of\nmass loss during the explosion is that the companion must have an overextended\nenvelope after the explosion. While this has been noted previously, it has not\nbeen strongly emphasized as an inevitable consequence. We calculate the future\nevolution of the companion by injecting 2-6 10^47 ergs into the stellar\nevolution model of a 1 Msun donor star based on the post-explosion progenitors\nseen in simulations. We find that, due to the Kelvin-Helmholtz collapse of the\nenvelope, the companion must become significantly more luminous (10 - 10^3\nLsun) for a long period of time (10^3 - 10^4 years). The lack of such a\nluminous \"leftover\" star in the LMC supernova remnant SNR 0609-67.5 provides\nanother piece of evidence against the SD scenario. We also show that none of\nthe stars proposed as the survivors of the Tycho supernova, including Tycho G,\ncould plausibly be the donor star. Additionally, luminous donors closer than\n~10 Mpc should be observable with the Hubble Space Telescope starting ~2 years\npost-peak. Such systems include SN 1937C, SN 1972E, SN 1986G, and SN 2011fe.\nThus, the SD channel is already ruled out for at least two nearby SNe Ia and\ncan be easily tested for a number of additional ones. We also discuss similar\nimplications for the companions of core-collapse SNe.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Search for Interstellar Carbon Chain Alcohol HC4OH in Star-Forming\n  Region L1527 and Dark Cloud TMC-1: We report a sensitive search for the rotational transitions of the carbon\nchain alcohol HC4OH in the frequency range of 21.2-46.7 GHz in the star-forming\nregion L1527 and the dark cloud TMC-1. The motivation was laboratory detection\nof HC4OH by microwave spectroscopy. Despite achieving rms noise levels of\nseveral millikelvin in the antenna temperature using the 45 m telescope at\nNobeyama Radio Observatory, the detection was not successful, leading to 3\nsigma upper limits corresponding to the column densities of 2.0 \\times 1012 and\n5.6 \\times 1012 cm-2 in L1527 and TMC-1, respectively. These upper limits\nindicate that [HC4OH]/[HC5N] ratios are less than 0.3 and 0.1 in L1527 and\nTMC-1, respectively, where HC5N is an HC4-chain cyanide and HC4OH is a\nhydroxide. These ratios suggest that the cyano carbon chain molecule dominates\nthe hydroxyl carbon chain molecule in L1527 and TMC-1. This is contrary to the\ncase of saturated compounds in hot cores, e.g., CH3OH and CH3CN, and can be a\nchemical feature of carbon chain molecules in L1527 and TMC-1. In addition, the\ncolumn densities of the \"unsubstituted\" carbon chain molecule C4H and the\nsulfur-bearing molecules SO and HCS+ were determined from detected lines in\nL1527.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Investigating Gaia EDR3 parallax systematics using asteroseismology of\n  Cool Giant Stars observed by Kepler, K2, and TESS I. Asteroseismic distances\n  to 12,500 red-giant stars: Gaia EDR3 has provided unprecedented data that generate a lot of interest in\nthe astrophysical community, despite the fact that systematics affect the\nreported parallaxes at the level of ~ 10 muas. Independent distance\nmeasurements are available from asteroseismology of red-giant stars with\nmeasurable parallaxes, whose magnitude and colour ranges more closely reflect\nthose of other stars of interest. In this paper, we determine distances to\nnearly 12,500 red-giant branch and red clump stars observed by Kepler, K2, and\nTESS. This is done via a grid-based modelling method, where global\nasteroseismic observables, constraints on the photospheric chemical\ncomposition, and on the unreddened photometry are used as observational inputs.\nThis large catalogue of asteroseismic distances allows us to provide a first\ncomparison with Gaia EDR3 parallaxes. Offset values estimated with\nasteroseismology show no clear trend with ecliptic latitude or magnitude, and\nthe trend whereby they increase (in absolute terms) as we move towards redder\ncolours is dominated by the brightest stars. The correction model proposed by\nLindegren et al. (2021) is not suitable for all the fields considered in this\nstudy. We find a good agreement between asteroseismic results and model\npredictions of the red clump magnitude. We discuss possible trends with the\nGaia scan law statistics, and show that two magnitude regimes exist where\neither asteroseismology or Gaia provides the best precision in parallax.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Diffraction-limited Visible Light Images of Orion Trapezium Cluster With\n  the Magellan Adaptive Secondary AO System (MagAO): We utilized the new high-order (250-378 mode) Magellan Adaptive Optics system\n(MagAO) to obtain very high spatial resolution observations in \"visible light\"\nwith MagAO's VisAO CCD camera. In the good-median seeing conditions of Magellan\n(0.5-0.7\") we find MagAO delivers individual short exposure images as good as\n19 mas optical resolution. Due to telescope vibrations, long exposure (60s) r'\n(0.63 micron) images are slightly coarser at FWHM=23-29 mas (Strehl ~28%) with\nbright (R<9 mag) guide stars. These are the highest resolution filled-aperture\nimages published to date. Images of the young (~1 Myr) Orion Trapezium Theta 1\nOri A, B, and C cluster members were obtained with VisAO. In particular, the 32\nmas binary Theta 1 Ori C1/C2 was easily resolved in non-interferometric images\nfor the first time. Relative positions of the bright trapezium binary stars\nwere measured with ~0.6-5 mas accuracy. We now are sensitive to relative proper\nmotions of just ~0.2 mas/yr (~0.4 km/s at 414 pc) - this is a ~2-10x\nimprovement in orbital velocity accuracy compared to previous efforts. For the\nfirst time, we see clear motion of the barycenter of Theta 1 Ori B2/B3 about\nTheta 1 Ori B1. All five members of the Theta 1 Ori B system appear likely a\ngravitationally bound \"mini-cluster\", but we find that not all the orbits can\nbe both circular and co-planar. The lowest mass member of the Theta 1 Ori B\nsystem (B4; mass ~0.2 Msun) has a very clearly detected motion (at 4.1+/-1.3\nkm/s; correlation=99.9%) w.r.t B1 and will likely be ejected in the future.\nThis \"ejection\" process of the lowest mass member of a \"mini-cluster\" could\nplay a major role in the formation of low mass stars and brown dwarfs.(slightly\nabridged abstract)",
        "category": "astro-ph_SR"
    },
    {
        "text": "Models of Individual Blue Stragglers: This chapter describes the current state of models of individual blue\nstragglers. Stellar collisions, binary mergers (or coalescence), and partial or\nongoing mass transfer have all been studied in some detail. The products of\nstellar collisions retain memory of their parent stars and are not fully mixed.\nVery high initial rotation rates must be reduced by an unknown process to allow\nthe stars to collapse to the main sequence. The more massive collision products\nhave shorter lifetimes than normal stars of the same mass, while products\nbetween low mass stars are long-lived and look very much like normal stars of\ntheir mass. Mass transfer can result in a merger, or can produce another binary\nsystem with a blue straggler and the remnant of the original primary. The\nproducts of binary mass transfer cover a larger portion of the colour-magnitude\ndiagram than collision products for two reasons: there are more possible\nconfigurations which produce blue stragglers, and there are differing\ncontributions to the blended light of the system. The effects of rotation may\nbe substantial in both collision and merger products, and could result in\nsignificant mixing unless angular momentum is lost shortly after the formation\nevent. Surface abundances may provide ways to distinguish between the formation\nmechanisms, but care must be taking to model the various mixing mechanisms\nproperly before drawing strong conclusions. Avenues for future work are\noutlined.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The quest for stable circumbinary companions to post-common envelope sdB\n  eclipsing binaries Does the observational evidence support their existence?: Context. Period variations have been detected in a number of eclipsing close\ncompact binary subdwarf B stars (sdBs) and these have often been interpreted as\ncaused by circumbinary massive planets or brown dwarfs. Various evolutionary\nscenarios have been proposed for these stars, but a definite mechanism remains\nto be established. Equally puzzling is the formation of these putative\ncircumbinary objects which must have formed either from the remaining post\ncommon envelope circumbinary disk or survived its evolution. Aims. In this\npaper we review the eclipse time variations (ETVs) exhibited by seven such\nsystems and explore if there is conclusive evidence that the ETVs observed over\nthe last two decades can reliably predict the presence of circumbinary bodies.\nMethods. We report 246 new observations of the seven sdB systems made between\n2013 September and 2017 July using a worldwide network of telescopes. We\ncombined our new data with previously published measurements to analyse the\nETVs of these systems. Results. Our data shows that period variations cannot be\nmodelled simply on the basis of circumbinary objects. This implies that more\ncomplex processes may be taking place in these systems. From eclipse time\nvariations, it has historically been suggested that five of the seven binary\nsystems reported herein had circumbinary objects. Based on our recent\nobservations and analysis only three systems remain serious contenders. We find\nagreement with other observers that at least a decade of observations is\nrequired to establish reliable ephemeris. With longer observational baselines\nit is quite conceivable that the data will support the circumbinary object\nhypothesis of these binary systems. Also we generally agree with other\nobservers that larger values of (O-C) residuals are found with secondary\ncompanions of spectral type M5/6 or earlier as a result of an Applegate type\nmechanism",
        "category": "astro-ph_SR"
    },
    {
        "text": "Zeeman Doppler maps. II: the perils of eschewing physics: For the observational modeling of horizontal abundance distributions and of\nmagnetic geometries in chemically peculiar (CP) stars, Zeeman Doppler mapping\n(ZDM) has become the method of choice. Comparisons between abundance maps\nobtained for CP stars and predictions from numerical simulations of atomic\ndiffusion have always proved unsatisfactory, with the blame routinely put on\ntheory. Expanding a previous study aimed at clarifying the question of the\nuniqueness of ZDM maps, this paper inverts the roles between observational\nmodeling and time-dependent diffusion results, casting a cold eye on essential\nassumptions and algorithms underlying ZDM, in particular the Tikhonov-style\nregularization functionals, from 1D to 3D. We show that these have been\nestablished solely for mathematical convenience, but that they in no way\nreflect the physical reality in the atmospheres of magnetic CP stars.\nRecognizing that the observed strong magnetic fields in most well-mapped stars\nrequire the field geometry to be force-free, we demonstrate that many published\nmaps do not meet this condition. There follows a discussion of the frequent\nchanges in magnetic and abundance maps of well observed stars and a caveat\nconcerning the use of least squares deconvolution in ZDM analyses. It emerges\nthat because of the complexity and non-linearity of the field-dependent\nchemical stratifications, Tikhonov based ZDM inversions cannot recover the true\nabundance and magnetic geometries. As our findings additionally show, there is\nno way to define a physically meaningful 3D regularization functional instead.\nZDM remains dysfunctional and does not provide any observational constraints\nfor the modeling of atomic diffusion.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Herschel Cold Debris Disks: Confusion with the Extragalactic\n  Background at 160 mu: The Herschel \"DUst around NEarby Stars (DUNES)\" survey has found a number of\ndebris disk candidates that are apparently very cold, with temperatures near\n22K. It has proven difficult to fit their spectral energy distributions with\nconventional models for debris disks. Given this issue we carefully examine the\nalternative explanation, that the detections arise from confusion with IR\ncirrus and/or background galaxies that are not physically associated with the\nforeground star. We find that such an explanation is consistent with all of\nthese detections.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Survey of Nearby FGK Stars at 160 microns with Spitzer: The Spitzer Space Telescope has advanced debris disk science tremendously\nwith a wealth of information on debris disks around nearby A, F, G, K and M\nstars at 24 and 70 microns with the MIPS photometer and at 8-34 microns with\nIRS. Here we present 160 micron observations of a small sub-set of these stars.\nAt this wavelength, the stellar photospheric emission is negligible and any\ndetected emission corresponds to cold dust in extended Kuiper belt analogs.\nHowever, the Spitzer 160 micron observations are limited in sensitivity by the\nlarge beam size which results in significant ''noise'' due to cirrus and\nextragalactic confusion. In addition, the 160 micron measurements suffer from\nthe added complication of a light leak next to the star's position whose flux\nis proportional to the near-infrared flux of the star. We are able to remove\nthe contamination from the leak and report 160 micron measurements or upper\nlimits for 24 stars. Three stars (HD 10647, HD 207129, and HD 115617) have\nexcesses at 160 micron that we use to constrain the properties of the debris\ndisks around them. A more detailed model of the spectral energy distribution of\nHD 10647 reveals that the 70 and 160 micron emission could be due to small\nwater ice particles at a distance of 100 AU consistent with Hubble Space\nTelescope optical imaging of circumstellar material in the system.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Binarity and Accretion in AGB Stars: HST/STIS Observations of UV\n  Flickering in Y Gem: Binarity is believed to dramatically affect the history and geometry of mass\nloss in AGB and post-AGB stars, but observational evidence of binarity is\nsorely lacking. As part of a project to search for hot binary companions to\ncool AGB stars using the GALEX archive, we discovered a late-M star, Y Gem, to\nbe a source of strong and variable UV and X-ray emission. Here we report UV\nspectroscopic observations of Y Gem obtained with the Hubble Space Telescope\nthat show strong flickering in the UV continuum on time-scales of <~20 s,\ncharacteristic of an active accretion disk. Several UV lines with P-Cygni-type\nprofiles from species such as Si IV and C IV are also observed, with emission\nand absorption features that are red- and blue- shifted by velocities of ~500\nkm/s from the systemic velocity. Our model for these (and previous)\nobservations is that material from the primary star is gravitationally captured\nby a companion, producing a hot accretion disk. The latter powers a fast\noutflow that produces blue-shifted features due to absorption of UV continuum\nemitted by the disk, whereas the red-shifted emission features arise in heated\ninfalling material from the primary. The outflow velocities support a previous\ninference by Sahai et al. (2015) that Y Gem's companion is a low-mass\nmain-sequence star. Blackbody fitting of the UV continuum implies an accretion\nluminosity of about 13 Lsun and thus a mass-accretion rate >5e-7 Msun/yr; we\ninfer that Roche lobe overflow is the most likely binary accretion mode for Y\nGem.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Magnetic field variations and the seismicity of solar active regions: Dynamical changes in the solar corona have proven to be very important in\ninducing seismic waves into the photosphere. Different mechanisms for their\ngeneration have been proposed. In this work, we explore the magnetic field\nforces as plausible mechanisms to generate sunquakes as proposed by Hudson,\nFisher and Welsch. We present a spatial and temporal analysis of the\nline-of-sight magnetic field variations induced by the seismically active 2003\nOctober 29 and 2005 January 15 solar flares and compare these results with\nother supporting observations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Properties of ion-cyclotron waves in the open solar corona: Remote observations of coronal holes have strongly implicated the resonant\ninteractions of ion-cyclotron waves with ions as a principal mechanism for\nplasma heating and acceleration of the fast solar wind. In order to study these\nwaves, a WKB (Wentzel-Kramers-Brillouin) linear perturbation analysis is used\nin the work frame of the collisionless multi-fluid model where we consider in\naddition to the protons a second ion component made of alpha particles. We\nconsider a non-uniform background plasma describing a funnel region in the open\ncoronal holes and we use the ray tracing Hamiltonian type equations to compute\nthe ray path of the waves and the spatial variation of their properties.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Multi-Survey Approach to White Dwarf Discovery: By selecting astrometric and photometric data from the Sloan Digital Sky\nSurvey (SDSS), the L{\\'e}pine & Shara Proper Motion North Catalog (LSPM-North),\nthe Two Micron All Sky Survey (2MASS), and the USNO-B1.0 catalog, we use a\nsuccession of methods to isolate white dwarf candidates for follow-up\nspectroscopy. Our methods include: reduced proper motion diagram cuts, color\ncuts, and atmospheric model adherence. We present spectroscopy of 26 white\ndwarfs obtained from the CTIO 4m and APO 3.5m telescopes. Additionally, we\nconfirm 28 white dwarfs with spectra available in the SDSS DR7 database but\nunpublished elsewhere, presenting a total of 54 WDs. We label one of these as a\nrecovered WD while the remaining 53 are new discoveries. We determine physical\nparameters and estimate distances based on atmospheric model analyses. Three\nnew white dwarfs are modeled to lie within 25 pc. Two additional white dwarfs\nare confirmed to be metal-polluted (DAZ). Follow-up time series photometry\nconfirms another object to be a pulsating ZZ Ceti white dwarf.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Most Lithium-rich Low-mass Evolved Stars Revealed as Red Clump stars by\n  Asteroseismology and Spectroscopy: Lithium has confused scientists for decades at almost each scale of the\nuniverse. Lithium-rich giants are peculiar stars with lithium abundances over\nmodel prediction. A large fraction of lithium-rich low-mass evolved stars are\ntraditionally supposed to be red giant branch (RGB) stars. Recent studies,\nhowever, report that red clump (RC) stars are more frequent than RGB. Here, we\npresent a uniquely large systematic study combining the direct asteroseismic\nanalysis with the spectroscopy on the lithium-rich stars. The majority of\nlithium-rich stars are confirmed to be RCs, whereas RGBs are minor. We reveal\nthat the distribution of lithium-rich RGBs steeply decline with the increasing\nlithium abundance, showing an upper limit around 2.6 dex, whereas the Li\nabundances of RCs extend to much higher values. We also find that the\ndistributions of mass and nitrogen abundance are notably different between RC\nand RGB stars. These findings indicate that there is still unknown process that\nsignificantly affects surface chemical composition in low-mass stellar\nevolution.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Characteristics of Flight Delays during Solar Flares: Solar flare is one of the severest solar activities on the sun, and it has\nmany important impacts on the near-earth space. It has been found that flight\narrival delays will increase during solar flare. However, the detailed\nintrinsic mechanism of how solar flares influence the delays is still unknown.\nBased on 5-years huge amount of flight data, here we comprehensively analyze\nthe flight departure delays during 57 solar flares. It is found that the\naveraged flight departure delay time during solar flares increased by 20.68%\n(7.67 min) compared to those during quiet periods. It is also shown that solar\nflare related flight delays reveal apparent time and latitude dependencies.\nFlight delays during dayside solar flares are more serious than those during\nnightside flares, and the longer (shorter) delays tend to occur in the lower\n(higher) latitude airport. Further analyses suggest that flight delay time and\ndelay rate would be directly modulated by the solar intensity (soft X-ray flux)\nand the Solar Zenith Angle. For the first time, these results indicate that the\ncommunication interferences caused by solar flares will directly affect flight\ndeparture delay time and delay rate. This work also expands our conventional\nunderstandings to the impacts of solar flares on human society, and it could\nalso provide us with brand new views to help prevent or cope with flight\ndelays.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Abundances in the Herbig Ae star HD 101412: Abundance anomalies; Lambda\n  Boo-Vega characteristics?: Context: Recent attention has been directed to abundance variations among\nvery young stars.\n  Aims: To perform a detailed abundance study of the Herbig Ae star HD 101412,\ntaking advantage of its unusually sharp spectral lines.\n  Methods: High-resolution spectra are measured for accurate wavelengths and\nequivalent widths. Balmer-line fits and ionization equlibria give a relation\nbetween Teff, and log(g). Abundance anomalies and uncertain reddening preclude\nthe use of spectral type or photometry to fix Teff. Excitation temperatures are\nused to break the degeneracy between Teff and log(g).\n  Results: Strong lines are subject to an anomalous saturation that cannot be\nremoved by assuming a low microturbulence. By restricting the analysis to weak\n(<= 20 m[A]) lines, we find consistent results for neutral and ionized species,\nbased on a model with Teff = 8300K, and log(g)=3.8. The photosphere is depleted\nin the most refractory elements, while volatiles are normal or, in the case of\nnitrogen, overabundant with respect to the sun. The anomalies are unlike those\nof Ap or Am stars.\n  Conclusions: We suggest the anomalous saturation of strong lines arises from\nheating of the upper atmospheric layers by infalling material from a disk. The\noverall abundance pattern may be related to those found for the Lambda Boo\nstars, though the depletions of the refractory elements are milder, more like\nthose of Vega. However, the intermediate volatile zinc is depleted, precluding\na straightforward interpretation of the abundance pattern in terms of gas-grain\nseparation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Six New Recycled Globular Cluster Pulsars Discovered with the Green Bank\n  Telescope: We have completed sensitive searches for new pulsars in seven globular\nclusters using the Robert C. Byrd Green Bank Telescope, and have discovered six\nnew recycled pulsars (four in NGC 6517 and two in M22), five of which are fully\nrecycled millisecond pulsars with P < 10 ms. We report full timing solutions\nfor all six new pulsars and provide estimates of their flux densities and\nspectral indices. None of the pulsars are detected in archival Chandra data\ndown to L_X~10^32 erg/s for NGC 6517 and L_X~10^31 erg/s for M22. One of the\nmillisecond pulsars in M22 appears to have a very low mass companion, and is\nlikely a new \"black widow\". A second binary pulsar in NGC 6517 is in a\nlong-period, mildly eccentric orbit. We are able to set some lower limits on\nthe age of the system, and find that it may be less than a few hundred million\nyears old, which would indicate recent pulsar recycling in NGC 6517. An\nisolated pulsar in NGC 6517 that lies about 20 core radii from the cluster\ncenter appears to have been ejected from the core by interacting with a massive\nbinary. By analyzing the luminosity function of the pulsars in NGC 6517, we\npredict the cluster to harbor roughly a dozen pulsars. We use the observed\nperiod derivatives of three pulsars to set lower limits on the mass-to-light\nratios in the cores of their host clusters, and find no evidence for large\namounts of low-luminosity matter. We also discuss reasons for non-detections in\nsome of the clusters we searched.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Partially-erupting prominences: a comparison between observations and\n  model-predicted observables: AIM: To investigate several partially-erupting prominences to study their\nrelationship with other CME-associated phenomena and to compare these\nobservations with observables predicted by a model of partially-expelled flux\nropes (Gibson & Fan, 2006a, b).\n  METHODS: We have studied 6 selected events with partially-erupting\nprominences using multi wavelength observations recorded by the\nExtreme-ultraviolet Imaging Telescope (EIT), Transition Region and Coronal\nExplorer (TRACE), Mauna Loa Solar Observatory (MLSO), Big Bear Solar\nObservatory (BBSO) and soft X-ray telescope (SXT). The observational features\nassociated with partially-erupting prominences were then compared with the\npredicted observables from the model.\n  RESULTS: The partially-expelled-flux-rope (PEFR) model of Gibson & Fan\n(2006a, b) can explain the partial eruption of these prominences, and in\naddition predicts a variety of other CME-related observables that provide\nevidence for internal reconnection during eruption. We find that all of the\npartially-erupting prominences studied in this paper exhibit indirect evidence\nfor internal reconnection. Moreover, all cases showed evidence of at least one\nobservable unique to the PEFR model, e.g., dimmings external to the source\nregion, and/or a soft X-ray cusp overlying a reformed sigmoid.\n  CONCLUSIONS: The PEFR model provides a plausible mechanism to explain the\nobserved evolution of partially-erupting-prominence-associated CMEs in our\nstudy.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Surface Magnetic Activity of the Weak-Line T Tauri Stars TWA 7 and\n  TWA 25: We present an analysis of spectropolarimetric observations of the low-mass\nweak-line T Tauri stars TWA 25 and TWA 7. The large-scale surface magnetic\nfields have been reconstructed for both stars using the technique of Zeeman\nDoppler imaging. Our surface maps reveal predominantly toroidal and\nnon-axisymmetric fields for both stars. These maps reinforce the wide range of\nsurface magnetic fields that have been recovered, particularly in pre-main\nsequence stars that have stopped accreting from the (now depleted) central\nregions of their discs. We reconstruct the large scale surface brightness\ndistributions for both stars, and use these reconstructions to filter out the\nactivity-induced radial velocity jitter, reducing the RMS of the radial\nvelocity variations from 495 m/s to 32 m/s for TWA 25, and from 127 m/s to 36\nm/s for TWA 7, ruling out the presence of close-in giant planets for both\nstars. The TWA 7 radial velocities provide an example of a case where the\nactivity-induced radial velocity variations mimic a Keplerian signal that is\nuncorrelated with the spectral activity indices. This shows the usefulness of\nlongitudinal magnetic field measurements in identifying activity-induced radial\nvelocity variations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Hydrodynamical instabilities induced by atomic diffusion in A stars and\n  their consequences: Aims. Atomic diffusion, including the effect of radiative accelerations on\nindividual elements, leads to important variations of the chemical composition\ninside the stars. The accumulation in specific layers of the elements, which\nare the main contributors of the local opacity, leads to hydrodynamical\ninstabilities that modify the internal stellar structure and surface\nabundances. Our aim is to study these effects and compare the resulting surface\nabundances with spectroscopic observations Methods. We computed the detailed\nstructure of A-type stars including these effects. We used the Toulouse-Geneva\nEvolution Code (TGEC), where radiative accelerations are computed using the\nSingle Valued Parameter (SVP) method, and we added double-diffusive convection\nwith mixing coefficients deduced from three-dimensional (3D) simulations.\nResults. We show that the modification of the initial chemical composition has\nimportant effects on the internal stellar mixing and leads to different surface\nabundances of the elements. The results fit the observed surface chemical\ncomposition well if the layers, which are individually mixed by\ndouble-diffusive convection, are connected.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Closest View of a Fast Coronal Mass Ejection: How Faulty Assumptions\n  near Perihelion Lead to Unrealistic Interpretations of PSP/WISPR Observations: We report on the closest view of a coronal mass ejection observed by the\nParker Solar Probe (PSP)/Wide-field Imager for {Parker} Solar PRobe (WISPR)\ninstrument on September 05, 2022, when PSP was traversing from a distance of\n15.3~to~13.5~R$_\\odot$ from the Sun. The CME leading edge and an arc-shaped\n{\\emph{concave-up} structure near the core} was tracked in WISPR~field of view\nusing the polar coordinate system, for the first time. Using the impact\ndistance on Thomson surface, we measured average speeds of CME leading edge and\nconcave-up structure as $\\approx$2500~$\\pm$~270\\,km\\,s$^{-1}$ and\n$\\approx$400~$\\pm$~70\\,km\\,s$^{-1}$ with a deceleration of\n$\\approx$20~m~s$^{-2}$ for the later. {The use of the plane-of-sky approach\nyielded an unrealistic speed of more than three times of this estimate.} We\nalso used single viewpoint STEREO/COR-2A images to fit the Graduated\nCylindrical Shell (GCS) model to the CME while incorporating the source region\nlocation from EUI of Solar Orbiter and estimated a 3D speed of\n$\\approx$2700\\,km\\,s$^{-1}$. We conclude that this CME exhibits the highest\nspeed during the ascending phase of solar cycle 25. This places it in the\ncategory of extreme speed CMEs, which account for only 0.15\\% of all CMEs\nlisted in the CDAW CME catalog.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Modeling Magnetically Channeled Winds in 3D: I. Isothermal Simulations\n  of a Magnetic O Supergiant: In this paper we present the first set of 3D magnetohydrodynamic (MHD)\nsimulations performed with the riemann geomesh code. We study the dynamics of\nthe magnetically channeled winds of magnetic massive stars in full three\ndimensions using a code that is uniquely suited to spherical problems.\nSpecifically, we perform isothermal simulations of a smooth wind on a rotating\nstar with a tilted, initially dipolar field. We compare the mass-loss, angular\nmomentum loss, and magnetospheric dynamics of a template star (with the\nproperties that are reminiscent of the O4 supergiant {\\zeta} Pup) over a range\nof rotation rates, magnetic field strengths, and magnetic tilt angles. The\nsimulations are run up to a quasi-steady state and the results are observed to\nbe consistent with the existing literature, showing the episodic centrifugal\nbreakout events of the mass outflow, confined by the magnetic field loops that\nform the closed magnetosphere of the star. The catalogued results provide\nperspective on how angular-momentum loss varies for different configurations of\nrotation rate, magnetic field strength, and large magnetic tilt angles. In\nagreement with previous 2D MHD studies, we find that high magnetic confinement\nreduces the overall mass-loss rate, and higher rotation increases the mass-loss\nrate. This and future studies will be used to estimate the angular-momentum\nevolution, spin-down time, and mass-loss evolution of magnetic massive stars as\na function of magnetic field strength, rotation rate, and dipole tilt.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Continuum and line emission of flares on red dwarf stars: The emission spectrum has been calculated of a homogeneous pure hydrogen\nlayer, which parameters are typical for a flare on a red dwarf. The ionization\nand excitation states were determined by the solution of steady-state equations\ntaking into account the continuum and all discrete hydrogen levels. We consider\nthe following elementary processes: electron-impact transitions, spontaneous\nand induced radiative transitions, and ionization by the bremsstrahlung and\nrecombination radiation of the layer itself. The Biberman--Holstein\napproximation was used to calculate the scattering of line radiation.\nAsymptotic formulae for the escape probability are obtained for a symmetric\nline profile taking into account the Stark and Doppler effects. The\napproximation for the core of the H$-\\alpha$ line by a gaussian curve has been\nsubstantiated.\n  The spectral intensity of the continuous spectrum, the intensity of the lines\nof the Balmer series and the magnitude of the Balmer jump have been calculated.\nThe conditions have been determined for which the Balmer jump and the emission\nline intensities above the continuum decrease to such low values that the\nemission spectrum can be assumed to be continuum as well as the conditions at\nwhich the emission spectrum becomes close to the blackbody.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Arrival time calculation for interplanetary coronal mass ejections with\n  circular fronts and application to STEREO observations of the 2009 February\n  13 eruption: A goal of the NASA STEREO mission is to study the feasibility of forecasting\nthe direction, arrival time and internal structure of solar coronal mass\nejections (CMEs) from a vantage point outside the Sun-Earth line. Through a\ncase study, we discuss the arrival time calculation of interplanetary CMEs\n(ICMEs) in the ecliptic plane using data from STEREO/SECCHI at large\nelongations from the Sun in combination with different geometric assumptions\nabout the ICME front shape (Fixed-\\Phi (FP): a point and harmonic Mean (HM): a\ncircle). These forecasting techniques use single-spacecraft imaging data and\nare based on the assumptions of constant velocity and direction. We show that\nfor the slow (350 km/s) ICME on 2009 February 13-18, observed at quadrature by\nthe two STEREO spacecraft, the results for the arrival time given by the HM\napproximation are more accurate by 12 hours than those for FP in comparison to\nin situ observations of solar wind plasma and magnetic field parameters by\nSTEREO/IMPACT/PLASTIC, and by 6 hours for the arrival time at Venus Express\n(MAG). We propose that the improvement is directly related to the ICME front\nshape being more accurately described by HM for an ICME with a low inclination\nof its symmetry axis to the ecliptic. In this case the ICME has to be tracked\nto > 30{\\deg} elongation to get arrival time errors < 5 hours. A newly derived\nformula for calculating arrival times with the HM method is also useful for a\ntriangulation technique assuming the same geometry.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Extended radio emission associated with a breakout eruption from the\n  back side of the Sun: Context. Coronal mass ejections (CMEs) on the Sun are the largest explosions\nin the Solar System that can drive powerful plasma shocks. The eruptions,\nshocks, and other processes associated to CMEs are efficient particle\naccelerators and the accelerated electrons in particular can produce radio\nbursts through the plasma emission mechanism. Aims. Coronal mass ejections and\nassociated radio bursts have been well studied in cases where the CME\noriginates close to the solar limb or within the frontside disc. Here, we study\nthe radio emission associated with a CME eruption on the back side of the Sun\non 22 July 2012. Methods. Using radio imaging from the Nan\\c{c}ay\nRadioheliograph, spectroscopic data from the Nan\\c{c}ay Decametric Array, and\nextreme-ultraviolet observations from the Solar Dynamics Observatory and Solar\nTerrestrial Relations Observatory spacecraft, we determine the nature of the\nobserved radio emission as well as the location and propagation of the CME.\nResults. We show that the observed low-intensity radio emission corresponds to\na type II radio burst or a short-duration type IV radio burst associated with a\nCME eruption due to breakout reconnection on the back side of the Sun, as\nsuggested by the pre-eruptive magnetic field configuration. The radio emission\nconsists of a large, extended structure, initially located ahead of the CME,\nthat corresponds to various electron acceleration locations. Conclusions. The\nobservations presented here are consistent with the breakout model of CME\neruptions. The extended radio emission coincides with the location of the\ncurrent sheet and quasi-separatrix boundary of the CME flux and the overlying\nhelmet streamer and also with that of a large shock expected to form ahead of\nthe CME in this configuration.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Uncovering new white dwarf - open cluster associations using Gaia DR3: Context: Open clusters (OCs) provide homogeneous samples of white dwarfs\n(WDs) with known distances, extinctions, and total ages. The unprecedented\nastrometric precision of \\textit{\\textit{Gaia}} allows us to identify many\nnovel OC--WD pairs. Studying WDs in the context of their parent OCs makes it\npossible to determine the properties of WD progenitors and study the\ninitial-final mass relation (IFMR). Aims: We seek to find potential new WD\nmembers of OCs in the solar vicinity. The analysis of OC members' parallaxes\nallows us to determine the OC distances to a high precision, which in turn\nenables us to calculate WD masses and cooling ages and to constrain the IFMR.\nMethods: We searched for new potential WD members of nearby OCs using the\ndensity-based machine learning clustering algorithm \\texttt{HDBSCAN}. The\nclustering analysis was applied in five astrometric dimensions -- positions in\nthe sky, proper motions and parallaxes -- and in three dimensions where the\npositional information was not considered in the clustering analysis. The\nidentified candidate OC WDs were further filtered using the photometric\ncriteria and properties of their putative host OCs. The masses and cooling ages\nof the WDs were calculated via a photometric method using all available\n\\textit{\\textit{Gaia}}, Pan-STARRS, SDSS, and GALEX photometry. The WD\nprogenitor masses were determined using the ages and metallicities of their\nhost OCs. Results: Altogether, 63 OC WD candidates were recovered, 27 of which\nare already known in the literature. We provide characterization for 36 novel\nWDs that have significant OC membership probabilities. Six of them fall into\nrelatively unconstrained sections of the IFMR where the relation seems to\nexhibit nonlinear behavior. We were not able to identify any WDs originating\nfrom massive progenitors that would even remotely approach the widely adopted\nWD progenitor mass limit. (abridged)",
        "category": "astro-ph_SR"
    },
    {
