Sazuna/UAT_keywords huggingface dataset:
Persistent Non-Gaussian Structure in the Image of Sagittarius A* at 86 GHz. Observations of the Galactic Center supermassive black hole Sagittarius A* (Sgr A*) with very long-baseline interferometry (VLBI) are affected by interstellar scattering along our line of sight. At long radio observing wavelengths (≲1 cm), the scattering heavily dominates image morphology. At 3.5 mm (86 GHz), the intrinsic source structure is no longer sub-dominant to scattering, and thus the intrinsic emission from Sgr A* is resolvable with the Global Millimeter VLBI Array (GMVA). Long-baseline detections to the phased Atacama Large Millimeter/submillimeter Array (ALMA) in 2017 provided new constraints on the intrinsic and scattering properties of Sgr A*, but the stochastic nature of the scattering Requires multiple observing epochs to reliably estimate its statistical properties. We present new observations with the GMVA+ALMA, taken in 2018, which confirm non-Gaussian structure in the scattered image seen in 2017. In particular, the ALMA-GBT baseline shows more flux density than expected for an anistropic Gaussian model, providing a tight constraint on the source size and an upper limit on the dissipation scale of interstellar turbulence. We find an intrinsic source extent along the minor axis of ~100 μas both via extrapolation of longer wavelength scattering constraints and direct modeling of the 3.5 mm observations. Simultaneously fitting for the scattering parameters, we find an at-most modestly asymmetrical (major-to-minor axis ratio of 1.5 ± 0.2) intrinsic source morphology for Sgr A*.

Keywords: Galactic center, Black holes, Very long baseline interferometry, Interstellar medium, Radio interferometry
Asteroseismic Inference of the Central Structure in a Subgiant Star. Asteroseismic measurements enable inferences of the underlying stellar structure, such as the density and the speed of sound at various points within the interior of the star. This provides an opportunity to test stellar evolution theory by assessing whether the predicted structure of a star agrees with the measured structure. Thus far, this kind of inverse analysis has only been applied to the Sun and three solar-like main-sequence stars. Here we extend the technique to stars on the subgiant branch, and apply it to one of the best-characterized subgiants of the Kepler mission, HR 7322. The observation of mixed oscillation modes in this star facilitates inferences of the conditions of its inert helium core, nuclear-burning hydrogen shell, and the deeper parts of its radiative envelope. We find that despite significant differences in the mode frequencies, the structure near to the center of this star does not differ significantly from the predicted structure.

Keywords: Stellar ages, Stellar properties, Stellar evolutionary models, Stellar physics, Stellar oscillations, Asteroseismology, Stellar evolution
Fast Dissipation of Colliding Alfvén Waves in a Magnetically Dominated Plasma. Magnetic energy around compact objects often dominates over plasma rest mass, and its dissipation can power the object's luminosity. We describe a dissipation mechanism that works faster than magnetic reconnection. The mechanism involves two strong Alfvén waves with anti-aligned magnetic fields B1 and B2 that propagate in opposite directions along the background magnetic field B0 and collide. The collision forms a thin current sheet perpendicular to B0, which absorbs the incoming waves. The current sheet is sustained by an electric field E breaking the magnetohydrodynamic condition E &lt; B and accelerating particles to high energies. We demonstrate this mechanism with kinetic plasma simulations using a simple setup of two symmetric plane waves with amplitude A = B1/B0 = B2/B0 propagating in a uniform B0. The mechanism is activated when A &gt; 1/2. It dissipates a large fraction of the wave energy, f = (2A - 1)/A2, reaching 100% when A = 1. The plane geometry allows one to see the dissipation process in a one-dimensional simulation. We also perform two-dimensional simulations, enabling spontaneous breaking of the plane symmetry by the tearing instability of the current sheet. At moderate A of main interest, the tearing instability is suppressed. Dissipation transitions to normal, slower, magnetic reconnection at A ≫ 1. The fast dissipation described in this paper may occur in various objects with perturbed magnetic fields, including magnetars, jets from accreting black holes, and pulsar wind nebulae.

