Sazuna/UAT_keywords huggingface dataset:
Collapse of Rotating Very Massive Stellar Cores Leading to a Black Hole and a Massive Disk as a Source of Gravitational Waves. We derive models of rotating very massive stellar cores with mass ≍102─104 M⊙ that are marginally stable to the pair-unstable collapse, assuming that the core is isentropic and composed primarily of oxygen. It is shown that cores with mass ≲103 M⊙ can form a massive disk with the mass more than 10% of the core mass around the formed black hole if the core is rotating with more than 30% of the Keplerian limit. We also indicate that the formation of rapidly spinning massive black holes such as the black holes of GW231123 naturally accompanies massive disk formation. By using the result of our previous study, which showed that the massive disk is unstable to the nonaxisymmetric deformation, we predict the amplitude and frequency of gravitational waves and show that the collapse of rotating very massive stellar cores can be a promising source of gravitational waves for the Einstein Telescope. The detection of such gravitational waves will provide us with important information about the formation process of intermediate mass black holes.

Keywords: Gravitational wave sources
Energy-containing Electrons in Solar Flares: Improving Hard X-Ray and EUV Diagnostics. Solar flares effectively accelerate particles to nonthermal energies. These accelerated electrons are responsible for energy transport and subsequent emissions in hard X-ray (HXR), radio, and UV/EUV radiation. Due to the steeply decreasing electron spectrum, the electron population and consequently the overall flare energetics are predominantly influenced by low-energy nonthermal electrons. However, deducing the electron distribution in this energy-containing range remains a significant challenge. In this study, we apply the warm-target HXR emission model with kappa-form injected electrons to two well-observed GOES M-class flares. Moreover, we utilize EUV observations to constrain the flaring plasma properties, which enables us to determine the characteristics of accelerated electrons across a range from a few keV to tens of keV. We demonstrate that the warm-target model (WTM) reliably constrains the properties of flare-associated electrons, even accounting for the uncertainties that had previously been unaddressed. The application of a kappa distribution for the accelerated electrons allows for meaningful comparisons with electron distributions inferred from EUV observations, specifically for energy ranges below the detection threshold of RHESSI. Our results indicate that the accelerated electrons constitute only a small fraction of the total electron population within the flaring region. Moreover, the physical parameters, such as electron escape time and acceleration timescale, inferred from both the WTM and the EUV observations further support the scenario in which electrons undergo thermalization within the corona. This study highlights the effectiveness of integrating the WTM with EUV observations to accurately characterize energy-containing electrons and their associated acceleration and transport processes.

Keywords: Solar flares, Solar extreme ultraviolet emission, Solar physics, Solar x-ray flares, Active solar corona
An Analytical Model for the Eccentricity Cascade: Hot Jupiter Formation via S-type Instability. A widely explored pathway for hot Jupiter (HJ) formation is high-eccentricity migration driven by von Zeipel─Lidov─Kozai cycles induced by an exterior companion. However, for a distant or low-mass companion, this mechanism typically demands that the planet's initial orbit be very nearly perpendicular to that of the companion. In previous work, we demonstrated that such fine-tuning can be circumvented in the HAT-P-7 system, due to the presence of an intermediate body that efficiently couples the orbits of the planet and the distant companion—a mechanism we termed the eccentricity cascade (EC). In this work, we analytically characterize the dynamics governing the EC and delineate the parameter space within which it effectively operates. Our qualitative results are as follows: (i) The proto-HJ's eccentricity is most efficiently excited when the inner triple is on the verge of dynamical instability, (ii) the addition of a distant fourth body allows this instability to be approached gradually, and (iii) the instability mechanism is closely related to the stability of circumstellar (S-type) planets in binaries. By deriving an analytic criterion for S-type instability, we obtain closed-form expressions describing the onset of the EC. Our results show that efficient HJ formation via the EC occurs across a broad range of intermediate perturbers, highlighting its potential as a robust migration channel.

