Sazuna/UAT_keywords huggingface dataset:
Prevalence of SED Turndown among Classical Be Stars: Are All Be Stars Close Binaries?. Rapid rotation is a fundamental characteristic of classical Be stars and a crucial property allowing for the formation of their circumstellar disks. Past evolution in a mass and angular momentum transferring binary system offers a plausible solution to how Be stars attained their fast rotation. Although the subdwarf remnants of mass donors in such systems should exist in abundance, only a few have been confirmed due to tight observational constraints. An indirect method of detecting otherwise hidden companions is offered by their effect on the outer parts of Be star disks, which are expected to be disrupted or truncated. In the context of the infrared and radio continuum excess radiation originating in the disk, the disk truncation can be revealed by a turndown in the spectral energy distribution due to reduced radio flux levels. In this work, we search for signs of spectral turndown in a sample of 57 classical Be stars with radio data, which include new data for 23 stars and the longest-wavelength detections so far (λ ≈ 10 cm) for two stars. We confidently detect the turndown for all 26 stars with sufficient data coverage (20 of which are not known to have close binary companions). For the remaining 31 stars, the data are inconclusive as to whether the turndown is present or not. The analysis suggests that many if not all Be stars have close companions influencing their outer disks. If confirmed to be subdwarf companions, the mass transfer spin-up scenario might explain the existence of the vast majority of classical Be stars.

Keywords: Binary stars, Subdwarf stars, Stellar rotation, Early-type emission stars, Infrared excess, Stellar astronomy, Circumstellar matter, Circumstellar disks, Radio continuum emission, Be stars, Radio astronomy, Stellar evolution
The Long-standing Closure Crisis in Coronal Plasmas. Coronal and solar wind physics have long used plasma fluid models to motivate physical explanations of observations; the hypothesized model is introduced into a fluid simulation to see if observations are reproduced. This procedure is called Verification of Mechanism (VoM) modeling; it is contingent on the self consistency of the closure that made the simulation possible. Inner corona VoMs typically assume weak gradient Spitzer-Braginskii closures. Four prominent coronal VoMs in place for decades are shown to contradict their closure hypotheses, demonstrably shaping coronal and solar wind research. These findings have been possible since 1953. This unchallenged evolution is worth understanding, so that similarly flawed VoMs do not continue to mislead new research. As a first step in this direction, this paper organizes four a posteriori quantitative tests for the purpose of easily screening the physical integrity of a proposed VoM. A fifth screen involving the thermal force, the tandem of the heat flux, has been shown to be mandatory when VoMs involve species-specific energy equations. VoM modeling will soon be required to advance Parker Solar Probe and Solar Orbiter science. Such modeling cannot advance the physical understanding sought by these missions unless the closures adopted (i) are demonstrated to be self consistent for the VoM plasma Knudsen numbers, (ii) are verified a posteriori as possessing nonnegative VDFs throughout the simulated volume, and (iii) include the physical completeness of thermal force physics when the VoM requires species-specific energy equations.

Keywords: Solar coronal heating, Solar wind, Astrophysical fluid dynamics, Solar corona
Measurement of the Extragalactic Background Light Spectral Energy Distribution with VERITAS. The extragalactic background light (EBL), a diffuse photon field in the optical and infrared range, is a record of radiative processes over the universe’s history. Spectral measurements of blazars at very high energies (&gt;100 GeV) enable the reconstruction of the spectral energy distribution (SED) of the EBL, as the blazar spectra are modified by redshift- and energy-dependent interactions of the gamma-ray photons with the EBL. The spectra of 14 VERITAS-detected blazars are included in a new measurement of the EBL SED that is independent of EBL SED models. The resulting SED covers an EBL wavelength range of 0.56-56 μm, and is in good agreement with lower limits obtained by assuming that the EBL is entirely due to radiation from cataloged galaxies.

