Sazuna/UAT_keywords huggingface dataset:
Cooling Models for the Most Massive White Dwarfs. We present a set of ultramassive white dwarf models, focused on masses above 1.3 M⊙. Given the uncertainties about the formation and compositions of such objects, we construct parameterized model sequences, guided by evolutionary calculations including both single star and double white dwarf merger formation channels. We demonstrate that the cooling of objects with central densities in excess of 109 g cm-3 is dominated by neutrino cooling via the Urca process in the first ≈100 Myr after formation. Our models indicate that the recently discovered ultramassive white dwarf ZTF J190132.9+145808.7 is likely to have experienced this Urca-dominated cooling regime. We also show that the high densities imply that diffusion is unlikely to significantly alter the core compositions of these objects before they crystallize.

Keywords: White dwarf stars, Stellar evolutionary models, Stellar mergers
On Stochastic Heating and Its Phase-space Signatures in Low-beta Kinetic Turbulence. We revisit the theory of stochastic heating of ions and investigate its phase-space signatures in kinetic turbulence of relevance to low-β portions of the solar wind. In particular, we retain a full scale-dependent approach in our treatment, and we explicitly consider the case in which electric-field fluctuations can be described by a generalized Ohm's law that includes Hall and thermoelectric effects. These two electric-field terms provide the dominant contributions to stochastic ion heating when the ion-Larmor scale is much smaller than the ion skin depth, ρi ≪ di, which is the case at β ≪ 1. Employing well-known spectral scaling laws for Alfvén-wave and kinetic-Alfvén-wave turbulent fluctuations, we obtain scaling relations characterizing the field-perpendicular particle-energization rate and energy diffusion coefficient associated with stochastic heating in these two regimes. Phase-space signatures of ion heating are then investigated using three-dimensional hybrid-kinetic simulations of continuously driven Alfvénic turbulence at low β (namely, βi = βe = 0.3 and βi = βe = 1/9). In these simulations, energization of ions parallel to the magnetic field is subdominant compared to its perpendicular counterpart (Q∥,i ≪ Q⊥,i), and the fraction of turbulent energy that goes into ion heating is ≈75% at βi = 0.3 and ≈40% at βi ≃ 0.1. The phase-space signatures of ion energization are consistent with Landau-resonant collisionless damping and a (β-dependent) combination of ion-cyclotron and stochastic heating. We demonstrate good agreement between our scale-dependent theory and various signatures associated with the stochastic portion of the heating. We discuss briefly the effect of intermittency on stochastic heating and the implications of our work for the interpretation of stochastic heating in solar-wind spacecraft data.

Keywords: Space plasmas, Solar wind, Plasma astrophysics, Plasma physics
A Preponderance of Perpendicular Planets. Observing the Rossiter-McLaughlin effect during a planetary transit allows the determination of the angle λ between the sky projections of the star's spin axis and the planet's orbital axis. Such observations have revealed a large population of well-aligned systems and a smaller population of misaligned systems, with values of λ ranging up to 180°. For a subset of 57 systems, we can now go beyond the sky projection and determine the 3D obliquity ψ by combining the Rossiter-McLaughlin data with constraints on the line-of-sight inclination of the spin axis. Here we show that the misaligned systems do not span the full range of obliquities; they show a preference for nearly perpendicular orbits (ψ = 80°-125°) that seems unlikely to be a statistical fluke. If confirmed by further observations, this pile-up of polar orbits is a clue about the unknown processes of obliquity excitation and evolution.