Keywords: High energy astrophysics, Alfven waves, Magnetic fields, Plasma astrophysics
AGILE Observations of Fast Radio Bursts. We report on a systematic search for hard X-ray and γ-ray emission in coincidence with fast radio bursts (FRBs) observed by the AGILE satellite. We used 13 yr of AGILE archival data searching for time coincidences between exposed FRBs and events detectable by the MCAL (0.4-100 MeV) and GRID (50 MeV-30 GeV) detectors at timescales ranging from milliseconds to days/weeks. The current AGILE sky coverage allowed us to extend the search for high-energy emission preceding and following the FRB occurrence. We considered all FRB sources currently included in catalogs and identified a subsample (15 events) for which a good AGILE exposure with either MCAL or GRID was obtained. In this paper we focus on nonrepeating FRBs, compared to a few nearby repeating sources. We did not detect significant MeV or GeV emission from any event. Our hard X-ray upper limits (ULs) in the MeV energy range were obtained for timescales from submillisecond to seconds, and in the GeV range from minutes to weeks around event times. We focus on a subset of five nonrepeating and two repeating FRB sources whose distances are most likely smaller than that of 180916.J0158+65 (150 Mpc). For these sources, our MeV ULs translate into ULs on the isotropically emitted energy of about 3 × 1046 erg, comparable to that observed in the 2004 giant flare from the Galactic magnetar SGR 1806-20. On average, these nearby FRBs emit radio pulses of energies significantly larger than the recently detected SGR 1935+2154 and are not yet associated with intense MeV flaring.

Keywords: Magnetars, X-ray transient sources, Gamma-ray transient sources, Radio transient sources
S<SUP>5</SUP>: The Destruction of a Bright Dwarf Galaxy as Revealed by the Chemistry of the Indus Stellar Stream. The recently discovered Indus stellar stream exhibits a diverse chemical signature compared to what is found for most other streams due to the abundances of two outlier stars, Indus_0 and Indus_13. Indus_13 exhibits an extreme enhancement in rapid neutron-capture (r-)process elements with [Eu/Fe] = + 1.81. It thus provides direct evidence of the accreted nature of r-process-enhanced stars. In this paper we present a detailed chemical analysis of the neutron-capture elements in Indus_13, revealing the star to be slightly actinide poor. The other outlier, Indus_0, displays a globular cluster-like signature with high N, Na, and Al abundances, while the rest of the Indus stars show abundances compatible with a dwarf galaxy origin. Hence, Indus_0 provides the first chemical evidence of a fully disrupted dwarf containing a globular cluster. We use the chemical signature of the Indus stars to discuss the nature of the stream progenitor which was likely a chemically evolved system, with a mass somewhere in the range from Ursa Minor to Fornax.

Keywords: Milky Way stellar halo, Stellar abundances, Dwarf galaxies, Globular star clusters
State-to-state Rate Constants for the H + LiH(v<SUB>0</SUB> = 0, j<SUB>0</SUB> = 0) Reaction: An Accurate Nonadiabatic Dynamical Study. The state-to-state rate constants for hydrogen abstraction, nonadiabatic hydrogen abstraction, and exchange channels of the H + LiH reaction have been studied in the temperature range from 10 to 5000 K by using the nonadiabatic time-dependent wave packet method. The total and vibrational state-resolved rate constants of the H + LiH (v0 = 0, j0 = 0) → Li(22S) + H2 reaction are calculated and compared with previous adiabatic values. The results indicated that adiabatic values always overestimate the rate constant due to the nonadiabatic effect not being considered. In addition, the ratio of adiabatic vibrational state-resolved rate constants versus that of nonadiabatic ones is calculated for the hydrogen abstraction channel. This reflects that the nonadiabatic effect is mainly focused on the low-lying vibrational states. Moreover, the rovibrational state-resolved rate constants show that the largest population of product is located at (v' = 2, j' = 11), (v' = 0, j' = 5), and (v' = 0, j' = 6) for the hydrogen abstraction, nonadiabatic hydrogen abstraction and exchange channels, respectively. The total and vibrational state-resolved rate constants of the Li(22P) → Li(22S) quenching process are also calculated in the temperature range up to 5000 K. The results show that when the temperature is lower than 200 K, the quenching efficiency increases rapidly, but with the further increase of temperature, the quenching efficiency hardly changes.

