adsabs/SciX_UAT_keywords huggingface dataset:
Updated Photometry of the Yellow Supergiant Progenitor and Late-time Observations of the Type IIb Supernova SN 2016gkg

We present Hubble Space Telescope (HST) observations of the Type IIb supernova (SN) SN 2016gkg at 652, 1698, and 1795 days from explosion with the Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3). Comparing to pre-explosion imaging from 2001 obtained with the Wide Field Planetary Camera 2, we demonstrate that SN 2016gkg is now fainter than its candidate counterpart in the latest WFC3 imaging, implying that the counterpart has disappeared and confirming that it was the SN progenitor star. We show the latest light curve and Keck spectroscopy of SN 2016gkg, which imply that SN 2016gkg is declining more slowly than the expected rate for 56Co decay during its nebular phase. We find that this emission is too luminous to be powered by other radioisotopes and infer that SN 2016gkg is entering a new phase in its evolution where it is powered primarily by interaction with circumstellar matter. Finally, we reanalyze the progenitor star spectral energy distribution and late-time limits in the context of binary evolution models. Including emission from a potential companion star, we find that all such predicted companion stars would be fainter than our limiting magnitudes.

Keywords: core-collapse supernovae; early stellar evolution
Thermophysical Model for Realistic Surface Layers on Airless Small Bodies: Applied to Study the Spin Orientation and Surface Dust Properties of (24) Themis from WISE/NEOWISE Multiepoch Thermal Light Curves

This work proposes a thermophysical model for realistic surface layers on airless small bodies (RSTPM) for the use of interpreting their multiepoch thermal light curves (e.g., WISE/NEOWISE). RSTPM considers the real orbital cycle, rotation cycle, rough surface, temperature-dependent thermal parameters, as well as contributions of sunlight reflection to observations. It is thus able to produce a precise temperature distribution and thermal emission of airless small bodies regarding the variations on orbital timescales. Details of the physics, mathematics, and numerical algorithms of RSTPM are presented. When used to interpret multiepoch thermal light curves by WISE/NEOWISE, RSTPM can give constraints on the spin orientation and surface physical properties, such as the mean thermal inertia or the mean size of dust grains, the roughness fraction, and the albedo via a radiometric procedure. As an application example, we apply this model to the main-belt object (24) Themis, the largest object of the Themis family, which is believed to be the source region of many main-belt comets. We find multiepoch (2010, 2014-2018) observations of Themis by WISE/NEOWISE, yielding 18 thermal light curves. By fitting these data with RSTPM, the best-fit spin orientation of Themis is derived to be (λ = 137°, β = 59°) in ecliptic coordinates, and the mean radius of dust grains on the surface is estimated to be $\tilde{b}=\,{140}_{-114}^{+500}(6\sim 640)$ μm, indicating that the surface thermal inertia varies from ∼3 Jm-2 s-0.5 K-1 to ∼60 Jm-2s-0.5 K-1 due to seasonal temperature variation. A more detailed analysis found that the thermal light curves of Themis show a weak feature that depends on the rotation phase, which is indicative of heterogeneous thermal properties or imperfections of the light-curve inversion shape model.

Keywords: infrared photometry; small solar system bodies; asteroid surfaces; computational methods
The RS CVn-type Star GT Mus Shows Most Energetic X-Ray Flares Throughout the 2010s

We report that the RS CVn-type star GT Mus (HR 4492, HD 101379+HD 101380) was the most active star in the X-ray sky in the last decade in terms of the scale of recurrent energetic flares. We detected 11 flares from GT Mus in 8 yr of observations with the Monitor of All-sky X-ray Image (MAXI) from 2009 August to 2017 August. The detected flare peak luminosities were 1-4 × 1033 erg s-1 in the 2.0-20.0 keV band for its distance of 109.6 pc. Our timing analysis showed long durations (τr + τd) of 2-6 days with long decay times (τd) of 1-4 days. The released energies during the decay phases of the flares in the 0.1-100 keV band were in the range of 1-11 × 1038 erg, which are at the upper end of the observed stellar flare. The released energies during the whole duration were in the range of 2-13 × 1038 erg in the same band. We carried out X-ray follow-up observations for one of the 11 flares with the Neutron star Interior Composition Explorer (NICER) on 2017 July 18 and found that the flare cooled quasi-statically. On the basis of a quasi-static cooling model, the flare loop length is derived to be 4 × 1012 cm (or 60 R⊙). The electron density is derived to be 1 × 1010 cm-3, which is consistent with the typical value of solar and stellar flares (1010-13 cm-3). The ratio of the cooling timescales between radiative (τrad) and conductive (τcond) cooling is estimated to be τrad ∼ 0.1 τcond from the temperature; thus, radiative cooling was dominant in this flare.