Keywords: Exoplanet dynamics, Exoplanet astronomy, Exoplanet tides, Planet hosting stars, Stellar rotation
A Circumplanetary Disk around PDS70c. PDS 70 is a unique system in which two protoplanets, PDS 70 b and c, have been discovered within the dust-depleted cavity of their disk, at ~22 and 34 au, respectively, by direct imaging at infrared wavelengths. Subsequent detection of the planets in the Hα line indicates that they are still accreting material through circumplanetary disks. In this Letter, we present new Atacama Large Millimeter/submillimeter Array (ALMA) observations of the dust continuum emission at 855 μm at high angular resolution (~20 mas, 2.3 au) that aim to resolve the circumplanetary disks and constrain their dust masses. Our observations confirm the presence of a compact source of emission co-located with PDS 70 c, spatially separated from the circumstellar disk and less extended than ~1.2 au in radius, a value close to the expected truncation radius of the circumplanetary disk at a third of the Hill radius. The emission around PDS 70 c has a peak intensity of ~86 ± 16 μJy beam-1, which corresponds to a dust mass of ~0.031 M⊕ or ~0.007 M⊕, assuming that it is only constituted of 1 μm or 1 mm sized grains, respectively. We also detect extended, low surface brightness continuum emission within the cavity near PDS 70 b. We observe an optically thin inner disk within 18 au of the star with an emission that could result from small micron-sized grains transported from the outer disk through the orbits of b and c. In addition, we find that the outer disk resolves into a narrow and bright ring with a faint inner shoulder.

Keywords: Protoplanetary disks
From Neutrino- to Photon-cooled in Three Years: Can Fallback Accretion Explain the X-Ray Excess in GW170817?. Excess X-ray emission from the neutron star merger GW170817 above the predicted afterglow was recently detected t ≈ 3.4 yr post-merger. One possible origin is accretion onto the newly unshrouded black hole (BH) remnant. While fallback of bound dynamical ejecta is insufficient to generate the excess luminosity, LX ~ 5 × 1038 erg s-1, fallback from the disk wind ejecta-due to their larger mass and lower velocity-remains a possibility. We present hydrodynamic α-viscosity simulations of the post-merger disk evolution that extend to timescales t ≈ 35 s post-merger, necessary to capture the asymptotic evolution into the radiatively inefficient regime. Due to inefficient neutrino cooling, the BH accretion rate decays rapidly at late times (${\dot{M}}_{\mathrm{bh}}\propto {t}^{-{\beta }_{\mathrm{bh}}}$, where βbh ≈ 2.4-2.8), which is incompatible with the late-time excess. However, matter falls back to the inner disk from the equatorial region more gradually, ${\dot{M}}_{\mathrm{fb}}\propto {t}^{-{\beta }_{\mathrm{fb}}}$ with βfb ≈ 1.43 in our α ≈ 0.03 simulations. By the present epoch t ≈ 3.4 yr, the fallback rate has become sub-Eddington and the disk can again accrete efficiently, i.e., ${\dot{M}}_{\mathrm{bh}}\approx {\dot{M}}_{\mathrm{fb}}$, this time due to photon instead of neutrino cooling. The predicted present-day X-ray accretion luminosity, ${L}_{{\rm{X}}}\approx 0.1{\dot{M}}_{\mathrm{bh}}{c}^{2}\approx (2\mbox{--}70)\times {10}^{38}$ erg s-1 for βfb ≈ 1.43-1.66, thus supports (with caveats) an accretion-powered origin for the X-ray excess in GW170817. The suppressed BH accretion rate prior to the sub-Eddington transition, weeks to months after the merger, is key to avoid overproducing the kilonova luminosity via reprocessing.

Keywords: Accretion, Neutron stars, X-ray transient sources, Compact radiation sources
Electron Heat Flux Instabilities in the Inner Heliosphere: Radial Distribution and Implication on the Evolution of the Electron Velocity Distribution Function. This Letter investigates the electron heat flux instability using the radial models of the magnetic field and plasma parameters in the inner heliosphere. Our results show that both the electron acoustic wave and the oblique whistler wave are unstable in the regime with large relative drift speed (ΔVe) between electron beam and core populations. Landau-resonant interactions of electron acoustic waves increase the electron parallel temperature that would lead to suppressing the electron acoustic instability and amplifying the growth of oblique whistler waves. Therefore, we propose that the electron heat flux can effectively drive oblique whistler waves in an anisotropic electron velocity distribution function. This study also finds that lower-hybrid waves and oblique Alfvén waves can be triggered in the solar atmosphere, and that the former instability is much stronger than the latter. Moreover, we clarify that the excitation of lower-hybrid waves mainly results from the transit-time interaction of beaming electrons with resonant velocities v∥ ~ ω/k∥, where ω and k∥ are the wave frequency and parallel wavenumber, respectively. In addition, this study shows that the instability of quasi-parallel whistler waves can dominate the regime with medium ΔVe at the heliocentric distance nearly larger than 10 times of the solar radius.

