adsabs/SciX_UAT_keywords huggingface dataset:
Case A or Case B? The Effective Recombination Coefficient in Gas Clouds of Arbitrary Optical Thickness

In calculations of the ionization state, one is often forced to choose between the Case A recombination coefficient α A (sum over recombinations to all hydrogen states) or the Case B recombination coefficient α B (sum over all hydrogen states except the ground state). If the cloud is optically thick to ionizing photons, α B is usually adopted on the basis of the "on-the-spot" approximation, wherein recombinations to the ground state are ignored because they produce ionizing photons absorbed nearby. In the opposite case of an optically thin cloud, one would expect the Case A recombination coefficient to better describe the effective recombination rate in the cloud. In this paper, I derive an analytical expression for the effective recombination coefficient in a gas cloud of arbitrary optical thickness which transitions from α A to α B as the optical thickness increases. The results can be readily implemented in numerical simulations and semi-analytical calculations.

Keywords: radiative recombination; photoionization; reionization; h ii regions; cosmology
A Catalog of Potential Post-Common Envelope Binaries

We present a catalog containing 839 candidate post-common envelope systems. Common envelope evolution is very important in stellar astrophysics, particularly in the context of very compact and short-period binaries, including cataclysmic variables, as progenitors of, e.g., supernovae Type Ia or mergers of black holes and/or neutron stars. At the same time, it is a barely understood process in binary evolution. Due to limitations, since partially remedied, on direct simulation, early investigations were mainly focused on providing analytic prescriptions of the outcome of common envelope evolution. In recent years, detailed hydrodynamical calculations have produced deeper insight into the previously elusive process of envelope ejection. However, a direct link between the observations and theory of this relatively short-lived phase in binary evolution has not been forthcoming. Therefore, the main insight to be gained from observations has to be derived from the current state of systems likely to have gone through a common envelope. Here we present an extensive catalog of such observations as found in the literature. The aim of this paper is to provide a reliable set of data, obtained from observations, to be used in the theoretical modeling of common envelope evolution. In this catalog, the former common envelope donor star is commonly observed as a white dwarf or hot subdwarf star. This catalog includes period and mass estimates wherever obtainable. Some binaries are borderline cases to allow an investigation of the transition between a common envelope formation and other mass-transfer processes.

Keywords: catalogs; common envelope binary stars; stellar masses; close binary stars; white dwarf stars; subdwarf stars; common envelope evolution; eclipsing binary stars; spectroscopic binary stars; cataclysmic variable stars
Evolution of the Ratio of Mg II Intensities during Solar Flares

The Mg II k and h line intensity ratios can be used to probe the characteristics of the plasma in the solar atmosphere. In this study, using the observations recorded by the Interface Region Imaging Spectrometer, we study the variation of the Mg II k and h intensity ratio for three flares belonging to X-class, M-class, and C-class, throughout their evolution. We also study the k-to-h intensity ratio as a function of magnetic flux density obtained from the line-of-sight magnetograms recorded by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. Our results reveal that, while the intensity ratios are independent of magnetic flux density, they show significant changes during the evolution of the C-class and M-class flares. The intensity ratios start to increase at the start of the flare and peak during the impulsive phase before the flare peak and decrease rapidly thereafter. The values of the ratios fall even below the preflare level during the peak and decline phases of the flare. These results are important in light of heating and cooling of localized plasma and provide further constraint on the understanding of flare physics.

Keywords: solar chromosphere; solar flares; solar ultraviolet emission
Using a Higher-order Numerical Scheme to Study the Hall Magnetic Reconnection

We use our recently developed higher-order conservation element and solution element scheme to investigate the evolutionary process of Hall magnetic reconnection. The purpose of this paper is twofold: (1) to take advantage of higher-order numerical schemes to capture some fine structures very well with fewer grid points and reduced computational cost; (2) to develop a better understanding of the magnetic reconnection described by Hall MHD; as Birn et al. pointed out, the Hall effect is a critical ingredient in determining collisionless reconnection rates in the magnetosphere. The contributions of this paper mainly include the following: (1) we capture a two-step magnetic reconnection process and describe the formation mechanism; (2) the simulations show complex formation and interaction of magnetic islands and we provide the ways by which the magnetic islands form and disappear; (3) we find an oscillatory nature of the reconnection and the transfer of energy from magnetic field to kinetic energy and thermal energy; (4) we identify the merging process of the central magnetic island and the outflow region magnetic island.