Keywords: stellar x-ray flares; stellar activity; time series analysis
A Comprehensive X-Ray Report on AT2019wey

Here, we present MAXI, Swift, NICER, NuSTAR, and Chandra observations of the X-ray transient AT2019wey (SRGA J043520.9+552226, SRGE J043523.3+552234). From spectral and timing analyses we classify it as a Galactic low-mass X-ray binary (LMXB) with a black hole (BH) or neutron star (NS) accretor. AT2019wey stayed in the low/hard state (LHS) from 2019 December to 2020 August 21, and the hard-intermediate state (HIMS) from 2020 August 21 to 2020 November. For the first six months of the LHS, AT2019wey had a flux of ~1 mCrab, and displayed a power-law X-ray spectrum with photon index Γ = 1.8. From 2020 June to August, it brightened to ~20 mCrab. Spectral features characteristic of relativistic reflection became prominent. On 2020 August 21, the source left the "hard line" on the rms-intensity diagram, and transitioned from LHS to HIMS. The thermal disk component became comparable to the power-law component. A low-frequency quasi-periodic oscillation (QPO) was observed. The QPO central frequency increased as the spectrum softened. No evidence of pulsation was detected. We are not able to decisively determine the nature of the accretor (BH or NS). However, the BH option is favored by the position of this source on the Γ-LX, Lradio-LX, and Lopt-LX diagrams. We find the BH candidate XTE J1752-223 to be an analog of AT2019wey. Both systems display outbursts with long plateau phases in the hard states. We conclude by noting the potential of SRG in finding new members of this emerging class of low luminosity and long-duration LMXB outbursts.

Keywords: accretion; black holes; low-mass x-ray binary stars; x-ray transient sources; transient sources; x-ray sources; x-ray binary stars
Accelerated Galaxy Growth and Environmental Quenching in a Protocluster at z = 3.24

We present a multiwavelength study of galaxies around D4UD01, a spectroscopically confirmed protocluster at z = 3.24, to investigate environmental trends. 450 galaxies are selected based on KS band detection with photometric redshifts (photo-z) at 3.0 < z < 3.4, among which ∼12% are classified as quiescent galaxies. The quiescent galaxies are among the most massive and reddest ones in the entire sample. We identify a large photo-z galaxy overdensity in the field, which lies close to the previously spectroscopically confirmed sources of the protocluster. We find that the quiescent galaxies are largely concentrated in the overdense protocluster region with a higher quiescent fraction, showing a sign of environmental quenching. Galaxies in the protocluster are forming faster than their field counterparts as seen in the stellar mass function, suggesting early and accelerated mass assembly in the overdense regions. Although weak evidence of suppressed star formation is found in the protocluster, the statistics are not significant enough to draw a definite conclusion. Our work sheds light on how the formation of massive galaxies is affected in the dense region of a protocluster when the universe was only 2 Gyr old.

Keywords: protoclusters
Dynamical Masses of 20 Asteroids Determined with Gaia DR3 Asteroid Observations

We combined the ground-based and Gaia Data Release 3 (DR3) asteroid observations to determine the masses of 20 asteroids with asteroid-asteroid close encounters. In order to take full advantage of the high-precision observations from Gaia, we use the Fisher information to select appropriate model parameters and the modified Encke's equation of motion to construct a dynamical model complete at the level of observation precision. With diameters from literature, bulk densities of 20 asteroids are derived. The results indicate that the utilization of Gaia DR3 provides substantial benefits in terms of improving mass precision. Among the 20 asteroids analyzed in our study, we find that 10 asteroids achieved a mass precision better than 5%, and 15 asteroids better than 10%.

