Sazuna/UAT_keywords huggingface dataset:
Cosmological Simulations of Stellar Halos with Gaia Sausage–Enceladus Analogs: Two Sausages, One Bun?. Observations of the Milky Way's stellar halo find that it is predominantly comprised of a radially biased population of stars, dubbed the Gaia Sausage–Enceladus, or GSE. These stars are thought to be debris from dwarf galaxy accretion early in the Milky Way's history. Though typically considered to be from a single merger, it is possible that the GSE debris has multiple sources. To investigate this possibility, we use the ILLUSTRISTNG50 simulation to identify stellar accretion histories in 98 Milky Way analogs—the largest sample for which such an identification has been performed—and find GSE-like debris in 32, with two-merger GSEs accounting for a third of these cases. Distinguishing single-merger GSEs from two-merger GSEs is difficult in common kinematic spaces, but differences are more evident through chemical abundances and star formation histories. This is because single-merger GSEs are typically accreted more recently than the galaxies in two-merger GSEs: the median infall times (with 16th and 84th percentiles) are   and   Gyr ago for single- and two-merger scenarios, respectively. The systematic shifts in abundances and ages that occur as a result suggest that efforts in modeling these aspects of the stellar halo prove ever-important in understanding its assembly.

Keywords: Stellar kinematics, Galactic archaeology, Galaxy stellar halos
Transition from Vortical to Alfvénic-like Fermi Electron Acceleration in Magnetic Reconnection with Increasing Guide Field. Using particle-in-cell simulations of magnetic reconnection (MR), we investigate how the changing magnetic guide field strength impacts the evolution of electron Kelvin–Helmholtz instability (EKHI) and the associated Fermi electron acceleration proposed by H. Che &amp; G. P. Zank. Through this investigation, an Alfvénic-like Fermi electron acceleration mechanism is discovered for strong guide field MR Bg/B0 &gt; 2.5, where Bg is the magnetic guide field. The electrons are accelerated by the intensive electric potential produced through  , where the ion velocity fluctuations   propagate parallel to the direction of the Alfvén-like waves. Differing from the two-stage second-order Fermi acceleration produced by the stochastic electric field of EKHI, the Alfvén-like wave mechanism is a much more efficient one-stage process that produces a much harder power-law electron energy spectrum, with an index ∼2, than that of the EKHI, with an index ∼4.

Keywords: Solar energetic particles, Solar flares, Solar magnetic reconnection
EPOCHS. XI. The Structure and Morphology of Galaxies in the Epoch of Reionization to z ∼ 12.5. We present a structural analysis of 520 galaxy candidates at 6.5 &lt; z &lt; 12.5 with a signal-to-noise ratio of &gt;10σ in the F444W filter taken from the EPOCHS v1 sample, consisting of uniformly reduced deep JWST NIRCam data covering the CEERS, JADES GOODS-S, NGDEEP, SMACS-0723, GLASS, and PEARLS surveys. We use standard software to fit single Sérsic models to each galaxy in the rest-frame optical and extract their parametric structural parameters (Sérsic index, half-light radius, and axis ratio) and Morfometryka to measure their nonparametric concentration and asymmetry parameters. We find a wide range of sizes for these early galaxies, with galaxy sizes overall continuing to become progressively smaller in the high-redshift regime, following   kpc. We further find a galaxy size–mass correlation up to z ∼ 12, with galaxies of a given mass also becoming smaller. Using nonparametric methods, we find that galaxy merger fractions, classified through asymmetry parameters, at these redshifts remain consistent with those in the literature, maintaining a value of fm ∼ 0.12 ± 0.07 showing little dependence with redshift when combined with the literature at z &gt; 4. We find that galaxies that are smaller in size also appear rounder, with an excess of high axis ratio objects. Finally, we artificially redshift a subsample of our objects to determine how robust the observational trends we see are, determining that the observed trends are due to real evolutionary effects, rather than being a consequence of redshift effects.

