adsabs/SciX_UAT_keywords huggingface dataset:
Stellar Revival and Repeated Flares in Deeply Plunging Tidal Disruption Events

Tidal disruption events with tidal radius r t and pericenter distance r p are characterized by the quantity β = r t/r p, and "deep encounters" have β ≫ 1. It has been assumed that there is a critical β ≡ β c ~ 1 that differentiates between partial and full disruption: for β < β c a fraction of the star survives the tidal interaction with the black hole, while for β > β c the star is completely destroyed, and hence all deep encounters should be full. Here we show that this assumption is incorrect by providing an example of a β = 16 encounter between a γ = 5/3, solar-like polytrope and a 106 M ⊙ black hole-for which previous investigations have found β c ≃ 0.9-that results in the reformation of a stellar core post-disruption that comprises approximately 25% of the original stellar mass. We propose that the core reforms under self-gravity, which remains important because of the compression of the gas both near pericenter, where the compression occurs out of the orbital plane, and substantially after pericenter, where compression is within the plane. We find that the core forms on a bound orbit about the black hole, and we discuss the corresponding implications of our findings in the context of recently observed, repeating nuclear transients.

Keywords: astrophysical black holes; black hole physics; hydrodynamical simulations; supermassive black holes; tidal disruption
Medium-resolution 0.97–5.3 μm Spectra of Very Young Benchmark Brown Dwarfs with NIRSpec on Board the James Webb Space Telescope

Spectra of young benchmark brown dwarfs with well-known ages are vital to characterize other brown dwarfs, for which ages are in general not known. These spectra are also crucial to test atmospheric models, which have the potential to provide detailed information about the atmospheres of these objects. However, to test atmospheric models optimally, medium-resolution, long-wavelength-coverage spectra with well-understood uncertainties are ideal, such as the spectra provided by the NIRSpec instrument on board the James Webb Space Telescope. In this paper, we present medium-resolution JWST/NIRSpec spectra of two young brown dwarfs, TWA 28 (M9.0) and TWA 27A (M9.0), and one planetary-mass object, TWA 27B (L6.0), members of the TW Hydrae Association (age ∼ 10 Myr). We show the richness of the atomic lines and molecular bands present in the spectra. All objects show signs of a circumstellar disk, via a near-infrared excess and/or via emission lines. We matched a set of cloudless atmospheric spectra (ATMO), and cloudy atmospheric spectra (BT-Settl) to our NIRSpec spectra, and analyzed which wavelength ranges and spectral features both models reproduce best. Both models derive consistent parameters for the three sources, and predict the existence of CH4 at 3.35 μm in TWA 27B. Nonetheless, in contrast to other slightly older objects with similar spectral type, like PSO 318.5–22 and VHS 1256b, this feature is not present in the spectrum of TWA 27B. The lack of the CH4 feature might suggest that the L–T transition of very young dwarfs starts at later spectral types than for older brown dwarfs.

Keywords: l dwarfs; m dwarf stars; spectroscopy
Polarization Leakage and the IXPE Point-spread Function

By measuring photoelectron tracks, the gas pixel detectors of the Imaging X-ray Polarimetry Explorer satellite provide estimates of the photon detection location and its electric vector position angle (EVPA). However, imperfections in reconstructing event positions blur the image, and EVPA-position correlations result in artificial polarized halos around bright sources. We introduce a new model describing this "polarization leakage" and use it to recover the on-orbit telescope point-spread functions, useful for faint-source detection and image reconstruction. These point-spread functions are more accurate than previous approximations or ground-calibrated products (Δχ 2 ≈ 3 × 104 and 4 × 104 respectively for a bright 106-count source). We also define an algorithm for polarization leakage correction substantially more accurate than existing prescriptions (Δχ 2 ≈ 1 × 103). These corrections depend on the reconstruction method, and we supply prescriptions for the mission-standard "Moments" methods as well as for "Neural Net" event reconstruction. Finally, we present a method to isolate leakage contributions to polarization observations of extended sources and show that an accurate PSF allows the extraction of sub-PSF-scale polarization patterns.