Keywords: Extragalactic astronomy, Diffuse radiation, Active galactic nuclei, Cosmology
Population Synthesis of Black Hole Binaries with Normal-star Companions. I. Detached Systems. Optical observations of normal stars in binary systems with massive unseen objects have been proposed to search for candidate black holes (BHs) and provide a direct measurement of their dynamical masses. In this paper, we have performed binary population synthesis calculations to simulate the potential population of detached binaries containing BHs and normal-star companions in the Galaxy. We focus on the influence of the BH progenitors. In the traditional model, BHs in binaries evolve from stars more massive than ∼25M ⊙. However, it is difficult for this model to produce BH low-mass X-ray binaries. Recent investigations of massive star evolution have suggested that the BH progenitors have masses as low as ∼15M ⊙. Based on this model, we provide the expected distributions of various parameters for detached BH binaries with normal-star companions, including the component masses, the orbital parameters of the binary systems, the radial velocity semi-amplitudes, and the astrometric signatures of the optical companions. Our calculations show that there are thousands of such detached binaries in the Galaxy, and hundreds of them are potentially observable systems with luminous companions brighter than 20 mag. In addition, detached BH binaries are dominated by those with main-sequence companions and only a small percent of them are expected to have giant companions.

Keywords: Binary stars, Black holes, Stellar evolution
Orbital Evolution of Eccentric Low-mass Companions Embedded in Gaseous Disks: Testing the Local Approximation. We study the tidal interaction between a low-mass companion (e.g., a protoplanet or a black hole) in orbit about a central mass, and the accretion disk within which it is submerged. We present results for a companion on a coplanar orbit with eccentricity, e, between 0.1 and 0.6. For these eccentricities, dynamical friction arguments in its local approximation, that is, ignoring differential rotation and the curvature of the orbit, provide simple analytical expressions for the rates of energy and angular momentum exchange between the disk and the companion. We examine the range of validity of the dynamical friction approach by conducting a series of hydrodynamical simulations of a perturber with softening radius R soft embedded in a two-dimensional disk. We find close agreement between predictions and the values in simulations provided that R soft is chosen sufficiently small, below a threshold value {\tilde{R}}soft}, which depends on the disk parameters and on e. We give {\tilde{R}}soft} for both razor-thin disks and disks with a finite scale height. For point-like perturbers, the local approximation is valid if the accretion radius is smaller than {\tilde{R}}soft}. This condition imposes an upper value on the mass of the perturber.

Keywords: Hydrodynamical simulations, Hydrodynamics, Galaxy accretion disks, Protoplanetary disks, Planetesimals
The M101 Satellite Luminosity Function and the Halo-Halo Scatter among Local Volume Hosts. We have obtained deep Hubble Space Telescope (HST) imaging of 19 dwarf galaxy candidates in the vicinity of M101. Advanced Camera for Surveys HST photometry for two of these objects showed resolved stellar populations and tip of the red giant branch derived distances (D ∼ 7 Mpc) consistent with M101 group membership. The remaining 17 were found to have no resolved stellar populations, meaning they are either part of the background NGC 5485 group or are distant low surface brightness (LSB) galaxies. It is noteworthy that many LSB objects that had previously been assumed to be M101 group members based on projection have been shown to be background objects, indicating the need for future diffuse dwarf surveys to be very careful in drawing conclusions about group membership without robust distance estimates. In this work we update the satellite luminosity function of M101 based on the presence of these new objects down to M  V  = -8.2. M101 is a sparsely populated system with only nine satellites down to M  V  ≈ -8, as compared with 26 for M31 and 24.5 ± 7.7 for the median host in the Local Volume. This makes M101 by far the sparsest group probed to this depth, although M94 is even sparser to the depth at which it has been examined (M  V  = -9.1). M101 and M94 share several properties that mark them as unusual compared with the other Local Volume galaxies examined: they have a very sparse satellite population but also have high star-forming fractions among these satellites; such properties are also found in the galaxies examined as part of the Satellites around Galactic Analogs survey. We suggest that these properties appear to be tied to the wider galactic environment, with more isolated galaxies showing sparse satellite populations that are more likely to have had recent star formation, while those in dense environments have more satellites that tend to have no ongoing star formation. Overall, our results show a level of halo-to-halo scatter between galaxies of similar mass that is larger than is predicted in the lambda cold dark matter model.