        "text": "Very fast variations of SiO maser emission in evolved stars: Context. Stars on the asymptotic giant branch (AGB) are long-period variables\nthat present strong flux variations at almost all wavelengths, including the\nSiO maser lines. The periods of these variations are of 300-500 days in\nMira-type stars and somewhat shorter in semi-regular variables. The variability\nof the SiO lines on short timescales has been investigated, but the data are\ninconclusive. Aims. We aim to study the time evolution of the SiO maser lines\nin Mira-type and semi-regular variables at short timescales. We also discuss\nthe origin of the observed fast variations. Methods. We observed the SiO maser\nlines at 7 mm (28SiO v=1,2 J=1-0) and 3 mm (28SiO v=1 J=2-1) using the 40 m\nYebes antenna and the 30 m IRAM telescope, respectively, with a minimum spacing\nof 1 day. We studied the semi-regular variables RX Boo and RT Vir and the\nMira-type variables U Her, R LMi, R Leo, and $\\chi$ Cyg. We performed a\ndetailed statistical analysis of the variations on different timescales.\nResults. RX Boo shows strong and fast variations in the intensity of the\ndifferent spectral features of the SiO lines at 7 mm and 3 mm. On a timescale\nof one day, we find variations of >10% in 25% of the cases. Variations of\ngreater than $\\sim$50% are often found when the observations are separated by 2\nor 3 days. A similar variation rate of the SiO lines at 7 mm is found for RT\nVir, but the observations of this object are less complete. On the contrary,\nthe variations of the SiO maser line intensity in the Mira-type variables are\nmoderate, with typical variation rates around <10% in 7 days. This phenomenon\ncan be explained by the presence of particularly small maser-emitting clumps in\nsemi-regular variables, which would lead to a strong dependence of the\nintensity on the density variations due to the passage of shocks.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Peeking Between the Pulses: The Far-UV Spectrum of the Previously Unseen\n  White Dwarf in AR Scorpii: The compact object in the interacting binary AR Sco has widely been presumed\nto be a rapidly rotating, magnetized white dwarf (WD), but it has never been\ndetected directly. Isolating its spectrum has proven difficult because the\nspin-down of the WD generates pulsed synchrotron radiation that far outshines\nthe WD's photosphere. As a result, a previous study of AR Sco was unable to\ndetect the WD in the averaged far-ultraviolet spectrum from a Hubble Space\nTelescope (HST) observation. In an effort to unveil the WD's spectrum, we\nreanalyze these HST observations by calculating the average spectrum in the\ntroughs between synchrotron pulses. We identify weak spectral features from the\npreviously unseen WD and estimate its surface temperature to be 11500$\\pm$500K.\nAdditionally, during the synchrotron pulses, we detect broad Lyman-$\\alpha$\nabsorption consistent with hot WD spectral models. We infer the presence of a\npair of hotspots, with temperatures between 23000K and 28000K, near the\nmagnetic poles of the WD. As the WD is not expected to be accreting from its\ncompanion, we describe two possible mechanisms for heating the magnetic poles.\nThe Lyman-$\\alpha$ absorption of the hotspots appears relatively undistorted by\nZeeman splitting, constraining the WD's field strength to be 100 MG, but the\ndata are insufficient to search for the subtle Zeeman splits expected at lower\nfield strengths.",
        "category": "astro-ph_SR"
    },
    {
        "text": "WISE Y Dwarfs As Probes of the Brown Dwarf-Exoplanet Connection: We have determined astrometric positions for 15 WISE-discovered late-type\nbrown dwarfs (6 T8-9 and 9 Y dwarfs) using the Keck II telescope, the Spitzer\nSpace Telescope, and the Hubble Space Telescope. Combining data from 8 to 20\nepochs we derive parallactic and proper motions for these objects which put the\nmajority within 15 parsecs. For ages greater than a few Gyr, as suggested from\nkinematic considerations, we find masses of 10-30 M$_{Jup}$ based on standard\nmodels for the evolution of low mass objects with a range of mass estimates for\nindividual objects depending on the model in question. Three of the coolest\nobjects have effective temperatures $\\sim$ 350 K and inferred masses of 10-15\nM$_{Jup}$. Our parallactic distances confirm earlier photometric estimates\n(Kirkpatrick et al. 2012) and direct measurements (Marsh et al. 2013, Beichman\net al. 2013, Dupuy and Krauss 2013) and suggest that the number of objects with\nmasses below about 15 M$_{Jup}$ must be flat or declining relative to higher\nmass objects. The masses of the coldest Y dwarfs may be similar to those\ninferred for recently imaged planet-mass companions to nearby young stars.\nObjects in this mass range, which appear to be rare in both the interstellar\nand proto-planetary environments, may both have formed via gravitational\nfragmentation: the brown dwarfs in interstellar clouds and companion objects in\na protoplanetary disk. In both cases, however, the fact that objects in this\nmass range are relatively infrequent, suggests that this mechanism must be\ninefficient in both environments.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Close Binary Companions to APOGEE DR16 Stars: 20,000 Binary-star Systems\n  Across the Color-Magnitude Diagram: Many problems in contemporary astrophysics---from understanding the formation\nof black holes to untangling the chemical evolution of galaxies---rely on\nknowledge about binary stars. This, in turn, depends on discovery and\ncharacterization of binary companions for large numbers of different kinds of\nstars in different chemical and dynamical environments. Current stellar\nspectroscopic surveys observe hundreds of thousands to millions of stars with\n(typically) few observational epochs, which allows binary discovery but makes\norbital characterization challenging. We use a custom Monte Carlo sampler (The\nJoker) to perform discovery and characterization of binary systems through\nradial-velocities, in the regime of sparse, noisy, and poorly sampled\nmulti-epoch data. We use it to generate posterior samplings in Keplerian\nparameters for 232,531 sources released in APOGEE Data Release 16. Our final\ncatalog contains 19,635 high-confidence close-binary (P < few years, a < few\nAU) systems that show interesting relationships between binary occurrence rate\nand location in the color-magnitude diagram. We find notable faint companions\nat high masses (black-hole candidates), at low masses (substellar candidates),\nand at very close separations (mass-transfer candidates). We also use the\nposterior samplings in a (toy) hierarchical inference to measure the\nlong-period binary-star eccentricity distribution. We release the full set of\nposterior samplings for the entire parent sample of 232,531 stars. This set of\nsamplings involves no heuristic \"discovery\" threshold and therefore can be used\nfor myriad statistical purposes, including hierarchical inferences about\nbinary-star populations and sub-threshold searches.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The timescale of low-mass proto-helium white dwarf evolution: A large number of low-mass (< 0.20 M_sun) helium white dwarfs (He WDs) have\nrecently been discovered. The majority of these are orbiting another WD or a\nmillisecond pulsar (MSP) in a close binary system; a few examples are found to\nshow pulsations or to have a main-sequence star companion. There appear to be\ndiscrepancies between the current theoretical modelling of such low-mass He WDs\nand a number of key observed cases, indicating that their formation scenario\nremains to be fully understood. Here we investigate the formation of detached\nproto-He WDs in close-orbit low-mass X-ray binaries (LMXBs). Our prime focus is\nto examine the thermal evolution and the contraction phase towards the WD\ncooling track and investigate how this evolution depends on the WD mass. Our\ncalculations are then compared to the most recent observational data. Numerical\ncalculations with a detailed stellar evolution code were used to trace the\nmass-transfer phase in a large number of close-orbit LMXBs. Subsequently, we\nfollowed the evolution of the detached low-mass proto-He WDs, including stages\nwith residual shell hydrogen burning and vigorous flashes caused by unstable\nCNO burning. We find that the time between Roche-lobe detachment until the\nlow-mass proto-He WD reaches the WD cooling track is typically Delta_t_proto =\n0.5 - 2 Gyr, depending systematically on the WD mass and therefore on its\nluminosity. The lowest WD mass for developing shell flashes is ~0.21 M_sun for\nprogenitor stars of mass M2 <= 1.5 M_sun (and ~0.18 M_sun for M2 = 1.6 M_sun).\nThe long timescale of low-mass proto-He WD evolution can explain a number of\nrecent observations, including some MSP systems hosting He WD companions with\nvery low surface gravities and high effective temperatures. We find no evidence\nfor Delta_t_proto to depend on the occurrence of flashes and thus question the\nsuggested dichotomy in thermal evolution of proto-WDs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Revealing the Two `Horns' of Taurus with GAIA DR2: We investigate the spatial properties of sources from the GAIA catalogue\npreviously identified as being members of the Taurus star forming region and\nwhich appear in the Spitzer catalogue. We study an area of sky of 10x15 deg,\ncentred on Right Ascension (2000.0) = 68.5 deg and Declination (2000.0)= 27.0\ndeg, this being an area surrounding the Taurus molecular cloud. We use data\nobtained from the GAIA DR2 release. By using an inversion of GAIA parallax\nmeasurements to obtain distance values and by defining limits to the proper\nmotions of the Taurus moving group, we are able to show that there are\nsubstantial differences in depth within the Taurus complex. Our results suggest\nthat the Taurus cloud has significant depth and that there are two main\nassociations centred at ~130.6+/-0.7 pc and ~160.2+/-0.9 pc at 1{\\sigma}. These\ntwo associations also have different proper motions, of 24.5+/-2.8 and\n20.1+/-2.4 mas/yr respectively. We here label them the 'Two Horns' of Taurus.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A cavity and further radial substructures in the disk around HD~97048: Context: Gaps, cavities and rings in circumstellar disks are signposts of\ndisk evolution and planet-disk interactions. We follow the recent suggestion\nthat Herbig Ae/Be disks with a flared disk harbour a cavity, and investigate\nthe disk around HD~97048.\n  Aims: We aim to resolve the 34$\\pm$ 4 au central cavity predicted by Maaskant\net al. (2013) and to investigate the structure of the disk.\n  Methods: We image the disk around HD~97048 using ALMA at 0.85~mm and 2.94~mm,\nand ATCA (multiple frequencies) observations. Our observations also include the\n12CO J=1-0, 12CO J=3-2 and HCO+ J=4-3 emission lines.\n  Results: A central cavity in the disk around HD~97048 is resolved with a\n40-46 au radius. Additional radial structure present in the surface brightness\nprofile can be accounted for either by an opacity gap at ~90 au or by an extra\nemitting ring at ~150 au. The continuum emission tracing the dust in the disk\nis detected out to 355 au. The 12CO J=3-2 disk is detected 2.4 times farther\nout. The 12CO emission can be traced down to $\\approx$ 10 au scales.\nNon-Keplerian kinematics are detected inside the cavity via the HCO+ J=4-3\nvelocity map. The mm spectral index measured from ATCA observations suggests\nthat grain growth has occurred in the HD~97048 disk. Finally, we resolve a\nhighly inclined disk out to 150 au around the nearby 0.5~$M_{\\odot}$ binary\nISO-ChaI 126.\n  Conclusions: The data presented here reveal a cavity in the disk of HD 97048,\nand prominent radial structure in the surface brightness. The cavity size\nvaries for different continuum frequencies and gas tracers. The gas inside the\ncavity follows non-Keplerian kinematics seen in HCO+ emission. The variable\ncavity size along with the kinematical signature suggests the presence of a\nsubstellar companion or massive planet inside the cavity.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Seismological determination of the Alfv\u00e9n speed and plasma-beta in\n  solar photospheric bright points: The Alfv\\'{e}n speed and plasma beta in photospheric bright points observed\nby the Broadband Filter Imager (BFI) of the Solar Optical Telescope (SOT) on\nboard the \\textit{Hinode} satellite, are estimated seismologically. The\ndiagnostics is based on the theory of slow magnetoacoustic waves in a\nnon-isothermally stratified photosphere with a uniform vertical magnetic field.\nWe identify and track bright points in a G-band movie by using the 3D region\ngrowing method, and align them with blue continuum images to derive their\nbrightness temperatures. From the Fourier power spectra of 118 continuum light\ncurves made in the bright points, we find that light curves of 91 bright points\nhave oscillations with properties which are significantly different from\noscillation in quiet regions, with the periods ranging 2.2--16.2~min. We find\nthat the model gives a moderate value of the plasma beta when $\\gamma$ lies at\naround 5/3. The calculated Alfv\\'{e}n speed is 9.68$\\pm$2.02~km~s$^{-1}$,\nranging in 6.3--17.4~km~s$^{-1}$. The plasma beta is estimated to be of\n0.93$\\pm$0.36, ranging in 0.2--1.9.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The L dwarf/T dwarf transition: multiplicity, magnetic activity and\n  mineral meteorology across the hydrogen burning limit: The transition between the L dwarf and T dwarf spectral classes is one of the\nmost remarkable along the stellar/brown dwarf Main Sequence, separating sources\nwith photospheres containing mineral condensate clouds from those containing\nmethane and ammonia gases. Unusual characteristics of this transition include a\n1 micron brightening between late L and early T dwarfs observed in both\nparallax samples and coeval binaries; a spike in the multiplicity fraction;\nevidence of increased photometric variability, possibly arising from patchy\ncloud structures; and a delayed transition for young, planetary-mass objects.\nAll of these features can be explained if this transition is governed by the\n\"rapid\" (nonequlibrium) rainout of clouds from the photosphere, triggered by\ntemperature, surface gravity, metallicity and (perhaps) rotational effects.\nWhile the underlying mechanism of this rainout remains under debate, the\ntransition is now being exploited to discover and precisely characterize tight\n(< 1 AU) very low-mass binaries that can be used to test brown dwarf\nevolutionary and atmospheric theories, and resolved binaries that further\nconstrain the properties of this remarkable transition.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Spectral analysis of 3D MHD models of coronal structures: We study extreme-ultraviolet emission line spectra derived from\nthree-dimensional magnetohydrodynamic models of structures in the corona. In\norder to investigate the effects of increased magnetic activity at photospheric\nlevels in a numerical experiment, a much higher magnetic flux density is\napplied at photospheric levels as compared to the Sun. Thus, we can expect our\nresults to highlight the differences between the Sun and more active, but still\nsolar-like stars. We discuss signatures seen in extreme-ultraviolet emission\nlines synthesized from these models and compare them to signatures found in the\nspatial distribution and temporal evolution of Doppler shifts in lines formed\nin the transition region and corona. This is of major interest to test the\nquality of the underlying magnetohydrodynamic model to heat the corona, i.e.\ncurrents in the corona driven by photospheric motions (flux braiding).",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Cepheid Impostor HD 18391 and its Anonymous Parent Cluster: New and existing photometry for the G0 Ia supergiant HD 18391 is analyzed in\norder to confirm the nature of the variablity previously detected in the star,\nwhich lies off the hot edge of the Cepheid instability strip. Small-amplitude\nvariability at a level of \\Delta V = 0.016+-0.002 is indicated, with a period\nof P=123.04+-0.06 d. A weaker second signal may be present at P=177.84+-0.18\nwith \\Delta V = 0.007+-0.002, likely corresponding to fundamental mode\npulsation if the primary signal represents overtone pulsation (123.04/177.84 =\n0.69). The star, with a spectroscopic reddening of E(B-V) = 1.02, is associated\nwith heavily-reddened B-type stars in its immediate vicinity that appear to be\noutlying members of an anonymous young cluster centered ~10 arcmin to the west\nand 1661+-73 pc distant. The cluster has nuclear and coronal radii of r_n=3.5\narcmin and R_c=14 arcmin, respectively, while the parameters for HD 18391\nderived from membership in the cluster with its outlying B stars are consistent\nwith those implied by its Cepheid-like pulsation, provided that it follows the\nsemi-period-luminosity relation expected of such objects. Its inferred\nluminosity as a cluster member is M_V=-7.76+-0.10, its age (9+-1)x10^6 years,\nand its evolutionary mass ~19 M_{\\sun}. HD 18391 is not a classical Cepheid,\nyet it follows the Cepheid period-luminosity relation closely, much like\nanother Cepheid impostor, V810 Cen.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The EBLM project -- IX. Five fully convective M-dwarfs, precisely\n  measured with CHEOPS and TESS light curves: Eclipsing binaries are important benchmark objects to test and calibrate\nstellar structure and evolution models. This is especially true for binaries\nwith a fully convective M-dwarf component for which direct measurements of\nthese stars' masses and radii are difficult using other techniques. Within the\npotential of M-dwarfs to be exoplanet host stars, the accuracy of theoretical\npredictions of their radius and effective temperature as a function of their\nmass is an active topic of discussion. Not only the parameters of transiting\nexoplanets but also the success of future atmospheric characterisation rely on\naccurate theoretical predictions. We present the analysis of five eclipsing\nbinaries with low-mass stellar companions out of a sub-sample of 23, for which\nwe obtained ultra high-precision light curves using the CHEOPS satellite. The\nobservation of their primary and secondary eclipses are combined with\nspectroscopic measurements to precisely model the primary parameters and derive\nthe M-dwarfs mass, radius, surface gravity, and effective temperature estimates\nusing the PYCHEOPS data analysis software. Combining these results to the same\nset of parameters derived from TESS light curves, we find very good agreement\n(better than 1\\% for radius and better than 0.2% for surface gravity). We also\nanalyse the importance of precise orbits from radial velocity measurements and\nfind them to be crucial to derive M-dwarf radii in a regime below 5% accuracy.\nThese results add five valuable data points to the mass-radius diagram of\nfully-convective M-dwarfs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "High Level Magnetic Activity Nature of an Eclipsing Binary KIC 12418816: We present comprehensive spectroscopic and photometric analysis of the\ndetached eclipsing binary KIC 12418816, which is composed of two very similar\nand young main sequence stars of spectral type K0 on a circular orbit.\nCombining spectroscopic and photometric modelling, we find masses and radii of\nthe components as 0.88+-0.06 Msun and 0.84+-0.05 Msun, and 0.85+-0.02 Rsun, and\n0.84+-0.02 Rsun for the primary and the secondary, respectively. Both\ncomponents exhibit narrow emission features superposed on the cores of the Ca\nII H&K lines, while H alpha and H beta photospheric absoprtion is more\ncompletely infilled by broader emission. Very high precision Kepler photometry\nreveals remarkable sinusoidal light variation at out-of-eclipse phases,\nindicating strong spot activity, presumably on the surface of the secondary\ncomponent. Spots on the secondary component appear to migrate towards\ndecreasing orbital phases with a migration period of 0.72+-0.05 year. Besides\nthe sinusoidal variation, we detect 81 flares, and find that both components\npossess flare activity. Our analysis shows that 25 flares among 81 come exhibit\nvery high energies with lower frequency, while the rest of them are very\nfrequent with lower energies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Initial Magnetic Field Distribution in AB Stars: Stars are born with magnetic fields, but the distribution of their initial\nfield strengths remains uncertain. We combine observations with theoretical\nmodels of magnetic field evolution to infer the initial distribution of\nmagnetic fields for AB stars in the mass range of 1.6 - 3.4 M$_{\\odot}$. We\ntested a variety of distributions with different shapes and found that a\ndistribution with a mean of $\\sim$800 G and a full width of $\\sim$600 G is most\nconsistent with the observed fraction of strongly magnetized stars as a\nfunction of mass. Our most-favored distribution is a Gaussian with a mean of\n$\\mu$ = 770 G and standard deviation of $\\sigma$ = 146 G. Independent\napproaches to measure the typical field strength suggest values closer to 2 - 3\nkG, a discrepancy that could suggest a mass-dependent and bimodal initial field\ndistribution, or an alternative theoretical picture for the origin of these\nmagnetic fields.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Updated High-Temperature Opacities for the Dartmouth Stellar Evolution\n  Program and their Effect on the Jao Gap Location: The Jao Gap, a 17 percent decrease in stellar density at $M_G \\sim10$\nidentified in both Gaia DR2 and EDR3 data, presents a new method to probe the\ninterior structure of stars near the fully convective transition mass. The Gap\nis believed to originate from convective kissing instability wherein asymmetric\nproduction of $^{3}$He causes the core convective zone of a star to\nperiodically expand and contract and consequently the stars' luminosity to\nvary. Modeling of the Gap has revealed a sensitivity in its magnitude to a\npopulation's metallicity primarily through opacity. Thus far, models of the Jao\nGap have relied on OPAL high-temperature radiative opacities. Here we present\nupdated synthetic population models tracing the Gap location modeled with the\nDartmouth stellar evolution code using the OPLIB high-temperature radiative\nopacities. Use of these updated opacities changes the predicted location of the\nJao Gap by $\\sim0.05$ mag as compared to models which use the OPAL opacities.\nThis difference is likely too small to be detectable in empirical data.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the circumstellar envelopes of semi-regular long-period variables: The mass loss process along the AGB phase is crucial for the formation of\ncircumstellar envelopes (CSEs), which in the post-AGB phase will evolve into\nplanetary nebulae (PNe). There are still important issues that need to be\nfurther explored in this field; in particular, the formation of axially\nsymmetric PNe from spherical CSEs. To address the problem, we have conducted\nhigh S/N IRAM 30m observations of 12CO J=1-0 and J=2-1, and 13CO J=1-0 in a\nvolume-limited unbiased sample of semi-regular variables (SRs). We also\nconducted Yebes 40m SiO J=1-0 observations in 1/2 of the sample in order to\ncomplement our 12CO observations. We report a moderate correlation between mass\nloss rates and the 12CO(1-0)-to-12CO(2-1) intensity ratio, introducing a\npossible new method for determining mass loss rates of SRs with short analysis\ntime. We also find that for several stars the SiO profiles are very similar to\nthe 12CO profiles, a totally unexpected result unless these are non-standard\nenvelopes.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Energetic consequences of flux emergence: When magnetic field in the solar convection zone buoyantly rises to pierce\nthe visible solar surface (photosphere), the atmosphere (corona) above this\nsurface must respond in some way. One response of the coronal field to\nphotospheric forcing is the creation of stress in the magnetic field,\ngenerating large currents and storing magnetic free energy. Using a topological\nmodel of the coronal magnetic field we will quantify this free energy. We find\nthe free energy just prior to major flares in active regions to be between 30%\nand 50% of the potential field energy. In a second way, the coronal field may\ntopologically restructure to form new magnetic connections with newly emerged\nfields. We use our topological model to quantify the rapid restructuring in the\ncase of solar flare and coronal mass ejections, finding that between 1% and 10%\nof total active region flux is exchanged. Finally, we use observational data to\nquantify the slow, quiescent reconnection with preexisting field, and find that\nfor small active regions between 20% and 40% of the total emerged flux may have\nreconnected at any given time.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The CIDA Variability Survey of Orion OB1 II: demographics of the young,\n  low-mass stellar populations: We present results of our large scale, optical, multi-epoch photometric\nsurvey of ~180 square degrees across the Orion OB1 association, complemented\nwith extensive follow up spectroscopy. We map and characterize in an uniform\nway the off-cloud, low-mass, pre-main sequence populations. We report 2064,\nmostly K and M-type, confirmed T Tauri members. Most (59%) are located in the\nOB1a subassociation, 27% in OB1b, and 14% within the confines of the A and B\nmolecular clouds. There is significant structure in the spatial distribution of\nthe young stars. We characterize two new clusterings of T Tauri stars, HD 35762\nand HR 1833 in the OB1a subassociation, and two stellar overdensities in OB1b.\nThere is indication of two populations of young stars in the OB1b region,\nlocated at two different distances, which may be due to the OB1a subassociation\noverlapping on front of OB1b. The various groups and regions can be ordered in\nan age sequence that agrees with the long standing picture of star formation\nstarting in Orion OB1a some 10-15 Myr ago.\n  We define a new type of T Tauri star, the C/W class, objects we propose may\nbe nearing the end of their accretion phase. We detect the observational\nsignature of Li depletion in young K and M stars with a timescale of 8.5 Myr.\nThe decline of the accretion fraction from ~2 - 10 Myr, implies an accretion\ne-folding timescale of 2.1 Myr. Finally, the median amplitude of the V-band\nvariability shows the decline of stellar activity, from accreting Classical T\nTauri stars to the least active field dwarfs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Abundances of Suprathermal Heavy Ions in CIRs on STEREO during the\n  Minimum of Solar Cycle 23: We examine the composition of the 0.1 - 1 MeV/n interplanetary heavy ions\nfrom H to Fe in corotating interaction regions (CIRs) measured by the SIT\n(Suprathermal Ion Telescope) instrument. We use observations taken on board the\ntwo STEREO spacecraft during the unusually long minimum of Solar Cycle 23 from\nJanuary 2007 through December 2010. During this period instruments on STEREO\nobserved more than 50 CIR events making it possible to investigate CIR ion\nabundances during solar minimum conditions with unprecedentedly high\nstatistics. The observations reveal annual variations of relative ion\nabundances in the CIRs during the 2007 - 2008 period. In 2010 the elemental\ncomposition in CIRs were influenced by solar energetic particle events.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Coronal radiation belts: The magnetic field of the solar corona has a large-scale dipole character,\nwhich maps into the bipolar field in the solar wind. Using standard\nrepresentations of the coronal field, we show that high-energy ions can be\ntrapped stably in these large-scale closed fields. The drift shells that\ndescribe the conservation of the third adiabatic invariant may have complicated\ngeometries. Particles trapped in these zones would resemble the Van Allen Belts\nand could have detectable consequences. We discuss potential sources of trapped\nparticles.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Hydrodynamic models of pulsation period evolution in R Hydrae: Pulsation period evolution during the helium-shell flash in the Mira variable\nR Hya is investigated using consistent stellar evolution and non-linear stellar\npulsation computations. The initial and time-dependent inner boundary\nconditions for the equations of radiation hydrodynamics describing non-linear\nstellar oscillations were determined using a grid of TP-AGB model sequences\nwith initial masses on the main sequence $1.5M_\\odot\\le M_\\mathrm{ZAMS}\\le\n5.0M_\\odot$ and the initial metallicity $Z=0.014$. The setup of initial\nconditions for hydrodynamic models corresponds to $\\approx 100$ yr prior to the\nmaximum of the helium-shell luminosity and ensures that the stellar envelope of\nthe evolution model is under both hydrostatic and thermal equilibrium. Solution\nof the equations of hydrodynamics allowed us to determine the temporal\nvariation of the pulsation period $\\Pi(t)$ during $\\approx 500$~yr. Within this\ntime interval R Hya is a fundamental mode pulsator. The period temporal\ndependencies $\\Pi(t)$ calculated for the AGB star models at the beginning of\nthe third dredge-up phase and with masses $4.4M_\\odot\\le M\\le 4.5M_\\odot$ are\nin agreement with observational estimates of the period of R Hya obtained\nduring last two centuries. The mean radius of R Hya pulsation models at the end\nof the XX century ($470 R_\\odot < \\bar{R} < 490 R_\\odot$) agrees with\nobservational estimates obtained using the interferometric angular diameter\nmeasurements.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Parameters of rotating neutron stars with and without hyperons: The discovery of a 2 Msun neutron star provided a robust constraint for the\ntheory of exotic dense matter, bringing into question the existence of strange\nbaryons in the interiors of neutron stars. Although many theories fail to\nreproduce this observational result, several equations of state containing\nhyperons are consistent with it. We study global properties of stars using\nequations of state containing hyperons, and compare them to those without\nhyperons to find similarities, differences, and limits that can be compared\nwith the astrophysical observations. Rotating, axisymmetric, and stationary\nstellar configurations in general relativity are obtained, and their global\nparameters are studied. Approximate formulae describing the behavior of the\nmaximum and minimum stellar mass, compactness, surface redshifts, and moments\nof inertia as functions of spin frequency are provided. We also study the thin\ndisk accretion and compare the spin-up evolution of stars with different\nmoments of inertia.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Observations and numerical models of solar coronal heating associated\n  with spicules: Spicules have been proposed as significant contributors to the mass and\nenergy balance of the corona. While previous observations have provided a\nglimpse of short-lived transient brightenings in the corona that are associated\nwith spicules, these observations have been contested and are the subject of a\nvigorous debate both on the modeling and the observational side. Therefore, it\nremains unclear whether plasma is heated to coronal temperatures in association\nwith spicules. We use high-resolution observations of the chromosphere and\ntransition region with the Interface Region Imaging Spectrograph (IRIS) and of\nthe corona with the Atmospheric Imaging Assembly (AIA) onboard the Solar\nDynamics Observatory (SDO) to show evidence of the formation of coronal\nstructures associated with spicular mass ejections and heating of plasma to\ntransition region and coronal temperatures. Our observations suggest that a\nsignificant fraction of the highly dynamic loop fan environment associated with\nplage regions may be the result of the formation of such new coronal strands, a\nprocess that previously had been interpreted as the propagation of transient\npropagating coronal disturbances (PCD)s. Our observations are supported by 2.5D\nradiative MHD simulations that show heating to coronal temperatures in\nassociation with spicules. Our results suggest that heating and strong flows\nplay an important role in maintaining the substructure of loop fans, in\naddition to the waves that permeate this low coronal environment.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The intermediate neutron-capture process and carbon-enhanced metal-poor\n  stars: Carbon-enhanced metal-poor (CEMP) stars in the Galactic Halo display\nenrichments in heavy elements associated with either the s (slow) or the r\n(rapid) neutron-capture process (e.g., barium and europium respectively), and\nin some cases they display evidence of both. The abundance patterns of these\nCEMP-s/r stars, which show both Ba and Eu enrichment, are particularly puzzling\nsince the s and the r processes require neutron densities that are more than\nten orders of magnitude apart, and hence are thought to occur in very different\nstellar sites with very different physical conditions. We investigate whether\nthe abundance patterns of CEMP-s/r stars can arise from the nucleosynthesis of\nthe intermediate neutron-capture process (the i process), which is\ncharacterised by neutron densities between those of the s and the r processes.\nUsing nuclear network calculations, we study neutron capture nucleosynthesis at\ndifferent constant neutron densities n ranging from $10^7$ to $10^{15}$\ncm$^{-3}$. With respect to the classical s process resulting from neutron\ndensities on the lowest side of this range, neutron densities on the highest\nside result in abundance patterns that show an increased production of heavy\ns-process and r-process elements but similar abundances of the light s-process\nelements. Such high values of n may occur in the thermal pulses of asymptotic\ngiant branch (AGB) stars due to proton ingestion episodes. Comparison to the\nsurface abundances of 20 CEMP-s/r stars show that our modelled i-process\nabundances successfully reproduce observed abundance patterns that could not be\npreviously explained by s-process nucleosynthesis. Because the i-process models\nfit the abundances of CEMP-s/r stars so well, we propose that this class should\nbe renamed as CEMP-i.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Episodic modulations in supernova radio light curves from luminous blue\n  variable supernova progenitor models: <Context> Ideally, one would like to know which type of core-collapse SNe is\nproduced by different progenitors and the channels of stellar evolution leading\nto these progenitors. These links have to be very well known to use the\nobserved frequency of different types of SN events for probing the star\nformation rate and massive star evolution in different types of galaxies.\n<Aims> We investigate the link between LBV as SN progenitors and the appearance\nof episodic radio light curve modulations of the SN event. <Methods> We use the\n20Msun and 25Msun models with rotation at solar metallicity, part of an\nextended grid of stellar models computed by the Geneva team. At their pre-SN\nstage, these two models have recently been shown to have spectra similar to\nthose of LBV stars and possibly explode as Type IIb SNe. Based on the wind\nproperties before the explosion, we derive the density structure of their\ncircumstellar medium. This structure is used as input for computing the SN\nradio light curve. <Results> We find that the 20Msun model shows radio light\ncurves with episodic luminosity modulations, similar to those observed in some\nType IIb SNe. This occurs because the evolution of the 20Msun model terminates\nin a region of the HR diagram where radiative stellar winds present strong\ndensity variations, caused by the bistability limit. The 25Msun model, ending\nits evolution in a zone of the HR diagram where no change of the mass-loss\nrates is expected, presents no such modulations in its radio SN light curve.\n<Conclusions> Our results reinforce the link between SN progenitors and LBV\nstars. We also confirm the existence of a physical mechanism for a single star\nto have episodic radio light curve modulations. In the case of the 25Msun\nprogenitors, we do not obtain modulations in the radio light curve, but our\nmodels may miss some outbursting behavior in the late stages of massive stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Determination of stellar atmospheric parameters for a sample of the\n  post-AGB stars: We report for the first time the stellar atmospheric parameters for a a set\nof post-AGB stars classified by Suarez et al. (2006). The stellar spectra were\ntaken from optical region, with low-resolution and have different spectral\nranges. We select a sample of 70 objects with A-K spectral types and\nluminosities I and Ie. The large majority of these objects have been scarcely\nstudied and are located toward the galactic south pole region. We employ a set\nof empirical relationships that use pseudo-equivalent widths like spectral\nfeature to estimate effective temperature, surface gravity and metallicity. The\ncriteria chosen for selection of absorption are similar to employed by MK\nclassification system.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Preliminary results of giant pulse investigations from Crab pulsar with\n  Radioastron: Giant pulses from Crab pulsar were observed together with Radioastron space\nradiotelescope, Global EVN radio telescopes and Kvazar-KVO VLBI stations to\nstudy the scattering effects in the ISM. Five observing sessions were conducted\nat 18 cm (EVN codes: EG060A, EG060B, EG067B, EG075) and one at 92 cm (EVN code:\nGS033A). We have estimated distance to the scattering screen, angular size of\nscattering disk, scattering time. All estimations were done in an assumption of\nsingle thin scattering screen. Also, a significant change in the shape of\ncross-correlation functions for space-ground baselines was found (starting from\n4 up to 12 Earth diameters).",
        "category": "astro-ph_SR"
    },
    {
        "text": "Oscillator Strength Measurements of Atomic Absorption Lines from Stellar\n  Spectra: We develop a new method to determine oscillator strength values of atomic\nabsorption lines with state-of-the-art detailed spectral synthesis calculations\nof the optical spectrum of the Sun and of standard spectral reference stars. We\nupdate the log(gf)-values of 911 neutral lines observed in the KPNO-FTS flux\nspectrum of the Sun and high-resolution echelle spectra (R=80,000) of Procyon\n(F5 IV-V) and {\\epsilon} Eri (K2 V) observed with large signal-to-noise (S/N)\nratios of ~2,000 using the new Mercator-Hermes spectrograph at La Palma\nObservatory (Spain). We find for 483 Fe I, 85 Ni I, and 51 Si I absorption\nlines in the sample a systematic over-estimation of the literature\nlog(gf)-values with central line depths below 15 %. We employ a curve-of-growth\nanalysis technique to test the accuracy of the new oscillator strength values\nand compare calculated equivalent line widths to the Moore, Minnaert, &\nHoutgast atlas of the Sun. The online SpectroWeb database at\nhttp://spectra.freeshell.org interactively displays the observed and synthetic\nspectra and provides the new log(gf)-values together with important atomic line\ndata. The graphical database is under development for stellar reference spectra\nof every spectral sub-class observed with large spectral resolution and S/N\nratios.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Search for aluminium monoxide in the winds of oxygen-rich AGB stars: Aluminium monoxide, AlO, is likely efficiently depleted from the gas around\noxygen-rich evolved stars to form alumina clusters and dust seeds. Its presence\nin the extended atmospheres of evolved stars has been derived from optical\nspectroscopy. More recently, AlO gas was also detected at long wavelengths\naround the supergiant VY CMa and the oxygen-rich asymptotic giant branch (AGB)\nstar o Cet (Mira A). In search of AlO, we mined data obtained with APEX, the\nIRAM 30m telescope, Herschel/HIFI, SMA, and ALMA, which were primarily aimed at\nstudying other molecular species. We report here on observations of AlO towards\na sample of eight oxygen-rich AGB stars in different rotational transitions, up\nto seven for some stars. We present definite detections of one rotational\ntransition of AlO for o Cet and R Aqr, and tentative detections of one\ntransition for R Dor and o Cet, and two for IK Tau and W Hya. The presented\nspectra of WX Psc, R Cas, and TX Cam show no signature of AlO. For o Cet, R\nAqr, and IK Tau, we find that the AlO(N=9-8) emission likely traces the inner\nparts of the wind, out to only a few tens of AU, where the gas has not yet\nreached its terminal velocity. The conclusive detections of AlO emission in the\ncase of o Cet and R Aqr confirm the presence of AlO gas in outflows of AGB\nstars. The tentative detections further support this. Since most of the\nobservations presented in this study were obtained with stronger emission from\nother species than AlO in mind, observations with higher sensitivity in\ncombination with high angular resolution will improve our understanding of the\npresence and behaviour of AlO. From the current data sets we cannot firmly\nconclude whether there is a direct correlation between the wind properties and\nthe detection rate of AlO emission. We hope that this study can serve as a\nstimulus to perform sample studies in search of AlO in oxygen-rich outflows.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Chemical Abundance Signature of J0023+0307 -- A Second-Generation\n  Main-Sequence Star with [Fe/H]<-6: We present a chemical abundance analysis of the faint halo metal-poor\nmain-sequence star J0023+0307, with [Fe/H]<-6.3, based on a high-resolution\n(R~35,000) Magellan/MIKE spectrum. The star was originally found to have\n[Fe/H]< -6.6 based on a Ca II K measurement in an R~2,500 spectrum. No iron\nlines could be detected in our MIKE spectrum. Spectral lines of Li, C, Na, Mg,\nAl, Si, and Ca were detected. The Li abundance is close to the Spite Plateau,\nA(Li) = 1.7, not unlike that of other metal-poor stars although in stark\ncontrast to the extremely low value found e.g., in HE~1327-2326 at a similar\n[Fe/H] value. The carbon G-band is detected and indicates strong C-enhancement,\nas is typical for stars with low Fe abundances. Elements from Na through Si\nshow a strong odd-even effect, and J0023+0307 displays the second-lowest known\n[Ca/H] abundance. Overall, the abundance pattern of J0023+0307 suggests that it\nis a second-generation star that formed from gas enriched by a massive\nPopulation III first star exploding as a fall-back supernova The inferred\ndilution mass of the ejecta is 10^(5+-0.5) Msun of hydrogen, strongly\nsuggesting J0023+0307 formed in a recollapsed minihalo. J0023+0307 is likely\nvery old because it has a very eccentric orbit with a pericenter in the\nGalactic bulge.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Protostellar Evolution in Serpens Main: Possible Origin of Disk-Size\n  Diversity: We have observed the submillimeter continuum condensations SMM2, SMM4, SMM9,\nand SMM11 in the star forming cluster Serpens Main using the Atacama Large\nMillimeter/submillimeter Array during Cycle 3 in the 1.3 mm continuum, 12CO\nJ=2-1, SO J_N=6_5-5_4, and C18O J=2-1 lines at an angular resolution of ~0.55\n(240 au). Sixteen sources have been detected in the 1.3 mm continuum, which can\nbe classified into three groups. Group 1 consists of six sources showing\nextended continuum emission and bipolar/monopolar 12CO outflows. Although all\nthe Group 1 members are classified as Class 0 protostars, our observations\nsuggest evolutionary trends among them in terms of 12CO outflow dynamical time,\nSO emission distribution, C18O fractional abundance, and continuum morphology.\nGroup 2 consists of four sources associated with a continuum filamentary\nstructure and no 12CO outflows. Central densities estimated from the 1.3 mm\ncontinuum intensity suggest that they are prestellar sources in a marginally\nJeans unstable state. Group 3 consists of six Spitzer sources showing\npoint-like 1.3 mm continuum emission and clumpy 12CO outflows. These features\nof Group 3 suggest envelope dissipation, preventing disk growth from the\npresent size, r <~ 60 au. The Group 3 members are protostars that may be\nprecursors to the T Tauri stars associated with small disks at tens-au radii\nidentified in recent surveys.",