Keywords: pulsar wind nebulae; x-ray astronomy; polarimetry; open source software; x-ray detectors
Hierarchical Bayesian Thermonuclear Rate for the 7Be(n,p)7Li Big Bang Nucleosynthesis Reaction

Big Bang nucleosynthesis provides the earliest probe of standard model physics, at a time when the universe was less than 1000 seconds old. It determines the abundances of the lightest nuclides, which give rise to the subsequent history of the visible matter in the universe. This work derives new 7Be(n,p)7Li thermonuclear reaction rates based on all available experimental information. This reaction sensitively impacts the primordial abundances of 7Be and 7Li during big bang nucleosynthesis. We critically evaluate all available data and disregard experimental results that are questionable. For the nuclear model, we adopt an incoherent sum of single-level, two-channel, R-matrix approximation expressions, which are implemented into a hierarchical Bayesian model, to analyze the remaining six data sets we deem most reliable. In the fitting of the data, we consistently model all known sources of uncertainty, including discrepant absolute normalizations of different data sets, and also take the variation of the neutron and proton channel radii into account, hence providing less biased estimates of the 7Be(n,p)7Li thermonuclear rates. From the resulting posteriors, we extract R-matrix parameters (Er, ${\gamma }_{n}^{2}$ , ${\gamma }_{p}^{2}$) and derive excitation energies and partial and total widths. Our fit is sensitive to the contributions of the first three levels above the neutron threshold. Reaction rates were computed by integrating 10,000 samples of the reduced cross section. Our 7Be(n,p)7Li thermonuclear rates have uncertainties between 1.5% and 2.0% at temperatures of ≤1 GK. We compare our rates to previous results and find that the 7Be(n,p)7Li rates most commonly used in big bang simulations have uncertainties that are too optimistic.

Keywords: big bang nucleosynthesis; nuclear astrophysics
Io's Optical Aurorae in Jupiter's Shadow

Decline and recovery timescales surrounding eclipse are indicative of the controlling physical processes in Io's atmosphere. Recent studies have established that the majority of Io's molecular atmosphere, SO2 and SO, condenses during its passage through Jupiter's shadow. The eclipse response of Io's atomic atmosphere is less certain, having been characterized solely by ultraviolet aurorae. Here we explore the response of optical aurorae for the first time. We find oxygen to be indifferent to the changing illumination, with [O I] brightness merely tracking the plasma density at Io's position in the torus. In shadow, line ratios confirm sparse SO2 coverage relative to O, since their collisions would otherwise quench the emission. Io's sodium aurora mostly disappears in eclipse and e-folding timescales, for decline and recovery differ sharply: ~10 minutes at ingress and nearly 2 hr at egress. Only ion chemistry can produce such a disparity; Io's molecular ionosphere is weaker at egress due to rapid recombination. Interruption of a NaCl+ photochemical pathway best explains Na behavior surrounding eclipse, implying that the role of electron impact ionization is minor relative to photons. Auroral emission is also evident from potassium, confirming K as the major source of far red emissions seen with spacecraft imaging at Jupiter. In all cases, direct electron impact on atomic gas is sufficient to explain the brightness without invoking significant dissociative excitation of molecules. Surprisingly, the nonresponse of O and rapid depletion of Na is opposite the temporal behavior of their SO2 and NaCl parent molecules during Io's eclipse phase.

Keywords: galilean satellites; eclipses; high resolution spectroscopy; aurorae; planetary magnetospheres
Stochastic and Quasi-adiabatic Electron Heating in Quasi-parallel Shocks

