adsabs/SciX_UAT_keywords huggingface dataset:
The Role of Outflows, Radiation Pressure, and Magnetic Fields in Massive Star Formation

Stellar feedback in the form of radiation pressure and magnetically driven collimated outflows may limit the maximum mass that a star can achieve and affect the star formation efficiency of massive prestellar cores. Here we present a series of 3D adaptive mesh refinement radiation-magnetohydrodynamic simulations of the collapse of initially turbulent, massive prestellar cores. Our simulations include radiative feedback from both the direct stellar and dust-reprocessed radiation fields, and collimated outflow feedback from the accreting stars. We find that protostellar outflows punch holes in the dusty circumstellar gas along the star's polar directions, thereby increasing the size of optically thin regions through which radiation can escape. Precession of the outflows as the star's spin axis changes due to the turbulent accretion flow further broadens the outflow, and causes more material to be entrained. Additionally, the presence of magnetic fields in the entrained material leads to broader entrained outflows that escape the core. We compare the injected and entrained outflow properties and find that the entrained outflow mass is a factor of ∼3 larger than the injected mass and the momentum and energy contained in the entrained material are ∼25% and ∼5% of the injected momentum and energy, respectively. As a result, we find that, when one includes both outflows and radiation pressure, the former are a much more effective and important feedback mechanism, even for massive stars with significant radiative outputs.

Keywords: star formation; radiative magnetohydrodynamics; massive stars; stellar jets; stellar feedback
Korg: Fitting, Model Atmosphere Interpolation, and Brackett Lines

We describe several updates to Korg, a package for 1D LTE spectral synthesis of FGKM stars. Built-in functions to fit observed spectra via synthesis or equivalent widths make it easy to take advantage of Korg's automatic differentiation. Comparison to a past analysis of 18 Sco shows that we obtain significantly reduced line-to-line abundance scatter with Korg. Fitting and synthesis are facilitated by a rigorously tested model atmosphere interpolation method, which introduces negligible error to synthesized spectra for stars with T eff ≳ 4000 K. For cooler stars, atmosphere interpolation is complicated by the presence of molecules, though we demonstrate an adequate method for cool dwarfs. The chemical equilibrium solver has been extended to include polyatomic and charged molecules, extending Korg's regime of applicability to M stars. We also discuss a common oversight regarding the synthesis of hydrogen lines in the infrared, and show that Korg's Brackett line profiles are a much closer match to observations than others available. Documentation, installation instructions, and tutorials are available at https://github.com/ajwheeler/Korg.jl.

Keywords: spectroscopy; stellar atmospheres; stellar photospheres; stellar spectral lines; stellar properties; astronomy software; open source software
Radio-loud Quasars above Redshift 4: Very Long Baseline Interferometry (VLBI) Imaging of an Extended Sample

High-redshift radio sources provide plentiful opportunities for studying the formation and evolution of early galaxies and supermassive black holes. However, the number of known radio-loud active galactic nuclei (AGN) above redshift 4 is rather limited. At high redshifts, it appears that blazars, with relativistically beamed jets pointing toward the observer, are in the majority compared to the radio-loud sources with jets misaligned with respect to the line of sight. To find more of these misaligned AGN, milliarcsecond-scale imaging studies carried out with very long baseline interferometry (VLBI) are needed, as they allow us to distinguish between compact-core-jet radio sources and those with more extended emission. Previous high-resolution VLBI studies revealed that some of the radio sources among blazar candidates in fact show unbeamed radio emission on milliarcsecond scales. The most accurate optical coordinates determined with the Gaia astrometric space mission are also useful in the classification process. Here, we report on dual-frequency imaging observations of 13 high-redshift (4 < z < 4.5) quasars at 1.7 and 5 GHz with the European VLBI Network. This sample increases the number of z > 4 radio sources for which VLBI observations are available by about a quarter. Using structural and physical properties, such as radio morphology, spectral index, variability, brightness temperature, as well as optical coordinates, we identified six blazars and six misaligned radio AGNs, with the remaining one tentatively identified as blazar.