Keywords: Early universe, Theoretical models, Galaxies, Recombination (cosmology)
Raman-scattered O VI Features in the Symbiotic Nova RR Telescopii. RR Telescopii (RR Tel) is an interacting binary system in which a hot white dwarf (WD) accretes matter from a Mira-type variable star via gravitational capture of its stellar wind. This symbiotic nova shows intense Raman-scattered O VI 1032 and 1038 features at 6825 and 7082 Å. We present high-resolution optical spectra of RR Tel taken in 2016 and 2017 with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph at the Magellan-Clay telescope, Chile. We aim to study the stellar wind accretion in RR Tel from the profile analysis of Raman O VI features. With an asymmetric O VI disk model, we derive a representative Keplerian speed of &gt;35 km s-1, and the corresponding scale &lt;0.8 au. The best fit for the Raman profiles is obtained with a mass-loss rate of the Mira $\dot{M}\sim 2\times {10}^{-6}\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$ and a wind terminal velocity v∞ ~ 20 km s-1. We compare the MIKE data with an archival spectrum taken in 2003 with the Fiber-fed Extended Range Optical Spectrograph at the MPG/ESO 2.2 m telescope. It allows us to highlight the profile variation of the Raman O VI features, indicative of a change in the density distribution of the O VI disk in the last two decades. We also report the detection of O VI recombination lines at 3811 and 3834 Å, which are blended with other emission lines. Our profile decomposition suggests that the recombination of O VII takes place nearer to the WD than the O VI 1032 and 1038 emission region.

Keywords: High resolution spectroscopy, Binary stars, Emission line stars, Spectral line identification, Interacting binary stars, Novae, Symbiotic novae, Radiative transfer, Stellar accretion, Symbiotic binary stars, Stellar spectral lines
Brightest Cluster Galaxies and Intracluster Light: Their Mass Distribution in the Innermost Regions of Groups and Clusters. We improve the model presented in Contini &amp; Gu that describes the radial mass distribution of brightest cluster galaxies (BCGs) and the diffuse component also known as intracluster light (ICL), by assuming that the global BCG+ICL radial mass distribution follows the sum of three profiles: Jaffe and exponential profiles for the bulge and disk of the BCG, respectively, and a modified version of an NFW profile for the ICL. We take advantage of a wide sample of BCG+ICL systems simulated with our state-of-the-art semianalytic model to (a) investigate the reliability of our BCG+ICL distribution by looking at several scaling relations between the BCG+ICL stellar mass within different apertures and the total BCG+ICL/halo mass, at different redshifts; (b) make a prediction of the distance where the radial distribution transitions from BCG to ICL dominated. We find that our model nicely reproduces all the observed scaling relations investigated at the present time with a compelling degree of precision, but slightly biased low with respect to observations at higher redshifts (z ≳ 0.5). The transition radius predicted by our model is in good agreement with recent observational results, and spans a range between ~15 kpc and ~100 kpc. It mostly depends on the morphology of the BCG, whether it is bulge or disk dominated, on the amount of ICL with respect to the bulge and/or disk, and on the dynamical state of the group/cluster.

Keywords: Galaxy clusters, Galaxy evolution
Amplification of Turbulence in Contracting Prestellar Cores in Primordial Minihalos. We investigate the amplification of turbulence through gravitational contraction of the primordial gas in minihalos. We perform numerical simulations to follow the cloud collapse, assuming polytropic equations of state for different initial turbulent Mach numbers and resolutions. We find that the turbulent velocity is amplified solely by gravitational contraction, and eventually becomes comparable to the sound speed, even for small initial turbulent Mach numbers ( ${{ \mathcal M }}_{0}\gtrsim 0.05$ ). We derive an analytic formula for the amplification of turbulent velocity in a collapsing cloud, and find that our numerical results are consistent with the formula. These results suggest that the turbulence can play an important role in collapsing clouds for general cases.