Using Magnetospheric Multiscale observations at the Earth's quasi-parallel bow shock we demonstrate that electrons are heated by two different mechanisms: a quasi-adiabatic heating process during magnetic field compression, characterized by the isotropic temperature relation $T/B={({T}_{0}/{B}_{0})({B}_{0}/B)}^{\alpha }$ with α = 2/3 when the electron heating function $| {\chi }_{e}| \lt 1$ , and a stochastic heating process when $| {\chi }_{e}| \gt 1$ . Both processes are controlled by the value of the stochastic heating function ${\chi }_{j}={m}_{j}{q}_{j}^{-1}{B}^{-2}\mathrm{div}({{\boldsymbol{E}}}_{\perp })$ for particles with mass mj and charge qj in the electric and magnetic fields ${\boldsymbol{E}}$ and ${\boldsymbol{B}}$ . Test-particle simulations are used to show that the stochastic electron heating and acceleration in the studied shock are accomplished by waves at frequencies (0.4-5) fce (electron gyrofrequency) for bulk heating, and waves $f\gt 5\,{f}_{{ce}}$ for acceleration of the tail of the distribution function. Stochastic heating can give rise to flat-top electron distribution functions, frequently observed near shocks. It is also shown that obliquely polarized electric fields of electron cyclotron drift and ion acoustic instabilities scatter the electrons into the parallel direction and keep the isotropy of the electron distribution. The results reported in this paper may be relevant to electron heating and acceleration at interplanetary shocks and other astrophysical shocks.

Keywords: planetary bow shocks
NASA's Double Asteroid Redirection Test (DART): Mutual Orbital Period Change Due to Reshaping in the Near-Earth Binary Asteroid System (65803) Didymos

The Double Asteroid Redirection Test (DART) is the first planetary defense mission to demonstrate the kinetic deflection technique. The DART spacecraft will collide with the asteroid Dimorphos, the smaller component of the binary asteroid system (65803) Didymos. The DART impact will excavate surface/subsurface materials of Dimorphos, leading to the formation of a crater and/or some magnitude of reshaping (i.e., shape change without significant mass loss). The ejecta may eventually hit Didymos's surface. If the kinetic energy delivered to the surface is high enough, reshaping may also occur in Didymos, given its near-critical spin rate. Reshaping on either body will modify the mutual gravitational field, leading to a reshaping-induced orbital period change, in addition to the impact-induced orbital period change. If left unaccounted for, this could lead to an erroneous interpretation of the effect of the kinetic deflection technique. Here we report the results of full two-body problem simulations that explore how reshaping influences the mutual dynamics. In general, we find that the orbital period becomes shorter linearly with increasing reshaping magnitude. If Didymos's shortest axis shrinks by ~0.7 m, or Dimorphos's intermediate axis shrinks by ~2 m, the orbital period change would be comparable to the Earth-based observation accuracy, ~7.3 s. Constraining the reshaping magnitude will decouple the reshaping- and impact-induced orbital period changes; Didymos's reshaping may be constrained by observing its spin period change, while Dimorphos's reshaping will likely be difficult to constrain but will be investigated by the ESA's Hera mission that will visit Didymos in late 2026.

Keywords: asteroid satellites; near-earth objects; asteroid dynamics; asteroid rotation; two-body problem; gravitational interaction; planetary structure; planetary science
A Massive Quiescent Galaxy in a Group Environment at z = 4.53

We report on the spectroscopic confirmation of a massive quiescent galaxy at z spec = 4.53 in the COSMOS field. The object was first identified as a galaxy with suppressed star formation at z phot ∼ 4.65 from the COSMOS2020 catalog. The follow-up spectroscopy with Keck/MOSFIRE in the K band reveals faint [O II] emission and the Balmer break, indicative of evolved stellar populations. We fit the spectral energy distribution using photometry and a spectrum to infer physical properties. The obtained stellar mass is high (M * ∼ 1010.8 M ⊙) and the current star formation rate is more than 1 dex below that of main-sequence galaxies at z = 4.5. Its star formation history suggests that this galaxy experienced rapid quenching from z ∼ 5. The galaxy is among the youngest quiescent galaxies confirmed so far at z spec > 3 with z form ∼ 5.2 (200 Myr ago), which is the epoch when 50% of the total stellar mass was formed. A unique aspect of the galaxy is that it is in an extremely dense region; there are four massive star-forming galaxies at 4.4 < z phot < 4.7 located within 150 physical kpc from the galaxy. Interestingly, three of them have virial radii that strongly overlap with that of the central quiescent galaxy (∼70 kpc), suggesting that the overdensity region is likely the highest-redshift candidate of a dense group with a spectroscopically confirmed quiescent galaxy at the center. The group provides us with a unique opportunity to gain insights into the role of the group environment in quenching at z ∼ 5, which corresponds to the formation epoch of massive elliptical galaxies in the local Universe.