Keywords: Exoplanet migration, Orbits, Hot Jupiters
IGR J17091−3624: Newly Formed Periodic Dips and Multiwavelength Activity during the 2025 Outburst. The black hole low-mass X-ray binary (LMXB) candidate IGR J17091─3624 experienced a hard-state-only outburst in 2025. In this paper, we show that IXPE detected a series of intermittent X-ray dips, spanning a total interval of ∼1 day. Subsequent observations with NICER, the Einstein Probe, NuSTAR, and Swift reveal that these dips recur with a period of 2.83 ± 0.07 days and are accompanied by an increase in spectral hardness. This is the first time such quasiperiodic dipping behavior has been observed in this target since discovery. Our spectral analysis shows that the dips can be explained by obscuration from an ionized absorber characterized by an ionization parameter of  logξ∼1 ─3 erg cm s−1 and an equivalent hydrogen column density of  NHzxipcf∼(1─30)×1022  cm−2. The periodic reappearance of the absorber is likely caused by obscuring material located in the outer accretion disk, modulated by the binary orbital period. If confirmed, this period would suggest that the donor star in IGR J17091─3624 has deviated from the standard main-sequence evolutionary path and is likely a (partially) stripped giant. In the optical band, no significant periodicity or correlation with the X-ray dips was detected, whereas the radio counterpart exhibited a flat to steep spectrum, in contrast to the inverted spectrum typically observed during the hard state of LMXBs.

Keywords: Accretion, Black hole physics, X-ray transient sources
The Triple System V1371 Tau: An Eclipsing Binary with an Outer Be Star. Although triple systems are common, their orbital dynamics and stellar evolution remain poorly understood. We investigated the V1371 Tau system using TESS photometry, multiepoch spectroscopy, and recent interferometric data, confirming it as a rare triple system consisting of a eclipsing binary orbited by a classical Be star, with a spectral classification of (B1V + B0V) + B0Ve. The eclipsing binary exhibits an orbital period of ≍34 days, and the Be star orbits the inner pair on a timescale of a few years. Weak Hα emission lines suggest the presence of a Keplerian disk with variability on a timescale of months around the Be star, and nearly constant V/R ratio no detectable asymmetry variations. Besides the eclipses, frequencies at 0.24 and 0.26 cycles day─1 dominate the photometric variability. Higher-frequency signals are present, which appear associated with nonradial pulsations. The eclipsing pair (i ≍ 90∘) shows projected rotational velocities of 160 and 200 km s−1. The Be star's measured  vsini≍250  km s−1 implies a critical rotation fraction between 0.44 and 0.76 for plausible inclinations, significantly faster than the eclipsing components. The shallower eclipses in the KELT data compared to TESS suggest a variation in orbital inclination, possibly induced by Kozai─Lidov cycles from the outer Be star. The evolution analysis suggests that all components are massive main-sequence stars, with the secondary star in the eclipsing binary being overluminous. This study emphasizes the complexity of triple systems with Be stars and provides a basis for future research on their formation, evolution, and dynamics.

Keywords: Supernova remnants, Non-radial pulsations, Eclipsing binary stars, Trinary stars, Be stars, Stellar evolution
Hierarchical Structure and Self-gravity in the Rosette Molecular Cloud. We analyze the hierarchical structure in the Rosette molecular cloud using 13CO J = 1─0 data from the Milky Way Imaging Scroll Painting survey with a nonbinary Dendrogram algorithm that allows multiple branches to emerge from parent structures. A total of 588 substructures are identified, including 458 leaves and 130 branches. The physical parameters of the substructures—including peak brightness temperature (Tpeak), brightness temperature difference (Tdiff), radius (R), mass (M), velocity dispersion (σv), and surface density (Σ)—are characterized. The Tpeak and Tdiff distributions follow exponential functions with characteristic values above 5σrms. The statistical properties and scaling relations—i.e., the σv─R, M─R, and σv─RΣ relations—are in general consistent with those from traditional segmentation methods. The mass and radius follow power-law distributions with exponents of 2.2─2.5, with slightly flatter slopes for substructures inside the H II region. The velocity dispersion scales weakly with radius (σv ∝ R0.45±0.03, r = 0.58) but shows a tighter correlation with the product of surface density and size (σv ∝ (ΣR)0.29±0.01, r = 0.73). Self-gravitating substructures are found across scales from ∼0.2 to 10 pc, and nearly all structures with peak brightness above 4 K are gravitationally bound (αvir &lt; 2). The fraction of bound structures increases with mass, size, and surface density, supporting the scenario of global hierarchical collapse for the evolution of molecular clouds, in which molecular clouds and their substructures are undergoing multiscale collapse.