Keywords: Galaxy structure, High-redshift galaxies, Galaxies
Alignment of Blue/Green and Red Early-type Galaxies with Large-scale Filaments Reveals Distinct Evolutionary Pathways. We investigate the alignment of nonred early-type galaxies (ETGs) with blue or green colors within large-scale filaments and compare this alignment pattern with that of red ETGs. Our analysis reveals a significant alignment of the major axes of red ETGs with the orientations of their host cosmic filaments, consistent with prior research. In contrast, nonred ETGs show no significant alignment signal. This divergence in alignment behavior between nonred and red ETGs plausibly suggests distinct evolutionary pathways for nonred and red ETGs.

Keywords: Photometry, Galaxy evolution, Early-type galaxies
Revisiting Physical Properties of Mock Globular Cluster Tidal Tails. In this work, we present results of the first in-depth analysis of extra-tidal mock stars of Milky Way globular clusters recently generated by S. M. Grondin et al. Particularly, we selected a sample of globular clusters with a general consensus of being formed in the bulge or in the disk of the Milky Way. From the catalog, we estimated the width and the dispersion in the z-component of the angular momentum and in the line-of-sight and tangential velocities of their tidal tails, and compared the results with those predicted by cosmological simulations of K. Malhan et al. and observations. We found that the resulting values of these four quantities are not in agreement with an in situ formation of the associated globular clusters. On average, the resulting widths agree with an in situ origin, while the dispersion in the z-component of the angular momentum, and the dispersion in the line-of-sight and tangential velocities fail in matching this formation scenario. The four quantities derived for globular clusters formed in the bulge or in the disk show similar correlations with the stream length, namely, the width and the dispersion in the z-component of the angular momentum increase with the stream length, while the bulk of dispersion values in the line-of-sight and tangential velocities is around 12 km s‑1 along the mock stream.

Keywords: Globular star clusters, Galaxy kinematics, Milky Way Galaxy
Hyperluminous Supersoft X-Ray Sources in the Chandra Catalog. Hyperluminous supersoft X-ray sources (HSSs), such as bright extragalactic sources characterized by particularly soft X-ray spectra, offer a unique opportunity to study accretion onto supermassive black holes in extreme conditions. Examples of hyperluminous supersoft sources are tidal disruption events (TDEs), systems exhibiting quasiperiodic eruptions, changing-look active galactic nuclei, and anomalous nuclear transients. Although these objects are rare phenomena among the population of X-ray sources, we developed an efficient algorithm to identify promising candidates exploiting archival observations. In this work, we present the results of a search for HSSs in the recently released Chandra catalog of serendipitous X-ray sources. This archival search has been performed via both a manual implementation of the algorithm we developed and a novel machine learning–based approach. This search identified a new TDE, which might have occurred in an intermediate-mass black hole. This event occurred between 2001 and 2002, making it one of the first TDEs ever observed by Chandra.

Keywords: Supermassive black holes, Tidal disruption, X-ray active galactic nuclei
Double Beam Instability for the Mercury Upstream Waves. Mercury shock-upstream region is plasma physically of great interest, as the solar wind plasma may encounter two ion beams, forming a double-beam plasma system. Properties of the double-beam instability are studied semianalytically using the magnetoionic theory (cold plasma waves including beams), such as the unstable mode, the resonance wavenumbers, and the growth rates, for various beam configurations. The cold plasma wave theory supports the idea that both the foreshock ions and the pickup ions can potentially drive the right-hand beam instability, and moreover, the instability may run simultaneously for the two beam species. Further nonlinear wave evolution scenarios are discussed, such as independent parametric instabilities and driven wave–wave couplings causing low-frequency and high-frequency splits of the waves. The double-beam instability is testable against numerical simulations of the plasma waves as well as magnetic field observations by the MESSENGER spacecraft and the upcoming BepiColombo spacecraft.