Keywords: active galactic nuclei; very long baseline interferometry; flat-spectrum radio quasars; radio loud quasars; relativistic jets
First X-Ray Polarization Measurement Confirms the Low Black Hole Spin in LMC X-3

X-ray polarization is a powerful tool to investigate the geometry of accreting material around black holes, allowing independent measurements of the black hole spin and orientation of the innermost parts of the accretion disk. We perform X-ray spectropolarimetric analysis of an X-ray binary system in the Large Magellanic Cloud, LMC X-3, that hosts a stellar-mass black hole, known to be persistently accreting since its discovery. We report the first detection of the X-ray polarization in LMC X-3 with the Imaging X-ray Polarimetry Explorer, and find the average polarization degree (PD) of 3.2% ± 0.6% and a constant polarization angle of -42° ± 6° over the 2-8 keV range. Using accompanying spectroscopic observations by NICER, NuSTAR, and the Neil Gehrels Swift observatories, we confirm previous measurements of the black hole spin via the X-ray continuum method, a ≈ 0.2. From polarization analysis only, we found consistent results with low black hole spin, with an upper limit of a < 0.7 at a 90% confidence level. A slight increase in the PD with energy, similar to other black hole X-ray binaries in the soft state, is suggested from the data but with a low statistical significance.

Keywords: x-ray binary stars; x-ray observatories; accretion; astrophysical black holes; black hole physics; spectropolarimetry; high mass x-ray binary stars; x-ray astronomy; x-ray detectors
CO Emission, Molecular Gas, and Metallicity in Main-sequence Star-forming Galaxies at z ∼ 2.3

We present observations of CO(3-2) in 13 main-sequence z = 2.0-2.5 star-forming galaxies at $\mathrm{log}({M}_{* }/{M}_{\odot })=10.2\mbox{--}10.6$ that span a wide range in metallicity (O/H) based on rest-optical spectroscopy. We find that ${L}_{\mathrm{CO}(3-2)}^{{\prime} }$ /SFR decreases with decreasing metallicity, implying that the CO luminosity per unit gas mass is lower in low-metallicity galaxies at z ~ 2. We constrain the CO-to-H2 conversion factor (α CO) and find that α CO inversely correlates with metallicity at z ~ 2. We derive molecular gas masses (M mol) and characterize the relations among M *, SFR, M mol, and metallicity. At z ~ 2, M mol increases and the molecular gas fraction (M mol/M *) decreases with increasing M *, with a significant secondary dependence on SFR. Galaxies at z ~ 2 lie on a near-linear molecular KS law that is well-described by a constant depletion time of 700 Myr. We find that the scatter about the mean SFR-M *, O/H-M *, and M mol-M * relations is correlated such that, at fixed M *, z ~ 2 galaxies with larger M mol have higher SFR and lower O/H. We thus confirm the existence of a fundamental metallicity relation at z ~ 2, where O/H is inversely correlated with both SFR and M mol at fixed M *. These results suggest that the scatter of the z ~ 2 star-forming main sequence, mass-metallicity relation, and M mol-M * relation are primarily driven by stochastic variations in gas inflow rates. We place constraints on the mass loading of galactic outflows and perform a metal budget analysis, finding that massive z ~ 2 star-forming galaxies retain only 30% of metals produced, implying that a large mass of metals resides in the circumgalactic medium. *The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

Keywords: molecular gas; co line emission; high-redshift galaxies; galaxy evolution; metallicity
Can Lense-Thirring Precession Produce QPOs in Supersonic Accretion Flows?

The timing properties of X-ray binaries are still not understood, particularly the presence of quasi-periodic oscillations (QPOs) in their X-ray power spectra. The solid-body regime of Lense-Thirring precession is one prominent model invoked to explain the most common type of QPOs, Type C. However, solid-body precession requires a specific structure that has not been examined in light of constrained properties of accretion flows. We assume in this paper, as solid-body precession requires, a disk separated into two flows at a transition radius rt: a cold outer flow and a hot inner flow (playing the role of the corona). We explore the physical structure of both flows using model-independent estimates of accretion parameters. We show that, in order to reproduce the observed X-ray spectra during luminous hard states, the hot flow must accrete at sonic to supersonic speeds, unreachable with typical viscous torques. As a result of this extreme accretion speed (or high α parameter), no region of the disk during these states lies in the "wave-like" regime required for solid-body precession. Furthermore, we expect the flow to align with the black hole spin axis via the Bardeen-Petterson effect inside a radius rbreak > rt. As a consequence, the hot inner flow cannot exhibit solid body precession—as currently pictured in the literature—during luminous hard states. Since Type C QPOs are prevalent in these states, we conclude that this mechanism is unlikely to be responsible for producing Type C QPOs around stellar mass black holes.

