adsabs/SciX_UAT_keywords huggingface dataset:
Abundances and Transients from Neutron Star-White Dwarf Mergers

We systematically investigate the mergers of neutron star-white dwarf binaries from beginning to end, with a focus on the properties of the inflows and outflows in accretion disks and their electromagnetic emissions. Using population synthesis models, we determine a subset of these binaries in which the white dwarf companion undergoes unstable mass transfer and complete tidal disruption, forming a large accretion disk around the neutron star. The material evolves according to a one-dimensional advection-dominated accretion-disk model with nuclear burning, neutrino emissions, and disk-surface wind ejection. The extreme dynamics of the entire process have proven difficult to analyze, and thus currently, the properties are poorly understood. The outflows from the mergers are iron- and nickel-rich, giving rise to optical and infrared emissions powered by the decay of the radioactive iron-type isotopes, calculated via the SuperNu light-curve code. We find these systems capable of powering bright, yet short-lived, optical transients with the potential to power gamma-ray bursts.

Keywords: white dwarf stars; neutron stars; stellar accretion disks; light curves; gamma-ray bursts
Performance of the Quasar Spectral Templates for the Dark Energy Spectroscopic Instrument

Millions of quasar spectra will be collected by the Dark Energy Spectroscopic Instrument (DESI), leading to a fourfold increase in the number of known quasars. High-accuracy quasar classification is essential to tighten constraints on cosmological parameters measured at the highest redshifts DESI observes (z > 2.0). We present spectral templates for identification and redshift estimation of quasars in the DESI Year 1 data release. The quasar templates are comprised of two quasar eigenspectra sets, trained on spectra from the Sloan Digital Sky Survey. The sets are specialized to reconstruct quasar spectral variation observed over separate yet overlapping redshift ranges and, together, are capable of identifying DESI quasars from 0.05 < z < 7.0. The new quasar templates show significant improvement over the previous DESI quasar templates regarding catastrophic failure rates, redshift precision and accuracy, quasar completeness, and the contamination fraction in the final quasar sample.

Keywords: quasars; cosmology; observational cosmology; redshift surveys
Chandra Observations of Spikey: A Possible Self-lensing Supermassive Black Hole Binary System

This work examines the recent X-ray observations of the active galactic nucleus KIC 11606854 (nicknamed "Spikey") by the Chandra space telescope. Based on previous observations of a symmetric flare in the system's light curve by the Kepler space telescope, Spikey has been proposed to be a self-lensing supermassive black hole binary system in which the more massive black hole gravitationally lenses the accretion flow of its smaller companion. The recent Chandra observations (2020 March-May) correspond to the time when the next pulse was expected to occur and were separated in enough time to observe the apparent relativistic Doppler boosting effect from the high orbital velocities of the black holes. We model the expected self-lensing plus Doppler boosting light curve using our wavelength-dependent extended source self-lensing model combined with our relativistic orbital motion code. This orbital motion code is capable of modeling the expected apsidal precession for Spikey, which can be used to predict future pulses. We show that the expected signal was undetectable in the Chandra data as the intrinsic X-ray variability associated with the system was large relative to the changes expected by self-lensing and Doppler boosting. Expected flux increases in more favorable wavelengths were also calculated using our wavelength-dependent self-lensing model, revealing a relationship between the observing wavelength and measured orbital inclination.

Keywords: black hole physics; gravitational microlensing; supermassive black holes
The Origin of the E/Z Isomer Ratio of Imines in the Interstellar Medium

