adsabs/SciX_UAT_keywords huggingface dataset:
Multiphase Gas and the Fractal Nature of Radiative Turbulent Mixing Layers

A common situation in galactic and intergalactic gas involves cold dense gas in motion relative to hot diffuse gas. Kelvin-Helmholtz instability creates a turbulent mixing layer and populates the intermediate-temperature phase, which often cools rapidly. The energy lost to cooling is balanced by the advection of hot high enthalpy gas into the mixing layer, resulting in growth and acceleration of the cold phase. This process may play a major role in determining the interstellar medium and circumgalactic medium phase structure, and accelerating cold gas in galactic winds and cosmic filaments. Cooling in these mixing layers occurs in a thin corrugated sheet, which we argue has an area with fractal dimension D = 5/2 and a thickness that adjusts to match the hot phase mixing time to the cooling time. These cooling sheet properties form the basis of a new model for how the cooling rate and hot gas inflow velocity depend on the size L, cooling time of the mixed phase ${t}_{\mathrm{cool}}$, relative velocity ${V}_{\mathrm{rel}}$, and density contrast ${\rho }_{\mathrm{cold}}/{\rho }_{\mathrm{hot}}$ of the system. Entrainment is expected to be enhanced in environments with short ${t}_{\mathrm{cool}}$, large ${V}_{\mathrm{rel}}$, and large ${\rho }_{\mathrm{cold}}/{\rho }_{\mathrm{hot}}$. Using a large suite of three-dimensional hydrodynamic simulations, we demonstrate that this fractal cooling layer model accurately captures the energetics and evolution of turbulent interfaces and can therefore be used as a foundation for understanding multiphase mixing with strong radiative cooling.

Keywords: astrophysical fluid dynamics; galaxy formation; galaxy evolution; galactic winds; circumgalactic medium; interstellar medium; intracluster medium; star formation
On the Formation of Over-ionized Plasma in Evolved Supernova Remnants

One of the outstanding mysteries surrounding the rich diversity found in supernova remnants (SNRs) is the recent discovery of over-ionized or recombining plasma from a number of dynamically evolved objects. To help decipher its formation mechanism, we have developed a new simulation framework capable of modeling the time evolution of the ionization state of the plasma in an SNR. The platform is based on a one-dimensional hydrodynamics code coupled to a fully time-dependent nonequilibrium ionization calculation, accompanied by a spectral synthesis code to generate space-resolved broadband X-ray spectra for SNRs at arbitrary ages. We perform a comprehensive parametric survey to investigate the effects of different circumstellar environments on the ionization state evolution in SNRs up to a few 104 yr. A two-dimensional parameter space, spanned by arrays of interstellar medium (ISM) densities and mass-loss rates of the progenitor, is used to create a grid of models for the surrounding environment, in which a core-collapse explosion is triggered. Our results show that a recombining plasma can be successfully reproduced in the case of a young SNR (a few 100 to 1000 yr old) expanding fast in a spatially extended low-density wind, an old SNR (>a few 1000 yr) expanding in a dense ISM, or an old SNR broken out from a confined dense wind region into a tenuous ISM. Finally, our models are confronted with observations of evolved SNRs, and an overall good agreement is found except for a couple of outliers.

Keywords: supernova remnants; interstellar medium; ionization
The Origin of Power-law Spectra in Relativistic Magnetic Reconnection

Magnetic reconnection is often invoked as a source of high-energy particles, and in relativistic astrophysical systems it is regarded as a prime candidate for powering fast and bright flares. We present a novel analytical model-supported and benchmarked with large-scale three-dimensional kinetic particle-in-cell simulations in electron-positron plasmas-that elucidates the physics governing the generation of power-law energy spectra in relativistic reconnection. Particles with Lorentz factor γ ≳ 3σ (here, σ is the magnetization) gain most of their energy in the inflow region, while meandering between the two sides of the reconnection layer. Their acceleration time is ${t}_{\mathrm{acc}}\sim \gamma \,{\eta }_{\mathrm{rec}}^{-1}{\omega }_{{\rm{c}}}^{-1}\simeq 20\,\gamma \,{\omega }_{{\rm{c}}}^{-1}$ , where η rec ≃ 0.06 is the inflow speed in units of the speed of light and ω c = eB 0/mc is the gyrofrequency in the upstream magnetic field. They leave the region of active energization after t esc, when they get captured by one of the outflowing flux ropes of reconnected plasma. We directly measure t esc in our simulations and find that t esc ~ t acc for σ ≳ few. This leads to a universal (i.e., σ-independent) power-law spectrum ${{dN}}_{\mathrm{free}}/d\gamma \propto {\gamma }^{-1}$ for the particles undergoing active acceleration, and ${dN}/d\gamma \propto {\gamma }^{-2}$ for the overall particle population. Our results help to shed light on the ubiquitous presence of power-law particle and photon spectra in astrophysical nonthermal sources.