Keywords: solar magnetic reconnection
The Strength and Shapes of Contact Binary Objects

While contact binary objects are common in the solar system, their formation mechanism is unclear. In this work we examine several contact binaries and calculate the necessary strength parameters that allow the two lobes to merge without the smaller of the two being gravitationally destroyed by the larger. We find a small but nonzero amount of cohesion or a large friction angle is required for the smaller lobe to survive the merging process, consistent with observations. This means it is possible for two previously separated rubble piles to experience a collapse of their mutual orbit and form a contact binary. The necessary strength required to survive this merger depends on the relative size, shape, and density of the body, with prolate shapes requiring more cohesion than oblate shapes.

Keywords: asteroids; small solar system bodies; comets; natural satellites (solar system)
Full-sky unWISE Coadds at Seven Years' Depth

unWISE is a wide-ranging data analysis effort aimed at repurposing the combined Wide-field Infrared Survey Explorer (WISE) plus NEOWISE imaging archive for Galactic and extragalactic astrophysics. We present updated static sky maps incorporating seven years of W1 (3.4 μm) and W2 (4.6 μm) single-exposure images, uniformly coadding nearly a quarter petabyte of input pixel data. These new unWISE atlases now constitute the deepest ever full-sky maps at wavelengths of 3-5 μm, and will be used to select millions of Dark Energy Spectroscopic Instrument quasar and luminous red galaxy targets.

Keywords: astronomy data reduction; astronomy data analysis; sky surveys; surveys; infrared astronomy
Superfluid Rivers in Spinning-down Neutron Stars

We study the motion of neutron superfluid vortices in a spinning-down neutron star, assuming axisymmetry of the flow and ignoring the motion of vortices about the rotation axis. We find that the vortex array, if initially rectilinear, is soon substantially deformed as the star spins down; vortices are swept outward by the Magnus force, accumulating in regions of the inner crust where they pin, accompanied by significant bending of the vortex array. As the star spins down to below a spin rate of ~20 Hz (twice the spin rate of the Vela pulsar), the Magnus and pinning forces gradually compress the vortex array into dense sheets that follow spherical shells. In some cases, the vortex array bends on itself and reconnects, forming one or more tori of vortex rings that contain superfluid "rivers" with significant angular momentum. Vortex sheets are likely to form near the base of the inner crust in the regime of nuclear pasta.

Keywords: neutron stars
Testing Evolution of LFQPOs with Mass Accretion Rate in GRS 1915+105 with Insight-HXMT

Using the Insight-HXMT observations of GRS 1915+105 when it exhibits low-frequency quasiperiodic oscillations (QPOs), we measure the evolution of the QPO frequency along with disk inner radius and mass accretion rate. We find a tight positive correlation between the QPO frequency and mass accretion rate. Our results extend the finding of previous work with AstroSat to a larger range of accretion rates with independent instruments and observations. Treating the QPO frequency of GRS 1915+105 as the relativistic dynamic frequency of a truncated disk, we are able to confirm the high spin nature of the black hole in GRS 1915+105. We also address the potential of our finding to test general relativity in the future.

Keywords: high energy astrophysics; x-ray astronomy; compact objects; stellar mass black holes
Ram Pressure Stripping in the EAGLE Simulation

Ram pressure stripping of satellite galaxies is thought to be a ubiquitous process in galaxy clusters, and a growing number of observations reveal satellites at different stages of stripping. However, in order to determine the fate of any individual galaxy, we turn to predictions from either simulations or analytic models. It is not well determined whether simulations and analytic models agree in their predictions, nor the causes of disagreement. Here we investigate ram pressure stripping in the reference EAGLE hydrodynamical cosmological simulation, and compare the results to predictions from analytic models. We track the evolution of galaxies with stellar mass M * > 109 M ⊙ and initial bound gas mass M gas > 109 M ⊙ that fall into galaxy clusters (M 200c > 1014 M ⊙) between z = 0.27 and z = 0. We divide each galaxy into its neutral gas disk and hot ionized gas halo and compare the evolution of the stripped gas fraction in the simulation to that predicted by analytic formulations for the two gas phases, as well as to a toy model that computes the motions of gas particles under the combined effects of gravity and a spatially uniform ram pressure. We find that the analytic models generally underpredict the stripping rate of neutral gas and overpredict that of ionized gas, with significant scatter between the model and simulation stripping timescales. This is due to opposing physical effects: the enhancement of ram pressure stripping by stellar feedback, and the suppression of stripping by the compaction of galactic gas.