        "category": "astro-ph_SR"
    },
    {
        "text": "3-D Kinematics of the near-IR HH 223 outflow in L723: In this work we derive the full 3-D kinematics of the near-infrared outflow\nHH 223, located in the dark cloud Lynds 723 (L723), where a well-defined\nquadrupolar CO outflow is found. HH 223 appears projected onto the two lobes of\nthe east-west CO outflow. The radio continuum source VLA 2, towards the centre\nof the CO outflow, harbours a multiple system of low-mass young stellar\nobjects. One of the components has been proposed to be the exciting source of\nthe east-west CO outflow.\n  From the analisys of the kinematics, we get further evidence on the\nrelationship between the near-infrared and CO outflows and on the location of\ntheir exciting source. The proper motions were derived using multi-epoch,\nnarrow-band H$_2$ (2.122 $\\mu$m line) images. Radial velocities were derived\nfrom the 2.122 $\\mu$m line of the spectra. Because of the extended (~5 arcmin),\nS-shaped morphology of the target, the spectra were obtained with the\nMulti-Object-Spectroscopy (MOS) observing mode using the instrument LIRIS at\nthe 4.2m William Herschel Telescope. To our knowledge, this work is the first\ntime that MOS observing mode has been successfully used in the near infrared\nrange for an extended target.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Mind the Gap I: H$\u03b1$ Activity of M Dwarfs Near the Partially/Fully\n  Convective Boundary and a New H$\u03b1$ Emission Deficiency Zone on the Main\n  Sequence: Since identifying the gap in the H-R Diagram (HRD) marking the transition\nbetween partially and fully convective interiors, a unique type of slowly\npulsating M dwarf has been proposed. These unstable M dwarfs provide new\nlaboratories in which to understand how changing interior structures can\nproduce potentially observable activity at the surface. In this work, we report\nthe results of the largest high-resolution spectroscopic H$\\alpha$ emission\nsurvey to date spanning this transition region, including 480 M dwarfs observed\nusing the CHIRON spectrograph at CTIO/SMARTS 1.5-m. We find that M dwarfs with\nH$\\alpha$ in emission are almost entirely found 0 to 0.5 magnitude above the\ntop edge of the gap in the HRD, whereas effectively no stars in and below the\ngap show emission. Thus, the top edge of the gap marks a relatively sharp\nactivity transition, and there is no anomalous H$\\alpha$ activity for stars in\nthe gap. We also identify a new region at 10.3 $<M_{G}<$ 10.8 on the main\nsequence where fewer M dwarfs exhibit H$\\alpha$ emission compared to M dwarfs\nabove and below this magnitude range. Careful evaluation of literature results\nindicates that 1) rotation and H$\\alpha$ activity distributions on the main\nsequence are closely related, and 2) fewer stars in this absolute magnitude\nrange rotate in less than $\\sim$13 days than populations surrounding this\nregion. This result suggests that the most massive fully convective stars lose\ntheir angular momentum faster than both partially convective stars and less\nmassive fully convective stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Non-Ideal Magnetohydrodynamic Simulations of the Two-Stage Fragmentation\n  Model for Cluster Formation: We model molecular cloud fragmentation with thin disk non-ideal\nmagnetohydrodynamic simulations that include ambipolar diffusion and partial\nionization that transitions from primarily ultraviolet dominated to cosmic ray\ndominated regimes. These simulations are used to determine the conditions\nrequired for star clusters to form through a two-stage fragmentation scenario.\nRecent linear analyses have shown that the fragmentation length and time scales\ncan undergo a dramatic drop across the column density boundary that separates\nthe ultraviolet and cosmic ray dominated ionization regimes. As found in\nearlier studies, the absence of an ionization drop and regular perturbations\nleads to a single-stage fragmentation on parsec scales in transcritical clouds,\nso that the nonlinear evolution yields the same fragment sizes as predicted by\nlinear theory. However, we find that a combination of initial transcritical\nmass-to-flux ratio, evolution through a column density regime in which the\nionization drop takes place, and regular small perturbations to the\nmass-to-flux ratio are sufficient to cause a second stage of fragmentation\nduring the nonlinear evolution. Cores of size ~0.1 pc are formed within an\ninitial fragment of ~ pc size. Regular perturbations to the mass-to-flux ratio\nalso accelerate the onset of runaway collapse.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Correlated Strontium and Barium Isotopic Compositions of Acid-Cleaned\n  Single Silicon Carbides from Murchison: We present strontium, barium, carbon, and silicon isotopic compositions of 61\nacid-cleaned presolar SiC grains from Murchison. Comparison with previous data\nshows that acid washing is highly effective in removing both strontium and\nbarium contamination. For the first time, by using correlated\n$^{88}Sr$/$^{86}Sr$ and $^{138}Ba$/$^{136}Ba$ ratios in mainstream SiC grains,\nwe are able to resolve the effect of $^{13}C$ concentration from that of\n$^{13}C$-pocket mass on s-process nucleosynthesis, which points towards the\nexistence of large $^{13}C$-pockets with low $^{13}C$ concentration in AGB\nstars. The presence of such large $^{13}$R-pockets with a variety of relatively\nlow $^{13}C$ concentrations seems to require multiple mixing processes in\nparent AGB stars of mainstream SiC grains.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Gravitational waves from pulsars in the context of magnetic ellipticity: In one of our previous articles we have considered the role of a time\ndependent magnetic ellipticity on pulsars' braking indices and on the putative\ngravitational waves these objects can emit. Since only nine of more than 2000\nknown pulsars have accurately measured braking indices, it is of interest to\nextend this study to all known pulsars, in particular to what concerns the\ngravitational waves generation. To do so, as shown in our previous article, we\nneed to know some pulsars' observable quantities such as: periods and their\ntime derivatives, and estimated distances to the Earth. Moreover, we also need\nto know the pulsars' masses and radii, for which, in here we are adopting\ncurrent fiducial values. Our results show that the gravitational wave amplitude\nis at best $h \\sim 10^{-28}$. This leads to a pessimistic prospect for the\ndetection of gravitational waves generated by these pulsars, even for Advanced\nLIGO and Advanced Virgo, and the planned Einstein Telescope, whether the\nellipticity has magnetic origin.",
        "category": "astro-ph_SR"
    },
    {
        "text": "HST Low Resolution Stellar Library: Hubble Space Telescope's (HST) Space Telescope Imaging Spectrograph (STIS)\ntargeted 556 stars in a long-running program called Next Generation Spectral\nLibrary (NGSL) via proposals GO9088, GO9786, GO10222, and GO13776. Exposures\nthrough three low resolution gratings provide wavelength coverage from 0.2 $<\n\\lambda <$ 1 $\\mu$m at $\\lambda/\\Delta\\lambda\\sim$ 1000, providing unique\ncoverage in the ultraviolet (UV). The UV grating (G230LB) scatters red light\nand this results in unwanted flux that becomes especially troubling for cool\nstars. We applied scattered light corrections based on\n\\cite{2022stis.rept....5W} and flux corrections arising from pointing errors\nrelative to the center of the 0\\farcs2 slit. We present 514 fully reduced\nspectra, fluxed, dereddened, and cross-correlated to zero velocity. Because of\nthe broad spectral range, we can simultaneously study H$\\alpha$ and Mg II\n$\\lambda$2800, indicators of chromospheric activity. Their behaviours are\ndecoupled. Besides three cool dwarfs and one giant with mild flares in\nH$\\alpha$, only Be stars show strong H$\\alpha$ emission. Mg2800 emission,\nhowever, strongly anti-correlates with temperature such that warm stars show\nabsorption and stars cooler than $5000 \\: \\! \\rm{K}$ universally show\nchromospheric emission regardless of dwarf/giant status or metallicity.\nTransformed to Mg2800 flux emerging from the stellar surface, we find a\ncorrelation with temperature with approximately symmetric astrophysical\nscatter, in contrast to other workers who find a basal level with asymmetric\nscatter to strong values. Unsurprisingly, we confirm that Mg2800 activity is\nvariable.",
        "category": "astro-ph_SR"
    },
    {
        "text": "HAZMAT VI: The Evolution of Extreme Ultraviolet Radiation Emitted from\n  Early M Star: Quantifying the evolution of stellar extreme ultraviolet (EUV, 100 -- 1000\n$\\overset{\\circ}{A}$) emission is critical for assessing the evolution of\nplanetary atmospheres and the habitability of M dwarf systems. Previous studies\nfrom the HAbitable Zones and M dwarf Activity across Time (HAZMAT) program\nshowed the far- and near-UV (FUV, NUV) emission from M stars at various stages\nof a stellar lifetime through photometric measurements from the Galaxy\nEvolution Explorer (GALEX). The results revealed increased levels of\nshort-wavelength emission that remain elevated for hundreds of millions of\nyears. The trend for EUV flux as a function of age could not be determined\nempirically because absorption by the interstellar medium prevents access to\nthe EUV wavelengths for the vast majority of stars. In this paper, we model the\nevolution of EUV flux from early M stars to address this observational gap. We\npresent synthetic spectra spanning EUV to infrared wavelengths of 0.4 $\\pm$\n0.05 M$_{\\odot}$ stars at five distinct ages between 10 and 5000 Myr, computed\nwith the PHOENIX atmosphere code and guided by the GALEX photometry. We model a\nrange of EUV fluxes spanning two orders of magnitude, consistent with the\nobserved spread in X-ray, FUV, and NUV flux at each epoch. Our results show\nthat the stellar EUV emission from young M stars is 100 times stronger than\nfield age M stars, and decreases as t$^{-1}$ after remaining constant for a few\nhundred million years. This decline stems from changes in the chromospheric\ntemperature structure, which steadily shifts outward with time. Our models\nreconstruct the full spectrally and temporally resolved history of an M star's\nUV radiation, including the unobservable EUV radiation, which drives planetary\natmospheric escape, directly impacting a planet's potential for habitability.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The (non-)variability of magnetic chemically peculiar candidates in the\n  Large Magellanic Cloud: The galactic magnetic chemically peculiar (mCP) stars of the upper main\nsequence are well known as periodic spectral and light variables. The observed\nvariability is obviously caused by the uneven distribution of overabundant\nchemical elements on the surfaces of rigidly rotating stars. The mechanism\ncausing the clustering of some chemical elements into disparate structures on\nmCP stars has not been fully understood up to now. The observations of light\nchanges of mCP candidates recently revealed in the nearby Large Magellanic\nCloud (LMC) should provide us with information about their rotational periods\nand about the distribution of optically active elements on mCP stars born in\nother galaxies. We queried for photometry at the Optical Gravitational Lensing\nExperiment (OGLE)-III survey of published mCP candidates selected because of\nthe presence of the characteristic 5200A flux depression. In total, the\nintersection of both sources resulted in twelve stars. We performed our own and\nstandard periodogram time series analyses of all available data. The final\nresults are, amongst others, the frequency of the maximum peak and the\nbootstrap probability of its reality. We detected that only two mCP candidates,\n190.1 1581 and 190.1 15527, may show some weak rotationally modulated light\nvariations with periods of 1.23 and 0.49 days; however, the 49% and 32%\nprobabilities of their reality are not very satisfying. The variability of the\nother 10 mCP candidates is too low to be detectable by their V and I OGLE\nphotometry. The relatively low amplitude variability of the studied LMC mCP\ncandidates sample can be explained by the absence of photometric spots of\noverabundant optically active chemical elements. The unexpected LMC mCPs\nbehaviour is probably caused by different conditions during the star formation\nin the LMC and the Galaxy.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Gaia18aen: First symbiotic star discovered by Gaia: Besides the astrometric mission of the Gaia satellite, its repeated and\nhigh-precision measurements serve also as an all-sky photometric transient\nsurvey. The sudden brightenings of the sources are published as Gaia\nPhotometric Science Alerts and are made publicly available allowing the\ncommunity to photometrically and spectroscopically follow-up the object. The\ngoal of this paper was to analyze the nature and derive the basic parameters of\nGaia18aen, transient detected at the beginning of 2018. It coincides with the\nposition of the emission line star WRAY 15-136. The brightening was classified\nas a \"nova?\" on the basis of subsequent spectroscopic observation. We have\nanalyzed two spectra of Gaia18aen and collected the available photometry of the\nobject covering the brightenings in 2018 and also the preceding and following\nperiods of quiescence. Based on this observational data, we have derived the\nparameters of Gaia18aen and discussed the nature of the object. Gaia18aen is\nthe first symbiotic star discovered by the Gaia satellite. The system is an\nS-type symbiotic star and consists of an M giant of a slightly super-solar\nmetallicity, with Teff ~3500 K, a radius of ~230 R$\\odot$, and a high\nluminosity L ~7400 L$\\odot$. The hot component is a hot white dwarf. We\ntentatively determined the orbital period of the system ~487 days. The main\noutburst of Gaia18aen in 2018 was accompanied by a decrease in the temperature\nof the hot component. The first phase of the outburst was characterized by the\nhigh luminosity L ~27000 L$\\odot$, which remained constant for about three\nweeks after the optical maximum, later followed by the gradual decline of\nluminosity and increase of temperature. Several re-brightenings have been\ndetected on the timescales of hundreds of days.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Transit Timing Observations of the Extrasolar Hot-Neptune Planet GL 436b: Gliese 436 is an M dwarf with a mass of 0.45 Msun and hosts the extrasolar\nplanet GL 436b [3, 6, 7, 2], which is currently the least massive transiting\nplanet with a mass of ~23.17 Mearth [10], and the only planet known to transit\nan M dwarf. GL 436b represents the first transiting detection of the class of\nextrasolar planets known as \"Hot Neptunes\" that have masses within a few times\nthat of Neptune's mass (~17 Mearth) and orbital semimajor axis <0.1 AU about\nthe host star. Unlike most other known transiting extrasolar planets, GL 436b\nhas a high eccentricity (e~0.16). This brings to light a new parameter space\nfor habitability zones of extrasolar planets with host star masses much smaller\nthan typical stars of roughly a solar mass. This unique system is an ideal\ncandidate for orbital perturbation and transit-time variation (TTV) studies to\ndetect smaller, possibly Earth-mass planets in the system. In April 2008 we\nbegan a long-term intensive campaign to obtain complete high-precision light\ncurves using the Apache Point Observatory's 3.5-meter telescope, NMSU's 1-meter\ntelescope (located at APO), and Sommers Bausch Observatory's 24\" telescope.\nThese light curves are being analyzed together, along with amateur and other\nprofessional astronomer observations. Results of our analysis are discussed.\nContinued measurements over the next few years are needed to determine if\nadditional planets reside in the system, and to study the impact of other\nmanifestations on the light curves, such as star spots and active regions.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Young Stars near Cometary Globule CG 30 in the Tumultuous Gum Nebula: We have conducted a high-dispersion ($R \\sim$ 34,000) optical spectroscopic\nstudy of 10 young stars near the cometary gloule CG 30 in the Gum Nebula, a\ndiffuse H \\textsc{ii} region home to at least 32 cometary globules. All 10\nspectroscopically observed stars at the nebula's northern edge are of low mass\n(spectral types M4.5 - K5), have broad H$\\alpha$ emission, and show spectral\nveiling. Eight of the 10 are classical T Tauri stars. We spectroscopically\nmeasure the photospheric properties of CG 30 IRS 4 inside CG 30. Though\nembedded, CG 30 IRS 4 is T Tauri-like, with relatively slow projected rotation\nand moderate veiling. Undepleted Li absorptions, strong H$\\alpha$ emissions,\nand positions well above the main sequence on an HR diagram suggest the 10\nstars are $\\lesssim$1 Myr old. Using our measurements, previous spectroscopy,\nand previous photometry of 11 other young stars in the area, we determine\nstellar, kinematic, and accretion properties of a total of 21 young stars.\nShared radial velocities, proper motions, distances, and ages suggest 14 of the\nyoung stars (including CG 30 IRS 4) are kinematically related to CG 30. From\n\\textit{Gaia} DR2 distances to 6 of these stars, we derive a distance of\n$358.1\\pm2.2$ pc to the cometary globule complex CG 30/31/38. The CG 30\nassociation has an accretor fraction of $29\\pm14$\\%, low for quiescent clusters\nof similar age but consistent with other irradiated clusters. The Gum Nebula's\nmoderate radiation environment ($G_0=6.6^{+3.2}_{-2.7}$ at CG 30) may be strong\nenough to shorten disk lifetimes.",
        "category": "astro-ph_SR"
    },
    {
        "text": "RR Lyrae stars in the globular cluster Palomar 2: A CCD VI imaging time-series over 11-year is employed to explore the light\ncurves of stars in the field of Palomar 2. We discovered 20 RRab and 1 RRc\nvariables. A revision of Gaia-DR3 data enabled us to identify 10 more variables\nand confirm the RRab nature of 6 of them and one RGB. The cluster membership is\ndiscussed and 18 variables are most likely cluster members. The Fourier light\ncurve decomposition for the 11 best quality light curves of cluster member\nstars leads to independent estimates of the cluster distance 27.2 +- 1.8 kpc\nand [Fe/H]ZW=-1.39 +- 0.55. We confirm the cluster as of the Oo I type.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Observed luminosity spread in young clusters and Fu Ori stars: a unified\n  picture: The idea that non steady accretion during the embedded phase of protostar\nevolution can produce the observed luminosity spread in the Herzsprung-Russell\ndiagram (HRD) of young clusters has recently been called into question.\nObservations of Fu Ori, for instance, suggest an expansion of the star during\nstrong accretion events whereas the luminosity spread implies a contraction of\nthe accreting objects, decreasing their radiating surface. In this paper, we\npresent a global scenario based on calculations coupling episodic accretion\nhistories derived from numerical simulations of collapsing cloud prestellar\ncores of various masses and subsequent protostar evolution. Our calculations\nshow that, assuming an initial protostar mass $\\mi \\sim 1\\,\\mjup$, typical of\nthe second Larson's core, both the luminosity spread in the HRD and the\ninferred properties of Fu Ori events (mass, radius, accretion rate) can be\nexplained by this scenario, providing two conditions. First, there must be some\nvariation within the fraction of accretion energy absorbed by the protostar\nduring the accretion process. Second the range of this variation should\nincrease with increasing accretion burst intensity, and thus with the initial\ncore mass and final star mass. The numerical hydrodynamics simulations of\ncollapsing cloud prestellar cores indeed show that the intensity of the\naccretion bursts correlates with the mass and initial angular momentum of the\nprestellar core. Massive prestellar cores with high initial angular momentum\nare found to produce intense bursts characteristic of Fu Ori like events. Our\nresults thus suggest a link between the burst intensities and the fraction of\naccretion energy absorbed by the protostar, with some threshold in the\naccretion rate, of the order of $10^{-5}\\msolyr$, delimitating the transition\nfrom \"cold\" to \"hot\" accretion. [Abridged]",
        "category": "astro-ph_SR"
    },
    {
        "text": "Large-amplitude longitudinal oscillations in solar prominences simulated\n  with different resolutions: Large-amplitude longitudinal oscillations (LALOs) in solar prominences have\nbeen widely studied in the last decades. However, their damping and\namplification mechanisms are not well understood. In this study, we investigate\nthe attenuation and amplification of LALOs using high-resolution numerical\nsimulations with progressively increasing spatial resolutions. We performed\ntime-dependent numerical simulations of LALOs using the 2D magnetic\nconfiguration that contains a dipped region. After the prominence mass loading\nin the magnetic dips, we triggered LALOs by perturbing the prominence mass\nalong the magnetic field. We performed the experiments with four values of\nspatial resolution. In the simulations with the highest resolution, the period\nshows a good agreement with the pendulum model. The convergence experiment\nrevealed that the damping time saturates at the bottom prominence region with\nimproving the resolution, indicating the existence of a physical reason for the\ndamping of oscillations. At the prominence top, the oscillations are amplified\nduring the first minutes and then are slowly attenuated. The characteristic\ntime suggests more significant amplification in the experiments with the\nhighest spatial resolution. The analysis revealed that the energy exchange\nbetween the bottom and top prominence regions is responsible for the\nattenuation and amplification of LALOs. The high-resolution experiments are\ncrucial for the study of the periods and the damping mechanism of LALOs. The\nperiod agrees with the pendulum model only when using high enough spatial\nresolution. The results suggest that numerical diffusion in simulations with\ninsufficient spatial resolution can hide important physical mechanisms, such as\namplification of oscillations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Optical and X-ray studies of three polars: RX J0859.1+0537, RX\n  J0749.1-0549, and RX J0649.8-0737: We present optical photometric and spectroscopic observations, and an\nanalysis of archival X-ray data of three polars: RX J0859.1+0537, RX\nJ0749.1-0549, and RX J0649.8-0737. Optical light curves of these three polars\nreveal eclipse features that are deep, total, and variable in shape. The\noptical and X-ray modulations of RX J0859.1+0537, RX J0749.1-0549, and RX\nJ0649.8-0737 are both found to occur at the orbital periods of 2.393$\\pm$0.003\nhrs, 3.672$\\pm$0.001 hrs, and 4.347$\\pm$0.001 hrs, respectively. RX\nJ0859.1+0537 is found to be an eclipsing polar which lies in the region of the\nperiod gap, whereas RX J0749.1-0549 and RX J0649.8-0737 are found to be\nlong-period eclipsing polars above the period gap. The eclipse length is found\nto be 61 min for RX J0749.1-0549 in the R-band, which is the highest among the\nlong period eclipsing polars. The radius of the eclipsed light source is found\nto be more than the actual size of the white dwarf for these three systems,\nindicating that the eclipsed component is not only the white dwarf but also\nappears to include the presence of an extended accretion region. Optical\nspectra of these systems show the presence of high ionization emission lines\nalong with the strong Balmer emission lines with an inverted Balmer decrement.\nCyclotron harmonics are also detected in the optical spectra from which we\ninfer magnetic field strength of the surface of the white dwarf to be 49$\\pm$2\nMG, 43.5$\\pm$1.4 MG, and 44$\\pm$1 MG for RX J0859.1+0537, RX J0749.1-0549, and\nRX J0649.8-0737, respectively.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the Performance of Multi-Instrument Solar Flare Observations During\n  Solar Cycle 24: The current fleet of space-based solar observatories offers us a wealth of\nopportunities to study solar flares over a range of wavelengths. Significant\nadvances in our understanding of flare physics often come from coordinated\nobservations between multiple instruments. Consequently, considerable efforts\nhave been, and continue to be made to coordinate observations among instruments\n(e.g. through the Max Millennium Program of Solar Flare Research). However,\nthere has been no study to date that quantifies how many flares have been\nobserved by combinations of various instruments. Here we describe a technique\nthat retrospectively searches archival databases for flares jointly observed by\nRHESSI, SDO/EVE (MEGS-A and -B), Hinode/(EIS, SOT, and XRT), and IRIS. Out of\nthe 6953 flares of GOES magnitude C1 or greater that we consider over the 6.5\nyears after the launch of SDO, 40 have been observed by six or more instruments\nsimultaneously. Using each instrument's individual rate of success in observing\nflares, we show that the numbers of flares co-observed by three or more\ninstruments are higher than the number expected under the assumption that the\ninstruments operated independently of one another. In particular, the number of\nflares observed by larger numbers of instruments is much higher than expected.\nOur study illustrates that these missions often acted in cooperation, or at\nleast had aligned goals. We also provide details on an interactive widget now\navailable in SSWIDL that allows a user to search for flaring events that have\nbeen observed by a chosen set of instruments. This provides access to a broader\nrange of events in order to answer specific science questions. The difficulty\nin scheduling coordinated observations for solar-flare research is discussed\nwith respect to instruments projected to begin operations during Solar Cycle\n25, such as DKIST, Solar Orbiter, and Parker Solar Probe.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Polarization due to rotational distortion in the bright star Regulus: Polarization in stars was first predicted by Chandrasekhar [1] who calculated\na substantial linear polarization at the stellar limb for a pure\nelectron-scattering atmosphere. This polarization will average to zero when\nintegrated over a spherical star but could be detected if the symmetry is\nbroken, for example by the eclipse of a binary companion. Nearly 50 years ago,\nHarrington and Collins [2] modeled another way of breaking the symmetry and\nproducing net polarization - the distortion of a rapidly rotating hot star.\nHere we report the first detection of this effect. Observations of the linear\npolarization of Regulus, with two different high-precision polarimeters, range\nfrom +42 parts-per-million (ppm) at a wavelength of 741 nm to -22 ppm at 395\nnm. The reversal from red to blue is a distinctive feature of rotation-induced\npolarization. Using a new set of models for the polarization of rapidly\nrotating stars we find that Regulus is rotating at 96.5$\\substack{+0.6-0.8}$%\nof its critical angular velocity for breakup, and has an inclination greater\nthan 76.5 degrees. The rotation axis of the star is at a position angle of\n79.5$\\pm$0.7 degrees. The conclusions are independent of, but in good agreement\nwith, the results of previously published interferometric observations of\nRegulus [3]. The accurate measurement of rotation in early-type stars is\nimportant for understanding their stellar environments [4], and course of their\nevolution [5].",
        "category": "astro-ph_SR"
    },
    {
        "text": "Sunspot Rotation as a Driver of Major Solar Eruptions in NOAA Active\n  Region 12158: We studied the developing conditions of sigmoid structure under the influence\nof magnetic non-potential characteristics of a rotating sunspot in the active\nregion (AR) 12158. Vector magnetic field measurements from Helioseismic\nMagnetic Imager and coronal EUV observations from Atmospheric Imaging Assembly\nreveal that the erupting inverse-S sigmoid had roots in the location of the\nrotating sunspot. Sunspot rotates at a rate of 0-5deg/h with increasing trend\nin the first half followed by a decrease. Time evolution of many non-potential\nparameters had a well correspondence with the sunspot rotation. The evolution\nof the AR magnetic structure is approximated by a time series of force free\nequilibria. The NLFFF magnetic structure around the sunspot manifests the\nobserved sigmoid structure. Field lines from the sunspot periphery constitute\nthe body of the sigmoid and those from interior overly the sigmoid similar to a\nfluxrope structure. While the sunspot is being rotating, two major CME\neruptions occurred in the AR. During the first (second) event, the coronal\ncurrent concentrations enhanced (degraded) consistent with the photospheric net\nvertical current, however the magnetic energy is released during both the\ncases. The analysis results suggest that the magnetic connections of the\nsigmoid are driven by slow motion of sunspot rotation, which transforms to a\nhighly twisted flux rope structure in a dynamical scenario. An exceeding\ncritical twist in the flux rope probably leads to the loss of equilibrium and\nthus triggering the onset of two eruptions.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Formation and Eruption of Hot Channels during an M6.5 Class Solar Flare: We investigate the formation and eruption of hot channels associated with the\nM6.5 class flare (SOL2015-06-22T18:23) occurring in NOAA AR 12371 on 2015 June\n22. Two flare precursors are observed before the flare main phase. Observations\nin 94 {\\AA} and 131 {\\AA} by SDO/AIA have revealed the early morphology of the\nfirst hot channel as a group of hot loops, which is termed as seed hot channel.\nA few seed hot channels are formed above the polarity inversion line (PIL) and\nthe formation is associated with footpoint brightenings' parallel motion along\nthe PIL, which proceeds into the early stage of the flare main phase. During\nthis process, seed hot channels build up and rise slowly, being accelerated at\nthe peak of the second precursor. They merge in the process of acceleration\nforming a larger hot channel, which then forms an \"inverted {\\gamma}\" shape\nkinking structure. Before the flare peak, the second kinking hot channel with\nnegative crossing appears near the first kinking hot channel that has erupted.\nThe eruption of these two hot channels produce two peaks on the main flare's\nGOES light curve. The footpoint brightenings' propagation along the PIL\nindicate that the first kinking hot channel may be formed due to zipper\nreconnection. The occurrence of merging between seed hot channels observed by\nAIA is supported by the extrapolated nonlinear force-free field models. The\nobserved writhing motion of the first kinking hot channel may be driven by the\nLorentz force.",
        "category": "astro-ph_SR"
    },
    {
        "text": "MIDI interferometry of massive YSOs: Updates on the MPIA programme: Information about the inner structure of young stellar objects is crucial for\nunderstanding how the central forming stars gain their mass. However,\nespecially for observations of (usually further away) high-mass young stellar\nobjects, conventional imaging has limitations in spatial resolution. By means\nof mid-infrared interferometry, we can peek deeply into the strongly\nextinguished central 100 AU of such objects. Here, we report on new data we\nobtained within our programme using MIDI at the VLTI. Shown are preliminary\nresults on the known outflow sources AFGL 2136 IRS 1 and Mon R2 IRS 3A. In\nparticular, we describe how quantities like differential phases give additional\ngeometrical structure information. We demonstrate how the combined\ninterpretation of single-telescope and interferometric data at different\nwavelength regimes can lead to a more complete picture of the nature of such\nMYSOs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evolutionary tracks of massive stars with different rotation and\n  metallicity in neutrino H-R diagram: Neutrino losses play a crucial role in the evolution of massive stars. We\nstudy the neutrino luminosity of stars ranging from 20 to 90 M_{\\odot} from\nZero Age Main Sequence (ZAMS) to Fe Core Collapse (FeCC) with different\nrotation and metallicity in a neutrino Hertzsprung-Russell diagram. In our\nsimulations, we consider {\\omega}/{\\omega}crit = 0 and 0.7 to represent\nnon-rotation and high rotation, respectively, and set the metallicities to\n0.014, 0.001, and 0.0001. During hydrogen burning stages, neutrino luminosity\nprimarily originates from CNO cycle, and increases with higher stellar mass\nwhile decreasing with increasing metallicity. For the high metallicity models\n(Z = 0.014) during the helium burning stage, the reduction of the hydrogen\nenvelope caused by a larger mass loss rate leads to a gradual decrease in\nneutrino luminosity. The rapid rotation results in extra mixing inside massive\nstars, which increases the neutrino luminosity during main sequence (MS), while\ndecreases the neutrino luminosity during helium burning phase. Simultaneously,\nthe rapid rotation also increases CO core mass, which enhances the neutrino\nluminosity during C and O burning phase. We also investigate the effect of\nneutrino magnetic moment (NMM) on the massive stars. We find that the energy\nloss caused by the NMM does not have effects on the evolutionary destiny of\nmassive stars, and it does not significant change the compactness at the time\nof Fe core collapse.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A New Multi-wavelength Solar Telescope: Optical and Near-infrared Solar\n  Eruption Tracer (ONSET): A new multi-wavelength solar telescope, Optical and Near-infrared Solar\nEruption Tracer (ONSET) of Nanjing University, was constructed, being\nfabricated by Nanjing Institute of Astronomical Optics & Technology and run in\ncooperation with Yunnan Astronomical Observatory. ONSET is able to observe the\nSun in three wavelength windows: He {\\small I} 10830 \\AA, H$\\alpha$, and\nwhite-light at 3600 \\AA and 4250 \\AA, which are selected in order to obtain the\ndynamics in the corona, chromosphere, and the photosphere simultaneously.\nFull-disk or partial-disk solar images with a field of 10 arcmin at three\nwavelengths can be obtained nearly simultaneously. It is designed to trace\nsolar eruptions with high spatial and temporal resolutions. This telescope was\ninstalled at a new solar observing site near Fuxian Lake in Yunnan Province,\nsouthwest China. The site is located at E102N24, with an altitude of 1722 m.\nThe seeing is stable and of high quality. We give a brief description of the\nscientific objectives and the basic structure of the ONSET. Some preliminary\nresults are also presented.",
        "category": "astro-ph_SR"
    },
    {
        "text": "PEPSI deep spectra. III. A chemical analysis of the ancient planet-host\n  star Kepler-444: We obtained an LBT/PEPSI spectrum with very high resolution and high\nsignal-to-noise ratio (S/N) of the K0V host Kepler-444, which is known to host\n5 sub-Earth size rocky planets. The spectrum has a resolution of R=250,000, a\ncontinuous wavelength coverage from 4230 to 9120A, and S/N between 150 and\n550:1 (blue to red). We performed a detailed chemical analysis to determine the\nphotospheric abundances of 18 chemical elements, in order to use the abundances\nto place constraints on the bulk composition of the five rocky planets. Our\nspectral analysis employs the equivalent width method for most of our spectral\nlines, but we used spectral synthesis to fit a small number of lines that\nrequire special care. In both cases, we derived our abundances using the MOOG\nspectral analysis package and Kurucz model atmospheres. We find no correlation\nbetween elemental abundance and condensation temperature among the refractory\nelements. In addition, using our spectroscopic stellar parameters and isochrone\nfitting, we find an age of 10+/-1.5 Gyr, which is consistent with the\nasteroseismic age of 11+/-1 Gyr. Finally, from the photospheric abundances of\nMg, Si, and Fe, we estimate that the typical Fe-core mass fraction for the\nrocky planets in the Kepler-444 system is approximately 24 per cent. If our\nestimate of the Fe-core mass fraction is confirmed by more detailed modeling of\nthe disk chemistry and simulations of planet formation and evolution in the\nKepler-444 system, then this would suggest that rocky planets in more\nmetal-poor and alpha-enhanced systems may tend to be less dense than their\ncounterparts of comparable size in more metal-rich systems.",
        "category": "astro-ph_SR"
    },
    {