Keywords: high energy astrophysics
Impact Crater Databases for Pluto and Charon, Version 2

Pluto and Charon are the farthest bodies from the Sun that have been visited by spacecraft where more than several dozen impact craters can be accurately identified and measured. These craters form a critical record of outer solar system impactors in the trans-Neptunian environment, so an accurate census of them is important for numerous dynamical applications seeking to understand the Kuiper Belt and its evolution. In 2017, a crater database was published based on several different researchers mapping on various image products from the New Horizons mission. To date, that remains the only work that has tried to provide a complete census of the craters on Pluto and Charon. Meanwhile, Singer et al. mapped on different, more final image products, while Robbins & Singer worked to reconcile their crater populations, demonstrate general agreement, and lay out issues with the previous work from 2017. However, neither attempted to make complete databases of Pluto's or Charon's craters. To come full circle, we present a short report on a revised crater database for both Pluto and Charon that incorporates revised interpretations of geologic features on both bodies; this work supersedes our other Pluto and Charon crater database work.

Keywords: pluto; plutonian satellites; craters
Using Machine Learning to Determine Morphologies of z < 1 AGN Host Galaxies in the Hyper Suprime-Cam Wide Survey

We present a machine-learning framework to accurately characterize the morphologies of active galactic nucleus (AGN) host galaxies within z < 1. We first use PSFGAN to decouple host galaxy light from the central point source, then we invoke the Galaxy Morphology Network (GAMORNET) to estimate whether the host galaxy is disk-dominated, bulge-dominated, or indeterminate. Using optical images from five bands of the HSC Wide Survey, we build models independently in three redshift bins: low (0 < z < 0.25), mid (0.25 < z < 0.5), and high (0.5 < z < 1.0). By first training on a large number of simulated galaxies, then fine-tuning using far fewer classified real galaxies, our framework predicts the actual morphology for ~60%-70% of the host galaxies from test sets, with a classification precision of ~80%-95%, depending on the redshift bin. Specifically, our models achieve a disk precision of 96%/82%/79% and bulge precision of 90%/90%/80% (for the three redshift bins) at thresholds corresponding to indeterminate fractions of 30%/43%/42%. The classification precision of our models has a noticeable dependency on host galaxy radius and magnitude. No strong dependency is observed on contrast ratio. Comparing classifications of real AGNs, our models agree well with traditional 2D fitting with GALFIT. The PSFGAN+GAMORNET framework does not depend on the choice of fitting functions or galaxy-related input parameters, runs orders of magnitude faster than GALFIT, and is easily generalizable via transfer learning, making it an ideal tool for studying AGN host galaxy morphology in forthcoming large imaging surveys.

Keywords: active galactic nuclei; galaxies; galaxy classification systems; astronomy data analysis; neural networks; convolutional neural networks; agn host galaxies
The First JWST Spectrum of a GRB Afterglow: No Bright Supernova in Observations of the Brightest GRB of all Time, GRB 221009A

We present James Webb Space Telescope (JWST) and Hubble Space Telescope (HST) observations of the afterglow of GRB 221009A, the brightest gamma-ray burst (GRB) ever observed. This includes the first mid-IR spectra of any GRB, obtained with JWST/Near Infrared Spectrograph (0.6-5.5 micron) and Mid-Infrared Instrument (5-12 micron), 12 days after the burst. Assuming that the intrinsic spectral slope is a single power law, with F ν ∝ ν -β , we obtain β ≈ 0.35, modified by substantial dust extinction with A V = 4.9. This suggests extinction above the notional Galactic value, possibly due to patchy extinction within the Milky Way or dust in the GRB host galaxy. It further implies that the X-ray and optical/IR regimes are not on the same segment of the synchrotron spectrum of the afterglow. If the cooling break lies between the X-ray and optical/IR, then the temporal decay rates would only match a post-jet-break model, with electron index p < 2, and with the jet expanding into a uniform ISM medium. The shape of the JWST spectrum is near-identical in the optical/near-IR to X-SHOOTER spectroscopy obtained at 0.5 days and to later time observations with HST. The lack of spectral evolution suggests that any accompanying supernova (SN) is either substantially fainter or bluer than SN 1998bw, the proto-type GRB-SN. Our HST observations also reveal a disk-like host galaxy, viewed close to edge-on, that further complicates the isolation of any SN component. The host galaxy appears rather typical among long-GRB hosts and suggests that the extreme properties of GRB 221009A are not directly tied to its galaxy-scale environment.