Keywords: asteroids; astrometry; celestial mechanics; close encounters
Probing the Earliest Phases in the Formation of Massive Galaxies with Simulated HST+JWST Imaging Data from Illustris

We use the Illustris-1 simulation to explore the capabilities of the Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) data to analyze the stellar populations in high-redshift galaxies, taking advantage of the combined depth, spatial resolution, and wavelength coverage. For that purpose, we use simulated broadband ACS, WFC3, and NIRCam data and two-dimensional stellar population synthesis (2D-SPS) to derive the integrated star formation history (SFH) of massive (M * > 1010 M ⊙) simulated galaxies at 1 < z < 4 that evolve into a local M * > 1011 M ⊙ galaxy. In particular, we explore the potential of HST and JWST data sets reaching a depth similar to those of the CANDELS and ongoing CEERS observations, respectively, and concentrate on determining the capabilities of this data set for characterizing the first episodes in the SFH of local M * > 1011 M ⊙ galaxies by studying their progenitors at z > 1. The 2D-SPS method presented in this paper has been calibrated to robustly recover the cosmic times when the first star formation episodes occurred in massive galaxies, i.e., the first stages in their integrated SFHs. In particular, we discuss the times when the first 1%-50% of their total stellar mass formed in the simulation. We demonstrate that we can recover these ages with typical median systematic offset of less than 5% and scatter around 20%-30%. According to our measurements on Illustris data, we are able to recover that local M * > 1011 M ⊙ galaxies would have started their formation by z = 16, forming the first 5% of their stellar mass present at z ~ 1 by z = 4.5, 10% by z = 3.7, and 25% by z = 2.7.

Keywords: astronomy data analysis; galaxy formation; galaxy evolution; high-redshift galaxies; stellar populations; broad band photometry; galaxy ages; james webb space telescope
Variability as a Predictor for the Hard-to-soft State Transition in GX 339-4

During the outbursts of black hole X-ray binaries (BHXRBs), their accretion flows transition through several states. The source luminosity rises in the hard state, dominated by nonthermal emission, before transitioning to the blackbody-dominated soft state. As the luminosity decreases, the source transitions back into the hard state and fades to quiescence. This picture does not always hold, as ≈40% of the outbursts never leave the hard state. Identifying the physics that govern state transitions remains one of the outstanding open questions in black hole astrophysics. In this paper we present an analysis of archival RXTE data of multiple outbursts of GX 339-4. We compare the properties of the X-ray variability and time-averaged energy spectrum and demonstrate that the variability (quantified by the power spectral hue) systematically evolves ≈10-40 days ahead of the canonical state transition (quantified by a change in spectral hardness); no such evolution is found in hard-state-only outbursts. This indicates that the X-ray variability can be used to predict if and when the hard-to-soft state transition will occur. Finally, we find a similar behavior in 10 outbursts of four additional BHXRBs with more sparse observational coverage. Based on these findings, we suggest that state transitions in BHXRBs might be driven by a change in the turbulence in the outer regions of the disk, leading to a dramatic change in variability. This change is only seen in the spectrum days to weeks later, as the fluctuations propagate inwards toward the corona.

Keywords: stellar mass black holes; relativistic binary stars; accretion
Unresolved Binary Exoplanet Host Stars Fit as Single Stars: Effects on the Stellar Parameters

In this work, we quantify the effect of an unresolved companion star on the derived stellar parameters of the primary star if a blended spectrum is fit assuming the star is single. Fitting tools that determine stellar parameters from spectra typically fit for a single star, but we know that up to half of all exoplanet host stars may have one or more companion stars. We use high-resolution spectra of planet host stars in the Kepler field from the California-Kepler Survey to create simulated binaries; we select eight stellar pairs and vary the contribution of the secondary star, then determine stellar parameters with SpecMatch-Emp and compare them to the parameters derived for the primary star alone. We find that, in most cases, the effective temperature, surface gravity, metallicity, and stellar radius derived from the composite spectrum are within 2-3σ of the values determined from the unblended spectrum, but the deviations depend on the properties of the two stars. Relatively bright companion stars that are similar to the primary star have the largest effect on the derived parameters; in these cases, the stellar radii can be overestimated by up to 60%. We find that metallicities are generally underestimated, with values up to eight times smaller than the typical uncertainty in [Fe/H]. Our study shows that follow-up observations are necessary to detect or set limits on stellar companions of planetary host stars so that stellar (and planet) parameters are as accurate as possible.