Keywords: galaxy evolution; high-redshift galaxies; galaxy quenching; quenched galaxies; galaxy groups; galaxy environments
Stellar X-Ray Activity Across the Hertzsprung-Russell Diagram. I. Catalogs

Stellar magnetic activity provides substantial information on the magnetic dynamo and the coronal heating process. We present a catalog of X-ray activity for about 6000 stars, based on the Chandra and Gaia DR2 data. We also classified more than 3000 stars as young stellar objects, dwarf stars, or giant stars. By using the stars with valid stellar parameters and classifications, we studied the distribution of X-ray luminosity (LX) and the ratio of X-ray-to-bolometric luminosities (RX), the positive relation between LX, RX, and hardness ratio, and the long-term X-ray variation. This catalog can be used to investigate some important scientific topics, including the activity-rotation relation, the comparison between different activity indicators, and the activities of interesting objects (e.g., A-type stars and giants). As an example, we use the catalog to study the activity-rotation relation, and find that the young stellar objects, dwarfs, and giants fall on a single sequence in the relation RX versus Rossby number, while the giants do not follow the relation RX versus ${P}_{\mathrm{rot}}^{-2}{R}^{-4}$ valid for dwarfs.

Keywords: late-type stars; stellar activity; x-ray stars; young stellar objects; giant stars
The Candidate Progenitor Companion Star of the Type Ib/c SN 2013ge

This Letter presents the detection of a source at the position of the Type Ib/c supernova (SN) 2013ge more than four years after the radioactive component is expected to have faded. This source could mark the first post-SN direct detection of a surviving companion to a stripped-envelope Type Ib/c explosion. We test this hypothesis and find the shape of the source's spectral energy distribution is most consistent with that of a B5 I supergiant. While binary models tend to predict OB-type stars for stripped-envelope companions, the location of the source on a color-magnitude diagram places it redward of its more likely position on the main sequence (MS). The source may be temporarily out of thermal equilibrium, or a cool and inflated non-MS companion, which is similar to the suggested companion of Type Ib SN 2019yvr that was constrained from pre-SN imaging. We also consider other possible physical scenarios for the source, including a fading SN, circumstellar shock interaction, line-of-sight coincidence, and an unresolved host star cluster, all of which will require future observations to more definitively rule out. Ultimately, the fraction of surviving companions ("binary fraction") will provide necessary constraints on binary evolution models and the underlying physics.

Keywords: type ic supernovae; type ib supernovae; core-collapse supernovae; close binary stars; massive stars
Starspot Mapping with Adaptive Parallel Tempering. II. Application to TESS Data for M-dwarf Flare Stars AU Microscopii, YZ Canis Minoris, and EV Lacertae

Starspots and stellar flares are indicators of stellar magnetic activity. The magnetic energy stored around spots is thought to be the origin of flares, but the connection is not completely understood. To investigate the relation between spot locations deduced from light curves and the occurrence of flares therein, we perform starspot modeling for the TESS light curves of three M-dwarf flare stars, AU Mic, YZ CMi, and EV Lac, using the code implemented in Paper I. The code enables us to deduce multiple stellar/spot parameters by the adaptive parallel tempering algorithm efficiently. We find that flare occurrence frequency is not necessarily correlated with the rotation phases of the light curve for each star. The result of starspot modeling shows that any spot is always visible to the line of sight in all phases, and we suggest that this can be one of the reasons why there is no or low correlation between rotation phases and flare frequency. In addition, the amplitude and shape of the light curve for AU Mic and YZ CMi have varied in two years between different TESS cycles. The result of starspot modeling suggests that this can be explained by the variations of spot size and latitude.