Keywords: gamma-ray bursts
The Behavior of Bp He-weak Stars in the Far UV-Paper III: HD 175362

The analysis of twelve spectra of the Bp He-weak Si star HD 175362 recorded by the SWP camera on board the International Ultraviolet Explorer reveals variations of the far-ultraviolet spectral energy distribution over a time interval of several years. Variations are present shortwards of 1700 Å which can be partly attributed to variations of lines and continuum of Si II. Other lines due to C II, Si III and Si IV also contribute to the opacity in this range too. The variations suggest that silicon and carbon are not distributed in a uniform manner over the surface.

Keywords: chemically peculiar stars; helium-weak stars
Tidal Disruptions of Main-sequence Stars. IV. Relativistic Effects and Dependence on Black Hole Mass

Using a suite of fully relativistic hydrodynamic simulations applied to main-sequence stars with realistic internal density profiles, we examine full and partial tidal disruptions across a wide range of black hole mass ( ${10}^{5}\leqslant {M}_{\mathrm{BH}}/{M}_{\odot }\leqslant 5\times {10}^{7}$ ) and stellar mass ( $0.3\leqslant {M}_{\star }/{M}_{\odot }\leqslant 3$ ) as larger MBH leads to stronger relativistic effects. For fixed M⋆, as MBH increases, the ratio of the maximum pericenter distance yielding full disruptions ( ${{ \mathcal R }}_{{\rm{t}}}$ ) to its Newtonian prediction rises rapidly, becoming triple the Newtonian value for ${M}_{\mathrm{BH}}=5\times {10}^{7}\,{M}_{\odot }$ , while the ratio of the energy width of the stellar debris for full disruptions to the Newtonian prediction decreases steeply, resulting in a factor of 2 correction at ${M}_{\mathrm{BH}}=5\times {10}^{7}\,{M}_{\odot }$ . We provide approximate formulae that express the relativistic corrections of both ${{ \mathcal R }}_{{\rm{t}}}$ and the energy width relative to their Newtonian approximate estimates. For partial disruptions, we find that the fractional remnant mass for a given ratio of the pericenter to ${{ \mathcal R }}_{{\rm{t}}}$ is higher for larger MBH. These results have several implications. As MBH increases above $\sim {10}^{7}\,{M}_{\odot }$ , the cross section for complete disruptions is suppressed by competition with direct capture. However, the cross-section ratio for partial to complete disruptions depends only weakly on MBH. The relativistic correction to the debris energy width delays the time of peak mass-return rate and diminishes the magnitude of the peak return rate. For ${M}_{\mathrm{BH}}\gtrsim {10}^{7}\,{M}_{\odot }$ , the MBH-dependence of the full disruption cross section and the peak mass-return rate and time is influenced more by relativistic effects than by Newtonian dynamics.

Keywords: black holes; galactic center; tidal disruption; general relativity; supermassive black holes; gravitation
First Demonstration of Early Warning Gravitational-wave Alerts

Gravitational-wave observations became commonplace in Advanced LIGO-Virgo's recently concluded third observing run. 56 nonretracted candidates were identified and publicly announced in near real time. Gravitational waves from binary neutron star mergers, however, remain of special interest since they can be precursors to high-energy astrophysical phenomena like γ-ray bursts and kilonovae. While late-time electromagnetic emissions provide important information about the astrophysical processes within, the prompt emission along with gravitational waves uniquely reveals the extreme matter and gravity during—and in the seconds following—merger. Rapid communication of source location and properties from the gravitational-wave data is crucial to facilitate multimessenger follow-up of such sources. This is especially enabled if the partner facilities are forewarned via an early warning (pre-merger) alert. Here we describe the commissioning and performance of such a low-latency infrastructure within LIGO-Virgo. We present results from an end-to-end mock data challenge that detects binary neutron star mergers and alerts partner facilities before merger. We set expectations for these alerts in future observing runs.