Keywords: galaxy evolution; galaxy physics; galaxy structure; hydrodynamical simulations; ram pressure stripped tails
A Variable X-Ray Source Close to the Magnetar SGR 1935+2154

The recent discovery of a millisecond radio burst from the Galactic magnetar SGR 1935+2154 has initiated an intense discussion about the connection between magnetars and fast radio bursts (FRBs). Although some properties of the radio burst from SGR 1935+2154are not the same as cosmological FRBs, there are theoretical models which propose a connection between magnetars and FRBs (see review by Margalit et al. 2020). In particular, the role of a magnetar wind nebula is included in some models, and therefore it is worthwhile to investigate the X-ray environment of SGR 1935+2154 in more detail. Here, we report on the discovery of an X-ray transient feature near SGR 1935+2154 using archival Chandra data and discuss its possible origin.

Keywords: x-ray transient sources; neutron stars; magnetars; radio bursts
Reconstructing the EUV Spectrum of Star-forming Regions from Millimeter Recombination Lines of H I, He I, and He II

The extreme ultraviolet (EUV) spectra of distant star-forming regions cannot be probed directly using either ground- or space-based telescopes due to the high cross section for interaction of EUV photons with the interstellar medium. This makes EUV spectra poorly constrained. The millimeter/submillimeter recombination lines of H and He, which can be observed from the ground, can serve as a reliable probe of the EUV. Here we present a study based on ALMA observations of three Galactic ultracompact H II regions and the starburst region Sgr B2(M), in which we reconstruct the key parameters of the EUV spectra using millimeter recombination lines of H I, He I, and He II. We find that in all cases the EUV spectra between 13.6 and 54.4 eV have similar frequency dependence: ${L}_{\nu }\propto {\nu }^{-4.5\pm 0.4}.$ We compare the inferred values of the EUV spectral slopes with the values expected for a purely single stellar evolution model (Starburst99) and the Binary Population and Spectral Synthesis code (BPASS). We find that the observed spectral slope differs from the model predictions. This may imply that the fraction of interacting binaries in H II regions is substantially lower than assumed in BPASS. The technique demonstrated here allows one to deduce the EUV spectra of star-forming regions, providing critical insight into photon production rates at $\lambda \leqslant 912\mathring{{\rm{A}}}$ , and can serve as calibration to starburst synthesis models, improving our understanding of star formation in the distant universe and the properties of ionizing flux during reionization.

Keywords: astrophysicists; interstellar medium; star formation; radiative recombination; compact h ii region
A Catalog of RV Variable Star Candidates from LAMOST

Radial velocity (RV) variable stars are important in astrophysics. The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectroscopic survey has provided ∼6.5 million stellar spectra in its Data Release 4 (DR4). During the survey ∼4.7 million unique sources were targeted and ∼1 million stars observed repeatedly. The probabilities of stars being RV variables are estimated by comparing the observed RV variations with simulated ones. We build a catalog of 80,702 RV variable candidates with probability greater than 0.60 by analyzing the multi-epoch sources covered by LAMOST DR4. Simulations and cross-identifications show that the purity of the catalog is higher than 80%. The catalog consists of 77% binary systems and 7% pulsating stars as well as 16% pollution by single stars. 3138 RV variables are classified through cross-identifications with published results in literatures. By using the 3138 sources common in both LAMOST and a collection of published RV variable catalogs, we are able to analyze LAMOST's RV variable detection rate. The efficiency of the method adopted in this work relies not only on the sampling frequency of observations but also periods and amplitudes of RV variables. With the progress of LAMOST, Gaia, and other surveys, more and more RV variables will be confirmed and classified. This catalog is valuable for other large-scale surveys, especially for RV variable searches. The catalog will be released according to the LAMOST Data Policy via http://dr4.lamost.org.