        "text": "New outburst of the symbiotic nova AG Peg after 165 years: AG Peg is known as the slowest symbiotic nova, which experienced its\nnova-like outburst around 1850. After 165 years, during June of 2015, it\nerupted again showing characteristics of the Z And-type outburst. The primary\nobjective is to determine basic characteristics, the nature and type of the\n2015 outburst of AG Peg. We achieved this aim by modelling the spectral energy\ndistribution using low-resolution spectroscopy (330-750 nm), medium-resolution\nspectroscopy (420-720 nm; R=11000), and $UBVR_{\\rm C}I_{\\rm C}$ photometry\ncovering the 2015 outburst with a high cadence. Optical observations were\ncomplemented with the archival HST and FUSE spectra from the preceding\nquiescence. During the outburst, the luminosity of the hot component was in the\nrange of 2-11$\\times 10^{37}(d/0.8{\\rm kpc})^2$ erg/s. To generate the maximum\nluminosity the white dwarf (WD) had to accrete at $\\sim 3\\times 10^{-7}$\nM$_{\\odot}yr^{-1}$, which exceeds the stable-burning limit and thus led to\nblowing optically thick wind from the WD. We determined its mass-loss rate to a\nfew $\\times 10^{-6}$ M$_{\\odot}yr^{-1}$. At the high temperature of the\nionising source, $1.5-2.3\\times 10^5$ K, the wind converted a fraction of the\nWD's photospheric radiation into the nebular emission that dominated the\noptical. A one order of magnitude increase of the emission measure, from a few\n$\\times 10^{59}(d/0.8 {\\rm kpc})^2$ cm$^{-3}$ during quiescence, to a few\n$\\times 10^{60}(d/0.8\\,{\\rm kpc})^2$ cm$^{-3}$ during the outburst, caused a 2\nmag brightening in the LC, which is classified as the Z And-type of the\noutburst. The very high nebular emission and the presence of a disk-like HI\nregion encompassing the WD, as indicated by a significant broadening and high\nflux of the Raman-scattered OVI 6825 \\AA\\ line during the outburst, is\nconsistent with the ionisation structure of hot components in symbiotic stars\nduring active phases.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Extraordinary Solar Modulation Effects On Galactic Cosmic Rays Observed\n  By V1 Near The Heliopause: We discuss two extraordinary increases of cosmic ray intensity observed by\nVoyager 1 in the last 1.1 AU before heliopause crossing, Aug 2012 at 121.7 AU.\nThe two increases are roughly similar in amplitude and result in a total\nincrease of cosmic ray nuclei around 1 GV of over 50 percent and of 0.01 GV\nelectrons of a factor about 2. During the first increase, the changes in the\nmagnetic, B, field are small. After the first increase, the B field variations\nand cosmic ray changes become large and during the second increase the B field\nvariations and the cosmic ray changes are correlated to within a day. The\nintensity variations of H and He nuclei during these time intervals are\nmeasured from 0.1 to over 1 GV. The total GCR increse over the two events\nresemble those expected from a simple force-field 'like' solar modulation model\nwith a modulation potential of about 80MV. This is nearly one third of the\ntotal modulation potential of about 250 GV required to produce the modulation\nof these particles observed at the earth at the 2009 sunspot minimum and adds a\nnew aspect to ideas about heliospheric modulation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "SN 2017ens: The Metamorphosis of a Luminous Broad-lined Type Ic\n  Supernova into an SN IIn: We present observations of supernova (SN) 2017ens, discovered by the ATLAS\nsurvey and identified as a hot blue object through the GREAT program. The\nredshift z=0.1086 implies a peak brightness of M_g=-21.1 mag, placing the\nobject within the regime of superluminous supernovae. We observe a dramatic\nspectral evolution, from initially being blue and featureless, to later\ndeveloping features similar to those of the broadlined Type Ic SN 1998bw, and\nfinally showing ~2000 km s^-1 wide H-alpha and H-beta emission. Relatively\nnarrow Balmer emission (reminiscent of a SN IIn) is present at all times. We\nalso detect coronal lines, indicative of a dense circumstellar medium. We\nconstrain the progenitor wind velocity to ~50-60 km s^-1 based on P-Cygni\nprofiles, which is far slower than those present in Wolf-Rayet stars. This may\nsuggest that the progenitor passed through a luminous blue variable phase, or\nthat the wind is instead from a binary companion red supergiant star. At late\ntimes we see the ~2000 km s^-1 wide H-alpha emission persisting at high\nluminosity (~3x10^40 erg s^-1) for at least 100 day, perhaps indicative of\nadditional mass loss at high velocities that could have been ejected by a\npulsational pair instability.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Rotating massive O stars with non-spherical 2D winds: We present solutions for the velocity field and mass-loss rates for 2D\naxisymmetric outflows, as well as for the case of mass accretion through the\nuse of the Lambert W-function. For the case of a rotating radiation-driven wind\nthe velocity field is obtained analytically using a parameterised description\nof the line acceleration that only depends on radius r at any given latitude\n$\\theta$. The line acceleration g(r) is obtained from Monte-Carlo multi-line\nradiative transfer calculations. The critical/sonic point of our equation of\nmotion varies with latitude $\\theta$. Furthermore, an approximate analytical\nsolution for the supersonic flow of a rotating wind is derived, which is found\nto closely resemble the exact solution. For the simultaneous solution of the\nmass-loss rate and velocity field, we use the iterative method of our 1D method\nextended to the non-spherical 2D case. We apply the new theoretical expressions\nwith our iterative method to the stellar wind from a differentially rotating 40\n$M_{sun}$ O5-V main sequence star as well as to a 60 $M_{sun}$ O-giant star,\nand we compare our results to previous studies that are extensions of the\nCastor et al. (1975, ApJ, 195, 157) CAK formalism. Next, we account for the\neffects of oblateness and gravity darkening. Our numerical results predict an\nequatorial decrease of the mass-loss rate, which would imply that\n(surface-averaged) total mass-loss rates are lower than for the spherical 1D\ncase, in contradiction to the Maeder & Meynet (2000, A&A, 361, 159) formalism\nthat is oftentimes employed in stellar evolution calculations for rotating\nmassive stars. To clarify the situation in nature we discuss observational\ntests to constrain the shapes of large-scale 2D stellar winds.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Influence of magnetic activity on the determination of stellar\n  parameters through asteroseismology: Magnetic activity changes the gravito-acoustic modes of solar-like stars and\nin particular their frequencies. There is an angular-degree dependence that is\nbelieved to be caused by the non-spherical nature of the magnetic activity in\nthe stellar convective envelope. These changes in the mode frequencies could\nmodify the small separation of low-degree modes (i.e. frequency difference\nbetween consecutive quadrupole and radial modes), which is sensitive to the\ncore structure and hence to the evolutionary stage of the star. Determining\nglobal stellar parameters such as the age using mode frequencies at a given\nmoment of the magnetic activity cycle could lead to biased results. Our\nestimations show that in general these errors are lower than other systematic\nuncertainties, but in some circumstances they can be as high as 10% in age and\nof a few percent in mass and radius. In addition, the frequency shifts caused\nby the magnetic activity are also frequency dependent. In the solar case this\nis a smooth function that will mostly be masked by the filtering of the\nso-called surface effects. However the observations of other stars suggest that\nthere is an oscillatory component with a period close to the one corresponding\nto the acoustic depth of the He II zone. This could give rise to a\nmisdetermination of some global stellar parameters, such as the helium\nabundance. Our computations show that the uncertainties introduced by this\neffect are lower than the 3% level.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Testing Disk-Locking in NGC 2264: We test analytic predictions from different models of magnetospheric\naccretion, which invoke disk-locking, using stellar and accretion parameters\nderived from models of low resolution optical spectra of 36 T Tauri stars\n(TTSs) in NGC 2264 (age~3 Myrs). Little evidence is found for models that\nassume purely dipolar field geometries; however, strong support is found in the\ndata for a modified version of the X-wind model (Shu et al. 1994) which allows\nfor non-dipolar field geometries. The trapped flux concept in the X-wind model\nis key to making the analytic predictions which appear supported in the data.\nBy extension, our analysis provides support for the outflows predicted by the\nX-wind as these also originate in the trapped flux region. In addition, we find\nno support in the data for accretion powered stellar winds from young stars. By\ncomparing the analysis presented here of NGC 2264 with a similar analysis of\nstars in Taurus (age~1-2 Myr), we find evidence that the equilibrium\ninteraction between the magnetic field and accretion disk in TTS systems\nevolves as the stars grow older, perhaps as the result of evolution of the\nstellar magnetic field geometry. We compare the accretion rates we derive with\naccretion rates based on U-band excess, finding good agreement. In addition, we\nuse our accretion parameters to determine the relationship between accretion\nand H-beta luminosity, again finding good agreement with previously published\nresults; however, we also find that care must be used when applying this\nrelationship due to strong chromospheric emission in young stars which can lead\nto erroneous results in some cases.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Oscillations above sunspots from the temperature minimum to the corona: Context. An analysis of the oscillations above sunspots was carried out using\nsimultaneous ground-based and Solar Dynamics Observatory (SDO) observations\n(SiI 10827A, HeI 10830A, FeI 6173A, 1700A, HeII 304A, FeIX 171A).\n  Aims. Investigation of the spatial distribution of oscillation power in the\nfrequency range 1-8 mHz for the different height levels of the solar\natmosphere. Measuring the time lags between the oscillations at the different\nlayers.\n  Methods. We used frequency filtration of the intensity and Doppler velocity\nvariations with Morlet wavelet to trace the wave propagation from the\nphotosphere to the chromosphere and the corona.\n  Results. The 15 min oscillations are concentrated near the outer penumbra in\nthe upper photosphere (1700 A), forming a ring, that expands in the transition\nzone. These oscillations propagate upward and reach the corona level, where\ntheir spatial distribution resembles a fan structure. The spatial distribution\nof the 5 min oscillation power looks like a circle-shape structure matching the\nsunspot umbra border at the photospheric level. The circle expands at the\nhigher levels (HeII 304A and FeIX 171A). This indicates that the low-frequency\noscillations propagate along the inclined magnetic tubes in the spot. We found\nthat the inclination of the tubes reaches 50--60 degrees in the upper\nchromosphere and the transition zone. The main oscillation power in the 5-8 mHz\nrange concentrates within the umbra boundaries at all the levels. The highest\nfrequency oscillations (8 mHz) are located in the peculiar points inside the\numbra. These points probably coincide with umbral dots. We deduced the\npropagation velocities to be 28+-15 km/s, 26+-15 km/s, and 55+-10 km/s for the\nSiI 10827A-HeI 10830A, 1700A-HeII 304A, and HeII 304A-FeIX 171A height levels,\nrespectively.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Calibrating the surface brightness - color relation for late-type red\n  giants stars in the visible domain using VEGA/CHARA interferometric\n  observations: The surface brightness - color relationship (SBCR) is a poweful tool for\ndetermining the angular diameter of stars from photometry. It was for instance\nused to derive the distance of eclipsing binaries in the Large Magellanic Cloud\n(LMC), which led to its distance determination with an accuracy of 1%. We\ncalibrate the SBCR for red giant stars in the 2.1 < V-K < 2.5 color range using\nhomogeneous VEGA/CHARA interferometric data secured in the visible domain, and\ncompare it to the relation based on infrared interferometric observations,\nwhich were used to derive the distance to the LMC. Observations of eight G-K\ngiants were obtained with the VEGA/CHARA instrument. The derived limb-darkened\nangular diameters were combined with a homogeneous set of infrared magnitudes\nin order to constrain the SBCR. The average precision we obtain on the\nlimb-darkened angular diameters of the eight stars in our sample is 2.4%. For\nthe four stars in common observed by both VEGA/CHARA and PIONIER/VLTI, we find\na 1 sigma agreement for the angular diameters. The SBCR we obtain in the\nvisible has a dispersion of 0.04 magnitude and is consistent with the one\nderived in the infrared (0.018 magnitude). The consistency of the infrared and\nvisible angular diameters and SBCR reinforces the result of 1% precision and\naccuracy recently achieved on the distance of the LMC using the\neclipsing-binary technique. It also indicates that it is possible to combine\ninterferometric observations at different wavelengths when the SBCR is\ncalibrated.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Wind mass-loss rates of stripped stars inferred from Cygnus X-1: Recent observations of the high-mass X-ray binary Cygnus X-1 have shown that\nboth the companion star (41 solar masses) and the black hole (21 solar masses)\nare more massive than previously estimated. Furthermore, the black hole appears\nto be nearly maximally spinning. Here we present a possible formation channel\nfor the Cygnus X-1 system that matches the observed system properties. In this\nformation channel, we find that the orbital parameters of Cygnus X-1, combined\nwith the observed metallicity of the companion, imply a significant reduction\nin mass loss through winds relative to commonly used prescriptions for stripped\nstars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Three-dimensional delayed-detonation models with nucleosynthesis for\n  Type Ia supernovae: We present results for a suite of fourteen three-dimensional, high resolution\nhydrodynamical simulations of delayed-detonation modelsof Type Ia supernova (SN\nIa) explosions. This model suite comprises the first set of three-dimensional\nSN Ia simulations with detailed isotopic yield information. As such, it may\nserve as a database for Chandrasekhar-mass delayed-detonation model\nnucleosynthetic yields and for deriving synthetic observables such as spectra\nand light curves. We employ a physically motivated, stochastic model based on\nturbulent velocity fluctuations and fuel density to calculate in situ the\ndeflagration to detonation transition (DDT) probabilities. To obtain different\nstrengths of the deflagration phase and thereby different degrees of\npre-expansion, we have chosen a sequence of initial models with 1, 3, 5, 10,\n20, 40, 100, 150, 200, 300, and 1600 (two different realizations) ignition\nkernels in a hydrostatic white dwarf with central density of 2.9 x 10^9 gcc,\nplus in addition one high central density (5.5 x 10^9 gcc), and one low central\ndensity (1.0 x 10^9 gcc) rendition of the 100 ignition kernel configuration.\nFor each simulation we determined detailed nucleosynthetic yields by\npost-processing 10^6 tracer particles with a 384 nuclide reaction network. All\ndelayed detonation models result in explosions unbinding the white dwarf,\nproducing a range of 56Ni masses from 0.32 to 1.11 solar masses. As a general\ntrend, the models predict that the stable neutron-rich iron group isotopes are\nnot found at the lowest velocities, but rather at intermediate velocities\n(~3,000 - 10,000 km/s) in a shell surrounding a 56Ni-rich core. The models\nfurther predict relatively low velocity oxygen and carbon, with typical minimum\nvelocities around 4,000 and 10,000 km/s, respectively.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The (non-)variability of magnetic chemically peculiar candidates in the\n  Large Magellanic Cloud: The galactic magnetic chemically peculiar (mCP) stars of the upper main\nsequence are well known as periodic spectral and light variables. The observed\nvariability is obviously caused by the uneven distribution of overabundant\nchemical elements on the surfaces of rigidly rotating stars. The mechanism\ncausing the clustering of some chemical elements into disparate structures on\nmCP stars has not been fully understood up to now. The observations of light\nchanges of mCP candidates recently revealed in the nearby Large Magellanic\nCloud (LMC) should provide us with information about their rotational periods\nand about the distribution of optically active elements on mCP stars born in\nother galaxies. We queried for photometry at the Optical Gravitational Lensing\nExperiment (OGLE)-III survey of published mCP candidates selected because of\nthe presence of the characteristic 5200A flux depression. In total, the\nintersection of both sources resulted in twelve stars. We performed our own and\nstandard periodogram time series analyses of all available data. The final\nresults are, amongst others, the frequency of the maximum peak and the\nbootstrap probability of its reality. We detected that only two mCP candidates,\n190.1 1581 and 190.1 15527, may show some weak rotationally modulated light\nvariations with periods of 1.23 and 0.49 days; however, the 49% and 32%\nprobabilities of their reality are not very satisfying. The variability of the\nother 10 mCP candidates is too low to be detectable by their V and I OGLE\nphotometry. The relatively low amplitude variability of the studied LMC mCP\ncandidates sample can be explained by the absence of photometric spots of\noverabundant optically active chemical elements. The unexpected LMC mCPs\nbehaviour is probably caused by different conditions during the star formation\nin the LMC and the Galaxy.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Imaging sub-milliarcsecond stellar features with intensity\n  interferometry using air Cherenkov telescope arrays: Recent proposals have been advanced to apply imaging air Cherenkov telescope\narrays to stellar intensity interferometry (SII). Of particular interest is the\npossibility of model-independent image recovery afforded by the good (u,\nv)-plane coverage of these arrays, as well as recent developments in phase\nretrieval techniques. The capabilities of these instruments used as SII\nreceivers have already been explored for simple stellar objects, and here the\nfocus is on reconstructing stellar images with non-uniform radiance\ndistributions. We find that hot stars (T > 6000 K) containing hot and/or cool\nlocalized regions (T \\sim 500 K) as small as \\sim 0.1 mas can be imaged at\nshort wavelengths ({\\lambda} = 400 nm).",
        "category": "astro-ph_SR"
    },
    {
        "text": "Beryllium detection in the very fast nova ASASSN-16kt (V407 Lupi): We present high-resolution spectroscopic observations of the fast nova\nASASSN-16kt (V407 Lup). A close inspection of spectra obtained at early stages\nhas revealed the presence of low-ionization lines, and among the others we have\nidentified the presence of the ionised $^7$Be doublet in a region relatively\nfree from possible contaminants. After studying their intensities, we have\ninferred that ASASSN-16kt has produced (5.9 - 7.7)$ \\times 10^{-9}$ M$_{\\odot}$\nof $^7$Be. The identification of bright Ne lines may suggest that the nova\nprogenitor is a massive (1.2 M$_{\\odot}$) oxygen-neon white dwarf. The high\noutburst frequency of oxygen-neon novae implies that they likely produce an\namount of Be similar, if not larger, to that produced by carbon-oxygen novae,\nthen confirming that classical novae are among the main factories of lithium in\nthe Galaxy.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Direct Imaging Explorations for Companions around Mid-Late M Stars from\n  the Subaru/IRD Strategic Program: The Subaru telescope is currently performing a strategic program (SSP) using\nthe high-precision near-infrared (NIR) spectrometer IRD to search for\nexoplanets around nearby mid/late-M~dwarfs via radial velocity (RV) monitoring.\nAs part of the observing strategy for the exoplanet survey, signatures of\nmassive companions such as RV trends are used to reduce the priority of those\nstars. However, this RV information remains useful for studying the stellar\nmultiplicity of nearby M~dwarfs. To search for companions around such\n``deprioritized\" M~dwarfs, we observed 14 IRD-SSP targets using Keck/NIRC2\nobservations with pyramid wavefront sensing at NIR wavelengths, leading to high\nsensitivity to substellar-mass companions within a few arcseconds. We detected\ntwo new companions (LSPM~J1002+1459~B and LSPM~J2204+1505~B) and two new\ncandidates that are likely companions (LSPM~J0825+6902~B and LSPM~J1645+0444~B)\nas well as one known companion. Including two known companions resolved by the\nIRD fiber injection module camera, we detected seven (four new) companions at\nprojected separations between $\\sim2-20$~au in total. A comparison of the\ncolors with the spectral library suggests that LSPM~J2204+1505~B and\nLSPM~J0825+6902~B are located at the boundary between late-M and early-L\nspectral types. Our deep high-contrast imaging for targets where no bright\ncompanions were resolved did not reveal any additional companion candidates.\nThe NIRC2 detection limits could constrain potential substellar-mass companions\n($\\sim10-75\\ M_{\\rm Jup}$) at 10~au or further. The failure with Keck/NIRC2\naround the IRD-SSP stars having significant RV trends makes these objects\npromising targets for further RV monitoring or deeper imaging with JWST to\nsearch for smaller-mass companions below the NIRC2 detection limits.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Asteroseismology of the $\u03b4$ Scuti star HD 50844: Aims. We aim to probe the internal structure and investigate more detailed\ninformation of the $\\delta$ Scuti star HD 50844 with asteroseismology. Methods.\nWe analyse the observed frequencies of the $\\delta$ Scuti star HD 50844\nobtained by Balona (2014), and search for possible multiplets based on the\nrotational splitting law of g-mode. We tried to disentangle the frequency\nspectra of HD 50844 by means of the rotational splitting only. We then compare\nthem with theoretical pulsation modes, which correspond to stellar evolutionary\nmodels with various sets of initial metallicity and stellar mass, to find the\nbest-fitting model. Results. There are three multiplets including two complete\ntriplets and one incomplete quintuplet, in which mode identifications for\nspherical harmonic degree $l$ and azimuthal number $m$ are unique. The\ncorresponding rotational period of HD 50844 is found to be\n2.44$^{+0.13}_{-0.08}$ days. The physical parameters of HD 50844 are well\nlimited in a small region by three modes identified as nonradial ones\n($f_{11}$, $f_{22}$, and $f_{29}$) and by the fundamental radial mode\n($f_{4}$). Our results show that the three nonradial modes ($f_{11}$, $f_{22}$,\nand $f_{29}$) are all mixed modes, which mainly represent the property of the\nhelium core. The fundamental radial mode ($f_{4}$) mainly represents the\nproperty of the stellar envelope. In order to fit these four pulsation modes,\nboth the helium core and the stellar envelope must be matched to the actual\nstructure of HD 50844. Finally, the mass of the helium core of HD 50844 is\nestimated to be 0.173 $\\pm$ 0.004 $M_{\\odot}$ for the first time. The physical\nparameters of HD 50844 are determined to be $M=$ 1.81 $\\pm$ 0.01 $M_{\\odot}$,\n$Z=$ 0.008 $\\pm$ 0.001. $T_{\\rm eff}=$ 7508 $\\pm$ 125 K, log$g=$ 3.658 $\\pm$\n0.004, $R=$ 3.300 $\\pm$ 0.023 $R_{\\odot}$, $L=$ 30.98 $\\pm$ 2.39 $L_{\\odot}$.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Simulations of Protoplanetary Disk Dispersal: Stellar Mass Dependence of\n  the Disk Lifetime: Recent infrared and submillimeter observations suggest that the\nprotoplanetary disk lifetime depends on the central stellar mass. The disk\ndispersal is thought to be driven by viscous accretion, magneto-hydrodynamics\n(MHD) winds, and photoevaporation by the central star. We perform a set of\none-dimensional simulations of long-term disk evolution that include all the\nthree processes. We vary the stellar mass in the range of 0.5-7M$_{\\odot}$, and\nstudy the mass dependence of the disk evolution. We show that a significant\nfraction of the disk gas is lost by MHD winds in the early stage, but the later\ndisk evolution is mainly governed by photoevaporation. The disk radius\ndecreases as photoevaporation clears out the gas in the outer disk efficiently.\nThe qualitative evolutionary trends of the disk mass are remarkably similar for\nthe wide range of the central stellar mass we consider, and the time evolution\nof the disk mass can be well fitted by a simple function. The dispersal time is\napproximately ten million years for low mass stars with weak mass dependence,\nbut gets as short as two million years around a 7M$_{\\odot}$ star. In the\nlatter case, a prominent inner hole is formed by the combined effect of\naccretion and MHD winds within about one million years. The strength of the MHD\nwind and viscous accretion controls the overall mass-loss rate, but does not\nalter the dependence of the dispersal timescale on the central stellar mass.",
        "category": "astro-ph_SR"
    },
    {
        "text": "From the atmosphere to the circumstellar environment in cool evolved\n  stars: We discuss and illustrate contributions that optical interferometry has made\non our current understanding of cool evolved stars. We include red giant branch\n(RGB) stars, asymptotic giant branch (AGB) stars, and red supergiants (RSGs).\nStudies using optical interferometry from visual to mid-infrared wavelengths\nhave greatly increased our knowledge of their atmospheres, extended molecular\nshells, dust formation, and winds. These processes and the morphology of the\ncircumstellar environment are important for the further evolution of these\nstars toward planetary nebulae (PNe) and core-collapse supernovae (SNe), and\nfor the return of material to the interstellar medium.",
        "category": "astro-ph_SR"
    },
    {
        "text": "ALMA Imaging of Millimeter/Submillimeter Continuum Emission in Orion KL: We have carried out high resolution observations with Atacama Large\nMillimeter/Submillimeter Array (ALMA) of continuum emission from Orion KL\nregion. We identify 11 compact sources at ALMA band 6 (245 GHz) and band 7 (339\nGHz), including Hot Core, Compact Ridge, SMA1, IRc4, IRc7, and a radio source I\n(Source I). Spectral energy distribution (SED) of each source is determined by\nusing previous 3 mm continuum emission data. Physical properties such as size,\nmass, hydrogen number density and column density are discussed based on the\ndust graybody SED. Among 11 identified sources, Source I, a massive protostar\ncandidate, is a dominant energy source in Orion KL. We extensively investigate\nits SED from centimeter to submillimeter wavelengths. The SED of Source I can\nbe fitted with a single power-law index of 1.97 suggesting an optically thick\nemission. We employ the H$^{-}$ free-free emission as an opacity source of this\noptically thick emission. The temperature, density, and mass of the\ncircumstellar disk associated with Source I are constrained by the SED of\nH$^{-}$ free-free emission. Still the fitting result shows a significant\ndeviation from the observed flux densities. Combined with the thermal dust\ngraybody SED to explain excess emission at higher frequency, a smaller\npower-law index of 1.60 for the H$^{-}$ free-free emission is obtained in the\nSED fitting. The power-law index smaller than 2 would suggest a compact source\nsize or a clumpy structure unresolved with the present study. Future higher\nresolution observations with ALMA are essential to reveal more detailed spatial\nstructure and physical properties of Source I.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Magnetic activities on two single-lined RS Canum Venaticorum binaries IM\n  Pegasi and $\u03c3$ Geminorum: We present the study on continuous high-resolution spectroscopic observations\nof two long-period single-lined RS Canum Venaticorum (RS CVn) binary stars IM\nPegasi (IM Peg) and $\\sigma$ Geminorum ($\\sigma$ Gem), obtained with the\nHertzsprung SONG telescope during the 2015-2016 season. Chromospheric activity\nindicators H$_{\\alpha}$, Na I D$_{1}$, D$_{2}$ doublet, He I D$_{3}$, and\nH$_{\\beta}$ lines have been analyzed by using the spectral subtraction\ntechnique. The expected chromospheric emission features in the H$_{\\alpha}$, Na\nI D$_{1}$, D$_{2}$ doublet, and H$_{\\beta}$ lines confirm that both of two\nstars are very active systems. In the spectra, the He I D$_{3}$ line had been\nalways detected in absorption feature. Although the behavior of chromospheric\nactivity indicators is very similar for both stars, the activity level of IM\nPeg is much stronger than that of $\\sigma$ Gem. Moreover, the EW variations of\nthe H$_{\\alpha}$, He I D$_{3}$, and H$_{\\beta}$ line subtractions correlate\nwell and show different behavior among different orbital cycles, which\nindicates the presence and evolution of activity longitudes over the surface of\ntwo stars. Furthermore, the subtracted H$_{\\alpha}$ line profile is usually\nasymmetric. The red-shifted excess absorption features could be interpreted as\na strong down-flow of cool absorbing material, while the blue-shifted emission\ncomponent is probably caused by up-flow of hot materials through microflare\nevents.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A very young, compact bipolar H$_2$O maser outflow in the\n  intermediate-mass star-forming LkH$\u03b1$ 234 region: We report multi-epoch VLBI H$_2$O maser observations towards the compact\ncluster of YSOs close to the Herbig Be star LkH$\\alpha$ 234. This cluster\nincludes LkH$\\alpha$ 234 and at least nine more YSOs that are formed within\nprojected distances of $\\sim$10 arcsec ($\\sim$9,000 au). We detect H$_2$O maser\nemission towards four of these YSOs. In particular, our VLBI observations\n(including proper motion measurements) reveal a remarkable very compact\n($\\sim$0.2 arcsec = $\\sim$180 au), bipolar H$_2$O maser outflow emerging from\nthe embedded YSO VLA 2. We estimate a kinematic age of $\\sim$40 yr for this\nbipolar outflow, with expanding velocities of $\\sim$20 km s$^{-1}$ and momentum\nrate $\\dot M_w V_w$ $\\simeq$ $10^{-4}-10^{-3}$ M$_{\\odot}$ yr$^{-1}$ km\ns$^{-1}$$\\times (\\Omega$/$4\\pi)$, powered by a YSO of a few solar masses. We\npropose that the outflow is produced by recurrent episodic jet ejections\nassociated with the formation of this YSO. Short-lived episodic ejection events\nhave previously been found towards high-mass YSOs. We show now that this\nbehaviour is also present in intermediate-mass YSOs. These short-lived episodic\nejections are probably related to episodic increases in the accretion rate, as\nobserved in low-mass YSOs. We predict the presence of an accretion disk\nassociated with VLA 2. If detected, this would represent one of the few known\nexamples of intermediate-mass stars with a disk-YSO-jet system at scales of a\nfew hundred au.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Emergence of magnetic flux generated in a solar convective dynamo. I:\n  Formation of Sunspots and Active regions, and Origin of Their Asymmetries: We present a realistic numerical model of sunspot and active region formation\nbased on the emergence of flux bundles generated in a solar convective dynamo.\nTo this end we use the magnetic and velocity fields in a horizontal layer near\nthe top boundary of the solar convective dynamo simulation to drive realistic\nradiative-magnetohydrodynamic simulations of the upper most layers of the\nconvection zone. The main results are: (1) The emerging flux bundles rise with\nthe mean speed of convective upflows, and fragment into small-scale magnetic\nelements that further rise to the photosphere, where bipolar sunspot pairs are\nformed through the coalescence of the small-scale magnetic elements. (2)\nFilamentary penumbral structures form when the sunspot is still growing through\nongoing flux emergence. In contrast to the classical Evershed effect, the\ninflow seems to prevail over the outflow in a large part of the penumbra. (3) A\nwell formed sunspot is a mostly monolithic magnetic structures that is anchored\nin a persistent deep-seated downdraft lane. The flow field outside the spot\nshows a giant vortex ring that comprises of an inflow below 15 Mm depth and an\noutflow above 15 Mm depth. (4) The sunspots successfully reproduce the\nfundamental properties of the observed solar active regions, including the more\ncoherent leading spots with a stronger field strength, and the correct tilts of\nbipolar sunspot pairs. These asymmetries can be linked to the intrinsic\nasymmetries in the magnetic and flow fields adapted from the convective dynamo\nsimulation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Solid-state production of complex organic molecules: H-atom addition\n  versus UV irradiation: Complex organic molecules (COMs) have been observed in comets, hot cores and\ncold dense regions of the interstellar medium. It is generally accepted that\nthese COMs form on icy dust grain through the recombination reaction of\nradicals triggered by either energetic UV- photon or non-energetic H-atom\naddition processing. In this work, we present for the first time laboratory\nstudies that allow for quantitative comparison of hydrogenation and UV-induced\nreactions as well as their cumulative effect in astronomically relevant\nCO:CH3OH=4:1 ice analogues. The formation of glycolaldehyde (GA) and ethylene\nglycol (EG) is confirmed in pure hydrogenation experiments at 14 K, except\nmethyl formate (MF), which is only clearly observed in photolysis. The\nfractions for MF:GA:EG are 0 : (0.2-0.4) : (0.8-0.6) for pure hydrogenation,\nand 0.2 : 0.3 : 0.5 for UV involving experiments and can offer a diagnostic\ntool to derive the chemical origin of these species. The GA/EG ratios in the\nlaboratory (0.3-1.5) compare well with observations toward different objects.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Envelope Convection, Surface Magnetism and Spots in A- and late B-type\n  Stars: Weak magnetic fields have recently been detected in a number of A-type stars,\nincluding Vega and Sirius. At the same time, space photometry observations of\nA- and late B-type stars from Kepler and TESS have highlighted the existence of\nrotational modulation of surface features akin to stellar spots. Here we\nexplore the possibility that surface magnetic spots might be caused by the\npresence of small envelope convective layers at or just below the stellar\nsurface, caused by recombination of H and He. Using 1D stellar evolution\ncalculations and assuming an equipartition dynamo, we make simple estimates of\nfield strength at the photosphere. For most models the largest effects are\ncaused by a convective layer driven by second helium ionization. While it is\ndifficult to predict the geometry of the magnetic field, we conclude that the\nmajority of intermediate-mass stars should have dynamo-generated magnetic\nfields of order a few gauss at the surface. These magnetic fields can appear at\nthe surface as bright spots, and cause photometric variability via rotational\nmodulation, which could also be wide-spread in A-stars. The amplitude of\nsurface magnetic fields and their associated photometric variability is\nexpected to decrease with increasing stellar mass and surface temperature, so\nthat magnetic spots and their observational effects should be much harder to\ndetect in late B-type stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Cluster AgeS Experiment (CASE). Variable stars in the field of the\n  globular cluster M10: The field of the globular cluster M10 (NGC 6254) was monitored between 1998\nand 2015 in a search for variable stars. V -light curves were derived for 40\nvariables or likely variables, most of which are new detections. Proper motions\nobtained within the CASE project indicate that 18 newly detected variables and\n14 previously known ones are members or likely members of the cluster,\nincluding one RRc-type, three type II Cepheids, and 14 SX Phe-type pulsators,\none contact binary, and six semi-regular red giants. As a byproduct of the\nsearch we discovered a candidate binary comprised of main sequence stars with\nthe record-short orbital period of 0.042 d. We also confirmed the photometric\nvariability of the red straggler M10-VLA1 hinted at by Shishkovsky et al.\n(2018), who discovered this object spectroscopically. In Appendix 1 we show\nthat CASE proper motion measurements are in a good agreement with those\nretrieved from the Gaia archive, while Appendix 2 presents evidence for low\nfrequency {\\gamma} Doradus-type oscillations in SX Phe stars belonging to M10.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Off-disk straylight measurements for the Swedish 1-meter Solar Telescope: Context. Accurate photometry with ground based solar telescopes requires\ncharacterization of straylight. Scattering in Earth's atmosphere and in the\ntelescope optics are potentially significant sources of straylight, for which\nthe point spread function (PSF) has wings that reach very far. This kind of\nstraylight produces an aureola, extending several solar radii off the solar\ndisk. Aims. We want to measure such straylight using the ordinary science\ninstrumentation. Methods. We scanned the intensity on and far off the solar\ndisk by use of the science cameras in several different wavelength bands on a\nday with low-dust conditions. We characterized the far wing straylight by\nfitting a model to the recorded intensities involving a multi-component\nstraylight PSF and the limb darkening of the disk. Results. The measured\nscattered light adds an approximately constant fraction of the local\ngranulation intensity to science images at any position on the disk. The\nfraction varied over the day but never exceeded a few percent. The PSFs have\nweak tails that extend to several solar radii but most of the scattered light\noriginates within ~1'. Conclusions. Far-wing scattered light contributes only a\nsmall amount of straylight in SST data. Other sources of straylight are\nprimarily responsible for the reduced contrast in SST images.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Measuring the Magnetic Field Strength of the Quiet Solar Corona Using\n  \"EIT Waves\": Variations in the propagation of globally-propagating disturbances (commonly\ncalled \"EIT waves\") through the low solar corona offer a unique opportunity to\nprobe the plasma parameters of the solar atmosphere. Here, high-cadence\nobservations of two \"EIT wave\" events taken using the Atmospheric Imaging\nAssembly (AIA) instrument onboard the Solar Dynamics Observatory (SDO) are\ncombined with spectroscopic measurements from the Extreme ultraviolet Imaging\nSpectrometer (EIS) onboard the Hinode spacecraft and used to examine the\nvariability of the quiet coronal magnetic-field strength. The combination of\npulse kinematics from SDO/AIA and plasma density from Hinode/EIS is used to\nshow that the magnetic-field strength is in the range ~2-6 G in the quiet\ncorona. The magnetic-field estimates are then used to determine the height of\nthe pulse, allowing a direct comparison with theoretical values obtained from\nmagnetic-field measurements from the Helioseismic and Magnetic Imager (HMI)\nonboard SDO using PFSS and local-domain extrapolations. While local-scale\nextrapolations predict heights inconsistent with prior measurements, the\nagreement between observations and the PFSS model indicates that \"EIT waves\"\nare a global phenomenon influenced by global-scale magnetic field.