Recent astronomical observations of both isomers E and Z of imines such as cyanomethanimine, ethanimine, and 2-propyn-1-imine have revealed that the abundances in the interstellar medium (ISM) of these isomers differ by factors of ∼3-10. Several theories have been proposed to explain the observed behavior, but none of them successfully explains the [E]/[Z] ratios. In this work we present a detailed study of the kinetics of the one-step E-Z isomerization reactions of cyanomethanimine, ethanimine, and 2-propyn-1-imine under interstellar conditions (in the 10-400 K temperature range). This reaction was previously thought to be nonviable in the ISM due to its associated high-energy barrier (about 13,000 K). In this Letter, we show that considering the multidimensional small curvature tunneling approximation, the tunneling effect enables the isomerization even at low temperatures. This is due to the fact that the representative tunneling energy lies in the vibrational ground state of the least stable isomer up to approximately 150 K, making the reaction constants of the isomerization from the least stable to the most stable isomer basically constant. The predicted [E]/[Z] ratios are almost the same as those reported from the astronomical observations for all imines observed. This study demonstrates that the [E]/[Z] ratio of imines in the ISM strongly depends on their relative stability.

Keywords: reaction rates; astrochemistry; pre-biotic astrochemistry; interstellar clouds
Predicting the Effect of Surface Properties on Enceladus for Landing

The prospect of landing on the surface of Enceladus comes with the question of whether the surface conditions permit selection and certification of one or more safe landing sites in an area of high science value. On Enceladus, the search for biosignatures in plume materials is a high science value objective that correlates with proximity to the south polar terrain, where the plume deposition rate is highest; however, such areas may be unsafe if unsintered particles make the landing site unstable. To investigate this, the surface of Enceladus was modeled using the level set discrete element method. This method models the kinetics and kinematics of large groups of individual ice particles both in contact and sintered together. Using the model, a rigid footpad was initialized at a 1 m s-1 descent just above the ice surface under Enceladus gravity. Parameters studied were the sintering amount, particle size distribution, footpad geometry, and surface slope. The model predicted that some sintering is required for the surface to support a lander; however, too much sintering can cause a lander to bounce. For tests on sloped surfaces, landing could be possible on slopes as steep as 20° for certain conditions, but it is safest to land in areas with a slope angle of 15° or less. While slope angle and sintering level were much more important than footpad geometry, the hemisphere footpad had the best performance (lowest slipping) in most cases compared to the cone or disk.

Keywords: landers; enceladus; surface ices; interdisciplinary astronomy
Gas-phase Synthesis of Silaformaldehyde (H2SiO) and Hydroxysilylene (HSiOH) in Outflows of Oxygen-rich Asymptotic Giant Branch Stars

Silicon- and oxygen-containing species such as silicon monoxide (SiO) and silicon dioxide (SiO2) represent basic molecular building blocks connected to the growth of silicate grains in outflows of oxygen-rich asymptotic giant branch (AGB) stars like R Doradus. Yet the fundamental mechanisms of the formation of silicate grains and the early processes that initiate the coupling of the silicon with the oxygen chemistries in circumstellar envelopes have remained obscure. Here, in a crossed molecular beams experiment combined with ab initio electronic structure calculations, we reveal that at least the d2-silaformaldehyde (D2SiO) and d2-hydroxysilylene (DSiOD) molecules-proxies for the astronomically elusive silaformaldehyde (H2SiO) and hydroxysilylene (HSiOH) molecules-can be synthesized via the reaction of the D1-silylidyne radical (SiD; X2Π) with D2-water (D2O) under single-collision conditions. This system represents a benchmark of a previously overlooked class of reactions, in which the silicon-oxygen bond coupling can be initiated by a reaction between the simplest silicon-bearing radical (silylidyne) and one of the most abundant species in the circumstellar envelopes of evolved oxygen-rich AGB stars (water). As supported by novel astrochemical modeling, considering that silicon- and oxygen-containing species like H2SiO and HSiOH might be photolyzed easily, they ultimately connect to simple molecular precursors such as SiO that drive a chain of reactions conceivably forming higher molecular weight silicon oxides and, ultimately, a population of silicates at high temperatures.