Keywords: Giant molecular clouds, Star forming regions, Interstellar clouds, Interstellar medium
Transformers for Stratified Spectropolarimetric Inversion: Proof of Concept. Solar spectropolarimetric inversion—inferring atmospheric conditions from the Stokes vector—is a key diagnostic tool for understanding solar magnetism, but traditional inversion methods are computationally expensive and sensitive to local minima. Advances in artificial intelligence offer faster solutions, but are often restricted to shallow models or a few spectral lines. We present a proof-of-concept study using a transformer machine learning model for multiline, full-Stokes inversion, to infer stratified parameters from synthetic spectra produced from 3D magnetohydrodynamic simulations. We synthesize a large set of Stokes vectors using forward modeling across 15 spectral lines spanning the deep photosphere toward the chromosphere. The model maps full-Stokes input to temperature, magnetic field strength, inclination, azimuth (encoded as  sin2ϕ ,  cos2ϕ ), and line-of-sight velocity as a function of optical depth. The transformer incorporates an attention mechanism that allows the model to focus on the most informative regions of the spectrum for each inferred parameter, and uses positional embedding to encode wavelength and depth order. We benchmark it against a multilayer perceptron (MLP), test robustness to noise, and assess generalization. The transformer outperforms the MLP, especially in the higher layers and for magnetic parameters, yielding higher correlations and more regularized stratifications. The model retains strong performance across a range of noise levels typical for real observations, with magnetic parameter inference degrading predictably while temperature and velocity remain stable. We explore attention maps, linking the transformer's learned behaviour to line-formation physics.

Keywords: Solar physics, Quiet sun, Solar magnetic fields, Magnetohydrodynamical simulations
Nonthermal Ionization of Kilonova Ejecta: Observable Impacts. The characteristic rapid rise and decline at optical wavelengths of a kilonova is the product of the low ejecta mass (≲0.05 M⊙) and high ejecta velocity (≳0.1c). We show that, even at very early times (≲2 days), regions of ejecta fall below critical density and temperature thresholds at which nonlocal thermodynamic equilibrium (NLTE) effects become important. Here, we present an approximate method for calculating the ionization state of the ejecta that accounts for the NLTE impact of high-energy electrons produced in the beta decay of freshly synthesized r-process elements. We find that incorporating ionization from high-energy electrons produces an "inverted" and "blended" ionization structure, where the most highly ionized species are located in the fastest moving homologous ejecta, and multiple ionization states coexist. In radiation transport calculations, the higher degree of ionization reduces line blanketing in optical bands, leading to improved agreement with the light-curve properties of AT 2017gfo, such as the duration, decay rates, brightness, and colors. Our quasi-NLTE implementation helps to alleviate tensions in kilonova modeling: for high-velocity (∼0.3 c) ejecta components, our models require less mass for a given peak brightness in optical bands, by as much as a factor of 3; our models can explain the presence of observed features associated to Sr II, W III, Se III, and Te III under conditions where LTE models would predict only neutral species; and we naturally predict the coexistence of species like Sr II and Ce III without the need for fine-tuning of the ejecta properties.