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Statistical Survey of Hard X-ray Spectral Characteristics of Solar\n  Flares with Two Footpoints: Using RHESSI data, we have analyzed some 172 hard X-ray peaks during 53 solar\nflares which exhibited a double-footpoint structure. Fitting both footpoints\nwith power-laws, we find that spectral index differences range mostly between 0\nto 0.6, and only rarely go beyond. Asymmetries between footpoints were not\nobserved to be significantly dependent on their mean heliographic position,\ntheir relative position with respect to each other, nor their orientation with\nrespect to the solar equator. Assuming a symmetric acceleration process, it is\nalso clear that differences in footpoint spectral indices and footpoint flux\nratios can seldom be attributed to a difference in column densities between the\ntwo legs of a coronal loop. Our results corroborate better the magnetic mirror\ntrap scenario. Moreover, footpoint asymmetries are more marked during times of\npeak HXR flux than when averaging over the whole HXR burst, suggesting that the\nmagnetic configuration evolves during individual HXR bursts. We observed also a\nlinear correlation between the peak 50-keV flux and the peak GOES 1-8A channel\nflux, and that HXR burst duration seem correlated with loop length.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Exploring the Nature of EUV Waves in a Radiative Magnetohydrodynamic\n  Simulation: Coronal extreme-ultraviolet (EUV) waves are large-scale disturbances\npropagating in the corona, whose physical nature and origin have been discussed\nfor decades. We report the first three dimensional (3D) radiative\nmagneto-hydrodynamic (RMHD) simulation of a coronal EUV wave and the\naccompanying quasi-periodic wave trains. The numerical experiment is conducted\nwith the MURaM code and simulates the formation of solar active regions through\nmagnetic flux emergence from the convection zone to the corona. The coronal EUV\nwave is driven by the eruption of a magnetic flux rope that also gives rise to\na C-class flare. It propagates in a semi-circular shape with an initial speed\nranging from about 550 to 700 km s$^{-1}$, which corresponds to an average Mach\nnumber (relative to fast magnetoacoustic waves) of about 1.2. Furthermore, the\nabrupt increase of the plasma density, pressure and tangential magnetic field\nat the wavefront confirms fast-mode shock nature of the coronal EUV wave.\nQuasi-periodic wave trains with a period of about 30 s are found as multiple\nsecondary wavefronts propagating behind the leading wavefront and ahead of the\nerupting magnetic flux rope. We also note that the true wavefront in the 3D\nspace can be very inhomogeneous, however, the line-of-sight integration of EUV\nemission significantly smoothes the sharp structures in 3D and leads to a more\ndiffuse wavefront.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Interpretation of CEMP(s) and CEMP(s + r) Stars with AGB Models: Asymptotic Giant Branch (AGB) stars play a fundamental role in the s-process\nnucleosynthesis during their thermal pulsing phase. The theoretical predictions\nobtained by AGB models at different masses, s-process efficiencies, dilution\nfactors and initial r-enrichment, are compared with spectroscopic observations\nof Carbon-Enhanced Metal-Poor stars enriched in s-process elements, CEMP(s),\ncollected from the literature. We discuss here five stars as example, CS\n22880-074, CS 22942-019, CS 29526-110, HE 0202-2204, and LP 625-44. All these\nobjects lie on the main-sequence or on the giant phase, clearly before the\nTP-AGB stage: the hypothesis of mass transfer from an AGB companion, would\nexplain the observed s-process enhancement. CS 29526-110 and LP 625-44 are\nCEMP(s+r) objects, and are interpreted assuming that the molecular cloud, from\nwhich the binary system formed, was already enriched in r-process elements by\nSNII pollution. In several cases, the observed s-process distribution may be\naccounted for AGB models of different initial masses with proper 13C-pocket\nefficiency and dilution factor. Na (and Mg), produced via the neutron capture\nchain starting from 22Ne, may provide an indicator of the initial AGB mass.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Stellar Yields of Rotating First Stars. II. Pair Instability Supernovae\n  and Comparison with Observations: Recent theory predicts that a first star is born with a massive initial mass\nof $\\gtrsim$ 100 $M_\\odot$. Pair instability supernova (PISN) is a common fate\nfor such a massive star. Our final goal is to prove the existence of PISN and\nthus the high mass nature of the initial mass function in the early universe by\nconducting {\\it abundance profiling}, in which properties of a hypothetical\nfirst star is constrained by metal-poor star abundances. In order to determine\nreliable and useful abundances, we investigate the PISN nucleosynthesis taking\nboth rotating and non-rotating progenitors for the first time. We show that the\ninitial and CO core mass ranges for PISNe depend on the envelope structures:\nnon-magnetic rotating models developing inflated envelopes have a lower-shifted\nCO mass range of $\\sim$ 70--125 $M_\\odot$, while non-rotating and magnetic\nrotating models with deflated envelopes have a range of $\\sim$ 80--135\n$M_\\odot$. However, we find no significant difference in explosive yields from\nrotating and non-rotating progenitors, except for large nitrogen production in\nnon-magnetic rotating models. Furthermore, we conduct the first systematic\ncomparison between theoretical yields and a large sample of metal-poor star\nabundances. We find that the predicted low [Na/Mg] $\\sim$ $-1.5$ and high\n[Ca/Mg] $\\sim$ $0.5$--$1.3$ abundance ratios are the most important to\ndiscriminate PISN signatures from normal metal-poor star abundances, and\nconfirm that no currently observed metal-poor star matches with the PISN\nabundance. Extensive discussion on the non-detection is finally made.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A New Look at the YY CrB Binary System: This study presented a new analysis for the TESS-observed W Ursae Majoris (W\nUMa) binary star YY Coronea Borealis (YY CrB). The light curve was analyzed by\nthe PHysics Of Eclipsing BinariEs (PHOEBE) Python version together with the\nMarkov chain Monte Carlo (MCMC) method. The light curve solutions required a\nhot spot and l3. New eclipse times from the TESS observations were extracted,\nand the O-C curve of primary and secondary minima showed an anti-correlated\nmanner. In order to study the O-C curve of minima, minima times between 1991\nand 2023 were collected. This investigation reported a new linear ephemeris and\nby fitting a quadratic function to the O-C curve of minima, calculated the\norbital period rate of \\mathop P\\limits^.\\approx 5.786*{10^{-8}} day/year.\nAssuming mass conservation, a mass exchange rate of\n\\mathop{{M_2}}\\limits^.=2.472*{10^{-8}} calculated from the more massive\ncomponent to the less massive one. Then, by using the light travel time\nfunction, the possible third body was determined in the binary and derived the\nmass of the third body as 0.498M_Sun with a period of \\simeq 7351.018 days. The\nO-C curve analysis and the quantity of mass indicate that the presence of a\nthird body is unlikely. This binary is expected to evolve into a broken-contact\nphase and is a good case to support the thermal relaxation oscillation model.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Searching For Wide Binary Stars with Non-coeval Components in the\n  Southern Sky: We have completed our observational program to search for wide binary systems\nwith non-coeval components in the southern sky and report our results here. The\nfinal set of four systems was spectroscopically investigated in this paper. No\nbinary systems with components of different ages were found among them. Taking\ninto account our previous studies, we estimate the fraction of such binaries\n(i.e., binaries formed, presumably, by capture) not higher than 0.06~\\%. The\nstudy will be continued on the northern sky.",
        "category": "astro-ph_SR"
    },
    {
        "text": "BD +44 493: A Ninth Magnitude Messenger from the Early Universe; Carbon\n  Enhanced and Beryllium Poor: We present a 1D LTE chemical abundance analysis of the very bright (V=9.1)\nCarbon-Enhanced Metal-Poor (CEMP) star BD +44 493, based on high-resolution,\nhigh signal-to-noise spectra obtained with Subaru/HDS. The star is shown to be\na subgiant with an extremely low iron abundance ([Fe/H]=-3.7), while it is rich\nin C ([C/Fe]=+1.3) and O ([O/Fe]=+1.6). Although astronomers have been\nsearching for extremely metal-poor stars for decades, this is the first star\nfound with [Fe/H]<-3.5 and an apparent magnitude V<12. Based on its low\nabundances of neutron-capture elements (e.g., [Ba/Fe]=-0.59), BD +44 493 is\nclassified as a \"CEMP-no\" star. Its abundance pattern implies that a\nfirst-generation faint supernova is the most likely origin of its carbon\nexcess, while scenarios related to mass loss from rapidly-rotating massive\nstars or mass transfer from an AGB companion star are not favored. From a\nhigh-quality spectrum in the near-UV region, we set an very low upper limit on\nthis star's beryllium abundance (A(Be)=log(Be/H)+12<-2.0), which indicates that\nthe decreasing trend of Be abundances with lower [Fe/H] still holds at\n[Fe/H]<-3.5. This is the first attempt to measure a Be abundance for a CEMP\nstar, and demonstrates that high C and O abundances do not necessarily imply\nhigh Be abundances.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The 12C + 12C reaction and the impact on nucleosynthesis in massive\n  stars: Despite much effort in the past decades, the C-burning reaction rate is\nuncertain by several orders of magnitude, and the relative strength between the\ndifferent channels 12C(12C,alpha)20Ne, 12C(12C,p)23Na and 12C(12C,n)23Mg is\npoorly determined. Additionally, in C-burning conditions a high 12C+12C rate\nmay lead to lower central C-burning temperatures and to 13C(alpha,n)16O\nemerging as a more dominant neutron source than 22Ne(alpha,n)25Mg, increasing\nsignificantly the s-process production. This is due to the rapid decrease of\nthe 13N(gamma,p)12C with decreasing temperature, causing the 13C production via\n13N(beta+)13C. Presented here is the impact of the 12C+12C reaction\nuncertainties on the s-process and on explosive p-process nucleosynthesis in\nmassive stars, including also fast rotating massive stars at low metallicity.\nUsing various 12C+12C rates, in particular an upper and lower rate limit of ~\n50000 higher and ~ 20 lower than the standard rate at 5*10^8 K, five 25 Msun\nstellar models are calculated. The enhanced s-process signature due to\n13C(alpha,n)16O activation is considered, taking into account the impact of the\nuncertainty of all three C-burning reaction branches. Consequently, we show\nthat the p-process abundances have an average production factor increased up to\nabout a factor of 8 compared to the standard case, efficiently producing the\nelusive Mo and Ru proton-rich isotopes. We also show that an s-process being\ndriven by 13C(alpha,n)16O is a secondary process, even though the abundance of\n13C does not depend on the initial metal content. Finally, implications for the\nSr-peak elements inventory in the Solar System and at low metallicity are\ndiscussed.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The VLT-FLAMES Tarantula Survey XXV. Surface nitrogen abundances of\n  O-type giants and supergiants: Theoretically, rotation-induced chemical mixing in massive stars has far\nreaching evolutionary consequences, affecting the sequence of morphological\nphases, lifetimes, nucleosynthesis, and supernova characteristics. Using a\nsample of 72 presumably single O-type giants to supergiants observed in the\ncontext of the VLT-FLAMES Tarantula Survey (VFTS), we aim to investigate\nrotational mixing in evolved core-hydrogen burning stars initially more massive\nthan $15\\,M_\\odot$ by analysing their surface nitrogen abundances. Using\nstellar and wind properties derived in a previous VFTS study, we constrained\nthe nitrogen abundance by fitting the equivalent widths of relatively strong\nlines that are sensitive to changes in the abundance of this element. Given the\nquality of the data, we constrained the nitrogen abundance in 38 cases; for 34\nstars only upper limits could be derived, which includes almost all stars\nrotating at $v_\\mathrm{e}\\sin i >200\\,\\mathrm{km s^{-1}}$. We analysed the\nnitrogen abundance as a function of projected rotation rate $v_\\mathrm{e}\\sin\ni$ and confronted it with predictions of rotational mixing. The upper limits on\nthe nitrogen abundance of the rapidly rotating stars are not in apparent\nviolation with theoretical expectations. However, we found a group of\nN-enhanced slowly-spinning stars that is not in accordance with predictions of\nrotational mixing in single stars. Among O-type stars with (rotation-corrected)\ngravities less than $\\log\\,g_c = 3.75$ this group constitutes 30$-$40 percent\nof the population. We found a correlation between nitrogen and helium abundance\nwhich is consistent with expectations, suggesting that, whatever the mechanism\nthat brings N to the surface, it displays CNO-processed material.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Search for Dormant Black Holes in Ellipsoidal Variables I. Revisiting\n  the Expected Amplitudes of the Photometric Modulation: Ellipsoidal variables present light-curve modulations caused by stellar\ndistortion, induced by tidal interaction with their companions. An analytical\napproximated model of the ellipsoidal modulation is given as a discrete Fourier\nseries by Morris and Naftilan 1993 (MN93). Based on numerical simulations using\nthe PHOEBE code we present here updated amplitudes of the first three harmonics\nof the model. The expected amplitudes are given as a function of the mass ratio\nand inclination of the binary system and the fillout factor of the\nprimary---the ratio between the stellar radius and that of its Roche lobe. The\ncorrections can get up to 30% relative to the MN93 model for fillout factors\nclose to unity. The updated model can be instrumental in searching for\nshort-period binaries with compact-object secondaries in large data sets of\nphotometric light curves. As shown in one OGLE light-curve example, the minimum\nmass ratio can be obtained by using only the amplitudes of the three harmonics\nand an estimation of the stellar temperature. High enough amplitudes can help\nto identify binaries with mass ratios larger than unity, some of which might\nhave compact companions.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Gas inside the 97 au cavity around the transition disk Sz\\,91: We present ALMA (Cycle 0) band-6 and band-3 observations of the transition\ndisk Sz\\,91. The disk inclination and position angle are determined to be\n$i=49.5\\degr\\pm3.5\\degr$ and $\\mathrm{PA}=18.2\\degr\\pm3.5\\degr$ and the dusty\nand gaseous disk are detected up to $\\sim220$ au and $\\sim400$ au from the\nstar, respectively. Most importantly, our continuum observations indicate that\nthe cavity size in the mm-sized dust distribution must be $\\sim97$ au in\nradius, the largest cavity observed around a T Tauri star. Our data clearly\nconfirms the presence of \\co(2-1) well inside the dust cavity. Based on these\nobservational constrains we developed a disk model that simultaneously accounts\nfor the \\co and continuum observations (i.e., gaseous and dusty disk).\nAccording to our model, most of the millimeter emission comes from a ring\nlocated between 97 and 140 au. We also find that the dust cavity is divided\ninto an innermost region largely depleted of dust particles ranging from the\ndust sublimation radius up to 85 au, and a second, moderately dust-depleted\nregion, extending from 85 to 97 au. The extremely large size of the dust\ncavity, the presence of gas and small dust particles within the cavity and the\naccretion rate of Sz\\,91 are consistent with the formation of multiple (giant)\nplanets.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Diagnosing the Clumpy Protoplanetary Disk of the UXor Type Young Star GM\n  Cephei: UX Orionis stars (UXors) are Herbig Ae/Be or T Tauri stars exhibiting\nsporadic occultation of stellar light by circumstellar dust. GM\\,Cephei is such\na UXor in the young ($\\sim4$~Myr) open cluster Trumpler\\,37, showing prominent\ninfrared excess, emission-line spectra, and flare activity. Our photometric\nmonitoring (2008--2018) detects (1)~an $\\sim$3.43~day period, likely arising\nfrom rotational modulation by surface starspots, (2)~sporadic brightening on\ntime scales of days due to accretion, (3)~irregular minor flux drops due to\ncircumstellar dust extinction, and (4)~major flux drops, each lasting for a\ncouple of months with a recurrence time, though not exactly periodic, of about\ntwo years. The star experiences normal reddening by large grains, i.e., redder\nwhen dimmer, but exhibits an unusual \"blueing\" phenomenon in that the star\nturns blue near brightness minima. The maximum extinction during relatively\nshort (lasting $\\leq 50$~days) events, is proportional to the duration, a\nconsequence of varying clump sizes. For longer events, the extinction is\nindependent of duration, suggestive of a transverse string distribution of\nclumps. Polarization monitoring indicates an optical polarization varying\n$\\sim3\\%$--8$\\%$, with the level anticorrelated with the slow brightness\nchange. Temporal variation of the unpolarized and polarized light sets\nconstraints on the size and orbital distance of the circumstellar clumps in the\ninterplay with the young star and scattering envelope. These transiting clumps\nare edge-on manifestations of the ring- or spiral-like structures found\nrecently in young stars with imaging in infrared of scattered light, or in\nsubmillimeter of thermalized dust emission.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Determining the ionization rates of interstellar neutral species using\n  direct-sampling observations of their direct and indirect beams: A good understanding of the ionization rates of neutral species in the\nheliosphere is important for studies of the heliosphere and planetary\natmospheres. So far, the intensities of the ionization reactions have been\nstudied based on observations of the contributing phenomena, such as the solar\nspectral flux in the EUV band and the flux of the solar wind protons, alpha\nparticles, and electrons. The results strongly depend on absolute calibration\nof these measurements, which, especially for the EUV measurements, is\nchallenging. Here, we propose a novel method of determining the ionization rate\nof neutral species based on direct sampling of interstellar neutral gas from\ntwo locations in space distant to each other. In particular, we suggest\nperforming observations from the vicinity of Earth's orbit and using ratios of\nfluxes of ISN He for the direct and indirect orbits of interstellar atoms. We\nidentify the most favorable conditions and observations geometries, suitable\nfor implementation on the forthcoming NASA mission Interstellar Mapping and\nAcceleration Probe.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Volume-limited Sample of 63 M7--M9.5 Dwarfs I. Space Motion, Kinematic\n  Age, and Lithium: In a volume-limited sample of 63 ultracool dwarfs of spectral type M7--M9.5,\nwe have obtained high-resolution spectroscopy with UVES at the Very Large\nTelescope and HIRES at Keck Observatory. In this first paper we introduce our\nvolume-complete sample from DENIS and 2MASS targets, and we derive radial\nvelocities and space motion. Kinematics of our sample are consistent with the\nstars being predominantly members of the young disk. The kinematic age of the\nsample is 3.1 Gyr. We find that six of our targets show strong Li lines\nimplying that they are brown dwarfs younger than several hundred million years.\nFive of the young brown dwarfs were unrecognized before. Comparing the fraction\nof Li detections to later spectral types, we see a hint of an unexpected local\nmaximum of this fraction at spectral type M9. It is not yet clear whether this\nmaximum is due to insufficient statistics, or to a combination of physical\neffects including spectral appearance of young brown dwarfs, Li line formation,\nand the star formation rate at low masses.",
        "category": "astro-ph_SR"
    },
    {
        "text": "NSCC -- A New Scheme of Classification of C-rich Stars Devised from\n  Optical and Infrared Observations: A new classification system for carbon-rich stars is presented based on an\nanalysis of 51 AGB carbon stars through the most relevant classifying indices\navailable. The extension incorporated, that also represents the major advantage\nof this new system, is the combination of the usual optical indices that\ndescribe the photospheres of the objects, with new infrared ones, which allow\nan interpretation of the circumstellar environment of the carbon-rich stars.\nThis new system is presented with the usual spectral subclasses and $C_2$-, j-,\nMS- and temperature indices, and also with the new SiC- (SiC/C.A. abundance\nestimation) and $\\tau$- (opacity) indices. The values for the infrared indices\nwere carried out through a Monte Carlo simulation of the radiative transfer in\nthe circumstellar envelopes of the stars. The full set of indices, when applied\nto our sample, resulted in a more efficient system of classification, since an\nexamination in a wide spectral range allows us to obtain a complete scenario\nfor carbon stars.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evidence for a physically bound third component in HD 150136: Context. HD150136 is one of the nearest systems harbouring an O3 star.\nAlthough this system was for a long time considered as binary, more recent\ninvestigations have suggested the possible existence of a third component.\nAims. We present a detailed analysis of HD 150136 to confirm the triple nature\nof this system. In addition, we investigate the physical properties of the\nindividual components of this system. Methods. We analysed high-resolution,\nhigh signal-to-noise data collected through multi-epoch runs spread over ten\nyears. We applied a disentangling program to refine the radial velocities and\nto obtain the individual spectra of each star. With the radial velocities, we\ncomputed the orbital solution of the inner system, and we describe the main\nproperties of the orbit of the outer star such as the preliminary mass ratio,\nthe eccentricity, and the orbital-period range. With the individual spectra, we\ndetermined the stellar parameters of each star by means of the CMFGEN\natmosphere code. Results. We offer clear evidence that HD 150136 is a triple\nsystem composed of an O3V((f\\ast))-3.5V((f+)), an O5.5-6V((f)), and an\nO6.5-7V((f)) star. The three stars are between 0-3 Myr old. We derive dynamical\nmasses of about 64, 40, and 35 Msun for the primary, the secondary and the\nthird components by assuming an inclination of 49{\\deg}. It currently\ncorresponds to one of the most massive systems in our galaxy. The third star\nmoves with a period in the range of 2950 to 5500 d on an outer orbit with an\neccentricity of at least 0.3. This discovery makes HD 150136 the first\nconfirmed triple system with an O3 primary star. However, because of the long\norbital period, our dataset is not sufficient to constrain the orbital solution\nof the tertiary component with high accuracy.",
        "category": "astro-ph_SR"
    },
    {
        "text": "When flux standards go wild: white dwarfs in the age of Kepler: White dwarf stars have been used as flux standards for decades, thanks to\ntheir staid simplicity. We have empirically tested their photometric stability\nby analyzing the light curves of 398 high-probability candidates and\nspectroscopically confirmed white dwarfs observed during the original Kepler\nmission and later with K2 Campaigns 0-8. We find that the vast majority (>97\nper cent) of non-pulsating and apparently isolated white dwarfs are stable to\nbetter than 1 per cent in the Kepler bandpass on 1-hr to 10-d timescales,\nconfirming that these stellar remnants are useful flux standards. From the\ncases that do exhibit significant variability, we caution that binarity,\nmagnetism, and pulsations are three important attributes to rule out when\nestablishing white dwarfs as flux standards, especially those hotter than\n30,000 K.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Properties and applications of a predicted population of runaway\n  He-sdO/B stars ejected from single degenerate He-donor SNe: This study builds on previous works, producing the most extensive prediction\nof the properties of such a hypothetical population to date, taking into\naccount both Chandrasekhar and non-Chandrasekhar mass events. These results are\nthen used to define criteria for membership of this population and characterise\nputative subpopulations\n  This study contains 6x10^6 individual ejection trajectories out of the\nGalactic plane calculated with the stellar kinematics framework SHyRT, which\nare analysed with regard to their bulk observational properties. These are then\nput into context with the only previously identified population member US\\,708\nand applied to a number of other possible candidate objects.\n  We find that two additional previously observed objects possess properties to\nwarrant a designation as candidate objects. Characterisation of these object\nwith respect to the predicted population finds all of them to be extreme in at\nleast one astrometric observable. We find that current observations support a\nGalactic SN rate on the order of ~3x10^-7/yr to ~2x10^-6/yr, three orders of\nmagnitude below the inferred Galactic SN Ia rate and two orders of magnitude\nbelow the formation rate of predicted He-donor progenitors. The number of\ncurrently observed population members suggests that the He-donor scenariois not\na dominant contributor to the number of observed SNe Ia. However, we find that,\neven at the low event rate suggested, the majority of possibly detectable\npopulation members is still undetected. The extreme nature of current\npopulation members suggests that a still larger number of objects has simply\nevaded detection up to this point, hinting at a higher contribution than is\ncurrently supported by observation. - abridged -",
        "category": "astro-ph_SR"
    },
    {
        "text": "The age-metallicity relation in the solar neighbourhood from a pilot\n  sample of white dwarf-main sequence binaries: The age-metallicity relation (AMR) is a fundamental observational constraint\nfor understanding how the Galactic disc formed and evolved chemically in time.\nHowever, there is not yet an agreement on the observational properties of the\nAMR for the solar neighbourhood, primarily due to the difficulty in obtaining\naccurate stellar ages for individual field stars. We have started an\nobservational campaign for providing the much needed observational input by\nusing wide white dwarf-main sequence (WDMS) binaries. White dwarfs are natural\nclocks and can be used to derive accurate ages. Metallicities can be obtained\nfrom the main sequence companions. Since the progenitors of white dwarfs and\nthe main sequence stars were born at the same time, WDMS binaries provide a\nunique opportunity to observationally constrain in a robust way the properties\nof the AMR. In this work we present the AMR derived from analysing a pilot\nsample of 23 WDMS binaries and provide clear observational evidence for the\nlack of correlation between age and metallicity at young and intermediate ages\n(0-7 Gyrs).",
        "category": "astro-ph_SR"
    },
    {
        "text": "Investigation of the Progenitors of Nova Explosions: Recurrent novae (RNe) play an important role as one of the suspected\nprogenitor systems of Type Ia supernovae (SNe) which are used as primary\ndistance indicators in cosmology. Thus, it is important to investigate the\nnature of their central binary systems to determine the relation between the\nparameters of the central system and outburst type, and finally ascertain the\npopulation of novae that might be available to give rise to the progenitors of\nType Ia SNe. The details of the preliminary investigation looking for\ncharacteristics that may distinguish recurrent and classical novae (CNe)\nsystems, the selection of initial targets for detailed study, and preliminary\nresults are presented. We identified 10 suspected RNe among the Galactic CNe\nfor investigation with the Liverpool Telescope population and our initial\nphotometric observations of their quiescent systems suggest 2 may belong to the\nRS Oph type and 2 to the U Sco type RNe. Spectroscopic follow-up is now\nunderway.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Release note: Massive peak bagging of red giants in the Kepler field: The NASA satellite Kepler has gathered about 1420 days-long photometric time\nseries for more than 20000 red giant stars. For about 6600 of them also APOGEE\nspectroscopic parameters are available, making the sample of high interest for\nvarious astrophysical investigations. To optimally exploit the full wealth of\nthe seismic information, extraction of mode parameters of all significant\nindividual frequencies is necessary. However, the complex structure of the\nmixed mode pattern makes it challenging to automate the peak bagging (i.e., the\nextraction of the individual mode parameters from the stars power density\nspectra). Even though several approaches have been successfully implemented,\nthe available results are still limited to a handful of stars. Here I present\nfrequencies, amplitudes, and lifetimes of more than a quarter of a million\noscillation modes of the spherical degree l=0 to 3, which have been observed in\n6179 Kepler red giants. The sample covers evolutionary stages from the lower\nred-giant branch to high up the asymptotic giant branch. The modes were\nextracted with the Automated Bayesian Peak-Bagging Algorithm (ABBA) and are\npublicly available at https://github.com/tkallinger/KeplerRGpeakbagging",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the relationship between G-band bright point dynamics and their\n  magnetic field strengths: G-band bright points (GBPs) are regarded as good manifestations of magnetic\nflux concentrations. We aim to investigate the relationship between the dynamic\nproperties of GBPs and their longitudinal magnetic field strengths. High\nspatial and temporal resolution observations were recorded simultaneously with\nG-band filtergrams and Narrow-band Filter Imager (NFI) Stokes I and V images\nwith Hinode /Solar Optical Telescope. The GBPs are identified and tracked in\nthe G-band images automatically, and the corresponding longitudinal magnetic\nfield strength of each GBP is extracted from the calibrated NFI magnetograms by\na point-to-point method. After categorizing the GBPs into five groups by their\nlongitudinal magnetic field strengths, we analyze the dynamics of GBPs of each\ngroup. The results suggest that with increasing longitudinal magnetic field\nstrengths of GBPs correspond to a decrease in their horizontal velocities and\nmotion ranges as well as by showing more complicated motion paths. This\nsuggests that magnetic elements showing weaker magnetic field strengths prefer\nto move faster and farther along straighter paths, while stronger ones move\nmore slowly in more erratic paths within a smaller region. The dynamic\nbehaviors of GBPs with different longitudinal magnetic field strengths can be\nexplained by that the stronger flux concentrations withstand the convective\nflows much better than weaker ones.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Solar-Stellar Connection: We discuss how recent advances in observations, theory and numerical\nsimulations have allowed the stellar community to progress in its understanding\nof stellar convection, rotation and magnetism and to assess the degree to which\nthe Sun and other stars share similar dynamical properties. Ensemble\nasteroseismology has become a reality with the advent of large time domain\nstudies, especially from space missions. This new capability has provided\nimproved constraints on stellar rotation and activity, over and above that\nobtained via traditional techniques such as spectropolarimetry or CaII H&K\nobservations. New data and surveys covering large mass and age ranges have\nprovided a wide parameter space to confront theories of stellar magnetism.\nThese new empirical databases are complemented by theoretical advances and\nimproved multi-D simulations of stellar dynamos. We trace these pathways\nthrough which a lucid and more detailed picture of magnetohydrodynamics of\nsolar-like stars is beginning to emerge and discuss future prospects.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Three ways to solve the orbit of KIC11558725: a 10 day beaming sdB+WD\n  binary with a pulsating subdwarf: The recently discovered subdwarf B (sdB) pulsator KIC11558725 features a rich\ng-mode frequency spectrum, with a few low-amplitude p-modes at short periods,\nand is a promising target for a seismic study aiming to constrain the internal\nstructure of this star, and of sdB stars in general.\n  We have obtained ground-based spectroscopic Balmer-line radial-velocity\nmeasurements of KIC11558725, spanning the 2010 and 2011 observing seasons. From\nthese data we have discovered that KIC11558725 is a binary with period P=10.05\nd, and that the radial-velocity amplitude of the sdB star is 58 km/s.\nConsequently the companion of the sdB star has a minimum mass of 0.63 M\\odot,\nand is therefore most likely an unseen white dwarf.\n  We analyse the near-continuous 2010-2011 Kepler light curve to reveal orbital\nDoppler-beaming light variations at the 238 ppm level, which is consistent with\nthe observed spectroscopic orbital radial-velocity amplitude of the subdwarf.\nWe use the strongest 70 pulsation frequencies in the Kepler light curve of the\nsubdwarf as clocks to derive a third consistent measurement of the orbital\nradial-velocity amplitude, from the orbital light-travel delay.\n  We use our high signal-to-noise average spectra to study the atmospheric\nparameters of the sdB star, deriving Teff = 27 910K and log g = 5.41 dex, and\nfind that carbon, nitrogen and oxygen are underabundant relative to the solar\nmixture.\n  Furthermore, we extract more than 160 significant frequencies from the Kepler\nlight curve. We investigate the pulsation frequencies for expected period\nspacings and rotational splittings. We find period-spacing sequences of\nspherical-harmonic degrees \\ell=1 and \\ell=2, and we associate a large fraction\nof the g-modes in KIC11558725 with these sequences. From frequency splittings\nwe conclude that the subdwarf is rotating subsynchronously with respect to the\norbit.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The GAPS programme at TNG LII. Spot modeling of V1298 Tau using SpotCCF\n  tool: The intrinsic variability due to the magnetic activity of young active stars\nis one of the main challenges in detecting and characterising exoplanets. We\npresent a method able to model the stellar photosphere and its surface\ninhomogeneities (starspots) in young/active and fast-rotating stars, based on\nthe cross-correlation function (CCF) technique, to extract information about\nthe spot configuration of the star. Within the Global Architecture of Planetary\nSystems (GAPS) Project at the Telescopio Nazionale Galileo, we analysed more\nthan 300 spectra of the young planet-hosting star V1298 Tau provided by HARPS-N\nhigh-resolution spectrograph. By applying the SpotCCF model to the CCFs we\nextracted the spot configuration (latitude, longitude and projected filling\nfactor) of this star, and also provided the new RVs time series of this target.\nWe find that the features identified in the CCF profiles of V1298 Tau are\nmodulated by the stellar rotation, supporting our assumption that they are\ncaused by starspots. The analysis suggests a differential rotation velocity of\nthe star with lower rotation at higher latitudes. Also, we find that SpotCCF\nprovides an improvement in RVs extraction with a significantly lower dispersion\nwith respect to the commonly used pipelines, with consequent mitigation of the\nstellar activity contribution modulated with stellar rotation. A detection\nsensitivity test, by the direct injection of a planetary signal into the data,\nconfirmed that the SpotCCF model improves the sensitivity and ability to\nrecover planetary signals. Our method enables the modelling of the stellar\nphotosphere, extracting the spot configuration of young/active and rapidly\nrotating stars. It also allows for the extraction of optimised RV time series,\nthereby enhancing our detection capabilities for new exoplanets and advancing\nour understanding of stellar activity.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Detection of transit timing variations in excess of one hour in the\n  Kepler multi-planet candidate system KOI 806 with the GTC: We report the detection of transit timing variations (TTVs) well in excess of\none hour in the Kepler multi-planet candidate system KOI 806. This system\nexhibits transits consistent with three separate planets -- a Super-Earth, a\nJupiter, and a Saturn -- lying very nearly in a 1:2:5 resonance, respectively.\nWe used the Kepler public data archive and observations with the Gran\nTelescopio de Canarias to compile the necessary photometry. For the largest\ncandidate planet (KOI 806.02) in this system, we detected a large transit\ntiming variation of -103.5$\\pm$6.9 minutes against previously published\nephemeris. We did not obtain a strong detection of a transit color signature\nconsistent with a planet-sized object; however, we did not detect a color\ndifference in transit depth, either. The large TTV is consistent with\ntheoretical predictions that exoplanets in resonance can produce large transit\ntiming variations, particularly if the orbits are eccentric. The presence of\nlarge TTVs among the bodies in this systems indicates that KOI806 is very\nlikely to be a planetary system. This is supported by the lack of a strong\ncolor dependence in the transit depth, which would suggest a blended eclipsing\nbinary.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Evolution of Circumstellar Disks Surrounding Intermediate Mass\n  Stars: IC 1805: We report the results of a study of the intermediate and high mass stars in\nthe young, rich star-forming complex IC 1805, based on a combination of\noptical, near-infrared, and mid-infrared photometry, and classification\nspectra. These data provide the basis for characterizing the masses and ages\nfor stars more massive than ~2 Msun and enable a study of the frequency and\ncharacter of circumstellar disks associated with intermediate- and high-mass\nstars. Optically thick accretion disks among stars with masses 2 < M/Msun <4\nare rare (~2% of members) and absent among more massive stars. A larger\nfraction (~10%) of stars with masses 2 < M/Msun < 4 appear to be surrounded by\ndisks that have evolved from the initial optically thick accretion phase. We\nidentify four classes of such disks. These classes are based on spectral energy\ndistributions (SEDs) of excess emsission above photospheric levels: disks that\nare (1) optically thin based on the magnitude of the observed excess emission\nfrom 2 to 24 um; (2) optically thin in their inner regions (r< 20 AU) and\noptically thick in their outer regions; (3) exhibit empty inner regions (r < 10\nAU) and optically thin emission in their outer regions; and (4) exhibit empty\ninner regions and optically thick outer regions. We discuss, and assess the\nmerits and liabilities of, proposed explanations for disks exhibiting these SED\ntypes and suggest additional observations that would test these proposals.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Binary interactions and UV photometry on photometric redshift: Using the Hyperz code (Bolzonella et al. 2000) we present photometric\nredshift estimates for a random sample of galaxies selected from the SDSS/DR7\nand GALEX/DR4, for which spectroscopic redshifts are also available.\n  We confirm that the inclusion of ultraviolet photometry improves the accuracy\nof photo-zs for those galaxies with g*-r* < 0.7 and z_spec < 0.2. We also\naddress the problem of how binary interactions can affect photo-z estimates,\nand find that their effect is negligible.