Keywords: starspots; stellar flares; m dwarf stars; markov chain monte carlo; importance sampling; model selection; astrostatistics
Accurate OH Maser Positions from the SPLASH Survey. III. The Final 96 deg2

We present high spatial resolution observations of ground-state OH masers achieved with the Australia Telescope Compact Array (ATCA). These observations targeted 253 pointing centers containing OH maser candidates at all four ground-state OH transitions identified in the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH) across 96 deg2 of the southern Galactic plane (332° < l < 334° and -2° < b < +2°, 344° < l < 355° and -2° < b < +2°, 358° < l < 4° and +2° < b < +6°, 5° < l < 10° and -2° < b < +2°). We detect maser emission toward 236 fields and suggest that 7 out of 17 nondetections are due to the slightly lower sensitivity of the ATCA observations, combined with some temporal variability. The superior resolution provided by the ATCA data has allowed us to identify 362 OH maser sites in the 236 target fields. Almost half (160 of 362) of these masers have been detected for the first time. Comparison between these 362 maser sites and information presented in the literature allowed us to categorize 238 sites as evolved star sites (66%), 63 as star formation (17%), 8 as supernova remnants, and 53 as unknown maser sites (15%). We present an analysis of the OH masers across the full SPLASH survey range (176 deg2) and find that the detection rate of 1.7 GHz radio continuum sources (18%) is lower than that previously found at 8.2 and 9.2 GHz (38%). We also find that the velocity separations of evolved star sites with symmetric 1612 MHz maser profiles are generally smaller than those with asymmetric profiles.

Keywords: catalogs; interstellar molecules; circumstellar masers; interstellar masers; star formation; asymptotic giant branch stars; post-asymptotic giant branch
Uncovering the Primary X-Ray Emission and Possible Starburst Component in the Polarized NLS1 Mrk 1239

X-ray observations of the unique NLS1 galaxy Mrk 1239 spanning 18 yr are presented. Data from XMM-Newton, Suzaku, Swift, and NuSTAR are combined to obtain a broadband, multi-epoch view of the source. There is spectral variability in the 3-10 keV band over the 18 yr. An analysis of the NuSTAR and Suzaku lightcurves also suggests rapid variability in the 3-10 keV band, which is consistent with the NLS1 definition of the source. However, no variability is seen below 3 keV on any timescale. Two distinct physical models are adopted to describe the data above and below $\sim 3\,\mathrm{keV}$ . The low energies are dominated by a hot, diffuse gas likely associated with a starburst component at large physical scales. The higher-energy spectrum is dominated by emission from the central region. Ionized partial covering and relativistic blurred reflection are considered for the central region emission. In both cases, the underlying power law has a photon index of Γ ∼ 2.3-2.4. A distant reflector, a neutral partial covering component with a covering fraction near ∼1, and contributions from starburst emission are always required. The blurred reflection model requires a reflection-dominated spectrum, which may be at odds with the low emissivity index and radio properties of the source. By contrast, the two absorption components required in the ionized partial covering model may correspond to the two distinct regions of polarization observed in the optical. Regardless of the physical model, spectral changes between epochs are driven by the absorption components—and on short timescales, by intrinsic active galactic nucleus variability.

Keywords: x-ray active galactic nuclei; agn host galaxies
Was There a 3.5 keV Line?

The 3.5 keV line is a purported emission line observed in galaxies, galaxy clusters, and the Milky Way whose origin is inconsistent with known atomic transitions and has previously been suggested to arise from dark matter decay. We systematically reexamine the bulk of the evidence for the 3.5 keV line, attempting to reproduce six previous analyses that found evidence for the line. We only reproduce one of the analyses; in the other five, we find no significant evidence for a 3.5 keV line when following the described analysis procedures on the original data sets. For example, previous results claimed 4σ evidence for a 3.5 keV line from the Perseus cluster; we dispute this claim, finding no evidence for a 3.5 keV line. We find evidence for background mismodeling in multiple analyses. We show that analyzing these data in narrower energy windows diminishes the effects of mismodeling but returns no evidence for a 3.5 keV line. We conclude that there is little robust evidence for the existence of the 3.5 keV line. Some of the discrepancy of our results from those of the original works may be due to the earlier reliance on local optimizers, which we demonstrate can lead to incorrect results. For ease of reproducibility, all code and data are publicly available.

Keywords: x-ray astronomy; astrostatistics; dark matter; cosmological neutrinos