Keywords: Space plasmas, Solar wind, Plasma astrophysics
Binary-stripped Stars as Core-collapse Supernovae Progenitors. Most massive stars experience binary interactions in their lifetimes that can alter both the surface and core structure of the stripped star with significant effects on their ultimate fate as core-collapse supernovae. However, core-collapse supernovae simulations to date have focused almost exclusively on the evolution of single stars. We present a systematic simulation study of single and binary-stripped stars with the same initial mass as candidates for core-collapse supernovae (11-21 M⊙). Generally, we find that binary-stripped stars core tend to have a smaller compactness parameter, with a more prominent, deeper silicon/oxygen interface, and explode preferentially to the corresponding single stars of the same initial mass. Such a dichotomy of behavior between these two modes of evolution would have important implications for supernovae statistics, including the final neutron star masses, explosion energies, and nucleosynthetic yields. Binary-stripped remnants are also well poised to populate the possible mass gap between the heaviest neutron stars and the lightest black holes. Our work presents an improvement along two fronts, as we self-consistently account for the pre-collapse stellar evolution and the subsequent explosion outcome. Even so, our results emphasize the need for more detailed stellar evolutionary models to capture the sensitive nature of explosion outcome.

Keywords: Close binary stars, Core-collapse supernovae
Discovery of Two TNO-like Bodies in the Asteroid Belt. Two extremely red main-belt asteroids, 203 Pompeja and 269 Justitia, were identified from combined visible and near-infrared spectroscopic observations collected at the Infra-red Telescope Facility (IRTF) and Seoul National University Astronomical Observatory (SAO). These two asteroids have a redder spectral slope than any other D-type body, which are the reddest objects in the asteroid belt, and similar to RR and IR-class objects found in the outer Solar System among trans-Neptunian objects (TNOs) and Centaurs. Spectroscopic results suggest the presence of complex organic materials on the surface layer of these asteroids, implying that they could have formed in the vicinity of Neptune and been transplanted to the main belt region during a phase of planetary migration. 203 Pompeja is the only very red asteroid known so far among the ~250 bodies with diameter larger than 110 km (i.e., presumably structurally intact) found in the asteroid belt. These discoveries provide more evidence that the main asteroid belt hosts a population of bodies that were formed in the outskirts of the Solar System.

Keywords: Asteroids, Solar system formation, Small Solar System bodies, Trans-Neptunian objects
Fermi/GBM Observations of the SGRJ1935+2154 Burst Forest. During 2020 April and May, SGR J1935+2154 emitted hundreds of short bursts and became one of the most prolific transient magnetars. At the onset of the active bursting period, a 130 s burst "forest," which included some bursts with peculiar time profiles, were observed with the Fermi/Gamma-ray Burst Monitor (GBM). In this Letter, we present the results of time-resolved spectral analysis of this burst "forest" episode, which occurred on 2020 April 27. We identify thermal spectral components prevalent during the entire 130 s episode; high-energy maxima appear during the photon flux peaks, which are modulated by the spin period of the source. Moreover, the evolution of the νFν spectral hardness (represented by Epeak or blackbody temperature) within the lightcurve peaks is anti-correlated with the pulse phases extrapolated from the pulsation observed within the persistent soft X-ray emission of the source six hours later. Throughout the episode, the emitting area of the high-energy (hotter) component is 1-2 orders of magnitude smaller than that for the low-energy component. We interpret this with a geometrical viewing angle scenario, inferring that the high-energy component likely originates from a low-altitude hotspot located within closed toroidal magnetic field lines.