Keywords: R-process, Radiative transfer, High energy astrophysics
3D Nonequilibrium Ionization and Spectroscopic Modeling of Coronal Mass Ejections. III. Charge State Evolution. The ionization state of plasmas leaving the Sun freezes in low in the solar corona and carries information about the inner corona through the heliosphere. In situ charge state measurements provide excellent diagnostic tools for studying the early evolution and energetics of the solar wind and coronal mass ejections (CMEs). Three-dimensional global magnetohydrodynamics models allow us to link the measured in situ charge states to the heating/cooling mechanisms in the low corona to understand its evolution. In this final paper of the series, we use the 2008 April 9 CME (the "Cartwheel CME") simulated with the Alfvén Wave Solar atmosphere Model to understand the evolution of the charge states. The highest-ionization material in the flux rope is caused by the high temperatures early in the eruption, which freezes in quickly due to the large adiabatic expansion rates. The prominence preserves low ionization state material to 1 au because of the high radiative cooling rates, but only occupies about 10% of the cross-sectional area of the CME, making it less likely to be observed in situ. We discuss in situ charge state distribution properties that can be used for understanding CME plasmas and their heating processes. We show that between the Extreme ultraviolet Imaging Spectrometer slit location (1.1 R⊙) and the freeze-in height, there is a significant amount of unobserved charge state evolution that would be lost without the use of models.

Keywords: Magnetohydrodynamics, Recombination, Chemical abundances, Solar coronal mass ejection shocks, Ionization
Hubble Constant Constraint Using 117 FRBs with a More Accurate Probability Density Function for DM<SUB>diff</SUB>. Fast radio bursts (FRBs) are among the most mysterious astronomical transients. Due to their short durations and cosmological distances, their dispersion measure (DM)─redshift (z) relation is useful for constraining cosmological parameters and detecting the baryons in the Universe. The increasing number of localized FRBs in recent years has provided more precise constraints on these parameters. However, the larger dataset reveals limitations in the widely used probability density function (pdiff) for DMdiff, which refers to the diffuse electron term of the FRB DM. In this project, we collect 117 of the latest localized FRBs; discuss the effect of a more accurate σdiff, which is a parameter in pdiff and once thought of as "effective standard deviation"; and more clearly rewrite their likelihood to better constrain the parameters above. We find that the widely used approximation  σdiff∼F/z  only works under contrived assumptions and shows the greatest deviation from the true standard deviation in low redshift. In general, one should use an accurate method to derive this parameter from pdiff. Our method yields better constraints on  H0Ωbfdiff=2.813−0.258+0.250kms-1Mpc-1  or  H0=66.889−5.459+6.754kms-1Mpc-1  when combining the FRB data with cosmic microwave background measurements and taking fdiff = 0.84. This fully analytical correction helps us better constrain cosmological parameters with the increasing number of localized FRBs available today.

Keywords: Galaxy dark matter halos, Hubble constant, Radio bursts, Intergalactic medium, Baryon density, Radio transient sources
The Spatial Distribution of Intracluster Globular Clusters in the Fornax Cluster. We investigate the spatial distribution of the intracluster globular clusters (ICGCs) detected in the core of the Fornax galaxy cluster. By separately modeling different components of the observed population of globular clusters (GCs), we confirm the existence of an abundant ICGC overdensity with a geometrically complex, elongated morphology roughly centered on the cluster dominant galaxy NGC 1399 and stretching along the E─W direction. We identify several areas in the ICGC distribution that deviate from a simple elliptical model and feature large density enhancements. These regions are characterized based on their statistical significance, GCs excess number, position, size, and location relative to the galaxies in their surroundings. The relations between the spatial distribution and features of the ICGCs structures, mostly populated by blue GCs, and properties of the intracluster light and dwarf galaxies detected in the core of the Fornax cluster, are described and discussed. The line-of-sight velocity distribution of spectroscopically confirmed GCs within the ICGC structures is compatible with the systemic velocities of nearby bright galaxies in the Fornax cluster, suggesting that the ICGC population is at least partially composed of GCs stripped from their hosts. We argue that the findings here presented suggest that, on subcluster scales, different mechanisms contribute to the growth of the ICGC. The western region of Fornax is likely associated with old merging events that predate the assembly of the Fornax cluster. The eastern side instead points to a mix of tidal disruption of dwarf galaxies and stripping from the GCs system of massive hosts, more typical of relaxed, high-density cluster environments.