Keywords: Space plasmas, Solar wind, Planetary bow shocks, Mercury (planet)
Observational Comparison between Confined and Eruptive Flares: Magnetohydrodynamics Instability Parameters in a Similar Magnetic Configuration. Unstable states of the solar coronal magnetic field structure result in various flare behaviors. In this study, we compared the confined and eruptive flares that occurred under similar magnetic circumstances in the active region 12673, on 2017 September 6, using the twist number, decay index, and height of magnetic field lines to identify observational behaviors of the flare eruption. We investigated the parameters from the magnetic field lines involved in an initial energy release, which were identified from the positions of the core of flare ribbons, i.e., flare kernels. The magnetic field lines were derived by nonlinear force-free field modeling calculated from the photospheric vector magnetic field obtained by the Solar Dynamics Observatory SDO/Helioseismic and Magnetic Imager, and flare kernels were identified from the 1600 Å data obtained by the SDO/Atmospheric Imaging Assembly. The twist number of all the magnetic field lines in the confined flare was below 0.6; however, the twist number in seven out of 24 magnetic field lines in the eruptive flare was greater than 0.6. These lines were tall. It is found that the decay index is not a clear discriminator of the confined and eruptive flares. Our study suggests that some magnetic field lines in the kink instability state may be important for eruptive flares, and that taller magnetic field lines may promote flare eruption.

Keywords: Magnetohydrodynamics, Solar flares, Solar magnetic reconnection, Solar physics, Solar coronal mass ejections
A Systematic NLTE Study of Very Metal-poor Stars with Metallicity down to ‑4.3 dex. I. Global Stellar Parameters Based on High-resolution Spectra. Very metal-poor (VMP) stars provide a record of the chemical composition and dynamics of the early Galaxy. Based on the high-resolution and high signal-to-noise ratio spectra from the Subaru Telescope for 103 VMP stars, in this series of papers we homogeneously investigate the nonlocal thermodynamic equilibrium (NLTE) abundances of important astrophysical elements. This sample covers a wide metallicity range from [Fe/H] ∼ ‑1.7 dex down to ‑4.3 dex, including 13 objects with [Fe/H] ≤ ‑3.0 dex. Here, we present a set of homogeneous stellar atmospheric parameters, including the effective temperature, surface gravity, metallicity, and microturbulence velocity with the spectroscopic method, and the NLTE line formation for both Fe I and Fe II in the classical one-dimensional model atmospheres have been considered. The NLTE effects of the Fe I lines range from ∼0.03 dex to ∼0.3 dex, and increase with decreasing metallicity. In addition, they depend on the surface gravity, which generally increases with decreasing  . The largest NLTE effects can be found for the giants of   ∼ 2.5 dex. For dwarfs and subgiants, our final effective temperatures are consistent with those derived from the TIRFM scales of A. Alonso et al. with a mean difference of 7.1 ± 100.2 K, while for giants, our results are slightly lower than those from the TIRFM scales of A. Alonso et al. with a mean difference of ‑69.5 ± 94.1 K. For dwarfs and subgiants, the spectroscopically derived surface gravity is consistent with that estimated based on the Gaia DR3 parallax; however, for giants, the former leads to a 0.2 dex lower surface gravity.

Keywords: Stellar abundances, Chemical abundances
The Spectral Variation Behavior of Fermi-LAT Blazars. In this study, we analyze the ∼16 yr Fermi-Large Area Telescope data of 160 blazars consisting of 92 flat-spectrum radio quasars (FSRQs), 64 BL Lacertae (BL Lac)–type objects, and 4 blazar-candidates-of-unknown-type objects, and investigate their spectral variation. By fitting a linear model (y = kx + b) to the monthly binned flux-index plots, we find that most of the FSRQs (∼89%) display a harder-when-brighter trend (k &gt; 0), while the majority of BL Lac–type objects (∼64%) exhibit a softer-when-brighter trend (k &lt; 0). By plotting their slope (k) versus their variability index and inverse Compton (IC) peak frequency, we find a moderate positive correlation between k and the variability index and a moderate negative correlation between k and the IC peak frequency, which we name as the gamma-ray variability sequence of blazars. Additionally, eight blazars may not follow the linear variation trend, particularly in PKS 0537-441, AP Librae, and PKS 1510-089. Additional very-high-energy gamma-ray emission components may be the cause of the nonlinear variation trend.