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the vertical-shear instability in astrophysical discs: We explore the linear stability of astrophysical discs exhibiting vertical\nshear, which arises when there is a radial variation in the temperature or\nentropy. Such discs are subject to a \"vertical-shear instability\", which recent\nnonlinear simulations have shown to drive hydrodynamic activity in the\nMRI-stable regions of protoplanetary discs. We first revisit locally isothermal\ndiscs using the quasi-global reduced model derived by Nelson et al. (2013).\nThis analysis is then extended to global axisymmetric perturbations in a\ncylindrical domain. We also derive and study a reduced model describing discs\nwith power law radial entropy profiles (\"locally polytropic discs\"), which are\nsomewhat more realistic in that they possess physical (as opposed to numerical)\nsurfaces. In all cases the fastest growing modes have very short wavelengths\nand are localised at the disc surfaces (if present), where the vertical shear\nis maximal. An additional class of modestly growing vertically global body\nmodes is excited, corresponding to destabilised classical inertial waves\n(\"r-modes\"). We discuss the properties of both types of modes, and stress that\nthose that grow fastest occur on the shortest available length scales\n(determined either by the numerical grid or the physical viscous length). This\nill-posedness makes simulations of the instability difficult to interpret. We\nend with some brief speculation on the nonlinear saturation and resulting\nangular momentum transport.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Radio light curves and imaging of the helium nova V445 Puppis reveal\n  seven years of synchrotron emission: V445 Puppis is the only helium nova observed to date; its eruption in late\n2000 showed high velocities up to 8500 km/s and a remarkable bipolar morphology\ncinched by an equatorial dust disc. Here we present multi-frequency radio\nobservations of V445 Pup obtained with the Very Large Array (VLA) spanning 1.5\nto 43.3 GHz, and between 2001 January and 2008 March (days 89 to 2700 after\neruption). The radio light curve is dominated by synchrotron emission over\nthese seven years, and shows four distinct radio flares. Resolved radio images\nobtained in the VLA A configuration show that the synchrotron emission hugs the\nequatorial disc, and comparisons to near-IR images of the nova clearly\ndemonstrate that it is the densest ejecta, not the fastest ejecta, that are the\nsites of the synchrotron emission in V445 Pup. The data are consistent with a\nmodel where the synchrotron emission is produced by a wind from the white dwarf\nimpacting the dense equatorial disc, resulting in shocks and particle\nacceleration. The individual synchrotron flares may be associated with density\nenhancements in the equatorial disc and/ or velocity variations in the wind\nfrom the white dwarf. This overall scenario is similar to a common picture of\nshock production in hydrogen-rich classical novae, but V445 Pup is remarkable\nin that these shocks persist for almost a decade, much longer than the weeks or\nmonths for which shocks are typically observed in classical novae.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The shear-Hall instability in newborn neutron stars: Aims. In the first few minutes of a newborn neutron star's life the Hall\neffect and differential rotation may both be important. We demonstrate that\nthese two ingredients are sufficient for generating a 'shear-Hall instability'\nand for studying its excitation conditions, growth rates, and characteristic\nmagnetic field patterns.\n  Methods. We numerically solve the induction equation in a spherical shell,\nwith a kinematically prescribed differential rotation profile {\\Omega}(s),\nwhere s is the cylindrical radius. The Hall term is linearized about an imposed\nuniform axial field. The linear stability of individual azimuthal modes, both\naxisymmetric and non-axisymmetric, is then investigated.\n  Results. For the shear-Hall instability to occur, the axial field must be\nparallel to the rotation axis if {\\Omega}(s) decreases outward, whereas if\n{\\Omega}(s) increases outward it must be anti-parallel. The instability draws\nits energy from the differential rotation, and occurs on the short rotational\ntimescale rather than on the much longer Hall timescale. It operates most\nefficiently if the Hall time is comparable to the diffusion time. Depending on\nthe precise field strengths B0, either axisymmetric or non-axisymmetric modes\nmay be the most unstable.\n  Conclusions. Even if the differential rotation in newborn neutron stars is\nquenched within minutes, the shear-Hall instability may nevertheless amplify\nany seed magnetic fields by many orders of magnitude.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Detection of Strong Millimeter Emission from the Circumstellar Dust Disk\n  Around V1094 Sco: Cold and Massive Disk around a T Tauri Star in a Quiescent\n  Accretion Phase?: We present the discovery of a cold massive dust disk around the T Tauri star\nV1094 Sco in the Lupus molecular cloud from the 1.1 millimeter continuum\nobservations with AzTEC on ASTE. A compact ($r\\lesssim$320 AU) continuum\nemission coincides with the stellar position having a flux density of 272 mJy\nwhich is largest among T Tauri stars in Lupus. We also present the detection of\nmolecular gas associated with the star in the five-point observations in\n$^{12}$CO J=3--2 and $^{13}$CO J=3--2. Since our $^{12}$CO and $^{13}$CO\nobservations did not show any signature of a large-scale outflow or a massive\nenvelope, the compact dust emission is likely to come from a disk around the\nstar. The observed SED of V1094 Sco shows no distinct turnover from near\ninfrared to millimeter wavelengths, which can be well described by a flattened\ndisk for the dust component, and no clear dip feature around 10 $\\micron$\nsuggestive of absence of an inner hole in the disk. We fit a simple power-law\ndisk model to the observed SED. The estimated disk mass ranges from 0.03 to\n$\\gtrsim$0.12 $M_\\sun$, which is one or two orders of magnitude larger than the\nmedian disk mass of T Tauri stars in Taurus.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Mapping the youngest and most massive stars in the Tarantula nebula with\n  MUSE-NFM: The evolution of the most massive stars is a puzzle with many missing pieces.\nStatistical analyses are the key to provide anchors to calibrate theory,\nhowever performing these studies is an arduous job. The state-of-the-art\nintegral field spectrograph MUSE has stirred up stellar astrophysicists who are\nexcited about the capability to take spectra of up to a thousand stars in a\nsingle exposure. The excitement was even higher with the commissioning of the\nMUSE narrow-field-mode (NFM) that has demonstrated angular resolutions akin to\nthe Hubble Space Telescope. We present the first mapping of the dense stellar\ncore R136 in the Tarantula nebula based on a MUSE-NFM mosaic. We aim to deliver\nthe first homogeneous analysis of the most massive stars in the local Universe\nand to explore the impact of these peculiar objects to the interstellar medium.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Short-term variability and mass loss in Be stars V. Space photometry and\n  ground-based spectroscopy of $\u03b3$ Cas: Context. Be stars are physically complex systems that continue to challenge\ntheory to understand their rapid rotation, complex variability and decretion\ndisks. $\\gamma$ Cassiopeiae ($\\gamma$ Cas) is one such star but is even more\ncurious because of its unexplained hard thermal X-ray emission. Aims. We aim to\nexamine the optical variability of $\\gamma$ Cas and thereby to shed more light\non its puzzling behaviour. Methods. Three hundred twenty-one archival H$\\alpha$\nspectra from 2006 to 2017 are analysed to search for frequencies corresponding\nto the 203.5 day orbit of the companion. Space photometry from the SMEI\nsatellite from 2003 to 2011 and the BRITE-Constellation of nano-satellites\nbetween 2015 and 2019 is investigated in the period range from a couple of\nhours to a few days. Results. The orbital period of the companion of 203.5 days\nis confirmed with independent measurements from the structure of the H$\\alpha$\nline emission. A strong blue/red asymmetry in the amplitude distribution across\nthe H$\\alpha$ emission line could hint at a spiral structure in the decretion\ndisk. With the space photometry, the known frequency of 0.82 d$^{-1}$ is\nconfirmed in data from the early 2000s. A higher frequency of 2.48 d$^{-1}$ is\npresent in the data from 2015 to 2019 and possibly also in the early 2000s. A\nthird frequency at 1.25 d$^{-1}$ is proposed to exist in both SMEI and BRITE\ndata. The only explanation covering all three rapid variations seems to be\nnonradial pulsation. The two higher frequencies are incompatible with rotation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Theory of fossil magnetic field: Theory of fossil magnetic field is based on the observations, analytical\nestimations and numerical simulations of magnetic flux evolution during star\nformation in the magnetized cores of molecular clouds. Basic goals, main\nfeatures of the theory and manifestations of MHD effects in young stellar\nobjects are discussed.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Silicate Extinction Profile Based on the Stellar Spectrum by Spitzer/IRS: The 9.7$\\mu m$ and 18$\\mu m$ interstellar spectral features, arising from the\nSi--O stretching and O--Si--O bending mode of amorphous silicate dust, are the\nstrongest extinction feature in the infrared. Here we use the \"pair method\" to\ndetermine the silicate extinction profile by comparing the \\emph{Spitzer}/IRS\nspectra of 49 target stars with obvious extinction with that of un-reddened\nstar of the same spectral type. The 9.7$\\mu m$ extinction profile is determined\nfrom all the 49 stars and the 18$\\mu m$ profile is determined from six stars.\nIt is found that the profile has the peak wavelength around $\\sim$9.2- 9.8$\\mu\nm$ and $\\sim$18-22$\\mu m$ respectively. The peak wavelength of the 9.7$\\mu m$\nfeature seems to become shorter from the stars of late spectral type, meanwhile\nthe FWHM seems irrelevant to the spectral type, which may be related to\ncircumstellar silicate emission. The silicate optical depth at 9.7$\\mu m$,\n$\\Delta\\tau_{9.7}$, mostly increases with the color excess in $J-K_S$ ($E_{\\rm\nJK_S}$). The mean ratio of the visual extinction to the 9.7$\\mu m$ silicate\nabsorption optical depth is $A_{\\rm V}/\\Delta\\tau_{9.7}\\approx 17.8$, in close\nagreement with that of the solar neighborhood diffuse ISM. When $E_{\\rm JK_S}$\n> 4, this proportionality changes. The correlation coefficient between the peak\nwavelength and FWHM of the 9.7$\\mu m$ feature is 0.4, which indicates a\npositive correlation considering the uncertainties of the parameters. The\nmethod is compared with replacing the reference star by an atmospheric model\nSED and no significant difference is present.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Helium-Star Models with Optically Thick Winds: Implications for the\n  Internal Structures and Mass-Loss Rates of Wolf-Rayet Stars: We construct helium (He) star models with optically thick winds and compare\nthem with the properties of Galactic Wolf-Rayet (WR) stars. Hydrostatic He-core\nsolutions are connected smoothly to trans-sonic wind solutions that satisfy the\nregularity conditions at the sonic point. Velocity structures in the supersonic\nparts are assumed by a simple beta-type law. By constructing a\ncenter-to-surface structure, a mass-loss rate can be obtained as an eigenvalue\nof the equations. Sonic points appear at temperatures ~ 1.8e5 - 2.8e5 K below\nthe Fe-group opacity peak, where the radiation force becomes comparable to the\nlocal gravity. Photospheres are located at radii 3-10 times larger than sonic\npoints. The obtained mass-loss rates are comparable to those of WR stars. Our\nmass-loss rate - luminosity relation agrees well with the relation recently\nobtained by Graefener et al. (2017). Photospheric temperatures of WR stars tend\nto be cooler than our predictions. We discuss the effects of stellar evolution,\ndetailed radiation transfer, and wind clumping, which are ignored in this\npaper.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The IACOB project VIII. Searching for empirical signatures of binarity\n  in fast-rotating O-type stars: The empirical distribution of projected rotational velocities (vsini) in\nmassive O-type stars is characterized by a dominant slow velocity component and\na tail of fast rotators. Binary interaction has been proposed to play a\ndominant role in the formation of this tail. We perform a complete and\nhomogeneous search for empirical signatures of binarity in a sample of 54\nfast-rotating stars with the aim of evaluating this hypothesis. This working\nsample has been extracted from a larger sample of 415 Galactic O-type stars\nwhich covers the full range of vsini values. We use new and archival\nmulti-epoch spectra in order to detect spectroscopic binary systems. We\ncomplement this information with Gaia proper motions and TESS photometric data\nto aid in the identification of runaway stars and eclipsing binaries,\nrespectively. The identified fraction of single-lined spectroscopic binary\n(SB1) systems and apparently single stars among the fast-rotating sample is\n$\\sim$18% and $\\sim$70%, respectively. When comparing these percentages with\nthose corresponding to the slow-rotating sample we find that our sample of fast\nrotators is characterized by a slightly larger percentage of SB1 systems\n($\\sim$18% vs. $\\sim$13%) and a considerably smaller fraction of clearly\ndetected SB2 systems (8% vs. 33%). Overall, there seems to be a clear deficit\nof spectroscopic binaries (SB1+SB2) among fast-rotating O-type stars ($\\sim$26%\nvs. $\\sim$46%). On the contrary, the fraction of runaway stars is significantly\nhigher in the fast-rotating domain ($\\sim$33-50%) than among those stars with\nvsini < 200 km/s. Lastly, almost 65% of the apparently single fast-rotating\nstars are runaways. Our empirical results seem to be in good agreement with the\nidea that the tail of fast-rotating O-type stars (with vsini > 200 km/s) is\nmostly populated by post-interaction binary products.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Color and Brightness of the F-Corona Inferred from the 2019 July 2\n  Total Solar Eclipse: Total solar eclipses (TSEs) provide a unique opportunity to quantify the\nproperties of the K-corona (electrons), F-corona (dust) and E-corona (ions)\ncontinuously from the solar surface out to a few solar radii. We apply a novel\ninversion method to separate emission from the K- and F-corona continua using\nunpolarized total brightness (tB) observations from five 0.5 nm bandpasses\nacquired during the 2019 July 2 TSE between 529.5 nm and 788.4 nm. The\nwavelength dependence relative to the photosphere (i.e., color) of the F-corona\nitself is used to infer the tB of the K- and F-corona for each line-of-sight.\nWe compare our K-corona emission results with the Mauna Loa Solar Observatory\n(MLSO) K-Cor polarized brightness (pB) observations from the day of the\neclipse, and the forward modeled K-corona intensity from the Predictive Science\nInc. (PSI) Magnetohydrodynamic (MHD) model prediction. Our results are\ngenerally consistent with previous work and match both the MLSO data and\nPSI-MHD predictions quite well, supporting the validity of our approach and of\nthe PSI-MHD model. However, we find that the tB of the F-corona is higher than\nexpected in the low corona, perhaps indicating that the F-corona is slightly\npolarized -- challenging the common assumption that the F-corona is entirely\nunpolarized.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Imaging from STIX visibility amplitudes: Aims: To provide the first demonstration of STIX Fourier-transform X-ray\nimaging using semi-calibrated (amplitude-only) visibility data acquired during\nthe Solar Orbiter's cruise phase. Methods: We use a parametric imaging approach\nby which STIX visibility amplitudes are fitted by means of two non-linear\noptimization methods: a fast meta-heuristic technique inspired by social\nbehavior, and a Bayesian Monte Carlo sampling method, which, although slower,\nprovides better quantification of uncertainties. Results: When applied to a set\nof solar flare visibility amplitudes recorded by STIX on November 18, 2020 the\ntwo parametric methods provide very coherent results. The analysis also\ndemonstrates the ability of STIX to reconstruct high time resolution\ninformation and, from a spectral viewpoint, shows the reliability of a\ndouble-source scenario consistent with a thermal versus nonthermal\ninterpretation. Conclusions: In this preliminary analysis of STIX imaging based\nonly on visibility amplitudes, we formulate the imaging problem as a non-linear\nparametric issue we addressed by means of two high-performance optimization\ntechniques that both showed the ability to sample the parametric space in an\neffective fashion, thus avoiding local minima.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Recycled Pulsars: Spins, Masses and Ages: Recycled pulsars are mainly characterized by their spin periods, B-fields and\nmasses. All these quantities are affected by previous interactions with a\ncompanion star in a binary system. Therefore, we can use these quantities as\nfossil records and learn about binary evolution. Here, I briefly review the\ndistribution of these observed quantities and summarize our current\nunderstanding of the pulsar recycling process.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Impacts of fragmented accretion streams onto Classical T Tauri Stars: UV\n  and X-ray emission lines: Context. The accretion process in Classical T Tauri Stars (CTTSs) can be\nstudied through the analysis of some UV and X-ray emission lines which trace\nhot gas flows and act as diagnostics of the post-shock downfalling plasma. In\nthe UV band, where higher spectral resolution is available, these lines are\ncharacterized by rather complex profiles whose origin is still not clear.\n  Aims. We investigate the origin of UV and X-ray emission at impact regions of\ndensity structured (fragmented) accretion streams.We study if and how the\nstream fragmentation and the resulting structure of the post-shock region\ndetermine the observed profiles of UV and X-ray emission lines.\n  Methods. We model the impact of an accretion stream consisting of a series of\ndense blobs onto the chromosphere of a CTTS through 2D MHD simulations. We\nexplore different levels of stream fragmentation and accretion rates. From the\nmodel results, we synthesize C IV (1550 {\\AA}) and OVIII (18.97 {\\AA}) line\nprofiles.\n  Results. The impacts of accreting blobs onto the stellar chromosphere produce\nreverse shocks propagating through the blobs and shocked upflows. These\nupflows, in turn, hit and shock the subsequent downfalling fragments. As a\nresult, several plasma components differing for the downfalling velocity,\ndensity, and temperature are present altoghether. The profiles of C IV doublet\nare characterized by two main components: one narrow and redshifted to speed\n$\\approx$ 50 km s$^{-1}$ and the other broader and consisting of subcomponents\nwith redshift to speed in the range 200 $\\approx$ 400 km s$^{-1}$. The profiles\nof OVIII lines appear more symmetric than C IV and are redshifted to speed\n$\\approx$ 150 km s$^{-1}$.\n  Conclusions. Our model predicts profiles of C IV line remarkably similar to\nthose observed and explains their origin in a natural way as due to stream\nfragmentation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Coronal Magnetic Field Topology From Total Solar Eclipse Observations: Measuring the global magnetic field of the solar corona remains exceptionally\nchallenging. The fine-scale density structures observed in white light images\ntaken during Total Solar Eclipses (TSEs) are currently the best proxy for\ninferring the magnetic field direction in the corona from the solar limb out to\nseveral solar radii (Rs). We present, for the first time, the topology of the\ncoronal magnetic field continuously between 1 and 6 Rs, as quantitatively\ninferred with the Rolling Hough Transform (RHT) for 14 unique eclipse coronae\nthat span almost two complete solar cycles. We find that the direction of the\ncoronal magnetic field does not become radial until at least 3 Rs, with a high\nvariance between 1.5 and 3 Rs at different latitudes and phases of the solar\ncycle. We find that the most non-radial coronal field topologies occur above\nregions with weaker magnetic field strengths in the photosphere, while stronger\nphotospheric fields are associated with highly radial field lines in the\ncorona. In addition, we find an abundance of field lines which extend\ncontinuously from the solar surface out to several solar radii at all\nlatitudes, regardless of the presence of coronal holes. These results have\nimplications for testing and constraining coronal magnetic field models, and\nfor linking in situ solar wind measurements to their sources at the Sun.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A method to identify and characterise binary candidates - a study of\n  CoRoT data: The analysis of the CoRoT space mission data was performed aiming to test a\nmethod that selects, among the several light curves observed, the transiting\nsystems that likely host a low-mass star orbiting the main target. The method\nidentifies stellar companions by fitting a model to the observed transits.\nApplying this model, that uses equations like Kepler's third law and an\nempirical mass-radius relation, it is possible to estimate the mass and radius\nof the primary and secondary objects as well as the semimajor axis and\ninclination angle of the orbit. We focus on how the method can be used in the\ncharacterisation of transiting systems having a low-mass stellar companion with\nno need to be monitored with radial-velocity measurements or ground-based\nphotometric observations. The model, which provides a good estimate of the\nsystem parameters, is also useful as a complementary approach to select\npossible planetary candidates. A list of confirmed binaries together with our\nestimate of their parameters are presented. The characterisation of the first\ntwelve detected CoRoT exoplanetary systems was also performed and agrees very\nwell with the results of their respective announcement papers. The comparison\nwith confirmed systems validates our method, specially when the radius of the\nsecondary companion is smaller than 1.5 Rjup, in the case of planets, or larger\nthan 2 Rjup, in the case of low-mass stars. Intermediate situations are not\nconclusive.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The PASTEL catalogue: 2016 version: The bibliographical compilation of stellar atmospheric parameters (Teff,\nlogg, [Fe/H]) relying on high-resolution, high signal-to-noise spectroscopy\nstarted in the eighties with the so-called [Fe/H] catalogue, and was continued\nin 2010 with the PASTEL catalogue, which also includes determinations of Teff\nalone, based on various methods. Here we present an update of the PASTEL\ncatalogue. The main journals and the CDS database have been surveyed to find\nrelevant publications presenting new determinations of atmospheric parameters.\nAs of February 2016, PASTEL includes 64082 determinations of either Teff or\n(Teff, logg, [Fe/H]) for 31401 stars, corresponding to 1142 bibliographical\nreferences. Some 11197 stars have a determination of the three parameters\n(Teff, logg, [Fe/H]) with a high-quality spectroscopic metallicity.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Budget equations and astrophysical nonlinear mean-field dynamos: Solar, stellar and galactic large-scale magnetic fields are originated due to\na combined action of non-uniform (differential) rotation and helical motions of\nplasma via mean-field dynamos. Usually, nonlinear mean-field dynamo theories\ntake into account algebraic and dynamic quenching of alpha effect and algebraic\nquenching of turbulent magnetic diffusivity. However, the theories of the\nalgebraic quenching do not take into account the effect of modification of the\nsource of turbulence by the growing large-scale magnetic field. This phenomenon\nis due to the dissipation of the strong large-scale magnetic field resulting in\nan increase of the total turbulent energy. This effect has been studied using\nthe budget equation for the total turbulent energy (which takes into account\nthe feedback of the generated large-scale magnetic field on the background\nturbulence) for (i) a forced turbulence, (ii) a shear-produced turbulence and\n(iii) a convective turbulence. As the result of this effect, a nonlinear dynamo\nnumber decreases with increase of the large-scale magnetic field, so that that\nthe mean-field $\\alpha\\Omega$, $\\alpha^2$ and $\\alpha^2\\Omega$ dynamo\ninstabilities are always saturated by the strong large-scale magnetic field.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Surface distribution of small-scale magnetic field on the active cool\n  star LQ Hydrae: It is well known that small-scale magnetism dominates the surface magnetic\nfield topologies of active late-type stars. However, little information is\navailable on the spatial distribution of this key magnetic field component.\nHere, we take advantage of the recently developed magnetic field diagnostic\nprocedure relying on the magnetic intensification of iron atomic lines in the\noptical. We extend this methodology from measuring a single average field\nstrength value to simultaneous Doppler imaging reconstruction of the\ntwo-dimensional maps of temperature and magnetic field strength. We applied\nthis novel surface mapping approach to two spectroscopic data sets of the young\nactive Sun-like star LQ Hya. For both epochs, we found a fairly uniform field\nstrength distribution, apart from a latitudinal trend of the field strength\nincreasing from 1.5-2.0 kG at low latitudes to 3.0-3.5 kG, close to the\nrotational poles. This distribution of the small-scale field does not display a\nclear correlation with the locations of temperature spots or the global\nmagnetic field structure reconstructed for the same epochs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Mid-IR observations of circumstellar disks -- Part III: A mixed sample\n  of PMS stars and Vega-type objects: We present new mid-infrared spectra for a sample of 15 targets (1 FU Orionis\nobject, 4 Herbig Ae stars, 5 T Tauri stars and 5 Vega type stars), obtained\nwith the TIMMI2 camera at La Silla Observatory (ESO). Three targets are members\nof the beta Pic moving group (HD 155555, HD 181296 and HD 319139). PAH bands\nare observed towards the T Tauri star HD 34700 and the Herbig Ae star PDS 144\nN. For HD 34700, the band profiles indicate processed PAHs. The spectrum of the\nVega-type object eta Corvi (HD 109085), for which a resolved disk at sub-mm\nwavelengths is known, is entirely stellar between 8--13 micron. Similarly, no\nindication for circumstellar matter at mid-infrared wavelengths is found\ntowards the Vega-like stars HD 3003, HD 80951, HD 181296 and, surprisingly, the\nT Tauri system HD 155555.\n  The silicate emission features of the remaining eight sources are modelled\nwith a mixture of silicates of different grain sizes and composition.\nUnprocessed dust dominates FU Ori, HD 143006 and CD-43 344. Large amorphous\ngrains are the main dust component around HD 190073, HD 319139, KK Oph and PDS\n144 S. Both small grains and crystalline dust is found for the Vega-type HD\n123356, with a dominance of small amorphous grains. We show that the infrared\nemission of the binary HD 123356 is dominated by its late-type secondary, but\noptical spectroscopy is still required to confirm the age of the system and the\nspectral class of the companion. For most targets this is their first\nmid-infrared spectroscopic observation. We investigate trends between stellar,\ndisk and silicate properties and confirm correlations of previous studies.\nSeveral objects present an exciting potential for follow-up high-resolution\ndisk studies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Metallicity estimation of MW, SMC and LMC classical Cepheids from the\n  shape of the $V$- and $I$-band light curves: Estimating metallicity of classical Cepheids is of prime importance for\nstudying metallicity effect on stellar evolution, chemical evolution of\ngalaxies, and ultimately its impact on period-luminosity relation used in the\nextragalactic distance scale. We aim at establishing new empirical relations\nfor estimating the iron content of classical Cepheids for short and\nlong-periods based on Fourier parameters from the $V$-band light curves. We\ncalibrate new interrelations of Fourier parameters to convert $V$-band\nempirical relations into the $I$-band. Then we apply these relation in $V$ and\n$I$-bands to Cepheids from Milky Way (MW), Small and Large Magellanic Clouds\n(SMC and LMC) available in the literature. Last, we map the metallicity\ndistribution in these galaxies for investigating potential application in\ngalactic archeology. These empirical relations in $V$ and $I$ bands are able to\nderive the mean metallicity of a sample of MW, SMC and LMC Cepheids in\nagreement with literature values within 1$\\sigma$. We also show that these\nrelations are precise enough to reconstruct the radial metallicity gradients\nwithin the MW from OGLE data. The empirical relations in the $V$ and $I$ bands\ncalibrated in this paper for short and long-period Cepheids provide a new\nuseful tool to estimate the metallicity of Cepheids which are not accessible by\nspectroscopy. The calibration can be improved with further high-resolution\nspectroscopic observations of metal-poor Cepheids and homogeneous photometries\nin $V$ and $I$ bands.",
        "category": "astro-ph_SR"
    },
    {
        "text": "On the Correlation between L Dwarf Optical and Infrared Variability and\n  Radio Aurorae: Photometric variability attributed to cloud phenomena is common in L/T\ntransition brown dwarfs. Recent studies show that such variability may also\ntrace aurorae, suggesting that localized magnetic heating may contribute to\nobserved brown dwarf photometric variability. We assess this potential\ncorrelation with a survey of 17 photometrically variable brown dwarfs using the\nKarl G. Jansky Very Large Array (VLA) at 4 -- 8 GHz. We detect quiescent and\nhighly circularly polarized flaring emission from one source, 2MASS\nJ17502484-0016151, which we attribute to auroral electron cyclotron maser\nemission. The detected auroral emission extends throughout the frequency band\nat $\\sim$5 -- 25$\\sigma$, and we do not detect evidence of a cutoff. Our\ndetection confirms that 2MASS J17502484-0016151 hosts a magnetic field strength\nof $\\geq$2.9 kG, similar to those of other radio-bright ultracool dwarfs. We\nshow that H$\\alpha$ emission continues to be an accurate tracer of auroral\nactivity in brown dwarfs. Supplementing our study with data from the\nliterature, we calculate the occurrence rates of quiescent emission in L dwarfs\nwith low- and high-amplitude variability and conclude that high amplitude O/IR\nvariability does not trace radio magnetic activity in L dwarfs.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Solar-like stars observed by Kepler: an incredible adventure: The NASA Kepler mission -in flight since March 2009- is producing an enormous\nnumber of high-quality continuous light curves. Now, and for the first time\never, we are able to do ensemble asteroseismology, i.e., to do an asteroseismic\nanalysis with a statistically significant sub-sample of solar-like stars\ncovering a wide range of stellar characteristics. In the present work, I\nhighlight some of the most recent studies carried out using these data.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Interacting winds in massive binaries: Massive stars feature highly energetic stellar winds that interact whenever\ntwo such stars are bound in a binary system. The signatures of these\ninteractions are nowadays found over a wide range of wavelengths, including the\nradio domain, the optical band, as well as X-rays and even gamma-rays. A proper\nunderstanding of these effects is thus important to derive the fundamental\nparameters of the components of massive binaries from spectroscopic and\nphotometric observations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Observations of microglitches in HartRAO radio pulsars: A detailed observation of microglitch phenomenon in relatively slow radio\npulsars is presented. Our analyses for these small amplitude jumps in pulse\nrotation frequency ($\\nu$) and/or spin down rate ($\\dot{\\nu}$) combine the\ntraditional manual detection method (which hinges on careful visual inspections\nof the residuals of pulse phase residuals) and a new, and perhaps more\nobjective, automated search technique (which exploits the power of the\ncomputer, rather than the eyes, for resolving discrete events in pulsar spin\nparameters). The results of the analyses of a sample of 26 radio pulsars reveal\nthat: (i) only 20 pulsars exhibit significant fluctuations in their arrival\ntimes to be considered suitable for meaningful microglitch analyses; (ii) a\nphenomenal 299 microglitch events were identified in $\\nu$ and/or $\\dot{\\nu}$:\n266 of these events were found to be simultaneously significant in $\\nu$ and\n$\\dot{\\nu}$, while 19 and 14 were noticeable only in $\\nu$ and $\\dot{\\nu}$,\nrespectively; (iii) irrespective of sign, the microglitches have fractional\nsizes which cover about 3 orders of magnitude in $\\nu$ and $\\dot{\\nu}$\n($10^{-11} < |\\Delta{\\nu}/\\nu| < 2.0\\times10^{-8}$ and $5.0\\times10^{-5} <\n|\\Delta{\\dot{\\nu}}/\\dot{\\nu}| < 2.0\\times10^{-2}$) with median values as\n$0.78\\times10^{-9}$ and $0.36\\times10^{-3}$, respectively.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Water around IRAS15398-3359 observed with ALMA: How protostars accrete mass is one of the fundamental problems of star\nformation. High column densities and complex kinematical structures make direct\nobservations challenging and they only provide a snapshot. Chemical tracers\nprovide an interesting alternative to characterise the infall histories of\nprotostars. Previous observations of H13CO+ towards the low-mass protostar\nIRAS15398-3359 showed a depression in the abundance. This is a sign of\ndestruction of HCO+ by an enhanced presence of gaseous water in an extended\nregion, possibly related to a recent burst in the accretion. Direct\nobservations of water vapour can determine the exact extent of the emission and\nconfirm the hypothesis that HCO+ is indeed a good tracer of the water\nsnow-line. IRAS15398 was observed using ALMA at 0.5\" resolution. Maps of\nHDO(101-000) and H218O(414-321) were taken simultaneously with observations of\nthe CS(8-7) and N2H+(5-4) lines and continuum at 0.65 and 0.75 mm. The maps\nwere interpreted using dust radiative transfer calculations of the protostellar\ninfalling envelope with an outflow cavity. HDO is clearly detected and extended\nover the scales of the H13CO+ depression, although it is displaced by ~500 AU\nin the direction of the outflow. H218O is tentatively detected towards the\nred-shifted outflow lobe, but otherwise it is absent from the mapped region,\nwhich suggests that temperatures are low. Based on the temperature structure\nobtained from dust radiative transfer models, we conclude that the water was\nmost likely released from the grains in an extended hour-glass configuration\nduring a recent accretion burst. HDO is only detected in the region closest to\nthe protostar, at distances of up to 500 AU. These signatures can only be\nexplained if the luminosity has recently been increased by orders of\nmagnitudes. Additionally, the densities in the outflow cones must be\nsufficiently low.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A survey of nulling pulsars using the Giant Meterwave Radio Telescope: Several pulsars show sudden cessation of pulsed emission, which is known as\npulsar nulling. In this paper, the nulling behaviour of 15 pulsars is\npresented. The nulling fractions of these pulsars, along with the degree of\nreduction in the pulse energy during the null phase, are reported for these\npulsars. A quasi-periodic null-burst pattern is reported for PSR J1738-2330.\nThe distributions of lengths of the null and the burst phases as well as the\ntypical nulling time scales are estimated for eight strong pulsars. The nulling\npattern of four pulsars with similar nulling fraction are found to be different\nfrom each other, suggesting that the fraction of null pulses does not quantify\nthe nulling behaviour of a pulsar in full detail. Analysis of these\ndistributions also indicate that while the null and the burst pulses occur in\ngroups, the underlying distribution of the interval between a transition from\nthe null to the burst phase and vice verse appears to be similar to that of a\nstochastic Poisson point process.",
        "category": "astro-ph_SR"
    },
    {