Keywords: variable stars; catalogs; radial velocity; extrinsic variable stars; intrinsic variable stars; binary stars; pulsating variable stars; star counts; stellar astronomy; astrostatistics; spectroscopy; surveys
Testing the Cosmic Distance Duality Relation with the Latest Strong Gravitational Lensing and Type Ia Supernovae

We perform a model-independent and comprehensive test on the cosmic distance duality relation (CDDR) by combining the latest observations of strong gravitational lensing (SGL) including a total of 161 well-measured systems from several surveys and observations of Type Ia supernovae (SNe Ia), I.e., the joint light-curve analysis of SNe Ia and the Pantheon SNe Ia. We parameterize the CDDR in the form of {D}{{A}}{≤ft(1+z\right)}2/{D}{{L}}=1+{η }0z, and also consider general lens mass models including the dependence on the lens redshift and surface mass density. First, we update tests using the new SGL and the two SNe Ia data sets for the singular isothermal sphere model. The constraint results suggest a moderate tension with the CDDR using the Pantheon SN Ia with a slightly negative η 0. We find that η 0 deviates significantly from the CDDR at more than the 3σ level if the lens mass model depends on redshift. Supplementary tests show that the error from aperture correction and the parameterization method of the CDDR can hardly justify the deviation. Several of the models investigated show some evidence for deviations from the CDDR. However, there is a significant scatter in the inferred level of the CDDR violation, depending on the model describing the population of strong lenses. This variance is too large for us to conclude yet that the CDDR is violated and needs further investigation and future measurements to be verified.

Keywords: cosmology; type ia supernovae; strong gravitational lensing; astronomy data analysis
Spectral Evolution Responsible for the Transition from Positive Lags to Negative Lags in Gamma-Ray Bursts

It is well known that most gamma-ray bursts (GRBs) are dominated by positive spectral lags, while a small fraction of GRBs show negative lags. However, Wei et al. first identified a well-defined transition from positive lags to negative lags in GRB 160625B, and then got robust limits on a possible violation of Lorentz invariance (LIV) based on the observation. Recently, such a transition has been found in three different emission episodes in GRB 190530A by Gunapati et al., which provides us a great opportunity to investigate whether the transition results from LIV-induced observed spectral lags. Our analysis shows that the LIV model cannot be compatible with the current observations, whereas, only the spectral evolution induced spectral lags could responsible for the transition. So, spectral evolution can also explain the positive to negative lag in GRB 190530A.

Keywords: particle astrophysics
International Asteroid Warning Network Timing Campaign: 2019 XS

As part of the International Asteroid Warning Network's observational exercises, we conducted a campaign to observe near-Earth asteroid 2019 XS around its close approach to Earth on 2021 November 9. The goal of the campaign was to characterize errors in the observation times reported to the Minor Planet Center, which become an increasingly important consideration as astrometric accuracy improves and more fast-moving asteroids are observed. As part of the exercise, a total of 957 astrometric observations of 2019 XS during the encounter were reported and subsequently were analyzed to obtain the corresponding residuals. While the timing errors are typically smaller than 1 s, the reported times appear to be negatively biased, i.e., they are generally earlier than they should be. We also compared the observer-provided position uncertainty with the cross-track residuals, which are independent of timing errors. A large fraction of the estimated uncertainties appear to be optimistic, especially when <0.″2. We compiled individual reports for each observer to help identify and remove the root cause of any possible timing error and improve the uncertainty quantification process. We suggest possible sources of timing errors and describe a simple procedure to derive reliable, conservative position uncertainties.

Keywords: asteroids; near-earth objects; optical telescopes; astrometry
Early Results from GLASS-JWST. XVIII. A First Morphological Atlas of the 1 < z < 5 Universe in the Rest-frame Optical