Keywords: astrochemistry; interstellar dust; laboratory astrophysics; circumstellar envelopes
Low-frequency Internal Gravity Waves Are Pseudo-incompressible

Starting from the fully compressible fluid equations in a plane-parallel atmosphere, we demonstrate that linear internal gravity waves are naturally pseudo-incompressible in the limit that the wave frequency ω is much less than that of surface gravity waves, i.e., $\omega \ll \sqrt{{{gk}}_{h}}$ , where g is the gravitational acceleration and k h is the horizontal wavenumber. We accomplish this by performing a formal expansion of the wave functions and the local dispersion relation in terms of a dimensionless frequency $\varepsilon =\omega /\sqrt{{{gk}}_{h}}$ . Further, we show that, in this same low-frequency limit, several forms of the anelastic approximation, including the Lantz-Braginsky-Roberts formulation, poorly reproduce the correct behavior of internal gravity waves. The pseudo-incompressible approximation is achieved by assuming that Eulerian fluctuations of the pressure are small in the continuity equation-whereas, in the anelastic approximation, Eulerian density fluctuations are ignored. In an adiabatic stratification, such as occurs in a convection zone, the two approximations become identical. However, in a stable stratification, the differences between the two approximations are stark and only the pseudo-incompressible approximation remains valid.

Keywords: internal waves; hydrodynamical simulations; astrophysical fluid dynamics; stellar convective zones; solar convective zone; stellar interiors; planetary interior; stellar oscillations
Stable Brine Layers beneath Europa's Chaos

The formation mechanism of Europa's large chaos terrain (>~100 km diameter) and associated lenticulae (<~10 km diameter) has been debated since their observations by the Galileo spacecraft. Their geomorphology and distribution suggest there may be reservoirs of saline liquid water 1-3 km beneath the surface-the "shallow water" model-generated by injection of ocean water or melting of the ice shell. Recent investigations on the evolution of small shallow-water bodies (≤103 km3) suggests that salts with a small effect on melting point (MgSO4) can extend the lifetime of saline bodies by ~5% compared to freshwater reservoirs. However, sodium chloride, identified as a potential oceanic salt, has a significantly stronger impact on the freezing point, suggesting a further extension of liquid lifetimes. Moreover, the substantial volumes of liquid water (~104 km3) beneath large chaos could be melted in situ rather than injected through a fracture, implying a distinct chemistry and formation environment. Here, we use numerical models to explore how the chemistry and disparate origins of shallow water control its evolution and lifetime. For small, injected sills, we find that NaCl can extend their liquid lifetime to ~140 kyr-up to a ~60% increase over freshwater sills. Saline melt lenses will last at least 175 kyr but, in contrast to sills, may persist as a stable layer of brine beneath the surface for over 500 kyr. Our results provide further support for the presence of liquid water at shallow depths within Europa's ice shell today.

Keywords: europa; planetary dynamics; planetary geology
Identification and Characterization of a Large Sample of Distant Active Dwarf Galaxies in XMM-SERVS

Active dwarf galaxies are important because they contribute to the evolution of dwarf galaxies and can reveal their hosted massive black holes. However, the sample size of such sources beyond the local universe is still highly limited. In this work, we search for active dwarf galaxies in the recently completed XMM-Spitzer Extragalactic Representative Volume Survey (XMM-SERVS). XMM-SERVS is currently the largest medium-depth X-ray survey covering 13 deg2 in three extragalactic fields, which all have well-characterized multiwavelength information. After considering several factors that may lead to misidentifications, we identify 73 active dwarf galaxies at z < 1, which constitutes the currently largest X-ray-selected sample beyond the local universe. Our sources are generally less obscured than predictions based on the massive-AGN (active galactic nucleus) X-ray luminosity function and have a low radio-excess fraction. We find that our sources reside in environments similar to those of inactive dwarf galaxies. We further quantify the accretion distribution of the dwarf-galaxy population after considering various selection effects and find that it decreases with X-ray luminosity, but redshift evolution cannot be statistically confirmed. Depending on how we define an AGN, the active fraction may or may not show a strong dependence on stellar mass. Their Eddington ratios and X-ray bolometric corrections significantly deviate from the expected relation, which is likely caused by several large underlying systematic biases when estimating the relevant parameters for dwarf galaxies. Throughout this work, we also highlight problems in reliably measuring photometric redshifts and overcoming strong selection effects for distant active dwarf galaxies.