Keywords: stellar spectral lines; stellar properties; fundamental parameters of stars; high resolution spectroscopy; binary stars; planet hosting stars
The Bluest Changing-Look QSO SDSS J224113-012108

In this manuscript, we report a new changing-look QSO (CLQSO) SDSS J2241 at z = 0.059. Based on the multi-epoch Sloan Digital Sky Survey spectra from 2011 to 2017, the flux ratio of broad Hα to broad Hβ has changed from 7 in 2011 to 2.7 in 2017, leading SDSS J2241 with a spectral index αλ ~ -5.21 ± 0.02 (λ < 4000) in 2017 to be, so far, the bluest CLQSO. Based on the SDSS spectrum in 2011, the host galaxy contribution with a stellar velocity dispersion of ~86 km s-1 can be well determined, leading to the M-sigma relation expected central black hole (BH) mass of ~3 × 106 M⊙. However, based on properties of the broad Hα emission, the virial BH mass is ~108 M⊙, about two magnitudes larger than the mass found through the M-sigma relation. The different BH masses found using different methods indicate that SDSS J2241 is one unique CLQSO. The long-term photometric light curve shows interesting variability that is not expected assuming the Damped Random Walk process commonly applied to active galactic nuclei but probably connected to a central tidal disruption event. Furthermore, based on continuum emission properties in 2017 with no dust obscuration, the moving dust clouds scenario alone cannot be preferred to explain the CLQSO SDSS J2241, because the expected intrinsic reddening-corrected continuum emission was unreasonably higher than the unobscured continuum emission in 2017.

Keywords: active galactic nuclei; active galaxies; quasars; emission line galaxies
Astrometry as a Tool for Discovering and Weighing Faint Companions to Nearby Stars

This tutorial covers the use of absolute astrometry, in particular from the combination of the Hipparcos and Gaia missions, to identify faint companions to nearby stars and to measure the masses and orbits of those companions. Absolute astrometry has been used with increasing success to discover new planets and brown dwarfs and to measure masses and orbits for systems with periods as long as centuries. This tutorial summarizes the nature of the underlying astrometric data, the approach typically used to fit orbits, and the assumptions about that data implicit throughout the process. It attempts to provide intuition for the sensitivity of astrometry as a function of stellar and companion properties and how the available constraints depend on the character and quantity of data available. This tutorial is written for someone with some background in astronomy but with no more than a minimal acquaintance with astrometry or orbit fitting.

Keywords: astrometry; orbit determination; exoplanet detection methods; brown dwarfs
Modules for Experiments in Stellar Astrophysics (MESA): Time-dependent Convection, Energy Conservation, Automatic Differentiation, and Infrastructure

We update the capabilities of the open-knowledge software instrument Modules for Experiments in Stellar Astrophysics (MESA). The new auto_diff module implements automatic differentiation in MESA, an enabling capability that alleviates the need for hard-coded analytic expressions or finite-difference approximations. We significantly enhance the treatment of the growth and decay of convection in MESA with a new model for time-dependent convection, which is particularly important during late-stage nuclear burning in massive stars and electron-degenerate ignition events. We strengthen MESA's implementation of the equation of state, and we quantify continued improvements to energy accounting and solver accuracy through a discussion of different energy equation features and enhancements. To improve the modeling of stars in MESA, we describe key updates to the treatment of stellar atmospheres, molecular opacities, Compton opacities, conductive opacities, element diffusion coefficients, and nuclear reaction rates. We introduce treatments of starspots, an important consideration for low-mass stars, and modifications for superadiabatic convection in radiation-dominated regions. We describe new approaches for increasing the efficiency of calculating monochromatic opacities and radiative levitation, and for increasing the efficiency of evolving the late stages of massive stars with a new operator-split nuclear burning mode. We close by discussing major updates to MESA's software infrastructure that enhance source code development and community engagement.