        "text": "White-light superflare and long-term activity of the nearby M7 type\n  binary EI~Cnc observed with GWAC system: Stellar white-light flares are believed to play an essential role on the\nphysical and chemical properties of the atmosphere of the surrounding\nexoplanets. Here we report an optical monitoring campaign on the nearby flaring\nsystem EI~Cnc carried out by the Ground-based Wide Angle Cameras (GWAC) and its\ndedicated follow-up telescope. A superflare, coming from the brighter component\nEI~CncA, was detected and observed, in which four components are required to\nproperly model the complex decay light curve. The lower limit of flare energy\nin the $R-$band is estimated to be $3.3\\times10^{32}$ ergs. 27 flares are\nadditionally detected from the GWAC archive data with a total duration of 290\nhours. The inferred cumulative flare frequency distribution follows a quite\nshallow power-law function with a slope of $\\beta=-0.50\\pm 0.03$ over the\nenergy range between $10^{30}$ and $10^{33}$ erg, which reinforces the trend\nthat stars cooler than M4 show enhanced superflare activity. The flares\nidentified in EI~Cnc enable us to extend the $\\tau-E$ relationship previously\nestablished in the white-light superflares of solar-type stars down to an\nenergy as low as $\\sim10^{30}$erg (i.e., by three orders): $\\tau\\propto\nE^{0.42\\pm0.02}$, which suggests a common flare mechanism for stars with a type\nfrom M to solar-like, and implies an invariant of $B^{1/3}\\upsilon_{\\rm A}$ in\nthe white-light flares.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Analytical maximum likelihood estimation of stellar magnetic fields: The polarised spectrum of stellar radiation encodes valuable information on\nthe conditions of stellar atmospheres and the magnetic fields that permeate\nthem. In this paper, we give explicit expressions to estimate the magnetic\nfield vector and its associated error from the observed Stokes parameters. We\nstudy the solar case where specific intensities are observed and then the\nstellar case, where we receive the polarised flux. In this second case, we\nconcentrate on the explicit expression for the case of a slow rotator with a\ndipolar magnetic field geometry. Moreover, we also give explicit formulae to\nretrieve the magnetic field vector from the LSD profiles without assuming mean\nvalues for the LSD artificial spectral line. The formulae have been obtained\nassuming that the spectral lines can be described in the weak field regime and\nusing a maximum likelihood approach. The errors are recovered by means of the\nhermitian matrix. The bias of the estimators are analysed in depth.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Bayesian Characterization of Main Sequence Binaries in the Old Open\n  Cluster NGC 188: The binary fractions of open and globular clusters yield powerful constraints\non their dynamical state and evolutionary history. We apply publicly available\nBayesian analysis code to a $UBVRIJHK_{S}$ photometric catalog of the old open\ncluster NGC 188 to detect and characterize photometric binaries along the\ncluster main sequence. This technique has the advantage that it\nself-consistently handles photometric errors, missing data in various\nbandpasses, and star-by-star prior constraints on cluster membership.\nSimulations are used to verify uncertainties and quantify selection biases in\nour analysis, illustrating that among binaries with mass ratios >0.5, we\nrecover the binary fraction to better than 7% in the mean, with no significant\ndependence on binary fraction and a mild dependence on assumed mass ratio\ndistribution. Using our photometric catalog, we recover the majority\n(65%$\\pm$11%) of spectroscopically identified main sequence binaries, including\n8 of the 9 with spectroscopically measured mass ratios. Accounting for\nincompleteness and systematics, we derive a mass ratio distribution that rises\ntoward lower mass ratios (within our $q >$0.5 analysis domain). We observe a\nraw binary fraction for solar-type main sequence stars with mass ratios $q\n>$0.5 of 42%$\\pm$4%, independent of the assumed mass ratio distribution to\nwithin its uncertainties, consistent with literature values for old open\nclusters but significantly higher than the field solar-type binary fraction. We\nconfirm that the binaries identified by our method are more concentrated than\nsingle stars, in agreement with previous studies, and we demonstrate that the\nbinary nature of those candidates which remain unidentified spectroscopically\nis strongly supported by photometry from Gaia DR2.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A key factor to the spin parameter of uniformly rotating compact stars:\n  crust structure: We study the key factor to determine the dimensionless spin parameter\n$j\\equiv cJ/(GM^2)$ of different kinds of uniformly rotating compact stars,\nincluding the traditional neutron stars, hyperonic neutron stars, and hybrid\nstars, and check the reliability of the results on various types of equations\nof state of dense matter. The equations of state from the relativistic mean\nfield theory and the MIT bag model are adopted to simulate compact stars.\nNumerical calculations of rigidly rotating neutron stars are performed using\nthe RNS code in the framework of general relativity by solving the Einstein\nequations for stationary axis-symmetric spacetime. The crust structure of\ncompact stars is found to be a key factor to determine the maximum value of the\nspin parameter $j_{\\rm max}$. For the stars with inclusion of the crust,\n$j_{\\rm max}\\sim 0.7$ is sustained for various kinds of compact stars with\n$M>0.5 M_{\\odot}$, and is found to be insensitive to the mass of star and\nselected equations of state. For the traditional neutron stars, hyperonic\nneutron stars and hybrid stars without crust structure, the value $j_{\\rm max}$\nlies in the range of $[0.7, 1.0]$. Thus, not $j>0.7$ but $j>1$ could be treated\nas the candidate criterion to distinguish the strange quark stars from the\nother kinds of compact stars. Furthermore, a universal formula\n$j=0.48(f/f_K)^3-0.42(f/f_K)^2+0.63(f/f_K)$ is suggested to calculate the spin\nparameter at any rotational frequency for all kinds of compact stars with crust\nstructure and $M>0.5M_{\\odot}$.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Massive Outflows Driven by Magnetic Effects II: Comparison with\n  Observations: The driving mechanism of massive outflows observed in high-mass star-forming\nregions is investigated using three-dimensional magnetohydrodynamics (MHD) and\nprotostellar evolution calculations. In our previous paper, we showed that the\nmass outflow rate depends strongly on the mass accretion rate onto the\ncircumstellar disk around a high-mass protostar, and massive outflows may be\ndriven by the magnetic effect in high-mass star-forming cores. In the present\nstudy, in order to verify that the MHD disk wind is the primary driving\nmechanism of massive outflows, we quantitatively compare outflow properties\nobtained through simulations and observations. Since the outflows obtained\nthrough simulations are slightly younger than those obtained through\nobservations, the time-integrated quantities of outflow mass, momentum, and\nkinetic energy are slightly smaller than those obtained through observations.\nOn the other hand, time-derivative quantities of mass ejection rate, outflow\nmomentum flux, and kinetic luminosity obtained through simulations are in very\ngood agreement with those obtained through observations. This indicates that\nthe MHD disk wind greatly contributes to the massive outflow driving from\nhigh-mass protostars, and the magnetic field might significantly control the\nhigh-mass star formation process.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Lithium in the Hyades L5 brown dwarf 2MASSJ04183483+2131275: From the luminosity, effective temperature, and age of the Hyades brown dwarf\n2MASSJ04183483+2131275 (2M0418), sub-stellar evolutionary models predict a mass\nin the range 39-55 Jupiter masses (M_Jup) which is insufficient to produce any\nsubstantial lithium burning except for the very upper range >53 M_Jup. Our goal\nis to measure the abundance of lithium in this object, test the consistency\nbetween models and observations and refine constraints on the mass and age of\nthe object.\n  We used the 10.4-m Gran Telescopio Canarias (GTC) with its low-dispersion\noptical spectrograph to obtain ten spectra of 2277s each covering the range\n6300-10300 Angstroms with a resolving power of R~500.\n  In the individual spectra, which span several months, we detect persistent\nunresolved H_alpha in emission with pseudo equivalent widths (pEW) in the range\n45-150 Angstroms and absorption lines of various alkalis with the typical\nstrengths found in objects of L5 spectral type. The lithium resonance line at\n6707.8 Angstroms is detected with pEW of 18+/-4 Angstroms in 2M0418 (L5).\n  We determine a lithium abundance of log N(Li) = 3.0+/-0.4 dex consistent with\na minimum preservation of 90% of this element which confirms 2M0418 as a brown\ndwarf with a maximum mass of 52 M_Jup. We infer a maximum age for the Hyades of\n775 Myr from a comparison with the BHAC15 models. Combining recent results from\nthe literature with our study, we constrain the mass of 2M0418 to 45-52 M_Jup\nand the age of the cluster to 580-775 Myr (1 sigma) based on the lithium\ndepletion boundary method.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Stability of the coronal magnetic field around large confined and\n  eruptive solar flares: In order to improve our understanding on the pre-requisites of eruptive solar\nflares, we study and compare different measures that characterize the eruptive\npotential of solar active regions - the critical height for torus instability\nas a local measure and the helicity ratio as a global measure - with the\nstructural properties of the underlying magnetic field, namely the altitude of\nthe center of the current-carrying magnetic structure. Using time series of 3D\noptimization-based nonlinear force-free magnetic field models for 10 different\nactive regions (ARs) around the time of large solar flares, we determine the\naltitudes of the current-weighted centers of the non-potential model\nstructures. Based on the potential magnetic field, we inspect the decay index,\n$n$, in multiple vertical planes oriented along of or perpendicular to the\nflare-relevant polarity inversion line, and estimate the critical height\n($h_{\\mathrm{crit}}$) for torus instability (TI) using different thresholds of\n$n$. The critical heights are interpreted with respect to the altitudes of the\ncurrent-weighted centers of the associated non-potential structures, as well as\nthe eruptive character of the associated flares, and the eruptive potential of\nthe host AR, as characterized by the helicity ratio. Our most important\nfindings are that (i) $h_{\\mathrm{crit}}$ is more segregated in terms of flare\ntype than the helicity ratio, and that (ii) coronal field configurations with a\nhigher eruptive potential (in terms of the helicity ratio) also appear to be\nmore prone to TI. Furthermore, we find no pronounced differences in the\naltitudes of the non-potential structures prior to confined and eruptive\nflares.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The past and future evolution of a star like Betelgeuse: We discuss the physics and the evolution of a typical massive star passing\nthrough an evolutionary stage similar to that of Betelgeuse. After a brief\nintroduction recalling various observed parameters of Betelgeuse, we discuss\nthe Pre-Main-Sequence phase (PMS), the Main-Sequence (MS) phase, the physics\ngoverning the duration of the first crossing of the HR diagram, the red\nsupergiant stage (RSG), the post-red supergiant phases and the final fate of\nsolar metallicity stars with masses between 9 and 25 M$_\\odot$. We examine the\nimpact of different initial rotation and of various prescriptions for the mass\nloss rates during the red supergiant phase. We show that, whatever the initial\nrotation rate (chosen between 0 and 0.7$\\times\\upsilon_{\\rm crit}$,\n$\\upsilon_{\\rm crit}$ being the surface equatorial velocity producing a\ncentrifugal acceleration balancing exactly the gravity) and the mass loss rates\nduring the RSG stage (varied between a standard value and 25 times that value),\na 15 M$_\\odot$ star always ends its lifetime as a RSG and explodes as a type\nII-P or II-L supernova.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Radial Speed Evolution of Interplanetary Coronal Mass Ejections During\n  Solar Cycle 23: We report radial speed evolution of interplanetary coronal mass ejections\n(ICMEs) detected by the SOHO/LASCO coronagraph, interplanetary scintillation\n(IPS) at 327 MHz, and in-situ observations. In this study, we analyze solar\nwind disturbance factor (g-value) data derived from IPS observations during\n1997-2009 covering nearly the whole period of Solar Cycle 23. By comparing\nobservations from the SOHO/LASCO, IPS, and in situ, we then identify 39 ICMEs\nthat could be analyzed carefully. Here, we define two speeds V_SOHO and V_bg,\nwhich are initial speed of ICME and the speed of the background solar wind,\nrespectively. Examinations of these speeds yield the following results; i) Fast\nICMEs (with V_SOHO - V_bg > 500 km/s) rapidly decelerate, moderate ICMEs (with\n0 km/s < V_SOHO - V_bg < 500 km/s) show either gradually decelerating or\nuniform motion, and slow ICMEs (with V_SOHO - V_bg < 0 km/s) accelerate. The\nradial speeds converge on the speed of background solar wind during their\noutward propagation. We subsequently find; ii) both the acceleration and\ndeceleration are nearly complete by 0.79 (+/- 0.04) AU, and those are ended\nwhen the ICME speed reaches a given speed. We find the value of that to be 480\n(+/- 21) km/s. iii) For the fast and moderate ICMEs, a linear equation with a\nconstant coefficient is more appropriate than a quadratic equation to describe\ntheir kinematics, because the chi-square for the linear equation satisfies the\nstatistical significance level of 0.05, while the quadratic one is not. These\nresults support the hypothesis that the radial motion of ICMEs is governed by a\ndrag force due to interaction with the background solar wind. These findings\nalso suggest that ICMEs propagating faster than background solar wind are\ncontrolled mainly by the hydrodynamic Stokes drag.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Discovery of three self-lensing binaries from Kepler: We report the discovery of three edge-on binaries with white dwarf companions\nthat gravitationally magnify (instead of eclipsing) the light of their stellar\nprimaries, as revealed by a systematic search for pulses with long periods in\nthe Kepler photometry. We jointly model the self-lensing light curves and\nradial-velocity orbits to derive the white dwarf masses, all of which are close\nto 0.6 Solar masses. The orbital periods are long, ranging from 419 to 728\ndays, and the eccentricities are low, all less than 0.2. These characteristics\nare reminiscent of the orbits found for many blue stragglers in open clusters\nand the field, for which stable mass transfer due to Roche-lobe overflow from\nan evolving primary (now a white dwarf) has been proposed as the formation\nmechanism. Because the actual masses for our three white dwarf companions have\nbeen accurately determined, these self-lensing systems would provide excellent\ntests for models of interacting binaries.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Astrometric detection of exoplanets from the ground: Astrometry is a powerful technique to study the populations of extrasolar\nplanets around nearby stars. It gives access to a unique parameter space and is\ntherefore required for obtaining a comprehensive picture of the properties,\nabundances, and architectures of exoplanetary systems. In this review, we\ndiscuss the scientific potential, present the available techniques and\ninstruments, and highlight a few results of astrometric planet searches, with\nan emphasis on observations from the ground. In particular, we discuss\nastrometric observations with the Very Large Telescope (VLT) Interferometer and\na programme employing optical imaging with a VLT camera, both aimed at the\nastrometric detection of exoplanets. Finally, we set these efforts into the\ncontext of Gaia, ESA's astrometry mission scheduled for launch in 2013, and\npresent an outlook on the future of astrometric exoplanet detection from the\nground.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Exploring the spectral \\textit{information content} in the LAMOST\n  medium-resolution survey (MRS): Low-resolution spectra are proved competitive to high-resolution spectra in\ndetermining many stellar labels at comparable precision. It is useful to\nconsider the spectral information content when assessing the capability of a\nstellar spectrum in deriving precise stellar labels. In this work, we quantify\nthe information content brought by the LAMOST-II medium-resolution\nspectroscopic survey (MRS) using the gradient spectra and the\ncoefficients-of-dependence (CODs). In general, the wavelength coverage of the\nMRS well constrains the stellar labels but the sensitivities of different\nstellar labels vary with spectral types and metallicity of the stars of\ninterest and, therefore, affect the performance of the stellar label\ndetermination from the MRS spectra. Applying the SLAM to the synthetic spectra\nwhich mimic the MRS data, we find the precision of the fundamental stellar\nparameters Teff, logg and [M/H] are better when combining both the blue and red\nbands of the MRS. This is especially important for warm stars since the\nH$\\alpha$ line located in the red part plays a more important role in\ndetermining the effective temperature for warm stars. With blue and red parts\ntogether, we are able to reach similar performance to the low-resolution\nspectra except for warm stars. However, at [M/H]$\\sim-2.0$ dex, the\nuncertainties of fundamental stellar labels estimated from MRS are\nsubstantially larger than those from low-resolution spectra. We also tested the\nuncertainties of Teff, logg and [M/H] of from MRS data induced from the radial\nvelocity mismatch and find that a mismatch of about 1 km s$^{-1}$, which is\ntypical for LAMOST MRS data, would not significantly affect the stellar label\nestimates. At last, reference precision limits are calculated using synthetic\ngradient spectra, according to which we expect abundances of at least 17\nelements to be measured precisely from MRS spectra.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Signatures of ubiquitous magnetic reconnection in the deep atmosphere of\n  sunspot penumbrae: Ellerman bombs are regions with enhanced Balmer line wing emission and mark\nmagnetic reconnection in the deep solar atmosphere in active regions and quiet\nSun. They are often found in regions where opposite magnetic polarities are in\nclose proximity. Recent high resolution observations suggest that Ellerman\nbombs are more prevalent than thought before. We aim to determine the\noccurrence of Ellerman bombs in the penumbra of sunspots. We analyze high\nspatial resolution observations of sunspots in the Balmer H-alpha and H-beta\nlines as well as auxiliary continuum channels obtained with the Swedish 1-m\nSolar Telescope and apply the k-means clustering technique to systematically\ndetect and characterize Ellerman Bombs. Features with all the defining\ncharacteristics of Ellerman bombs are found in large numbers over the entire\npenumbra. The true prevalence of these events is only fully appreciated in the\nH-beta line due to highest spatial resolution and lower chromospheric opacity.\nWe find that the penumbra hosts some of the highest Ellerman bomb densities,\nonly surpassed by the moat in the immediate surroundings of the sunspot. Some\npenumbral Ellerman bombs show flame morphology and rapid dynamical evolution.\nMany penumbral Ellerman bombs are fast moving with typical speed of 3.7 km/s\nand sometimes more than 10 km/s. Many penumbral Ellerman bombs migrate from the\ninner to the outer penumbra over hundreds of km and some continue moving beyond\nthe outer penumbral boundary into the moat. Many penumbral Ellerman bombs are\nfound in the vicinity of regions with opposite magnetic polarity. We conclude\nthat reconnection is a near continuous process in the low atmosphere of the\npenumbra of sunspots as manifest in the form of penumbral Ellerman bombs. These\nare so prevalent that they may be a major sink of sunspot magnetic energy.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The process of data formation for the Spectrometer/Telescope for Imaging\n  X-rays (STIX) in Solar Orbiter: The Spectrometer/Telescope for Imaging X-rays (STIX) is a hard X-ray imaging\nspectroscopy device to be mounted in the Solar Orbiter cluster with the aim of\nproviding images and spectra of solar flaring regions at different photon\nenergies in the range from a few keV to around 150 keV. The imaging modality of\nthis telescope is based on the Moire pattern concept and utilizes 30\nsub-collimators, each one containing a pair of co-axial grids. This paper\napplies Fourier analysis to provide the first rigorous description of the data\nformation process in STIX. Specifically, we show that, under first harmonic\napproximation, the integrated counts measured by STIX sub-collimators can be\ninterpreted as specific spatial Fourier components of the incoming photon flux,\nnamed visibilities. Fourier analysis also allows the quantitative assessment of\nthe reliability of such interpretation. The description of STIX data in terms\nof visibilities has a notable impact on the image reconstruction process, since\nit fosters the application of Fourier-based imaging algorithms.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Dust-temperature of an isolated star-forming cloud: Herschel\n  observations of the Bok globule CB244: We present Herschel observations of the isolated, low-mass star-forming Bok\nglobule CB244. It contains two cold sources, a low-mass Class 0 protostar and a\nstarless core, which is likely to be prestellar in nature, separated by 90\narcsec (~ 18000 AU). The Herschel data sample the peak of the Planck spectrum\nfor these sources, and are therefore ideal for dust-temperature and column\ndensity modeling. With these data and a near-IR extinction map, the MIPS 70\nmicron mosaic, the SCUBA 850 micron map, and the IRAM 1.3 mm map, we model the\ndust-temperature and column density of CB244 and present the first measured\ndust-temperature map of an entire star-forming molecular cloud. We find that\nthe column-averaged dust-temperature near the protostar is ~ 17.7 K, while for\nthe starless core it is ~ 10.6K, and that the effect of external heating causes\nthe cloud dust-temperature to rise to ~ 17 K where the hydrogen column density\ndrops below 10^21 cm^-2. The total hydrogen mass of CB244 (assuming a distance\nof 200 pc) is 15 +/- 5 M_sun. The mass of the protostellar core is 1.6 +/- 0.1\nM_sun and the mass of the starless core is 5 +/- 2 M_sun, indicating that ~ 45%\nof the mass in the globule is participating in the star-formation process.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A new extremely low-mass white dwarf in the NLTT catalogue: We report on the discovery of the extremely low-mass, hydrogen-rich white\ndwarf, NLTT 11748. Based on measurements of the effective temperature\n(8540+/-50 K) and surface gravity (log g = 6.20+/-0.15) obtained by fitting the\nobserved Balmer line profiles with synthetic spectra, we derive a mass of\n0.167+/-0.005 M_solar. This object is one of only a handful of white dwarfs\nwith masses below 0.2 M_solar that are believed to be the product of close\nbinary evolution with an episode of Roche lobe overflow onto a degenerate\ncompanion (neutron star or white dwarf). Assuming membership in the halo\npopulation, as suggested by the kinematics and adopting a cooling age of 4.0 -\n6.3 Gyrs for the white dwarf, we infer a progenitor mass of 0.87 - 0.93\nM_solar. The likely companion has yet to be identified, but a search for radial\nvelocity variations may help constrain its nature.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Formation of Extremely Low-mass White Dwarfs Binaries: Motivated by the discovery of a handful of pulsating, extremely low mass\nwhite dwarfs (ELM WDs, mass $M \\lesssim 0.17\\, M_\\odot$) which likely have WD\ncompanions, this paper discusses binary formation models for these systems.\nFormation of an ELM WD by unstable mass transfer (MT) or a common envelope (CE)\nevent is unlikely. Stable Roche-lobe overflow with conservative MT produces\nonly $M \\gtrsim 0.2\\, M_\\odot$. This paper discusses the formation of ELM WD\nusing angular momentum losses by magnetic braking. The initially more massive\nstar becomes the companion WD through a CE event. The initially less massive\nstar becomes the ELM WD as an evolved donor in a Cataclysmic Variable binary.\nEvolutionary models are constructed using the Modules for Experiments in\nStellar Astrophysics (MESA), with ELM WD progenitors in the range\n$M_d=1.0-1.5\\, M_\\odot$ and WD companions in the range $M_{\\rm a} \\simeq\n0.4-0.9\\, M_\\odot$. A prescription to reduce magnetic braking for thin surface\nconvection zones is included. Upon the thinning of the evolved donor's\nenvelope, the donor star shrinks out of contact and MT ceases, revealing the\nELM WD. Systems with small masses have previously been suggested as possible AM\nCVN's. Systems with large masses, up to the limit $M \\simeq 0.18\\, M_\\odot$ at\nwhich shell flashes occur on the WD cooling track, will tend to expand out to\norbital periods $P_{\\rm orb} \\gtrsim 15\\, {\\rm hr}$. In between this range, ELM\nWD may become pulsators both as pre-WD and on the WD cooling track. Brickhill's\ncriterion for convective mode driving is used to estimate the location of the\nblue edge of the g-mode instability strip.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Neutrino Quantum Kinetic Equations: Neutrinos propagate in astrophysical and cosmological environments modifying\ntheir flavor in intriguing ways. The study of neutrino propagation in media is\nbased on the mean-field, extended mean-field and Boltzmann equations. We\nsummarise salient features of these evolution equations and the methods\nemployed so far to derive them. We emphasize applications to situations of\nobservational interest.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The nature of Els\u00e4sser variables in compressible MHD: The Els\\\"{a}sser variables are often used in studies of plasma turbulence, in\nhelping differentiate between MHD waves propagating parallel or anti-parallel\nto the main magnetic field. While for pure Alfv\\'en waves in a homogeneous\nplasma the method is strictly valid, we show that compressible, magnetoacoustic\nwaves are in general described by both Els\\\"{a}sser variables. Furthermore, in\na compressible and inhomogeneous plasma, the pure MHD waves (Alfv\\'en, fast and\nslow) are no longer normal modes, but waves become linearly coupled or display\nmixed properties of Alfv\\'en and magnetoacoustic nature. These waves are\nnecessarily described by both Els\\\"{a}sser variables and therefore the\nEls\\\"{a}sser formalism cannot be used to strictly separate parallel and\nanti-parallel propagating waves. Nevertheless, even in an inhomogeneous plasma,\nfor a highly Alfv\\'enic wave the Els\\\"{a}sser variable corresponding to the\npropagation direction appears still dominating. We suggest that for Alfv\\'enic\nwaves, the relative amplitude of Els\\\"{a}sser variables depends on the local\ndegree of inhomogeneity and other plasma and wave properties. This finding has\nimplications for turbulence studies in inhomogeneous and compressible plasmas,\nsuch as the solar corona and solar wind.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Algebraic and machine learning approach to hierarchical triple-star\n  stability: We present two approaches to determine the dynamical stability of a\nhierarchical triple-star system. The first is an improvement on the\nMardling-Aarseth stability formula from 2001, where we introduce a dependence\non inner orbital eccentricity and improve the dependence on mutual orbital\ninclination. The second involves a machine learning approach, where we use a\nmultilayer perceptron (MLP) to classify triple-star systems as `stable' and\n`unstable'. To achieve this, we generate a large training data set of 10^6\nhierarchical triples using the N-body code MSTAR. Both our approaches perform\nbetter than previous stability criteria, with the MLP model performing the\nbest. The improved stability formula and the machine learning model have\noverall classification accuracies of 93 % and 95 % respectively. Our MLP model,\nwhich accurately predicts the stability of any hierarchical triple-star system\nwithin the parameter ranges studied with almost no computation required, is\npublicly available on Github in the form of an easy-to-use Python script.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Colors of c-type RR Lyrae Stars and Interstellar Reddening: RR Lyrae stars pulsating in the fundamental mode have long been used to\nmeasure interstellar reddening, based on their observed uniformity of $B-V$\ncolor at minimum light after small corrections for metallicity and period are\napplied. However, little attention has been paid to the first overtone\npulsators (RRc or RR1). We present new $V-I$ observations of field RRc stars,\nsupplemented with published data from uncrowded RRc in globular clusters.\nPreliminary results indicate the RRc colors are correlated with period, but\nappear to be independent of the stars' metallicity. The scatter around the\nperiod-color relation is slightly larger than a comparable relation for RRab.\nThus, RRc can be useful indicators of line of sight reddening toward old\nstellar systems, particularly when multiple stars are available as in\nOosterhoff II globular clusters and metal-poor galaxies.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Chromospheric magnetic field: A comparison of He I 10830 A observations\n  with nonlinear force-free field extrapolation: The nonlinear force-free field (NLFFF) modeling has been extensively used to\ninfer the three-dimensional (3D) magnetic field in the solar corona. One of the\nassumptions in the NLFFF extrapolation is that the plasma beta is low, but this\ncondition is considered to be incorrect in the photosphere. We examine direct\nmeasurements of the chromospheric magnetic field in two active regions through\nspectropolarimetric observations at He I 10830 A, which are compared with the\npotential fields and NLFFFs extrapolated from the photosphere. The comparisons\nallow quantitative estimation of the uncertainty in the NLFFF extrapolation\nfrom the photosphere. Our analysis shows that observed chromospheric magnetic\nfield may have larger non-potentiality compared to the photospheric magnetic\nfield. Moreover, the large non-potentiality in the chromospheric height may not\nbe reproduced by the NLFFF extrapolation from the photospheric magnetic field.\nThe magnitude of the underestimation of the non-potentiality at chromospheric\nheights may reach 30-40 degree in shear signed angle in some locations. This\ndeviation may be caused by the non-force-freeness in the photosphere. Our study\nsuggests the importance of the inclusion of measured chromospheric magnetic\nfields in the NLFFF modeling for the improvement of the coronal extrapolation.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The geometry of the close environment of SV Psc as probed by VLTI/MIDI: Context. SV Psc is an asymptotic giant branch (AGB) star surrounded by an\noxygen-rich dust envelope. The mm-CO line profile of the object's outflow shows\na clear double-component structure. Because of the high angular resolution,\nmid-IR interferometry may give strong constraints on the origin of this\ncomposite profile.\n  Aims. The aim of this work is to investigate the morphology of the\nenvironment around SV Psc using high-angular resolution interferometry\nobservations in the mid-IR with the Very Large Telescope MID-infrared\nInterferometric instrument (VLTI/MIDI).\n  Methods. Interferometric data in the N-band taken at different baseline\nlengths (ranging from 32-64 m) and position angles (73- 142{\\deg}) allow a\nstudy of the morphology of the circumstellar environment close to the star. The\ndata are interpreted on the basis of 2-dimensional, chromatic geometrical\nmodels using the fitting software tool GEM-FIND developed for this purpose.\n  Results. The results favor two scenarios: (i) the presence of a highly\ninclined, optically thin, dusty disk surrounding the central star; (ii) the\npresence of an unresolved binary companion at a separation of 13.7 AU and a\nposition angle of 121.8{\\deg} NE. The derived orbital period of the binary is\n38.1 yr. This detection is in good agreement with hydrodynamic simulations\nshowing that a close companion could be responsible for the entrainment of the\ngas and dust into a circumbinary structure.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Asteroseismic stellar activity relations: In asteroseismology an important diagnostic of the evolutionary status of a\nstar is the small frequency separation which is sensitive to the gradient of\nthe mean molecular weight in the stellar interior. It is thus interesting to\ndiscuss the classical age-activity relations in terms of this quantity.\nMoreover, as the photospheric magnetic field tends to suppress the amplitudes\nof acoustic oscillations, it is important to quantify the importance of this\neffect by considering various activity indicators. We propose a new class of\nage-activity relations that connects the Mt. Wilson $S$ index and the average\nscatter in the light curve with the small frequency separation and the\namplitude of the p-mode oscillations. We used a Bayesian inference to compute\nthe posterior probability of various empirical laws for a sample of 19\nsolar-like active stars observed by the Kepler telescope. We demonstrate the\npresence of a clear correlation between the Mt. Wilson $S$ index and the\nrelative age of the stars as indicated by the small frequency separation, as\nwell as an anti-correlation between the $S$ index and the oscillation\namplitudes. We argue that the average activity level of the stars shows a\nstronger correlation with the small frequency separation than with the absolute\nage that is often considered in the literature. The phenomenological laws\ndiscovered in this paper have the potential to become new important diagnostics\nto link stellar evolution theory with the dynamics of global magnetic fields.\nIn particular we argue that the relation between the Mt. Wilson $S$ index and\nthe oscillation amplitudes is in good agreement with the findings of direct\nnumerical simulations of magneto-convection.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The rapid dispersal of low-mass virialised clusters: Infant mortality brought about by the expulsion of a star cluster's natal gas\nis widely invoked to explain cluster statistics at different ages. While a well\nstudied problem, most recent studies of gas expulsion's effect on a cluster\nhave focused on massive clusters, with stellar counts of order $10^4$. Here we\nargue that the evolutionary timescales associated with the compact low-mass\nclusters typical of the median cluster in the Solar neighborhood are short\nenough that significant dynamical evolution can take place over the ages\nusually associated with gas expulsion. To test this we perform {\\it N}-body\nsimulations of the dynamics of a very young star forming region, with initial\nconditions drawn from a large-scale hydrodynamic simulation of gravitational\ncollapse and fragmentation. The subclusters we analyse, with populations of a\nfew hundred stars, have high local star formation efficiencies and are roughly\nvirialised even after the gas is removed. Over 10 Myr they expand to a similar\ndegree as would be expected from gas expulsion if they were initially gas-rich,\nbut the expansion is purely due to the internal stellar dynamics of the young\nclusters. The expansion is such that the stellar densities at 2 Myr match those\nof YSOs in the Solar neighborhood. We argue that at the low-mass end of the\ncluster mass spectrum, a deficit of clusters at 10s of Myr does not necessarily\nimply gas expulsion as a disruption mechanism.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Variability in Hot Carbon-Dominated Atmosphere (hot DQ) White Dwarfs:\n  Rapid Rotation?: Hot white dwarfs with carbon-dominated atmospheres (hot DQs) are a cryptic\nclass of white dwarfs. In addition to their deficiency of hydrogen and helium,\nmost of these stars are highly magnetic, and a large fraction vary in\nluminosity. This variability has been ascribed to nonradial pulsations, but\nincreasing data call this explanation into question. We present studies of\nshort-term variability in seven hot DQ white dwarfs. Three (SDSS J1426+5752,\nSDSS J2200-0741, and SDSS J2348-0942) were known to be variable. Their\nphotometric modulations are coherent over at least two years, and we find no\nevidence for variability at frequencies that are not harmonics. We present the\nfirst time-series photometry for three additional hot DQs (SDSS J0236-0734,\nSDSS J1402+3818, and SDSS J1615+4543); none are observed to vary, but the\nsignal-to-noise is low. Finally, we present high speed photometry for SDSS\nJ0005-1002, known to exhibit a 2.1 d photometric variation; we do not observe\nany short-term variability. Monoperiodicity is rare among pulsating white\ndwarfs, so we contemplate whether the photometric variability is due to\nrotation rather than pulsations; similar hypotheses have been raised by other\nresearchers. If the variability is due to rotation, then hot DQ white dwarfs as\na class contain many rapid rotators. Given the lack of companions to these\nstars, the origin of any fast rotation is unclear -- both massive progenitor\nstars and double degenerate merger remnants are possibilities. We end with\nsuggestions on future work that would best clarify the nature of these rare,\nintriguing objects.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The gravitational redshift of solar-type stars from Gaia DR3 wide\n  binaries: Light escaping from a gravitational potential suffers a redshift with\nmagnitude proportional to the depth of the potential. This \"gravitational\nredshift\" is easily measurable in dense stars such as white dwarfs, but is much\nweaker and has evaded unambiguous detection in main-sequence stars. I show that\nthe effect is directly measurable in the Gaia DR3 radial velocities (RVs) of\nthe components of wide binary stars. In a sample of $\\sim$500 wide binaries\ncontaining a solar-type main-sequence star and a red giant or red clump\ncompanion, the apparent RV of the giant is on average $0.49 \\pm 0.02 \\,\\, \\rm\nkm\\,s^{-1}$ lower than that of the main-sequence star. This owes primarily to\nthe giants' weaker gravitational fields and is in reasonably good agreement\nwith the value expected from general relativity.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Consecutive Narrow and Broad Quasi-periodic Fast-propagating Wave Trains\n  Associated with a Flare: The excitation mechanism of coronal quasi-period fast-propagating (QFP) wave\ntrains remains unresolved. Using Atmospheric Imaging Assembly onboard the Solar\nDynamics Observatory observations, we study a narrow and a broad QFP wave train\nexcited one after another during the successive eruptions of filaments hosted\nwithin a fan-spine magnetic system on 2013 October 20. The consecutive\noccurrence of these two types of QFP wave trains in the same event provides an\nexcellent opportunity to explore their excitation mechanisms and compare their\nphysical parameters. Our observational results reveal that narrow and broad QFP\nwave trains exhibit distinct speeds, periods, energy fluxes, and relative\nintensity amplitudes, although originating from the same active region and\nbeing associated with the same {\\em GOES} C2.9 flare. Using wavelet analysis,\nwe find that the narrow QFP wave train shares a similar period with the flare\nitself, suggesting its possible excitation through the pulsed energy release in\nthe magnetic reconnection process that generated the accompanying flare. On the\nother hand, the broad QFP wave train appears to be associated with the energy\npulses released by the successive expansion and unwinding of filament threads.\nAdditionally, it is plausible that the broad QFP wave train was also excited by\nthe sequential stretching of closed magnetic field lines driven by the erupting\nfilament. These findings shed light on the different excitation mechanisms and\norigins of the QFP wave trains.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Panchromatic evolution of three luminous red novae: Forbidden hugs in\n  pandemic times -- IV: We present photometric and spectroscopic data on three extragalactic luminous\nred novae (LRNe): AT2018bwo, AT2021afy, and AT2021blu. AT2018bwo was discovered\nin NGC45 (at 6.8 Mpc) a few weeks after the outburst onset. During the\nmonitoring period, the transient reached a peak luminosity of 10^40 erg/s.\nAT2021afy, hosted by UGC10043 (49.2 Mpc), showed a double-peaked light curve,\nwith the two peaks reaching a similar luminosity of 2.1(+-0.6)x10^41 erg/s. For\nAT2021blu in UGC5829, (8.6 Mpc), the pre-outburst phase was well-monitored by\nseveral photometric surveys, and the object showed a slow luminosity rise\nbefore the outburst. The light curve of AT2021blu was sampled with an\nunprecedented cadence until the object disappeared behind the Sun, and it was\nthen recovered at late phases. The light curve of AT2021blu shows a double\npeak, with a prominent early maximum reaching a luminosity of 6.5x10^40 erg/s,\nwhich is half of that of AT2021afy. The spectra of AT2021afy and AT2021blu\ndisplay the expected evolution for LRNe: a blue continuum dominated by\nprominent Balmer lines in emission during the first peak, and a redder\ncontinuum consistent with that of a K-type star with narrow absorption metal\nlines during the second, broad maximum. The spectra of AT2018bwo are markedly\ndifferent, with a very red continuum dominated by broad molecular features in\nabsorption. As these spectra closely resemble those of LRNe after the second\npeak, AT2018bwo was probably discovered at the very late evolutionary stages.\nThis would explain its fast evolution and the spectral properties compatible\nwith that of an M-type star. From the analysis of deep frames of the LRN sites\nyears before the outburst, and considerations of the light curves, the\nquiescent progenitor systems of the three LRNe were likely massive, with\nprimaries ranging from 13Mo for AT2018bwo, to 13-18Mo for AT2021blu, and over\n40Mo for AT2021afy.",
        "category": "astro-ph_SR"
    },
    {