Keywords: Luminosity function, Dwarf galaxies, HST photometry, Galaxy stellar halos, Galaxy evolution, Galaxy groups
Spectral Diagnostics of Cool Flare Loops Observed by the SST. I. Inversion of the Ca II 8542 Å and Hβ Lines. Flare loops form an integral part of eruptive events, being detected in the range of temperatures from X-rays down to cool chromospheric-like plasmas. While hot loops are routinely observed by the Solar Dynamics Observatory’s Atmospheric Imaging Assembly, cool loops seen off-limb are rare. In this paper we employ unique observations of the SOL2017-09-10T16:06 X8.2-class flare which produced an extended arcade of loops. The Swedish 1 m Solar Telescope made a series of spectral images of the cool off-limb loops in the Ca II 8542 Å and the hydrogen Hβ lines. Our focus is on the loop apices. Non-local thermal equilibrium (non-LTE; i.e., departures from LTE) spectral inversion is achieved through the construction of extended grids of models covering a realistic range of plasma parameters. The Multilevel Accelerated Lambda Iterations code solves the non-LTE radiative-transfer problem in a 1D externally illuminated slab, approximating the studied loop segment. Inversion of the Ca II 8542 Å and Hβ lines yields two similar solutions, both indicating high electron densities around 2 × 1012 cm-3 and relatively large microturbulence around 25 km s-1. These are in reasonable agreement with other independent studies of the same or similar events. In particular, the high electron densities in the range 1012-1013 cm-3 are consistent with those derived from the Solar Dynamics Observatory’s Helioseismic and Magnetic Imager white-light observations. The presence of such high densities in solar eruptive flares supports the loop interpretation of the optical continuum emission of stars which manifest superflares.

Keywords: Solar activity, Stellar flares, Solar flares, Solar flare spectra
Chromospheric UV Bursts and Turbulent-driven Magnetic Reconnection. We use Interface Region Imaging Spectrograph (IRIS) spacecraft data to study a group of Chromospheric ultraviolet bursts (UVBs) associated with an active region. We classify the UVBs into two types: smaller ones that can only be measured once by the scanning slit, and larger UVBs that are measured twice by the slit. The UVBs’ optically thin Si IV 1402.77 Å line profiles are studied intensively. By fitting the smaller UVBs’ lines with 1-2 Gaussians, we obtain a variety of line-of-sight flow measurements that hint various 3D orientations of small-scale magnetic reconnections, each associated with a UVB. The larger UVBs are, however, unique in a way that they each have two sets of measurements at two slit locations. This makes it possible to unambiguously detect two oppositely directed heated flows jetting out of a single UVB, a signature of magnetic reconnection operating at the heart of the UVB. Here, we report on the first of such an observation. Additionally, all the optically thin Si IV 1402.77 Å line profiles from those UVBs consistently demonstrate excessive broadening, an order of magnitude larger than would be expected from thermal broadening, suggesting that those small-scale reconnections could be driven by large scale (macroscale) turbulence in the active region.

Keywords: Interplanetary turbulence, Solar ultraviolet emission, Solar chromospheric heating, Solar magnetic reconnection
Exploring the Asymmetry of the Solar Corona Electron Density with Very Long Baseline Interferometry. The Sun’s corona has interested researchers for multiple reasons, including the search for a solution to the famous coronal heating problem and a purely practical consideration of predicting geomagnetic storms on Earth. There exist numerous different theories regarding the solar corona; therefore, it is important to be able to perform comparative analysis and validation of those theories. One way that could help us move toward the answers to those problems is the search for observational methods that could obtain information about the physical properties of the solar corona and provide means for comparing different solar corona models. In this work we present evidence that very long baseline interferometry (VLBI) observations are, in certain conditions, sensitive to the electron density of the solar corona and are able to distinguish between different electron density models, which makes the technique of VLBI valuable for solar corona investigations. Recent works on the subject used a symmetric power-law model of the electron density in solar plasma; in this work, an improvement is proposed based on a three-dimensional numerical model.