Keywords: Magnetars, Soft gamma-ray repeaters, X-ray bursters
Observational Consequences of Shallow-water Magnetohydrodynamics on Hot Jupiters. We use results of shallow-water magnetohydrodynamics to place estimates on the minimum magnetic field strengths required to cause atmospheric wind variations (and therefore westward-venturing hotspots) for a data set of hot Jupiters (HJs), including HAT-P-7b, CoRoT-2b, Kepler-76, WASP-12b, and WASP-33b, on which westward hotspots have been observationally inferred. For HAT-P-7b and CoRoT-2b our estimates agree with past results; for Kepler-76b we find that the critical dipolar magnetic field strength, over which the observed wind variations can be explained by magnetism, lies between 4 G and 19 G; for WASP-12b and WASP-33b westward hotspots can be explained by 1 G and 2 G dipolar fields, respectively. Additionally, to guide future observational missions, we identify 61 further HJs that are likely to exhibit magnetically driven atmospheric wind variations and predict these variations are highly likely in ~40 of the hottest HJs.

Keywords: Exoplanet dynamics, Magnetohydrodynamics, Hot Jupiters, Exoplanet atmospheric variability, Astrophysical fluid dynamics, Exoplanet atmospheres
Can Stochastic Resonance Explain Recurrence of Grand Minima?. The amplitude of the 11 yr solar cycle is well known to be subject to long-term modulation, including sustained periods of very low activity known as Grand Minima. Stable long-period cycles found in proxies of solar activity have given new momentum to the debate about a possible influence of the tiny planetary tidal forcing. Here, we study the solar cycle by means of a simple zero-dimensional dynamo model, which includes a delay caused by meridional circulation as well as a quenching of the α-effect at toroidal magnetic fields exceeding an upper threshold. Fitting this model to the sunspot record, we find a set of parameters close to the bifurcation point at which two stable oscillatory modes emerge. One mode is a limit cycle resembling normal solar activity including a characteristic kink in the decaying limb of the cycle. The other mode is a weak sub-threshold cycle that could be interpreted as Grand Minimum activity. Adding noise to the model, we show that it exhibits Stochastic Resonance, which means that a weak external modulation can toss the dynamo back and forth between these two modes, whereby the periodicities of the modulation get strongly amplified.

Keywords: Solar cycle, Solar dynamo
Fast Magnetic Wave Could Heat the Solar Low-beta Chromosphere. Magnetohydrodynamic (MHD) waves are candidates for heating the solar chromosphere, although it is still unclear which mode of the wave is dominant in heating. We perform two-dimensional radiative MHD simulation to investigate the propagation of MHD waves in the quiet region of the solar chromosphere. We identify the mode of the shock waves by using the relationship between gas pressure and magnetic pressure across the shock front and calculate their corresponding heating rate through the entropy jump to obtain a quantitative understanding of the wave-heating process in the chromosphere. Our result shows that the fast magnetic wave is significant in heating the low-beta chromosphere. The low-beta fast magnetic waves are generated from high-beta fast acoustic waves via mode conversion crossing the equipartition layer. Efficient mode conversion is achieved by large attacking angles between the propagation direction of the shock waves and the chromospheric magnetic field.

Keywords: Solar chromospheric heating, Solar chromosphere
The Second Discovery from the COCONUTS Program: A Cold Wide-orbit Exoplanet around a Young Field M Dwarf at 10.9 pc. We present the identification of the second discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) program, the COCONUTS-2 system, composed of the M3 dwarf L 34-26 and the T9 dwarf WISEPA J075108.79-763449.6. Given their common proper motions and parallaxes, these two field objects constitute a physically bound pair with a projected separation of 594″ (6471au). The primary star COCONUTS-2A has strong stellar activity (Hα, X-ray, and ultraviolet emission) and is rapidly rotating (Prot = 2.83 days), from which we estimate an age of 150-800 Myr. Comparing equatorial rotational velocity derived from the Transiting Exoplanet Survey Satellite (TESS) light curve to spectroscopic vsini, we find that COCONUTS-2A has a nearly edge-on inclination. The wide exoplanet COCONUTS-2b has an effective temperature of Teff = 434 ± 9 K, a surface gravity of $\mathrm{log}g={4.11}_{-0.18}^{+0.11}$ dex, and a mass of M = ${6.3}_{-1.9}^{+1.5}$ MJup based on hot-start evolutionary models, leading to a mass ratio of ${0.016}_{-0.005}^{+0.004}$ for the COCONUTS-2 system. COCONUTS-2b is the second coldest (after WD 0806-661B), and the second widest (after TYC 9486-927-1 b) exoplanet imaged to date. Comparison of COCONUTS-2b's infrared photometry with ultracool model atmospheres suggests the presence of both condensate clouds and non-equilibrium chemistry in its photosphere. Similar to 51 Eri b, COCONUTS-2b has a sufficiently low luminosity ($\mathrm{log}({L}_{\mathrm{bol}}/{L}_{\odot })=-6.384\pm 0.028$ dex) to be consistent with the cold-start process that may form gas-giant (exo)planets, though its large separation means that such formation would not have occurred in situ. Finally, at a distance of 10.9 pc, COCONUTS-2b is the nearest imaged exoplanet to Earth known to date.