        "text": "EPIC 216747137: a new HW Vir eclipsing binary with a massive sdOB\n  primary and a low-mass M-dwarf companion: EPIC 216747137 is a new HW~Virginis system discovered by the Kepler\nspacecraft during its K2 \"second life\". Like the other HW Vir systems, EPIC\n216747137 is a post-common-envelope eclipsing binary consisting of a hot\nsubluminous star and a cool low-mass companion. The short orbital period of\n3.87 hours produces a strong reflection effect from the secondary (~9% in the R\nband). Together with AA Dor and V1828 Aql, EPIC 216747137 belongs to a small\nsubgroup of HW Vir systems with a hot evolved sdOB primary. We find the\nfollowing atmospheric parameters for the hot component: Teff=40400$\\pm$1000 K,\nlogg=5.56$\\pm$0.06, log(N(He)/N(H))=$-$2.59$\\pm$0.05. The sdOB rotational\nvelocity vsini=51$\\pm$10 km/s implies that the stellar rotation is slower than\nthe orbital revolution and the system is not synchronized. When we combine\nphotometric and spectroscopic results with the Gaia parallax, the best solution\nfor the system corresponds to a primary with a mass of about 0.62 Msun close\nto, and likely beyond, the central helium exhaustion, while the cool M-dwarf\ncompanion has a mass of about 0.11 Msun.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Damping of filament thread oscillations: effect of the slow continuum: Transverse oscillations of small amplitude are commonly seen in\nhigh-resolution observations of filament threads, i.e. the fine-structures of\nsolar filaments/prominences, and are typically damped in a few periods. Kink\nwave modes supported by the thread body offer a consistent explanation of these\nobserved oscillations. Among the proposed mechanisms to explain the kink mode\ndamping, resonant absorption in the Alfven continuum seems to be the most\nefficient as it produces damping times of about 3 periods. However, for a\nnonzero-beta plasma and typical prominence conditions, the kink mode is also\nresonantly coupled to slow (or cusp) continuum modes, which could further\nreduce the damping time. In this Letter, we explore for the first time both\nanalytically and numerically the effect of the slow continuum on the damping of\ntransverse thread oscillations. The thread model is composed of a homogeneous\nand straight cylindrical plasma, an inhomogeneous transitional layer, and the\nhomogeneous coronal plasma. We find that the damping of the kink mode due to\nthe slow resonance is much less efficient than that due to the Alfven\nresonance.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Radial velocity photon limits for the dwarf stars of spectral classes\n  F--M: The determination of extrasolar planet masses with the radial velocity (RV)\ntechnique requires spectroscopic Doppler information from the planet's host\nstar, which varies with stellar brightness and temperature. We analyze Doppler\ninformation in spectra of F--M dwarfs utilizing empirical information from\nHARPS and CARMENES, and from model spectra. We come to the conclusions that an\noptical setup ($BVR$-bands) is more efficient that a near-infrared one ($YJHK$)\nin dwarf stars hotter than 3200\\,K. We publish a catalogue of 46,480\nwell-studied F--M dwarfs in the solar neighborhood and compare their\ndistribution to more than one million stars from Gaia DR2. For all stars, we\nestimate the RV photon noise achievable in typical observations assuming no\nactivity jitter and slow rotation. We find that with an ESPRESSO-like\ninstrument at an 8m-telescope, a photon noise limit of 10\\,cm\\,s$^{-1}$ or\nlower can be reached in more than 280 stars in a 5\\,min observation. At\n4m-telescopes, a photon noise limit of 1\\,m\\,s$^{-1}$ can be reached in a\n10\\,min exposure in approx.\\ 10,000 predominantly sun-like stars with a\nHARPS-like (optical) instrument. The same applies to $\\sim$3000 stars for a\nred-optical setup covering the $RIz$-bands, and to $\\sim$700 stars for a\nnear-infrared instrument. For the latter two, many of the targets are nearby M\ndwarfs. Finally, we identify targets in which Earth-mass planets within the\nliquid water habitable zone can cause RV amplitudes comparable to the RV photon\nnoise. Assuming the same exposure times, we find that an ESPRESSO-like\ninstrument can reach this limit for 1\\,M$_\\Earth$ planets in more than 1000\nstars. The optical, red-optical, and near-infrared configurations reach the\nlimit for 2\\,M$_\\Earth$ planets in approximately 500, 700, and 200 stars,\nrespectively.",
        "category": "astro-ph_SR"
    },
    {
        "text": "CSI 2264: Characterizing Young Stars in NGC 2264 with Stochastically\n  Varying Light Curves: We provide CoRoT and Spitzer light curves, as well as broad-band\nmulti-wavelength photometry and high resolution, multi- and single-epoch\nspectroscopy for 17 classical T Tauris in NGC 2264 whose CoRoT light curves\n(LCs) exemplify the \"stochastic\" LC class as defined in Cody et al. (2014). The\nmost probable physical mechanism to explain the optical variability in this LC\nclass is time-dependent mass accretion onto the stellar photosphere, producing\ntransient hot spots. As evidence in favor of this hypothesis, multi-epoch high\nresolution spectra for a subset of these stars shows that their veiling levels\nalso vary in time and that this veiling variability is consistent in both\namplitude and timescale with the optical LC morphology. Furthermore, the\nveiling variability is well-correlated with the strength of the HeI 6678A\nemission line, a feature predicted by models to arise in accretion shocks on or\nnear the stellar photosphere. Stars with accretion burst LC morphology\n(Stauffer et al. 2014) are also attributed to variable mass accretion. Both the\nstochastic and accretion burst LCs can be explained by a simple model of\nrandomly occurring flux bursts, with the stochastic LC class having a higher\nfrequency of lower amplitude events. Based on their UV excesses, veiling, and\nmean Ha equivalent widths, members of the stochastic LC class have only\nmoderate time-averaged mass accretion rates. The most common feature of their\nHa profiles is for them to exhibit blue-shifted absorption features, most\nlikely originating in a disk wind. The lack of periodic signatures in the LCs\nsuggests that little of the variability is due to long-lived hot spots rotating\ninto or out of our line of sight; instead, the primary driver of the observed\nphotometric variability is likely to be instabilities in the inner disk that\nlead to variable mass accretion.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Classical Cepheids Require Enhanced Mass Loss: Measurements of rates of period change of Classical Cepheids probe stellar\nphysics and evolution. Additionally, better understanding of Cepheid structure\nand evolution provides greater insight into their use as standard candles and\ntools for measuring the Hubble constant. Our recent study of the period change\nof the nearest Cepheid, Polaris, suggested that it is undergoing enhanced mass\nloss when compared to canonical stellar evolution model predictions. In this\nwork, we expand the analysis to rates of period change measured for about 200\nGalactic Cepheids and compare them to population synthesis models of Cepheids\nincluding convective core overshooting and enhanced mass loss. Rates of period\nchange predicted from stellar evolution models without mass loss do not agree\nwith observed rates whereas including enhanced mass loss yields predicted rates\nin better agreement with observations. This is the first evidence that enhanced\nmass loss as suggested previously for Polaris and delta Cephei must be a\nubiquitous property of Classical Cepheids.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The quasi-biennial periodicity as a window on the solar magnetic dynamo\n  configuration: Manifestations of the solar magnetic activity through periodicities of about\n11 and 2 years are now clearly seen in all solar activity indices.In this\npaper, we add information about the mechanism driving the 2 year period by\nstudying the time and latitudinal properties of acoustic modes that are\nsensitive probes of the subsurface layers. We use almost 17 years of high\nquality resolved data provided by the Global Oscillation Network Group (GONG)\nto investigate the solar cycle changes in p-mode frequencies for spherical\ndegrees l from 0 to 120 and 1.6 mHz < nu < 3.5 mHz. For both periodic\ncomponents of solar activity, we locate the origin of the frequency shift in\nthe subsurface layers and put in evidence for a sudden enhancement in amplitude\njust in the last few hundred kilometers. We also show that, in both cases, the\nsize of the shift increases towards equatorial latitudes and from minimum to\nmaximum of solar activity, but, in agreement with previous findings,the\nquasi-biennial periodicity (QBP) causes a weaker shift in mode frequencies and\na slower enhancement than the one caused by the 11 year cycle. We compare our\nobservational findings with the features predicted by different models that try\nto explain the origin of this QBP and conclude that the observed\npropertiescould result from the beating between a dipole and quadrupole\nmagnetic configuration of the dynamo.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Nature of grand minima and maxima from fully non-linear Flux-Transport\n  Dynamos: We aim to investigate the nature and occurrence characteristics of grand\nsolar minimum and maximum periods, which are observed in the solar proxy\nrecords such as 10Be and 14C, using a fully non-linear Babcock-Leighton type\nflux-transport dynamo including momentum and entropy equations. The\ndifferential rotation and meridional circulation are generated from the effect\nof turbulent Reynolds stress and are subjected to back-reaction from the\nmagnetic field. To generate grand minimum and maximum-like periods in our\nsimulations, we used random fluctuations in the angular momentum transport\nprocess, namely the Lambda-mechanism, and in the Babcock-Leighton mechanism. To\ncharacterise the nature and occurrences of the identified grand minima and\nmaxima in our simulations, we used the waiting time distribution analyses,\nwhich reflects whether the underlying distribution arises from a random or a\nmemory-bearing process. The results show that, in majority of the cases, the\ndistributions of grand minima and maxima reveal that the nature of these events\noriginates from memoryless processes. We also found that in our simulations the\nmeridional circulation speed tends to be smaller during grand maximum, while it\nis faster during grand minimum periods. The radial differential rotation tend\nto be larger during grand maxima, while it is smaller during grand minima. The\nlatitudinal differential rotation on the other hand is found to be larger\nduring grand minima.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Envelope Kinematics and a Possible Disk Around the Class 0 Protostar\n  within BHR7: We present a characterization of the protostar embedded within the BHR7 dark\ncloud, based on both photometric measurements from the near-infrared to\nmillimeter and interferometric continuum and molecular line observations at\nmillimeter wavelengths. We find that this protostar is a Class 0 system, the\nyoungest class of protostars, measuring its bolometric temperature to be\n50.5~K, with a bolometric luminosity of 9.3~L$_{\\odot}$. The near-infrared and\n\\textit{Spitzer} imaging show a prominent dark lane from dust extinction\nseparating clear bipolar outflow cavities. Observations of $^{13}$CO\n($J=2\\rightarrow1$), C$^{18}$O ($J=2\\rightarrow1$), and other molecular lines\nwith the Submillimeter Array (SMA) exhibit a clear rotation signature on scales\n$<$1300~AU. The rotation can be traced to an inner radius of $\\sim$170~AU and\nthe rotation curve is consistent with an R$^{-1}$ profile, implying that\nangular momentum is being conserved. Observations of the 1.3~mm dust continuum\nwith the SMA reveal a resolved continuum source, extended in the direction of\nthe dark lane, orthogonal to the outflow. The deconvolved size of the continuum\nindicates a radius of $\\sim$100~AU for the continuum source at the assumed\ndistance of 400~pc. The visibility amplitude profile of the continuum emission\ncannot be reproduced by an envelope alone and needs a compact component. Thus,\nwe posit that the resolved continuum source could be tracing a Keplerian disk\nin this very young system. If we assume that the continuum radius traces a\nKeplerian disk (R$\\sim$120~AU) the observed rotation profile is consistent with\na protostar mass of 1.0~$M_{\\odot}$.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Tests of sunspot number sequences: 4. Discontinuities around 1946 in\n  various sunspot number and sunspot group number reconstructions: We use 5 test data series to quantify putative discontinuities around 1946 in\n5 annual-mean sunspot number or group number sequences. The series tested are:\nthe original and new versions of the Wolf/Zurich/International sunspot number\ncomposite [$R_{ISNv1}$ and $R_{ISNv2}$] ; the corrected version of $R_{ISNv1}$\n[$R_C$]; the backbone group number [$R_{BB}$]; and the group number composite\n[$R_{UEA}$]. The test data are: the group number $N_G$ and total sunspot area\n$A_G$ from the RGO photoheliographic data; the CaK index from re-analysis of\nMWO CaII K spectroheliograms; the group number from the MWO sunspot drawings,\n$N_{MWO}$; and ionospheric critical frequencies at Slough [$foF2$]. The test\ndata all vary with sunspot numbers, in some cases non-linearly. Tests use both\nbefore-and-after fit-residual comparison and correlation methods, applied to\nintervals iterated to minimise errors and eliminate the effect of the\ndiscontinuity date. It is not assumed that the correction required is by a\nconstant factor, nor even linear in sunspot number. A non-linear correction is\nrequired by $R_C$, $R_{BB}$ and $R_{ISNv1}$, but not by $R_{ISNv2}$ or\n$R_{UEA}$. The test datasets give very similar results in all cases. By\nmultiplying the probability distribution functions together we obtain the\noptimum correction for each data series that must be applied to\npre-discontinuity data to make them consistent with the post-discontinuity\ndata. It is shown that, on average, values for 1932-1943 are too small\n(relative to later values) by about 12.3% for $R_{ISNv1}$ but are too large for\n$R_{ISNv2}$ and $R_{BB}$ by 3.8% and 5.2%. The correction applied to generate\n$R_C$ from $R_{ISNv1}$ reduces this average factor to 0.5% but does not remove\nthe non-linear variation, and other errors remain uncorrected. A test is\nprovided by $R_{UEA}$, which is identical to the RGO $N_G$ values over the\ninterval used.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A Theory about Electric Current and Heating in Plasma: The traditional generalized Ohm's law in MHD do not explicitly present the\nrelation of electric currents and electric fields in fully ionized plasma, and\nlead to some unexpected concepts, such as \"the magnetic frozen-in plasma\",\nmagnetic reconnection etc. In the present paper, we solve the balance equation\ncan give exact solution of the velocities of electrons and ions, and then\nderived the electric current in fully ionized plasma. In the case ignoring\nboundary condition, there is no electric current in the plane perpendicular to\nthe magnetic field when external forces are ignored. The electric field in the\nplane perpendicular to magnetic field do not contribute to the electric\ncurrents, so do the induced electric field from the motion of the plasma across\nmagnetic field. The lack of induced electric current will keep magnetic field\nin space unaffected. The velocity of the bulk velocity of the plasma\nperpendicular to magnetic field is not free, it is decided by electromagnetic\nfield and the external forces. We conclude that the bulk velocity of the fully\nionized plasma is not coupled with the magnetic field. The motion of the plasma\ndo not change the magnetic field in space, but the plasma will be confined by\nmagnetic field.\n  Due to the confinement of magnetic field, the plasma kinetic energy will be\ntransformed into plasma thermal energy by the Lamor motion and collisions\nbetween the same species of particles inside plasma. Because the electric field\nperpendicular to magnetic field do not contribute electric current, the\nvariation of magnetic field will transfer energy directly into the plasma\nthermal energy by induced electric field. The heating of plasma could be from\nthe kinetic energy and the variation of magnetic field.",
        "category": "astro-ph_SR"
    },
    {
        "text": "A blueprint of state-of-the-art techniques for detecting quasi-periodic\n  pulsations in solar and stellar flares: Quasi-periodic pulsations (QPPs) appear to be a common feature observed in\nthe light curves of both solar and stellar flares. However, their\nquasi-periodic nature, along with the fact that they can be small in amplitude\nand short-lived, makes QPPs difficult to unequivocally detect. In this paper,\nwe test the strengths and limitations of state-of-the-art methods for detecting\nQPPs using a series of hare-and-hounds exercises. The hare simulated a set of\nflares, both with and without QPPs of a variety of forms, while the hounds\nattempted to detect QPPs in blind tests. We use the results of these exercises\nto create a blueprint for anyone who wishes to detect QPPs in real solar and\nstellar data. We present eight clear recommendations to be kept in mind for\nfuture QPP detections, with the plethora of solar and stellar flare data from\nnew and future satellites. These recommendations address the key pitfalls in\nQPP detection, including detrending, trimming data, accounting for colored\nnoise, detecting stationary-period QPPs, detecting QPPs with nonstationary\nperiods, and ensuring that detections are robust and false detections are\nminimized. We find that QPPs can be detected reliably and robustly by a variety\nof methods, which are clearly identified and described, if the appropriate care\nand due diligence are taken.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Lyman-alpha Emission in Solar Flares. I. a Statistical Study on Its\n  Relationship with the 1--8 \u00c5 Soft X-ray Emission: We statistically study the relationship between the Lyman-alpha (\\lya) and\n1--8 \\AA\\ soft X-ray (SXR) emissions from 658 M- and X-class solar flares\nobserved by the {\\em Geostationary Operational Environmental Satellite} during\n2006--2016. Based on the peak times of the two waveband emissions, we divide\nthe flares into three types. Type I (III) has an earlier (a later) peak time in\nthe \\lya\\ emission than that in the SXR emission, while type II has nearly a\nsame peak time (within the time resolution of 10 s) between the \\lya\\ and SXR\nemissions. In these 658 flares, we find that there are 505 (76.8\\%) type I\nflares, 10 (1.5\\%) type II flares, and 143 (21.7\\%) type III flares, and that\nthe three types appear to have no dependence on the flare duration, flare\nlocation, or solar cycle. Besides the main peak, the \\lya\\ emission of the\nthree type flares also shows sub-peaks which can appear in the impulsive or\ngradual phase of the flare. It is found that the main-peak (for type I) and\nsub-peak (for type III) emissions of \\lya\\ that appear in the impulsive phase\nfollow the Neupert effect in general. This indicates that such \\lya\\ emissions\nare related to the nonthermal electron beam heating. While the main-peak (for\ntype III) and sub-peak (for type I) emissions of \\lya\\ that appear in the\ngradual phase are supposed to be primarily contributed by the thermal plasma\nthat cools down.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Coronal Hole and Solar Global Magnetic Field Evolution in 1976-2012: Coronal hole spatial-temporal evolution is studied and comparison made with\nthat of the solar global magnetic field in cycles 21-23 (1976-2012). The\nlatitude-longitude distribution dynamics of coronal holes and the regularities\nin the global magnetic field associated with the solar polar field reversal are\nanalyzed. Polar and non-polar coronal hole populations are considered. The\ninvestigation reveals some temporal and spatial regularities in coronal hole\ndistributions that match well the global magnetic-field cycle evolution. The\nresults show that the non-polar coronal hole longitudinal distribution follows\nall configuration changes in the global magnetic-field structure.\nReorganizations of the global magnetic-field and coronal hole distributions\noccur simultaneously during a time interval of a few solar rotations. The cycle\nevolution of the non-polar coronal holes reflects the transition of the solar\nglobal magnetic field from the zonal structure to sectorial andvice versa. Two\ndifferent type waves of non-polar coronal holes are revealed from their\nlatitudinal distribution. The first one is short poleward waves. They trace the\npoleward motion of the unipolar photospheric magnetic fields from approximately\n35 deg to the associated pole in each hemisphere and the redevelopment of a\nnew-polarity polar CH. Although they start the poleward movement before the\nchange of the polar magnetic field in the associated hemisphere, they reach the\npole after the polar reversal. The other type of non-polar CH wave forms two\nsinusoidal branches associated with the positive- and negative-polarity\nmagnetic fields. The complete period of the wave was equal to approximately 268\nCRs (22 years). These wave CHs arrive at high latitudes during declining phases\nwhen the new polarity polar CHs are already completely formed.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Absolute dimensions of detached eclipsing binaries. II. The\n  metallic-lined system XY Ceti: We present phase-resolved spectroscopy and extensive survey photometry of the\ndetached eclipsing binary system XY Cet, which is composed of two\nmetallic-lined stars. We measure their masses to be 1.773 +/- 0.016 and 1.615\n+/- 0.014 Msun and their radii to be 1.873 +/- 0.035 and 1.773 +/- 0.029 Rsun,\nresulting in logarithmic surface gravities of 4.142 +/- 0.016 and 4.149 +/-\n0.014 (cgs). We determine effective temperatures of 7870 +/- 115 and 7620 +/-\n125 K. The projected rotational velocities are 34.4 +/- 0.4 and 34.1 +/- 0.4\nkm/s, which are close to synchronous. Theoretical models cannot match all of\nthese properties, but come closest for a solar helium and metal abundance and\nan age in the region of 850 Myr. We obtain the individual spectra of the two\nstars by the spectral disentangling method, and compare them to synthetic\nspectra calculated for the measured effective temperatures and a solar chemical\ncomposition. Both stars show enhanced abundances of iron-group elements and\nclear deficiencies of Ca I and Sc II, confirming their classification as Am\nstars. We also find strong overabundances of Zr II and the rare-earth species\nLa II, Ce II and Nd II, a hallmark of chemically peculiar A-stars. XY Cet is a\nprime candidate for detailed spectroscopic analyses of metallic-lined stars\nwhose masses and radii are known to accuracies of 1-2%.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Disk Evolution in OB Associations - Deep Spitzer/IRAC Observations of IC\n  1795: We present a deep Spitzer/IRAC survey of the OB association IC 1795 carried\nout to investigate the evolution of protoplanetary disks in regions of massive\nstar formation. Combining Spitzer/IRAC data with Chandra/ACIS observations, we\nfind 289 cluster members. An additional 340 sources with an infrared excess,\nbut without X-ray counterpart, are classified as cluster member candidates.\nBoth surveys are complete down to stellar masses of about 1 Msun. We present\npre-main sequence isochrones computed for the first time in the Spitzer/IRAC\ncolors. The age of the cluster, determined via the location of the Class III\nsources in the [3.6]-[4.5]/[3.6] color-magnitude diagram, is in the range of 3\n- 5 Myr. As theoretically expected, we do not find any systematic variation in\nthe spatial distribution of disks within 0.6 pc of either O-type star in the\nassociation. However, the disk fraction in IC 1795 does depend on the stellar\nmass: sources with masses >2 Msun have a disk fraction of ~20%, while lower\nmass objects (2-0.8 Msun) have a disk fraction of ~50%. This implies that disks\naround massive stars have a shorter dissipation timescale.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The Intricate Structure of HH 508, the Brightest Microjet in the Orion\n  Nebula: We present Magellan adaptive optics H$\\alpha$ imaging of HH 508, which has\nthe highest surface brightness among protostellar jets in the Orion Nebula. We\nfind that HH 508 actually has a shorter component to the west, and a longer and\nknotty component to the east. The east component has a kink at 0.3\" from the\njet-driving star $\\theta^1$ Ori B2, so it may have been deflected by the\nwind/radiation from the nearby $\\theta^1$ Ori B1B5. The origin of both\ncomponents is unclear, but if each of them is a separate jet, then $\\theta^1$\nOri B2 may be a tight binary. Alternatively, HH 508 may be a slow-moving\noutflow, and each component represents an illuminated cavity wall. The\nionization front surrounding $\\theta^1$ Ori B2B3 does not directly face\n$\\theta^1$ Ori B1B5, suggesting that the EUV radiation from $\\theta^1$ Ori C\nplays a dominant role in affecting the morphology of proplyds even in the\nvicinity of $\\theta^1$ Ori B1B5. Finally, we report an H$\\alpha$ blob that\nmight be ejected by the binary proplyd LV 1.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Dynamics of fast and slow magnetoacoustic waves in plasma slabs with\n  thermal misbalance: Non-uniformity of the solar atmosphere along with the presence of\nnon-adiabatic processes such as radiation cooling and unspecified heating can\nsignificantly affect the dynamics and properties of magnetoacoustic (MA) waves.\nTo address the co-influence of these factors on the dispersion properties of MA\nwaves, we considered a single magnetic slab composed of the thermally active\nplasma. Using the perturbation theory, we obtained a differential equation that\ndetermines the dynamics of the two-dimensional perturbations. Applying the\nassumption of strong magnetic structuring, we derived the dispersion relations\nfor the sausage and kink MA modes. The numerical solution of the dispersion\nrelations for the coronal conditions was performed to investigate the interplay\nbetween the non-uniformity and the thermal misbalance. For the heating scenario\nconsidered, it was obtained that the phase speed of both the sausage and kink\nslow MA waves is highly affected by the thermal misbalance in the long\nwavelength limit. The obtained characteristic timescales of the slow waves\ndissipation coincide with the periods of waves observed in the corona.\nSimultaneously, the phase speed of the fast waves is not affected by the\nthermal misbalance. The geometry of the magnetic structure still remains the\nmain dispersion mechanism for the fast waves. Our estimation reveals that\ndissipation of the fast waves is weaker than dissipation of the slow waves in\nthe coronal conditions. The obtained results are of importance for using the\nmagnetoacoustic waves not only as a tool for estimating plasma parameters, but\nalso as a tool for estimating the non-adiabatic processes.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Forward seismic modelling of the pulsating magnetic B-type star HD 43317: The large-scale magnetic fields detected at the surface of about 10% of hot\nstars extend into the stellar interior, where they may alter the structure.\nDeep inner regions of stars are only observable using asteroseismology. Here,\nwe investigated the pulsating magnetic B3.5V star HD43317, inferred its\ninterior properties and assessed whether the dipolar magnetic field with a\nsurface strength of $B_p = 1312 \\pm 332$G caused different properties compared\nto those of non-magnetic stars. We analysed the latest version of the star's\n150d CoRoT light curve and extracted 35 significant frequencies, 28 of which\nwere determined to be independent and not related to the known surface rotation\nperiod of $P_{\\rm rot} = 0.897673$d. We performed forward seismic modelling\nbased on non-magnetic, non-rotating 1D MESA models and the adiabatic module of\nthe pulsation code GYRE, utilizing a grid-based approach. Our aim was to\nestimate the stellar mass, age, and convective core overshooting. The GYRE\ncalculations were done for uniform rotation with $P_{\\rm rot}$. This modelling\nwas able to explain 16 of the 28 frequencies as gravity modes belonging to\nretrograde modes with $(\\ell, m) = (1, -1)$ and $(2, -1)$ period spacing\npatterns and one distinct prograde $(2,2)$ mode. The modelling resulted in a\nstellar mass $M_{\\star} = 5.8^{+0.1}_{-0.2}$$\\mathrm{M_{\\odot}}$, a central\nhydrogen mass fraction $X_c = 0.54^{+0.01}_{-0.02}$, and exponential convective\ncore overshooting parameter $f_{\\rm ov} = 0.004^{+0.014}_{-0.002}$. The low\nvalue for $f_{\\rm ov}$ is compatible with the suppression of near-core mixing\ndue to a magnetic field but the uncertainties are too large to pinpoint such\nsuppression as the sole physical interpretation. $[...]$",
        "category": "astro-ph_SR"
    },
    {
        "text": "The magnetic field of the double-lined spectroscopic binary system HD\n  5550: (Abridged) In the framework of the BinaMicS project, we have begun a study of\nthe magnetic properties of a sample of intermediate-mass and massive\nshort-period binary systems, as a function of binarity properties. We report in\nthis paper the characterisation of the magnetic field of HD 5550, a\ndouble-lined spectroscopic binary system of intermediate-mass, using\nhigh-resolution spectropolarimetric Narval observations of HD 5550. We first\nfit the intensity spectra using Zeeman/ATLAS9 LTE synthetic spectra to estimate\nthe effective temperatures, microturbulent velocities, and the abundances of\nsome elements of both components, as well as the light-ratio of the system. We\nthen fit the least-square deconvolved $I$ profiles to determine the radial and\nprojected rotational velocities of both stars. We then analysed the shape and\nevolution of the LSD $V$ profiles using the oblique rotator model to\ncharacterise the magnetic fields of both stars.\n  We confirm the Ap nature of the primary, previously reported in the\nliterature, and find that the secondary displays spectral characteristics\ntypical of an Am star. While a magnetic field is clearly detected in the lines\nof the primary, no magnetic field is detected in the secondary, in any of our\nobservation. If a dipolar field were present at the surface of the Am star, its\npolar strength must be below 40 G. The faint variability observed in the Stokes\n$V$ profiles of the Ap star allowed us to propose a rotation period of\n$6.84_{-0.39}^{+0.61}$ d, close to the orbital period ($\\sim$6.82 d),\nsuggesting that the star is synchronised with its orbit. By fitting the\nvariability of the $V$ profiles, we propose that the Ap component hosts a\ndipolar field inclined with the rotation axis at an angle $\\beta=156\\pm17$\n$^{\\circ}$ and a polar strength $B_{\\rm d}=65 \\pm 20$ G. The field strength is\nthe weakest known for an Ap star.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Evolving waves and turbulence in the outer corona and inner heliosphere:\n  the accelerating expanding box: Alfv\\'enic fluctuations in the solar wind display many properties reflecting\nan ongoing nonlinear cascade, e.g. a well-defined spectrum in frequency,\ntogether with some characteristics more commonly associated with the linear\npropagation of waves from the Sun, such as the variation of fluctuation\namplitude with distance, dominated by solar wind expansion effects. Therefore\nboth nonlinearities and expansion must be included simultaneously in any\nsuccessful model of solar wind turbulence evolution. Because of the disparate\nspatial scales involved, direct numerical simulations of turbulence in the\nsolar wind represent an arduous task, especially if one wants to go beyond the\nincompressible approximation. Indeed, most simulations neglect solar wind\nexpansion effects entirely. Here we develop a numerical model to simulate\nturbulent fluctuations from the outer corona to 1 AU and beyond, including the\nsub-Alfv\\'enic corona. The accelerating expanding box (AEB) extends the\nvalidity of previous expanding box models by taking into account both the\nacceleration of the solar wind and the inhomogeneity of background density and\nmagnetic field. Our method incorporates a background accelerating wind within a\nmagnetic field that naturally follows the Parker spiral evolution using a\ntwo-scale analysis in which the macroscopic spatial effect coupling\nfluctuations with background gradients becomes a time-dependent coupling term\nin a homogeneous box. In this paper we describe the AEB model in detail and\ndiscuss its main properties, illustrating its validity by studying Alfv\\'en\nwave propagation across the Alfv\\'en critical point.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Time delay between outer heliosheath crossing and observation of\n  interstellar neutral atoms: In situ measurements of the heliospheric particle populations by the Voyager\nspacecraft can only be put in an appropriate context with remote-sensing\nobservations of energetic and interstellar neutral atoms (ENAs and ISN,\nrespectively) at 1 au when the time delay between the production and the\nobservation times is taken into account. ENA times of flight from the\nproduction regions in the heliosheath are relatively easy to estimate because\nthese atoms follow almost constant speed, force-free trajectories. For the ISN\npopulations, dynamical and ballistic selection effects are important, and times\nof flight are much longer. We estimate these times for ISN He and H atoms\nobserved by IBEX and in the future by IMAP using the WTPM model with synthesis\nmethod. We show that for the primary population atoms, the times of flight are\non the order of three solar cycle periods, with a spread equivalent to one\nsolar cycle. For the secondary populations, the times of flight are on the\norder of ten solar cycle periods, and during the past ten years of\nobservations, IBEX has been collecting secondary He atoms produced in the OHS\nduring almost entire 19th century. ISN atoms penetrating the heliopause at the\ntime of Voyager crossing will become gradually visible about 2027, during the\nplanned IMAP observations. Hypothetical variations in the ISN flow in the Local\nInterstellar Medium are currently not detectable. Nevertheless, we expect\nsteady-state heliosphere models used with appropriately averaged solar wind\nparameters to be suitable for understanding the ISN observations.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Explaining the Early Exit of Eta Carinae from its 2009 X-Ray Minimum\n  with the Accretion Model: We use the accretion model to explain the early exit of Eta Carinae from its\n2009 X-ray minimum. In the accretion model the secondary star accretes mass\nfrom the primary wind near periastron passage, a process that suppresses the\nsecondary wind. As the shocked secondary wind is responsible for most of the\nX-ray emission, the accretion process accounts for the X-ray minimum. The early\nexit from the 2009 X-ray minimum after four weeks, instead of ten weeks as in\nthe two previous minima, is attributed to the primary wind that during the last\nminimum was somewhat faster and of lower mass loss rate than during the two\nprevious X-ray minima. This results in a much lower mass accretion rate during\nthe X-ray minimum. We show that using fluctuations in these quantities that are\nwithin the range deduced from fluctuations in the X-ray flux outside the\nminimum, can account for the short duration of the last X-ray minimum. The\nshorter X-ray minimum may have further implications on the recovery of the\nsystem from the spectroscopic event.",
        "category": "astro-ph_SR"
    },
    {
        "text": "The VMC Survey - XIV. First results on the look-back time star-formation\n  rate tomography of the Small Magellanic Cloud: We analyse deep images from the VISTA survey of the Magellanic Clouds in the\nYJKs filters, covering 14 sqrdeg (10 tiles), split into 120 subregions, and\ncomprising the main body and Wing of the Small Magellanic Cloud (SMC). We apply\na colour--magnitude diagram reconstruction method that returns their\nbest-fitting star formation rate SFR(t), age-metallicity relation (AMR),\ndistance and mean reddening, together with 68% confidence intervals. The\ndistance data can be approximated by a plane tilted in the East-West direction\nwith a mean inclination of 39 deg, although deviations of up to 3 kpc suggest a\ndistorted and warped disk. After assigning to every observed star a probability\nof belonging to a given age-metallicity interval, we build high-resolution\npopulation maps. These dramatically reveal the flocculent nature of the young\nstar-forming regions and the nearly smooth features traced by older stellar\ngenerations. They document the formation of the SMC Wing at ages <0.2 Gyr and\nthe peak of star formation in the SMC Bar at 40 Myr. We clearly detect periods\nof enhanced star formation at 1.5 Gyr and 5 Gyr. The former is possibly related\nto a new feature found in the AMR, which suggests ingestion of metal-poor gas\nat ages slightly larger than 1 Gyr. The latter constitutes a major period of\nstellar mass formation. We confirm that the SFR(t) was moderately low at even\nolder ages.",
        "category": "astro-ph_SR"
    },
    {
        "text": "Destination exoplanet: Habitability conditions influenced bystellar\n  winds properties: The cumulative effect of the magnetized stellar winds on exoplanets dominates\nover other forms of star-planet interactions. When combined with\nphotoevaporation, these winds will lead to atmospheric erosion. This is\ndirectly connected with the concept of Habitable Zone (HZ) planets around\nlate-type stars. Our knowledge of these magnetized winds is limited, making\nnumerical models useful tools to explore them. In this preliminary study, we\nfocus on solar-like stars exploring how different stellar wind properties scale\nwith one another. We used one of the most detailed physics-based models, the 3D\nAlfv\\'en Wave Solar Model part of the Space Weather ModelingFramework, and\napplied it to the stellar winds domain. Our simulations showed that the\nmagnetic field topology on the star surface plays a fundamental role in shaping\nthe different stellar wind properties (wind speed, mass loss rate, angular\nmomentum loss rate). We conclude that a characterization of the Alfv\\'en\nsurface is crucial when studying star-planet interaction as it can serve as an\ninner-boundary of the HZ",
        "category": "astro-ph_SR"
    },
    {
        "text": "Convective scale and subadiabatic layers in simulations of rotating\n  compressible convection: (abridged) Context: Rotation is thought to influence the size of convective\neddies and the efficiency of convective energy transport in the deep convection\nzones of stars. Rotationally constrained convection has been invoked to explain\nthe lack of large-scale power in observations of solar flows. Aims: The main\naims are to quantify the effects of rotation on the scale of convective eddies\nand velocity, the depths of convective overshoot, and the subadiabatic\nDeardorff layers. Methods: Three-dimensional hydrodynamic simulations of\nrotating convection in Cartesian domains were run. The results were compared\nwith theoretical scaling results that assume a balance between Coriolis,\ninertial, and buoyancy (Archemedean) forces (CIA balance). Results: The scale\nof convective eddies decreases as rotation increases, and ultimately reaches a\nrotationally constrained regime consistent with the CIA balance. Using a new\nmeasure of the rotational influence on the system, it is shown that even the\ndeep parts of the solar convection zone are not in the rotationally constrained\nregime. The simulations capture the slowly and rapidly rotating scaling laws\npredicted by theory, and the Sun appears to be in between these two regimes.\nBoth, the overshooting depth and the extent of the Deardorff layer, decrease as\nrotation becomes more rapid. For sufficiently rapid rotation the Deardorff\nlayer is absent. Conclusions: Relating the simulations with the Sun suggests\nthat the convective scale even in the deep parts of the Sun is only mildly\naffected by rotation and that some other mechanism is needed to explain the\nlack of strong large-scale flows in the Sun. Taking the current results at face\nvalue, the overshoot and Deardorff layers are estimated to span roughly five\nper cent of the pressure scale height at the base of the convection zone in the\nSun.",
        "category": "astro-ph_SR"
    }
]