Keywords: stellar physics; stellar evolution; stellar evolutionary models; computational methods
The Physical Drivers and Observational Tracers of CO-to-H2 Conversion Factor Variations in Nearby Barred Galaxy Centers

The CO-to-H2 conversion factor (α CO) is central to measuring the amount and properties of molecular gas. It is known to vary with environmental conditions, and previous studies have revealed lower α CO in the centers of some barred galaxies on kiloparsec scales. To unveil the physical drivers of such variations, we obtained Atacama Large Millimeter/submillimeter Array bands (3), (6), and (7) observations toward the inner ~2 kpc of NGC 3627 and NGC 4321 tracing 12CO, 13CO, and C18O lines on ~100 pc scales. Our multiline modeling and Bayesian likelihood analysis of these data sets reveal variations of molecular gas density, temperature, optical depth, and velocity dispersion, which are among the key drivers of α CO. The central 300 pc nuclei in both galaxies show strong enhancement of temperature T k ≳ 100 K and density ${n}_{{{\rm{H}}}_{2}}\gt {10}^{3}$ cm-3. Assuming a CO-to-H2 abundance of 3 × 10-4, we derive 4-15 times lower α CO than the Galactic value across our maps, which agrees well with previous kiloparsec-scale measurements. Combining the results with our previous work on NGC 3351, we find a strong correlation of α CO with low-J 12CO optical depths (τ CO), as well as an anticorrelation with T k. The τ CO correlation explains most of the α CO variation in the three galaxy centers, whereas changes in T k influence α CO to second order. Overall, the observed line width and 12CO/13CO 2-1 line ratio correlate with τ CO variation in these centers, and thus they are useful observational indicators for α CO variation. We also test current simulation-based α CO prescriptions and find a systematic overprediction, which likely originates from the mismatch of gas conditions between our data and the simulations.

Keywords: barred spiral galaxies; co line emission; galaxy nuclei; molecular gas; star forming regions
A Model of Homologous Confined and Ejective Eruptions Involving Kink Instability and Flux Cancellation

In this study, we model a sequence of a confined and a full eruption, employing the relaxed end state of the confined eruption of a kink-unstable flux rope as the initial condition for the ejective one. The full eruption, a model of a coronal mass ejection, develops as a result of converging motions imposed at the photospheric boundary, which drive flux cancellation. In this process, parts of the positive and negative external flux converge toward the polarity inversion line, reconnect, and cancel each other. Flux of the same amount as the canceled flux transfers to a flux rope, increasing the free magnetic energy of the coronal field. With sustained flux cancellation and the associated progressive weakening of the magnetic tension of the overlying flux, we find that a flux reduction of ≈11% initiates the torus instability of the flux rope, which leads to a full eruption. These results demonstrate that a homologous full eruption, following a confined one, can be driven by flux cancellation.

Keywords: active solar corona; solar coronal mass ejections; solar flares; solar magnetic fields; solar magnetic reconnection
Earth-based Stellar Occultation Predictions for Jupiter, Saturn, Uranus, Neptune, Titan, and Triton: 2023-2050

In support of studies of decadal-timescale evolution of outer solar system atmospheres and ring systems, we present detailed Earth-based stellar occultation predictions for Jupiter, Saturn, Uranus, Neptune, Titan, and Triton for 2023-2050, based on the Gaia Data Release 3 star catalog and near-IR K-band photometry from the Two Micron All Sky Survey catalog. We tabulate the number of observable events by year and magnitude interval, reflecting the highly variable frequency of high-signal-to-noise ratio (S/N) events depending on the target's path relative to the star-rich regions of the Milky Way. We identify regions on Earth where each event is potentially observable, and for atmospheric occultations we determine the latitude of the ingress and egress events. For Saturn, Uranus, and Neptune, we also compute the predicted ring occultation event times. We present representative subsets of the predicted events and highlight particularly promising events. Jupiter occultations with K ≤ 7 occur at a cadence of about one per year, with bright events at higher frequency in 2031 and 2043. Saturn occultations are much rarer, with only two predicted events with K ≤ 5 in 2032 and 2047. Ten Uranus ring occultations are predicted with K ≤ 10 for the period 2023-2050. Neptune traverses star-poor regions of the sky until 2068, resulting in only 13 predicted occultations for K ≤ 12 between 2023 and 2050. Titan has several high-S/N events between 2029 and 2031, whereas Triton is limited to a total of 22 occultations with K ≤ 15 between 2023 and 2050. Details of all predicted events are included in the Supplementary Online Material.

Keywords: planetary rings; jupiter; saturn; uranus; neptune; titan; triton