Keywords: Blazars
The Critical Role of Dust on the [O III] Planetary Nebula Luminosity Function's Bright-end Cutoff. We examine the relationship between circumnebular extinction and core mass for sets of [O III]-bright planetary nebulae (PNe) in the Large Magellanic Cloud and M31. We confirm that for PNe within 1 magnitude of the planetary nebula luminosity function's (PNLF's) bright-end cutoff magnitude (M*), higher core-mass PNe are disproportionally affected by greater circumnebular extinction. We show that this result can explain why the PNLF cutoff is so insensitive to population age. In younger populations, the higher-mass, higher-luminosity cores experience greater circumnebular extinction from the dust created by their asymptotic giant branch (AGB) progenitors compared to the lower-mass cores. We further show that when our core-mass–nebular extinction law is combined with post-AGB stellar evolutionary models, the result is a large range of population ages where the brightest PNe all have nearly identical [O III] luminosities. Finally, we note that while there is some uncertainty about whether the oldest stellar populations can produce PNe as bright as M*, this issue is resolved if the initial–final mass relation (IFMR) for the lowest-mass stars results in slightly more massive cores, as observed in some clusters. Alternatively, introducing a small amount of intrinsic scatter (0.022 M⊙) into the IFMR also addresses this uncertainty.

Keywords: Planetary nebulae nuclei, Circumstellar dust, Galaxy distances, Late stellar evolution
Spectral Line Search in Bright Gamma-Ray Bursts Observed by Fermi/GBM. The recently reported discoveries of emission lines with power-law decay from ∼37 to ∼6 MeV in the gamma-ray burst (GRB) 221009A have greatly contributed to probing the physics of GRBs, which also raises the natural question of whether there are more emission lines in bright GRBs. In this work, we search for emission lines in the top 30 highest fluence GRBs in the Gamma-Ray Burst Catalog observed by the Fermi/Gamma-ray Burst Monitor. Although no new lines are confirmed, there are a portion of GRBs that seem to show emission line features in their spectra with mean values of 61 keV, 110 keV, and 2.1 MeV, respectively. However, considering that the redshifts of these GRBs are different, the observed emission line features have almost identical central energy, thus the ∼110 keV spectral line should be caused by instrumental effects rather than GRB or other astronomical sources. The ∼61 keV and ∼2.1 MeV are comparable to 57.6 keV and 2.2 MeV, which are spectral lines confirmed to come from the excited energy level of 127I and the neutron capture line, respectively, as reported by previous studies. Our findings indicate that various effects may produce emission line-like features in the GRB spectrum and caution should be taken to carefully identify whether they are from GRB, other astronomical sources, or instrumental effects. Finally, the lack of spectral lines searched in other bright GRBs implies the special radiation mechanism of the spectral lines in GRB 221009A.

Keywords: Gamma-ray detectors, Gamma-ray bursts
Modeling the UV/Optical Light Curve of Re-brightening Tidal Disruption Events. In recent years, a new subclass of tidal disruption events (TDEs) has been reported in the literature. The light curve of these TDEs shows a re-brightening feature in the decline phase after the first peak, which then leads to a second flare. The re-brightening TDEs challenge the existing light-curve fitting tools designed to handle a single flare. In this work, we present a model aimed at reproducing the light curve of the re-brightening TDEs, based on the scenario that the consecutive flares are produced by the same star that experienced two partial TDEs. We also developed a fitting code from this model and applied it to two re-brightening TDEs: AT 2022dbl and AT 2023adr. The light curves of both TDEs are well-fitted. Finally, we forecast the time and peak brightness of the next flare for these two TDEs so that observers can prepare to examine our model.

Keywords: Supermassive black holes, Tidal disruption, Transient sources
Frequency-resolved Time Lags due to X-Ray Disk Reprocessing in AGN. Over the last years, a number of broadband reverberation mapping campaigns have been conducted to explore the short-term UV and optical variability of nearby AGN. Despite the extensive data collected, the origin of the observed variability is still debated in the literature. Frequency-resolved time lags offer a promising approach to distinguish between different scenarios as they probe variability on different timescales. In this study, we present the expected frequency-resolved lags resulting from X-ray reprocessing in the accretion disk. The predicted lags are found to feature a general shape that resembles that of observational measurements while exhibiting strong dependence on various physical parameters. Additionally, we compare our model predictions to observational data for the case of NGC 5548, concluding that the X-ray illumination of the disk can effectively account for the observed frequency-resolved lags and power spectra in a self-consistent way. To date, X-ray disk reprocessing is the only physical model that has successfully reproduced the observed multiwavelength variability, in both amplitude and time delays, across a range of temporal frequencies.