Keywords: gravitational waves; gravitational wave astronomy; neutron stars; high energy astrophysics
Monte Carlo Calculations of Helium Escape on Mars via Energy Transfer from Hot Oxygen Atoms

Understanding He escape is crucial for deciphering the evolution of the He budget on Mars. A number of viable mechanisms have been proposed to drive He escape, with energy transfer from hot O generally thought to be the dominant one. This study is devoted to a state-of-the-art evaluation of the above process assuming hot O is exclusively produced from the dissociative recombination of O2+ in the Martian ionosphere. A Monte Carlo model is constructed, with model inputs optimized by the recent Mars Atmosphere and Volatile Evolution measurements. The model calculations reveal a dayside He escape flux of (1-2) × 106 cm-2 s-1 referred to the surface, for a possible range of H2 mixing ratio at an altitude of 80 km from 0 to 40 ppm. The computed He escape flux increases with increasing nascent O energy and decreasing atmospheric H2 and H abundances. The portion of the atmosphere below the exobase is found to make an exceptionally large contribution of 95% to He escape.

Keywords: mars; planetary atmospheres; upper atmosphere; exosphere; aeronomy
Carbon- and Oxygen-rich Asymptotic Giant Branch (AGB) Stars in the Bulge Asymmetries and Dynamical Evolution (BAaDE) Survey

Detections of SiO masers from the Bulge Asymmetries and Dynamical Evolution (BAaDE) survey more tightly define the region where Oxygen-rich (O) Asymptotic Giant Branch (AGB) stars reside in multiple infrared (IR) color-color diagrams. Using Midcourse Space eXperiment (MSX) and Two Micron All Sky Survey (2MASS) data along with radio spectra from the BAaDE survey, we find that three main populations were observed in the BAaDE survey: O-rich AGB stars of which about 73% host SiO masers, Carbon-rich (C) AGB stars that do not host these masers, and a small contaminating set of possible Young Stellar Objects (YSOs). The distinction between YSOs and AGB stars can be drawn using only MSX data, specifically the [D]-[E] color, while the shorter wavelengths provided by 2MASS are necessary to divide potential C- and O-rich AGB stars. Divisions similar to these have been seen in multiple earlier IR-studies, but BAaDE currently provides a sample of ∼15,000 sources, which far exceeds previous studies in sample size, and, therefore, provides much more distinct divisions. With these IR distinctions in place, we discuss the sources that are exceptions in either their molecular detections or IR colors, as well as the distribution of the three populations in Galactic coordinates.

Keywords: silicon monoxide masers; circumstellar masers; asymptotic giant branch stars; galactic bulge; carbon stars
A Selection of Hα Emitters at z = 2.1-2.5 Using the K s -band Photometry of ZFOURGE

Large and less-biased samples of star-forming galaxies are essential to investigate galaxy evolution. Hα emission line is one of the most reliable tracers of star-forming galaxies because its strength is directly related to recent star formation. However, it is observationally expensive to construct large samples of Hα emitters by spectroscopic or narrowband imaging survey at high redshifts. In this work, we demonstrate a method to extract Hα fluxes of galaxies at z = 2.1-2.5 from K s broadband photometry of ZFOURGE catalog. Combined with 25-39 other filters, we estimate the emission-line fluxes by SED fitting with stellar population models that incorporate emission-line strengths. 2005 galaxies are selected as Hα emitters by our method and their fluxes show good agreement with previous measurements in the literature. On the other hand, there are more Hα luminous galaxies than previously reported. The discrepancy can be explained by extended Hα profiles of massive galaxies and a luminosity dependence of dust attenuation, which are not taken into account in the previous work. We also find that there are a large number of low-mass galaxies with a much higher specific star formation rate (sSFR) than expected from the extrapolated star formation main sequence. Such low-mass galaxies exhibit larger ratios between Hα and UV fluxes compared to more massive high sSFR galaxies. This result implies that a "starburst" mode may differ among galaxies: low-mass galaxies appear to assemble their stellar mass via short-duration bursts, while more massive galaxies tend to experience longer-duration (>10 Myr) bursts.