Keywords: dwarf galaxies; intermediate-mass black holes; x-ray active galactic nuclei
Growth of a Massive Black Hole in a Dense Star Cluster Via Tidal Disruption Accretion

Stars that are tidally disrupted by a massive black hole (MBH) may contribute significantly to the growth of the MBH, especially in dense nuclear star clusters. Yet, this tidal disruption accretion (TDA) of stars onto the MBH has largely been overlooked compared to the gas accretion (GA) channel in most numerical experiments until now. In this work, we implement a black hole growth channel via TDA in the high-resolution adaptive mesh refinement code ENZO to investigate its influence on an MBH seed's early evolution. We find that an MBH seed grows rapidly from 103 to ≳106 M ⊙ in 200 Myr in some of the tested simulations. Compared to an MBH seed that grows only via GA, TDA can enhance the MBH's growth rate by more than 1 order of magnitude. However, as predicted, TDA mainly helps the early growth of the MBH (from 103-4 to ≲105 M ⊙) while the later evolution is generally dominated by GA. We also observe that the star formation near the MBH is suppressed when TDA is the most active, sometimes with a visible cavity in gas (of size ~ a few pc) created in the vicinity of the MBH. It is because the MBH may grow expeditiously with both GA and TDA, and the massive MBH could consume its neighboring gas faster than being replenished by gas inflows. Our study demonstrates the need to consider different channels of black hole accretion that may provide clues for the existence of supermassive black holes at high redshifts.

Keywords: astrophysical black holes
Internal 1000 au Scale Structures of the R CrA Cluster-forming Cloud. I. Filamentary Structures

We report Atacama Large Millimeter/submillimeter Array/Atacama Compact Array observations of a high-density region of the Corona Australis cloud forming a young star cluster, and the results of resolving internal structures. In addition to embedded Class 0/I protostars in the continuum, a number of complex dense filamentary structures are detected in the C18O and SO lines by the 7 m array. These are substructures of the molecular clump that are detected by the total power array as extended emission. We identify 101 and 37 filamentary structures with widths of a few thousand astronomical units in C18O and SO, respectively, which are called feathers. The typical column density of the feathers in C18O is about 1022 cm‑2, and the volume density and line mass are ∼105 cm‑3 and a few M ☉ pc‑1, respectively. This line mass is significantly smaller than the critical line mass expected for cold and dense gas. These structures have complex velocity fields, indicating a turbulent interior. The number of feathers associated with Class 0/I protostars is only ∼10, indicating that most of them do not form stars but rather are transient structures. The formation of feathers can be interpreted as a result of colliding gas flow because the morphology is well reproduced by MHD simulations, and this is supported by the presence of H I shells in the vicinity. The colliding gas flows may accumulate gas and form filaments and feathers, and trigger the active star formation of the R CrA cluster.

Keywords: interstellar medium; interstellar filaments; interstellar clouds; dark interstellar clouds; star formation; young star clusters; star clusters; young stellar objects; dense interstellar clouds; protostars
The Black Hole Candidate Swift J1728.9-3613 and the Supernova Remnant G351.9-0.9

A number of neutron stars have been observed within the remnants of the core-collapse supernova explosions that created them. In contrast, black holes are not yet clearly associated with supernova remnants (SNRs). Indeed, some observations suggest that black holes are "born in the dark," i.e., without a supernova explosion. Herein, we present a multiwavelength analysis of the X-ray transient Swift J1728.9-3613, based on observations made with Chandra, ESO-VISTA, MeerKAT, NICER, NuSTAR, Swift, and XMM-Newton. Three independent diagnostics indicate that the system likely harbors a black hole primary. Infrared imaging signals a massive companion star that is broadly consistent with an A or B spectral type. Most importantly, the X-ray binary lies within the central region of the cataloged SNR G351.9-0.9. Our deep MeerKAT image at 1.28 GHz signals that the remnant is in the Sedov phase; this fact and the nondetection of the soft X-ray emission expected from such a remnant argue that it lies at a distance that could coincide with the black hole. Utilizing a formal measurement of the distance to Swift J1728.9-3613 (d = 8.4 ± 0.8 kpc), a lower limit on the distance to G351.9-0.9 (d ≥ 7.5 kpc), and the number and distribution of black holes and SNRs within the Milky Way, extensive simulations suggest that the probability of a chance superposition is <1.7% (99.7% credible interval). The discovery of a black hole within an SNR would support numerical simulations that produce black holes and remnants, and thus provide clear observational evidence of distinct black hole formation channels. We discuss the robustness of our analysis and some challenges to this interpretation.