Keywords: Globular star clusters, Galaxy clusters
The AURORA Survey: Robust Helium Abundances at High Redshift Reveal a Subpopulation of Helium-enhanced Galaxies in the Early Universe. We present the first robust helium (He) abundance measurements in star-forming galaxies at redshifts 1.6 ≲ z ≲ 3.3 using deep, moderate-resolution JWST/NIRSpec spectroscopy from the AURORA survey. We establish a high-z He sample consisting of 20 galaxies with multiple high-signal-to-noise-ratio (&gt;5σ) He I emission-line detections, including the critical near-infrared λ10833 line. This is the first study at high redshift leveraging λ10833 to break degeneracies between temperature, electron density, optical depth, and He+/H+, enabling reliable He abundance determinations in the early Universe. We use a custom Markov Chain Monte Carlo framework incorporating direct-method electron temperature priors, extended optical depth (τλ3890) model grids up to densities of 106 cm−3, and simultaneous fits of the physical conditions and He I/H I line ratios to derive ionic He+/H+ abundances. Most of the AURORA galaxies follow the extrapolated z ∼ 0 He/H─O/H trend, indicating modest He enrichment by z ∼ 2─3. However, we identify a subpopulation of four galaxies that exhibit elevated He mass fractions (∆Y &gt; 0.03) without corresponding enhancements in N/O or α-elements (∼20% of the sample). This abundance pattern is inconsistent with enrichment from asymptotic giant branch stars, but favors early He enrichment from very massive stars (M ≳ 100M⊙), which can eject He-rich, N-poor material via stellar winds and binary stripping in young stellar populations. We speculate that these elevated-He systems may represent an early phase of globular cluster (GC) formation where N enrichment is still lagging behind He production. This work demonstrates the power of JWST multi-line He I spectroscopy for tracing early stellar feedback, enrichment pathways, and GC progenitor signatures in the high-redshift Universe.

Keywords: Globular star clusters, Interstellar line emission, Galaxy chemical evolution, Chemical abundances, Chemical enrichment, Radiative transfer, Interstellar medium, High-redshift galaxies
The White Dwarf Initial─Final Mass Relation from Open Clusters in Gaia DR3. The initial─final mass relation (IFMR) links a star's birth mass to the mass of its white dwarf (WD) remnant, providing key constraints on stellar evolution. Open clusters offer the most straightforward way to empirically determine the IFMR, as their well-defined ages allow for direct progenitor lifetime estimates. We construct the most comprehensive open cluster WD IFMR to date by combining new spectroscopy of 22 WDs with an extensive literature review of WDs with strong cluster associations. To minimize systematics, we restrict our analysis to spectroscopically confirmed hydrogen-atmosphere (DA) WDs consistent with single-stellar origins. We separately analyze a subset with reliable Gaia-based astrometric membership assessments, as well as a full sample that adds WDs with strong cluster associations whose membership cannot be reliably assessed with Gaia. The Gaia-based sample includes 69 spectroscopically confirmed DA WDs, more than doubling the sample size of previous Gaia-based open cluster IFMRs. The full sample, which includes 53 additional literature WDs, increases the total number of cluster WDs by over 50% relative to earlier works. We provide functional forms for both the Gaia-based and full-sample IFMRs. The Gaia-based result useful for Mi ≥ 2.67 M⊙ is  Mf=0.179−0.100H(Mi−3.84M⊙)×(Mi−3.84M⊙)+0.628M⊙ , where H(x) is the Heaviside step function. Comparing our IFMR to recent literature, we identify significant deviations from best-fit IFMRs derived from both Gaia-based volume-limited samples of field WDs and double WD binaries, with the largest discrepancy occurring for initial masses of about 5 M⊙.

Keywords: Open star clusters, White dwarf stars, Compact objects, Stellar evolution