Keywords: Early universe, Population III stars, Hydrodynamics, Collapsing clouds
Gravitational-wave Asteroseismology with f-modes from Neutron Star Binaries at the Merger Phase. Gravitational-wave signals from coalescing binary neutron stars (BNSs) can yield important information about the properties of nuclear-matter equation of state. We investigate a direct link between the frequency of the quadrupolar 2f-mode oscillation (f2f) of nonrotating and rotating neutron stars calculated by a nonlinear hydrodynamics code in the conformally flat approximation and the gravitational-wave frequency associated with the peak amplitude ( ${f}_{\max }$ ) of binary neutron stars from a set of publicly available simulations. We find that ${f}_{\max }$ and f2f differ by about 1%, on average, across 45 equal-mass systems with different total mass and equations of state. Interestingly, assuming that the gravitational-wave frequency is still approximately equal to twice the orbital frequency forb near the merger, the result indicates that the condition for tidal resonance ${(| m| {f}_{\mathrm{orb}})}_{m=2}={f}_{2f}$ is satisfied to high accuracy near the merger. Moreover, the well-established universal relation between ${f}_{\max }$ and the tidal deformability of equal-mass binary systems can now be explained by a similar relation between f2f and the tidal deformability of isolated neutron stars. For unequal-mass binaries, ${f}_{\max }$ increasingly deviates from f2f of the two stars as the mass ratio decreases from unity. Therefore, it is possible to relate the gravitational-wave signal at the merger of a BNS system directly to the fundamental oscillation modes and the mass ratio. This work potentially brings gravitational-wave asteroseismology to the late-inspiral and merger phases of BNSs.

Keywords: Neutron stars, Asteroseismology, Compact binary stars, Gravitational waves
Mass and Radius Relations of Quarkyonic Stars Using an Excluded-volume Model. Inspired by the excluded-volume model for isospin symmetric quarkyonic matter of Jeong et al., we construct an excluded-volume model for a charge-neutral quarkyonic phase whose hadronic sector contains only neutrons. We refer to this model as quarkyonic neutron matter. We compute the equation of state for this model and solve the Tolman-Oppenhermer-Volkoff equations to obtain mass and radius relations relevant for neutron stars. The most straightforward extension of the model for symmetric quarkyonic matter of Jeong et al. to quarkyonic neutron matter does not satisfy the mass-radius constraints from neutron star measurements. However, we show that by incorporating appropriate nuclear interactions in the excluded-volume model, one can produce mass-radius relations that lie within the constraints obtained from gravitational waves of binary neutron star mergers and maximum-mass measurements of neutron stars.

Keywords: Neutron stars, High energy astrophysics, Nuclear physics
On the Evolution of Supermassive Primordial Stars in Cosmological Flows. Primordial supermassive stars (SMSs) formed in atomic-cooling halos at z ~ 15-20 are leading candidates for the seeds of the first quasars. Past numerical studies of the evolution of SMSs have typically assumed constant accretion rates rather than the highly variable flows in which they form. We model the evolution of SMSs in the cosmological flows that create them using the Kepler stellar evolution and implicit hydrodynamics code. We find that they reach masses of 1 - 2 × 105 M⊙ before undergoing direct collapse to black holes (DCBHs) during or at the end of their main-sequence hydrogen burning, at 1-1.5 Myr, regardless of halo mass, spin, or merger history. We also find that realistic, highly variable accretion histories allow for a much greater diversity of supermassive stellar structures, including in some cases largely thermally relaxed objects, which may provide a significant source of radiative feedback. Our models indicate that the accretion histories predicted for purely atomic-cooling halos may impose a narrow spectrum of masses on the seeds of the first massive quasars; however, further studies incorporating realistic feedback will be essential in order to confirm whether or not this holds true in all cases. Our results also indicate that multiple SMSs at disparate stages of evolution can form in these halos, raising the possibility of SMS binaries and supermassive X-ray binaries, as well as DCBH mergers that could be detected by LISA.