Keywords: Space plasmas, Very long baseline interferometry, Solar wind, Solar physics, Solar corona, Radio interferometry
A Cepheid-based Distance to the Seyfert Galaxy NGC 6814. We present a Cepheid-based distance to the nearby Seyfert galaxy NGC 6814 from Hubble Space Telescope observations. We obtained F555W and F814W imaging over the course of 12 visits with logarithmic time spacing in 2013 August-October. We detected and made photometric measurements for 16,469 unique sources across all images in both filters, from which we identify 90 excellent Cepheid candidates spanning a range of periods of 13-84 days. We find evidence for incompleteness in the detection of candidates at periods &lt;21 days. Based on the analysis of Cepheid candidates above the incompleteness limit, we determine a distance modulus for NGC 6814 relative to the LMC of {μ }rel{LMC}}={13.200}-0.031+0.031 mag. Adopting the recent constraint of the distance modulus to the LMC determined by Pietrzyński et al., we find m-M={31.677}-0.041+0.041 which gives a distance of 21.65 ± 0.41 Mpc to NGC 6814.

Keywords: AGN host galaxies, Seyfert galaxies, Cepheid distance, Spiral galaxies
VER J2227+608: A Hadronic PeVatron Pulsar Wind Nebula?. We report the detection of GeV γ-ray emission from the very-high-energy γ-ray source VER J2227+608 associated with the “tail” region of supernova remnant (SNR) G106.3+2.7. The GeV γ-ray emission is extended and spatially coincident with molecular clouds traced by CO emission. The broadband GeV to TeV emission of VER J2227+608 can be well fitted by a single power-law function with an index of 1.90 ± 0.04, without obvious indication of spectral cutoff toward high energies. The pure leptonic model for the γ-ray emission can be marginally ruled out by the X-ray and TeV data. In the hadronic model, the low energy content of CRs and the hard γ-ray spectrum, in combination with the center bright source structure, suggest that VER J2227+608 may be powered by the Pulsar wind nebula instead of shocks of the SNR. And the cutoff energy of the proton distribution needs to be higher than ∼400 TeV, which makes it an attractive PeVatron candidate. Future observations by the upcoming Large High Altitude Air Shower Observatory and the Cherenkov Telescope Array in the north could distinguish these models and constrain the maximum energy of cosmic rays in SNRs.

Keywords: Extrasolar radiation, Gamma-ray sources, Extended radiation sources, Cosmic ray astronomy, Astronomical radiation sources, Supernova remnants, Diffuse nebulae, Non-thermal radiation sources, Cosmic ray sources, Gamma-rays, Cosmic rays
Oscillator Strengths for the Lyman and Werner Bands of Molecular Hydrogen Studied by the Dipole (γ, γ) Method. Absolute optical oscillator strength (OOS) plays an extremely important role in molecular astrophysics research, especially for hydrogen. In this work, a novel dipole (γ, γ) method is used to determine the absolute OOSs for the Lyman and Werner bands of H2 with a high energy resolution of 25 meV. A detailed comparison between the obtained results and earlier experimental and theoretical data is presented, and great agreement between the present work and the calculated results is achieved by taking the non-adiabatic effect into account. The total integrated OOSs are also obtained and discussed. The absolute OOSs for the Lyman and Werner bands of H2 reported in this work can serve as a benchmark to test various theoretical methods and can be applied in molecular astrophysics.

Keywords: Molecular physics, Laboratory astrophysics, Astronomy databases
Coronae of Zero/Low-metal, Low-mass Stars. Recent theoretical studies suggest the existence of low-mass, zero-metal stars in the current universe. To study the basic properties of the atmosphere of low-mass first stars, we perform one-dimensional magnetohydrodynamical simulations for the heating of coronal loops on low-mass stars with various metallicities. While the simulated loops are heated up to ≥106 K by the dissipation of Alfvénic waves originating from the convective motion irrespective of metallicity, the coronal properties sensitively depend on the metallicity. Lower-metal stars create hotter and denser coronae because the radiative cooling is suppressed. The zero-metal star gives more than 40 times higher coronal density than the solar-metallicity counterpart, and as a result, the UV and X-ray fluxes from the loop are several times higher than those of the solar-metallicity star. We also discuss the dependence of the coronal properties on the length of the simulated coronal loops.