Keywords: astrophysical black holes; black holes; stellar mass black holes; x-ray binary stars; core-collapse supernovae; supernova remnants; compact objects; compact binary stars; rotating black holes; late stellar evolution
Far-ultraviolet Spectroscopy of Active Galactic Nuclei with ASTROSAT/UVIT

We study accretion disk emission from eight Seyfert 1-1.5 active galactic nuclei (AGN) using far-ultraviolet (FUV) (1300-1800 Å) slitless grating spectra acquired with AstroSat/UVIT. We correct for the Galactic and intrinsic extinction, contamination from the host galaxies, narrow and broad-line regions, Fe II emission, and Balmer continuum, and derive the intrinsic continua. We use Hubble Space Telescope COS/FOS spectra to account for the emission/absorption lines in the low-resolution UVIT spectra. We find generally redder power-law (f ν ∝ ν α ) slopes (α ~ -1.1 to 0.3) in the FUV band than predicted by the standard accretion disk model in the optical/UV band. We fit accretion disk models such as the multitemperature disk blackbody (DISKBB) and relativistic disk (ZKERRBB, OPTXAGNF) models to the observed intrinsic continuum emission. We measure the inner disk temperatures using the DISKBB model for seven AGN. These temperatures in the range ~3.6-5.8 eV are lower than the peak temperatures predicted for standard disks around maximally spinning supermassive black holes accreting at Eddington rates. The inner disks in two AGN, NGC 7469, and Mrk 352, appear to be truncated at ~35-125 and 50-135 r g , respectively. While our results show that the intrinsic FUV emission from the AGN is consistent with the standard disks, it is possible that UV continua may be affected by the presence of soft X-ray excess emission, X-ray reprocessing, and thermal Comptonization in the hot corona. Joint spectral modeling of simultaneously acquired UV/X-ray data may be necessary to further investigate the nature of accretion disks in AGN.

Keywords: active galactic nuclei; ultraviolet spectroscopy; extragalactic astronomy; spectral index
The Inhomogeneity Effect. II. Rotational and Orbital States Impact Planetary Cooling

We generalize the theory of the inhomogeneity effect to enable comparison among different inhomogeneous planets. A metric of inhomogeneity based on the cumulative distribution function is applied to investigate the dependence of planetary cooling on previously overlooked parameters. The mean surface temperature of airless planets increases with rotational rate and surface thermal inertia, which bounds the value in the tidally locked configuration and the equilibrium temperature. Using an analytical model, we demonstrate that the internal heat flux of giant planets exhibits significant spatial variability, primarily emitted from the nightside and high-latitude regions acting as radiator fins. Given a horizontally uniform interior temperature in the convective zone, the outgoing internal flux increases up to several folds as the inhomogeneity of the incoming stellar flux increases. The enhancement decreases with increasing heat redistribution through planetary dynamics or rotation. The outgoing internal flux on rapidly rotating planets generally increases with planetary obliquity and orbital eccentricity. The radiative timescale and true anomaly of the vernal equinox also play significant roles. If the radiative timescale is long, the outgoing internal flux shows a slightly decreasing but nonlinear trend with obliquity. Our findings indicate that rotational and orbital states greatly influence the cooling of planets and impact the interior evolution of giant planets, particularly for tidally locked planets and planets with high eccentricity and obliquity (such as Uranus), as well as the spatial and temporal variations of their cooling fluxes.

Keywords: planetary atmospheres; exoplanet atmospheres; atmospheric evolution