We present a rest-frame optical morphological analysis of galaxies observed with the NIRCam imager on the James Webb Space Telescope (JWST) as part of the GLASS-JWST Early Release Science program. We select 388 sources at redshifts 0.8 < z < 5.4 and use the seven 0.9-5 μm NIRCam filters to generate rest-frame gri composite color images, and conduct visual morphological classification. Compared to Hubble Space Telescope (HST)-based work we find a higher incidence of disks and bulges than expected at z > 1.5, revealed by rest-frame optical imaging. We detect 123 clear disks (58 at z > 1.5) of which 76 have bulges. No evolution of bulge fraction with redshift is evident: 61% at z < 2 (N = 110) versus 60% at z ≥ 2 (N = 13). A stellar mass dependence is evident, with bulges visible in 80% of all disk galaxies with mass >109.5 M ⊙ (N = 41) but only 52% at M < 109.5 M ⊙ (N = 82). We supplement visual morphologies with nonparametric measurements of Gini and asymmetry coefficients in the rest-frame i band. Our sources are more asymmetric than local galaxies, with slightly higher Gini values. When compared to high-z rest-frame ultraviolet measurements with HST, JWST shows more regular morphological types such as disks, bulges, and spiral arms at z > 1.5, with smoother (i.e., lower Gini) and more symmetrical light distributions.

Keywords: galaxy evolution; infrared astronomy; galaxy classification systems; james webb space telescope
Variable Hα Emission in the Nebular Spectra of the Low-luminosity Type Ia SN2018cqj/ATLAS18qtd

We present optical photometry and spectroscopy of the Type Ia supernova SN2018cqj/ATLAS18qtd. The supernova exploded in an isolated region at ∼65 kpc from the S0 galaxy IC 550 at z = 0.0165 (D ≈ 74 Mpc) and has a redshift consistent with a physical association to this galaxy. Multicolor photometry show that SN2018cqj/ATLAS18qtd is a low-luminosity ( ${M}_{{B}_{\max }}\approx -17.9$ mag), fast-declining Type Ia, with color stretch sBV ≈ 0.6 and B-band decline rate Δm15(B) ≈ 1.77 mag. Two nebular-phase spectra obtained as part of the 100IAS survey at +193 and +307 days after peak show the clear detection of a narrow Hα line in emission that is resolved in the first spectrum with FWHM ≈ 1200 km s-1 and LHα ≈ 3.8 × 1037 erg s-1. The detection of a resolved Hα line with a declining luminosity is broadly consistent with recent models where hydrogen is stripped from the nondegenerate companion in a single-degenerate progenitor system. However, the amount of hydrogen consistent with the luminosities of the Hα line would be ∼10-3 M⊙, which is significantly less than theoretical model predictions in the classical single-degenerate progenitor systems. SN2018cqj/ATLAS18qtd is the second low-luminosity, fast-declining SN Ia after SN2018fhw/ASASSN-18tb that shows narrow Hα in emission in its nebular-phase spectra.

Keywords: supernovae; type ia supernovae; high energy astrophysics; transient sources
An Updated Reference Frame for the Galactic Inner Parsec

Infrared observations of stellar orbits about Sgr A* probe the mass distribution in the inner parsec of the Galaxy and provide definitive evidence for the existence of a massive black hole. However, the infrared astrometry is relative and is tied to the radio emission from Sgr A* using stellar SiO masers that coincide with infrared-bright stars. To support and improve this two-step astrometry, we present new astrometric observations of 15 stellar SiO masers within 2 pc of Sgr A*. Combined with legacy observations spanning 25.8 yr, we reanalyze the relative offsets of these masers from Sgr A* and measure positions and proper motions that are significantly improved compared to the previously published reference frame. Maser positions are corrected for epoch-specific differential aberration, precession, nutation, and solar gravitational deflection. Omitting the supergiant IRS 7, the mean position uncertainties are 0.46 mas and 0.84 mas in R.A. and decl., and the mean proper motion uncertainties are 0.07 mas yr-1 and 0.12 mas yr-1, respectively. At a distance of 8.2 kpc, these correspond to position uncertainties of 3.7 and 6.9 au and proper motion uncertainties of 2.7 and 4.6 km s-1. The reference frame stability, the uncertainty in the variance-weighted mean proper motion of the maser ensemble, is 8 μas yr-1 (0.30 km s-1) in R.A. and 11 μas yr-1 (0.44 km s-1) in decl., which represents a 2.3-fold improvement over previous work and a new benchmark for the maser-based reference frame.

Keywords: astrophysical masers; circumstellar masers; silicon monoxide masers; galactic center; astrophysical black holes; supermassive black holes; astronomical coordinate systems; radio astrometry; astrometry; proper motions; stellar kinematics; stellar motion