Keywords: Extrasolar gaseous giant planets, Exoplanets, T dwarfs
The Changing Lightcurve of the Double-mode RR Lyrae Variable Star V338 Boo. We present an analysis of the lightcurve extracted from Transiting Exoplanet Survey Satellite (TESS) Full Frame Images (FFIs) of the double-mode RR Lyrae V338 Boo. We find that the fundamental mode pulsation is changing in amplitude across the 54 days of observations. The first overtone mode pulsation also changes, but on a much smaller scale. Harmonics and combinations of the primary pulsation modes also exhibit unusual behavior. Possible connections with other changes in RR Lyrae pulsations are discussed, but a full understanding of the cause of the changes seen in V338 Boo should shed light on some of the most difficult and unanswered questions in stellar pulsation theory, and astrophysics more generally.

Keywords: Pulsating variable stars, Light curves, RRd variable stars, RR Lyrae variable stars, Space telescopes
Fallback Accretion Model for the Years-to-decades X-Ray Counterpart to GW170817. A new component was reported in the X-ray counterpart to the binary neutron star merger and gravitational-wave event GW170817, exceeding the afterglow emission from an off-axis structured jet. The afterglow emission from the kilonova/macronova ejecta may explain the X-ray excess but exceeds the radio observations if the spectrum is the same. We propose a fallback accretion model that a part of ejecta from the neutron star merger falls back and forms a disk around the central compact object. In the super-Eddington accretion phase, the X-ray luminosity stays near the Eddington limit of a few solar masses and the radio is weak, as observed. This will be followed by a power-law decay. The duration of the constant luminosity phase conveys the initial fallback timescale t0 in the past. The current multiyear duration requires t0 &gt; 3-30 s, suggesting that the disk wind rather than the dynamical ejecta falls back after the jet launch. Future observations in the next decades will probe the timescale of t0 ~ 10-104 s, around the time of extended emission in short gamma-ray bursts. The fallback accretion has not been halted by the r-process heating, implying that fission is weak on the year scale. We predict that the X-ray counterpart will disappear in a few decades due to the r-process halting or the depletion of fallback matter.

Keywords: Accretion, Gamma-ray bursts, Hydrodynamics, Nucleosynthesis, R-process, Stellar accretion disks, Astrophysical black holes
Probabilistic Reconstruction of Type Ia Supernova SN 2002bo. Manual fits to spectral times series of Type Ia supernovae have provided a method of reconstructing the explosion from a parametric model but due to lack of information about model uncertainties or parameter degeneracies direct comparison between theory and observation is difficult. In order to mitigate this important problem we present a new way to probabilistically reconstruct the outer ejecta of the normal Type Ia supernova SN 2002bo. A single epoch spectrum, taken 10 days before maximum light, is fit by a 13-parameter model describing the elemental composition of the ejecta and the explosion physics (density, temperature, velocity, and explosion epoch). Model evaluation is performed through the application of a novel rapid spectral synthesis technique in which the radiative transfer code, TARDIS, is accelerated by a machine-learning framework. Analysis of the posterior distribution reveals a complex and degenerate parameter space and allows direct comparison to various hydrodynamic models. Our analysis favors detonation over deflagration scenarios and we find that our technique offers a novel way to compare simulation to observation.

Keywords: Bayesian statistics, Type Ia supernovae, Supernovae, Radiative transfer