Keywords: Magnetohydrodynamics, Population III stars, Low mass stars, Stellar coronal loops, Stellar coronae
Investigating Early-type Galaxy Evolution with a Multiwavelength Approach. III. Insights from SPH Simulations with Chemophotometric Implementation. We are exploring galaxy evolution in low-density environments exploiting smooth particle hydrodynamic simulations, including chemophotometric implementation. From a large grid of simulations of galaxy encounters and mergers starting from triaxial halos of gas and dark matter, we single out the simulations matching the global properties of our targets. These simulations are used to give insights into their evolution. We focus on 11 early-type galaxies selected because of their nearly passive stage of evolution in the nuclear region. However, a variety of UV features are detected in more than half of these galaxies. We find no significant differences in the formation mechanisms between galaxies with or without UV features. Major and minor mergers are able to reproduce their peculiar UV morphologies, and galaxy encounters are more suitable for “normal” early-type galaxies. Their star formation rate self-quenches several gigayears later than the merger/encounter occurred via gas exhaustion and stellar feedback, moving the galaxy from blue to red colors and driving the galaxy transformation. The length of the quenching is mass-dependent and lasts from 1 to 5 Gyr or more in the less massive systems. All of our targets are gas-rich at redshift 1. Three of them assembled at most 40% of their current stellar mass at z &gt; 1, and seven assembled more than 40% between redshift 0.5 and 1. Their stellar mass grows by 4% by crossing the green valley before reaching their current position on the NUV-r versus M  r  diagram.

Keywords: Galaxy dark matter halos, Galaxy formation, Galaxy interactions, Galaxy quenching, Galaxy ages, Computational methods, Galaxy evolution, Galaxy masses, Early-type galaxies
Diagnostics of Collisions between Electrons and Water Molecules in Near-ultraviolet and Visible Wavelengths. We studied the dissociation reactions of electron impact on water vapor for several fragment species at optical and near-ultraviolet wavelengths (200-850 nm). The resulting spectrum is dominated by the hydrogen Balmer series, by the OH (A 2Σ+ - X 2Π) band, and by the emission of ionic H2O+(A 2A1 - X 2B1) and OH+(A 3Π - X 3Σ-) band systems. Emission cross sections and reaction channel thresholds were determined for energies between 5 and 100 eV. We find that the electron impact dissociation of H2O results in an emission spectrum of the OH (A 2Σ+ - X 2Π) band that is distinctly different from the emission spectra from other excitation mechanisms seen in planetary astronomy. We attribute the change to a strongly non-thermal population of rotational states seen in planetary astronomy. This difference can be utilized for remote probing of the contribution of different physical reactions in astrophysical environments.

Keywords: Small Solar System bodies, Laboratory astrophysics, Molecular spectroscopy, Atomic spectroscopy, Excitation rates, Collision physics, Electron impact ionization
A NICER Thermonuclear Burst from the Millisecond X-Ray Pulsar SAX J1808.4-3658. The Neutron Star Interior Composition Explorer (NICER) has extensively monitored the 2019 August outburst of the 401 Hz millisecond X-ray pulsar SAX J1808.4-3658. In this Letter, we report on the detection of a bright helium-fueled Type I X-ray burst. With a bolometric peak flux of (2.3 ± 0.1) × 10-7 erg s-1 cm-2, this was the brightest X-ray burst among all bursting sources observed with NICER to date. The burst shows a remarkable two-stage evolution in flux, emission lines at 1.0 and 6.7 keV, and burst oscillations at the known pulsar spin frequency, with ≈4% fractional sinusoidal amplitude. We interpret the burst flux evolution as the detection of the local Eddington limits associated with the hydrogen and helium layers of the neutron star envelope. The emission lines are likely associated with Fe, due to reprocessing of the burst emission in the accretion disk.

Keywords: X-ray bursts, Millisecond pulsars
Inferring the Presence of Tides in Detached White Dwarf Binaries. Tidal interactions can play an important role as compact white dwarf (WD) binaries are driven together by gravitational waves (GWs). This will modify the strain evolution measured by future space-based GW detectors and impact the potential outcome of the mergers. Surveys now and in the near future will generate an unprecedented population of detached WD binaries to constrain tidal interactions. Motivated by this, I summarize the deviations between a binary evolving under the influence of only GW emission and a binary that is also experiencing some degree of tidal locking. I present analytic relations for the first and second derivative of the orbital period and braking index. Measurements of these quantities will allow the inference of tidal interactions, even when the masses of the component WDs are not well constrained. Finally, I discuss tidal heating and how it can provide complimentary information.

Keywords: Close binary stars, White dwarf stars, Eclipsing binary stars, Gravitational waves