Keywords: Supermassive black holes, Population III stars, Quasars, Massive stars, High-redshift galaxies
Enhanced Doppler Beaming for Dust-enshrouded Objects and Pulsars in the Galactic Center. Stars within the innermost part of the nuclear star cluster can reach orbital velocities up to a few percent of the light speed. As analyzed by Rafikov, Doppler boosting of stellar light may be of relevance at the pericenter of stellar orbits, especially with the upcoming high-precision photometry in the near- and mid-infrared bands. Here we analyze the previously neglected effect of the infrared spectral index of monitored objects on the Doppler-boosted continuum emission in a narrow photometric band. In contrast to main-sequence stars, the detected compact infrared-excess dust-enshrouded objects have an enhanced Doppler-boosting effect by as much as an order of magnitude, with the variability amplitude of the order of 10% for the most eccentric orbits. In a similar way, pulsars dominated by nonthermal synchrotron emission are also expected to exhibit a stronger Doppler-boosted signal by a factor of at least 4 in comparison with canonical S stars. In case the stellar orbit is robustly determined, the relative flux variation can thus provide hints about the nature of the objects. For extended dust-enshrouded objects, such as G1, that are variable due to tidal, ellipsoidal, bow-shock, and irradiation effects, the subtraction of the expected Doppler-boosting variations will help to better comprehend their internal physics. In addition, the relative flux variability due to higher-order relativistic effects is also modified for different negative spectral indices in a way that it can obtain both positive and negative values with the relative variability of the order of 1%.

Keywords: Stellar photometry, Galactic center, Relativistic binary stars
On the Origin of Stochastic, Low-Frequency Photometric Variability in Massive Stars. High-precision photometric observations have revealed ubiquitous stochastic low-frequency photometric variability in early-type stars. It has been suggested that this variability arises due to either subsurface convection or internal gravity waves launched by the convective core. Here we show that relevant properties of convection in subsurface convective layers correlate very well with the timescale and amplitude of stochastic low-frequency photometric variability, as well as with the amplitude of macroturbulence. We suggest that low-frequency, stochastic photometric variability and surface turbulence in massive stars are caused by the presence of subsurface convection. We show that an explanation for the observed surface photometric variability and macroturbulence relying on convective core driven internal gravity waves encounters a number of difficulties and seems unlikely to be able to explain the observed trends.

Keywords: Stellar convective zones, Stellar structures, Surface variability, Stellar convection envelopes, Internal waves, Stellar pulsations, Massive stars, Stellar magnetic fields, Non-radial pulsations, Asteroseismology, Stellar evolution
The Colliding Winds of WR 25 in High-resolution X-Rays. WR 25 is a colliding-wind binary star system comprising a very massive O2.5If*/WN6 primary and an O-star secondary in an eccentric orbit with a 208 day period. These hot stars have strong, highly supersonic winds that interact to form a bright X-ray source from wind-collision shocks whose conditions change with stellar separation. Different views through the winds of Wolf-Rayet and O stars are afforded with orbital phase as the stars move about their orbits, allowing for exploration of wind structure in ways not easy or even possible for single stars. We have analyzed an on-axis Chandra/HETGS spectrum of WR 25 obtained shortly before periastron when the X-rays emanating from the system are the brightest. From the on-axis observations, we constrain the line fluxes, centroids, and widths of various emission lines, including He triplets of Si XIII and Mg XI. We have also been able to include several serendipitous off-axis HETG spectra from the archive and study their flux variation with phase. This is the first report on high-resolution spectral studies of WR 25 in X-rays.

Keywords: Wolf-Rayet stars, Massive stars, X-ray astronomy
Huffing and Puffing and Blowing Your House In: Strong Stellar Wind Interaction with a Supermassive Black Hole. We present analytical and numerical models of a cluster wind flow resulting from the interaction of stellar winds of massive stars with a supermassive black hole (SMBH). We consider the motion of the stars as well as the gravitational force of the SMBH. In the numerical simulations we consider two cases: the first one with the stars in circular orbits around the SMBH, and the second one with the stars in eccentric orbits. We found that after the system reaches an equilibrium, the circular and elliptical cases are very similar. We found a very good agreement between the analytical and numerical results, not only from our numerical simulations but also from other high-resolution numerical calculations. The analytical models are very interesting, since the properties of such complex systems involving strong winds and a massive compact object can be rapidly inferred without the need of a numerical calculation.

Keywords: Wolf-Rayet stars, Hydrodynamical simulations, Stellar winds, Black holes