Keywords: Accretion, Reverberation mapping, Seyfert galaxies, Active galactic nuclei
Variation of Dense Gas Mass–Luminosity Conversion Factor with Metallicity in the Milky Way. HCN and HCO+ are the most common dense gas tracers used both in the Milky Way and external galaxies. The luminosity of HCN and HCO+ J = 1 → 0 lines are converted to a dense gas mass by the conversion factor, αQ. Traditionally, this αQ has been considered constant throughout the Galaxy and in other galaxies, regardless of the environment. We analyzed 17 outer Galaxy clouds and five inner Galaxy clouds with metallicities ranging from 0.38 Z⊙ to 1.29 Z⊙. Our analysis indicates that αQ is not constant; instead, it varies with metallicity. The metallicity-corrected αQ derived from the HCN luminosity of the entire cloud is almost three times higher in the outer Galaxy than in the inner Galaxy. In contrast, HCO+ seems less sensitive to metallicity. We recommend using the metallicity-corrected dense gas conversion factors   and   for extragalactic studies. Radiation from nearby stars has an effect on the conversion factor of a similar magnitude as that of the metallicity. If we extend the metallicity-corrected scaling relation for HCN to the Central Molecular Zone (CMZ), the value of   becomes one-third to one-half of the local values. This effect could partially account for the low star formation rate per dense gas mass observed in the CMZ.

Keywords: Gas-to-dust ratio, Molecular data, Star forming regions, Extinction, Interstellar medium, Metallicity
An Equilibrium Model of the Galaxy Determined by Element Abundance Gradients. We present a method to determine the gravitational potential of the Milky Way from measurements of the locations, velocities, and element abundances of stars in the Galactic disk. The method relies on the assumption that the Galaxy is axisymmetric and stationary and that element abundance ratios such as [Fe/H] and [Mg/Fe] are smooth functions of three isolating integrals of motion. We use Fisher Information theory to predict the efficacy of individual abundance ratios to constrain the potential. We also use N-body simulations to test the extent by which secular evolution in the disk introduces statistical and systematic errors into the analysis. We apply our method to a sample of stars from Gaia Data Release 3 and the APOGEE survey and infer the vertical force profile at the position of the Sun and the rotation curve in the midplane and find good agreement with previously published results. The residuals of the model show corrugations in [Fe/H] as a function of Lz, the angular momentum component along the spin axis of the Galaxy. Using the correspondence between Lz and Galactocentric cylindrical radius, we show that these features line up with the four spiral arms closest to the Sun.

Keywords: Galaxy structure, Galaxy abundances, Galaxy kinematics
Convective Overstability in Radially Global Protoplanetary Disks. II. Impact on Planetesimal Formation. The convective overstability (COS) is a hydrodynamic instability occurring in protoplanetary disk (PPD) regions with an adverse radial entropy gradient. It is a potential driver of turbulence and may influence planetesimal formation. In this second paper of our series, we study the effects of the COS on dust dynamics in radially global PPD simulations, focusing on the midplane region, where vertical gravity on the dust is included. Axisymmetric 2D simulations show susceptibility to both the COS and the vertically shearing streaming instability. For a Stokes number τ = 0.1, strong dust clumping occurs only for highly supersolar initial metallicities Z ≳ 0.05. In 3D nonaxisymmetric simulations, the COS generates large-scale, long-lived vortices that have the potential to efficiently concentrate dust, with dust accumulation strengthening as τ increases. For τ = 0.01, no strong clumping occurs even at metallicities as high as Z = 0.1, and vortices remain robust and long-lived. At τ ≈ 0.04, strong dust clumping is observed for solar metallicity (Z = 0.01) and higher. For τ = 0.1, clumping occurs even at strongly subsolar metallicities (Z ≳ 0.004), peaking at Z ∼ 0.01–0.03, including solar values. Under these conditions, vortices weaken significantly. At higher metallicities (Z ≳ 0.04) with τ = 0.1, large-scale vortex formation is suppressed, leading to nearly axisymmetric dust rings, which can still undergo clumping via the classical streaming instability.

Keywords: Protoplanetary disks, Planetesimals, Astrophysical fluid dynamics, Planet formation, Magnetohydrodynamical simulations