Keywords: galaxy evolution; galaxy formation; galaxies; broad band photometry; multi-color photometry; emission line galaxies; high-redshift galaxies
Radio, X-Ray, and Extreme-ultraviolet Observations of Weak Energy Releases in the "Quiet" Sun

We analyzed ground-based low frequency (<100 MHz) radio spectral and imaging data of the solar corona obtained with the facilities in the Gauribidanur observatory during the same time as the very weak soft X-ray flares (sub-A-class, flux <10-7Wm-2 in the 1-8 Å wavelength range) from the quiet Sun observed with the X-ray Solar Monitor (XSM) on board Chandrayaan-2 during the recent solar minimum. Nonthermal type I radio burst activity was noticed in close temporal association with the X-ray events. The estimated brightness temperature (Tb) of the bursts at a typical frequency like 80 MHz is ≈3 × 105 K. Extreme-ultraviolet (EUV) observations at 94 Å with the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) revealed a brightening close to the same location and time as the type I radio bursts. As far as we know reports of simultaneous observations of X-ray and/or EUV counterpart to weak transient radio emission at low frequencies from the quiet Sun in particular are rare. Considering this and the fact that low frequency radio observations are sensitive to weak energy releases in the solar atmosphere, the results indicate that coordinated observations of similar events would be useful to understand transient activities in the quiet Sun.

Keywords: quiet sun; solar radio emission; solar x-ray flares; solar corona; solar activity; solar flares
Quantum Tunneling of Fuzzy Dark Matter out of Satellite Galaxies

I show that the tidal gravitational potential of the Milky Way galaxy removes fuzzy dark matter from its satellite dwarf galaxies through quantum-mechanical tunneling. The existence of dark matter in satellites rules-out ultra-light axions as dark matter with a particle mass, m a < 2 × 10-21 eV. This limit exceeds the canonical mass range proposed as a solution to the small-scale challenges of the cold-dark-matter paradigm.

Keywords: cold dark matter
Statistical Analysis of the Radial Evolution of the Solar Winds between 0.1 and 1 au and Their Semiempirical Isopoly Fluid Modeling

Statistical classification of the Helios solar wind observations into several populations sorted by bulk speed has revealed an outward acceleration of the wind. The faster the wind, the smaller this acceleration in the 0.3-1 au radial range. In this paper, we show that recent measurements from the Parker Solar Probe (PSP) are compatible with an extension closer to the Sun of the latter Helios classification. For instance, the well-established bulk speed/proton temperature (u, T p) correlation and bulk speed/electron temperature (u, T e) anticorrelation, together with the acceleration of the slowest winds, are verified in PSP data. We also model the combined PSP and Helios data using empirical Parker-like models for which the solar wind undergoes an "isopoly" expansion: isothermal in the corona, then polytropic at distances larger than the sonic point radius. The polytropic indices are derived from the observed temperature and density gradients. Our modeling reveals that the electron thermal pressure has a major contribution in the acceleration process of slow and intermediate winds (in the range of 300-500 km s-1 at 1 au) over a broad range of distances and that the global (electron and proton) thermal energy alone is able to explain the acceleration profiles. Moreover, we show that the very slow solar wind requires, in addition to the observed pressure gradients, another source of acceleration.

Keywords: solar wind; interplanetary particle acceleration; slow solar wind; astronomy data modeling
Impact of Microlensing on Observational Strategies for Gravitational Time-delay Measurements

Microlensing not only allows for extra magnification lightcurves on top of the intrinsic ones but also shifts them in time domain, causing the actual time delays between images of strongly lensed active galactic nucleus to change on the ∼day(s) light-crossing timescale of the emission region. The microlensing-induced time delays would bias strong lens time-delay cosmography if unaccounted for. However, due to the uncertainties of the disk size and the disk model, the impact is hard to accurately estimate. In this work, we study how to reduce the bias with a designed observation strategy based on a standard disk model. We find that long time monitoring of the images could alleviate the impact since it averages the microlensing time-lag maps due to the peculiar motion of the source relative to the lens galaxy. In addition, images in bluer bands correspond to smaller disk sizes and therefore benefit time-delay measurements as well. We conduct a simulation based on a PG 1115+080-like lensed quasar. The results show that the time-delay dispersions caused by microlensing can be reduced by ∼40% with 20 yr lightcurves, while u band relative to r band reduces ∼75% of the dispersions. Nevertheless, such an effect cannot be totally eliminated in any cases. Further studies are still needed to appropriately incorporate this effect into inferences of an accurate Hubble constant.

Keywords: gravitational microlensing; strong gravitational lensing; quasar microlensing