Keywords: black hole physics; high energy astrophysics; time domain astronomy; x-ray binary stars; x-ray transient sources; x-ray astronomy; transient sources; accretion; astrophysical black holes; stellar mass black holes
Blue Stragglers as Tracers of the Dynamical State of Two Clusters in the Small Magellanic Cloud: NGC 339 and NGC 419

The level of central segregation of Blue Straggler stars has proved to be an excellent tracer of the dynamical evolution of old star clusters (the so-called "dynamical clock"), both in the Milky Way and in the Large Magellanic Cloud. The A + parameter, used to measure the Blue Stragglers degree of segregation, has in fact been found to strongly correlate with the parent cluster central relaxation time. Here, we have studied the Blue Straggler population of two young stellar systems in the Small Magellanic Cloud, namely NGC 339 (which is 6 Gyr old) and NGC 419 (with an age of only 1.5 Gyr), in order to study their dynamical state. Thanks to multi-epoch, high angular resolution Hubble Space Telescope observations available for both clusters, we took advantage of the stellar proper motions measured in the regions of the two systems and we selected a population of likely cluster members, removing the strong contamination from Small Magellanic Cloud stars. This enabled us to study, with unprecedented accuracy, the radial distribution of Blue Stragglers in these two extra-Galactic clusters and to measure their dynamical age. As expected for such young clusters, we found that both systems are poorly evolved from the dynamical point of view, also fully confirming that the A + parameter is a sensitive "clock hand" even in the dynamically young regime.

Keywords: star clusters; blue straggler stars; photometry; proper motions
A Deep Learning Method for AGILE-GRID Gamma-Ray Burst Detection

The follow-up of external science alerts received from gamma-ray burst (GRB) and gravitational wave detectors is one of the AGILE Team's current major activities. The AGILE team developed an automated real-time analysis pipeline to analyze AGILE Gamma-Ray Imaging Detector (GRID) data to detect possible counterparts in the energy range 0.1-10 GeV. This work presents a new approach for detecting GRBs using a convolutional neural network (CNN) to classify the AGILE-GRID intensity maps by improving the GRB detection capability over the Li & Ma method, currently used by the AGILE team. The CNN is trained with large simulated data sets of intensity maps. The AGILE complex observing pattern due to the so-called "spinning mode" is studied to prepare data sets to test and evaluate the CNN. A GRB emission model is defined from the second Fermi-LAT GRB catalog and convoluted with the AGILE observing pattern. Different p-value distributions are calculated, evaluating, using the CNN, millions of background-only maps simulated by varying the background level. The CNN is then used on real data to analyze the AGILE-GRID data archive, searching for GRB detections using the trigger time and position taken from the Swift-BAT, Fermi-GBM, and Fermi-LAT GRB catalogs. From these catalogs, the CNN detects 21 GRBs with a significance of ≥3&sgr;, while the Li & Ma method detects only two GRBs. The results shown in this work demonstrate that the CNN is more effective in detecting GRBs than the Li & Ma method in this context and can be implemented into the AGILE-GRID real-time analysis pipeline.

Keywords: gamma-ray astronomy; gamma-ray bursts; convolutional neural networks; neural networks
Self-similar Solution of Rotating Eruptive Outflows on Its Equatorial Plane

We construct axisymmetric self-similar solutions of transonic outflows emanating from a point source including the effect of the rotation. The solutions are constructed exclusively on the equatorial plane. The features of solutions are determined by three parameters: the adiabatic index γ, the dimensionless coordinate of the transonic point, and the dimensionless azimuthal velocity at the transonic point. We classify the solutions into five groups according to asymptotic behaviors. We find that the behaviors of the self-similar solutions change at γ = 11/9. In addition, some solutions show double-power-law density profiles, which are usually seen in ejecta from a binary merger or nova-like explosion. Thus, our self-similar solutions can be applied not only to the outflow blowing from the central spinning objects but also to the ejecta erupted from the binary merger or nova-like explosion.

Keywords: stellar winds; solar wind
Quantitative Age Estimation of Supernova Remnants and Associated Pulsars

The age of a supernova remnant (SNR) is, though undoubtedly one of the most important properties for study of its evolution, difficult to estimate reliably in most cases. In this study, we compare the dynamical and plasma ages of the SNRs and characteristic ages of their associated pulsars with the corresponding SNRs' ages that are generally thought to be reliable (tr): historical and light-echo ages of the SNRs, kinematic ages of the ejecta knots and kinematic ages of the associated neutron stars (NS). The kinematic age of ejecta knots or an NS is the time that they have taken to reach the current positions from the explosion center. We use all of the available 24 systems for which tr is already available (historical, light-echo, and ejecta kinematic ages) or measurable (NS kinematic age). We estimate the NS kinematic ages for eight SNR-NS systems by determining quantitatively the geometric centers of the SNR shells. The obtained tr ranges from 33 yr to ≈400 kyr. We find that the two SNR ages, dynamical and plasma ages, are consistent with tr within a factor of four, whereas the characteristic ages of the pulsars differ from tr by more than a factor of four in some systems. Using the tr summarized in this work, we present the initial spin periods of the associated pulsars, which are more strictly constrained than the previous works, as well.

Keywords: pulsars; supernova remnants; neutron stars; x-ray astronomy; radio continuum emission
The SDSS-V Black Hole Mapper Reverberation Mapping Project: C IV Broad Absorption Line Acceleration in the Quasar SBS 1408+544

We present the results of an investigation of a highly variable C IV broad absorption line (BAL) feature in spectra of the quasar SBS 1408+544 (z = 2.337) that shows a significant shift in velocity over time. This source was observed as a part of the Sloan Digital Sky Survey (SDSS) Reverberation Mapping project and the SDSS-V Black Hole Mapper Reverberation Mapping project, and has been included in two previous studies, both of which identified significant variability in a high-velocity C IV BAL on timescales of just a few days in the quasar rest frame. Using ∼130 spectra acquired over 8 yr of spectroscopic monitoring with SDSS, we have determined that this BAL is not only varying in strength, but is also systematically shifting to higher velocities. Using cross-correlation methods, we measure the velocity shifts (and corresponding acceleration) of the BAL over a wide range of timescales, measuring an overall velocity shift of km s‑1 over the 8 yr monitoring period. This corresponds to an average rest-frame acceleration of a = 1.04 cm s‑2, though the magnitude of the acceleration on shorter timescales is not constant throughout. We place our measurements in the context of BAL-acceleration models and examine various possible causes of the observed velocity shift.

Keywords: broad-absorption line quasar; quasars; active galactic nuclei
Performance Verification of the EXtreme PREcision Spectrograph

The EXtreme PREcision Spectrograph (EXPRES) is a new Doppler spectrograph designed to reach a radial-velocity measurement precision sufficient to detect Earth-like exoplanets orbiting nearby, bright stars. We report on extensive laboratory testing and on-sky observations to quantitatively assess the instrumental radial-velocity measurement precision of EXPRES, with a focused discussion of individual terms in the instrument error budget. We find that EXPRES can reach a single-measurement instrument calibration precision better than 10 cm s-1, not including photon noise from stellar observations. We also report on the performance of the various environmental, mechanical, and optical subsystems of EXPRES, assessing any contributions to radial-velocity error. For atmospheric and telescope related effects, this includes the fast tip-tilt guiding system, atmospheric dispersion compensation, and the chromatic exposure meter. For instrument calibration, this includes the laser fRequency comb (LFC), flat-field light source, CCD detector, and effects in the optical fibers. Modal noise is mitigated to a negligible level via a chaotic fiber agitator, which is especially important for wavelength calibration with the LFC. Regarding detector effects, we empirically assess the impact on the radial-velocity precision due to pixel-position nonuniformities and charge transfer inefficiency (CTI). EXPRES has begun its science survey to discover exoplanets orbiting G-dwarf and K-dwarf stars, in addition to transit spectroscopy and measurements of the Rossiter-McLaughlin effect.

Keywords: astronomical instrumentation; high resolution spectroscopy
An Argument in Favor of Magnetic Polarity Reversals Due to Heat Flux Variations in Fully Convective Stars and Planets

Low-mass M dwarf stars, T Tauri stars, as well as planets such as the Earth and Jupiter are permeated by large-scale magnetic fields generated by the convection-driven dynamo operating in their convection zones. These magnetic fields are often characterized by a significant time variability, most prominently expressed by the inversions of their polarity, denoted as reversals, whose mechanism has not been completely understood. This work aims to gain some insights into the mechanism that generates these reversals. With this purpose, a simplified nonlinear model is developed to investigate the role played in polarity reversals by the convective heat transfer occurring in stellar and planetary convection zones. A model result is the enhancement of the global heat transport before polarity reversals, showing the crucial role that heat transport might play in their occurrence. This role is elucidated by considering that a reversal has a greater than 70% probability of occurring during a burst of convective heat transport. This high probability has been found in 94 out of 101 numerical simulations obtained by changing characteristic model parameters. Moreover, the causal relationship between the convective heat flux growth and the magnetic field variations is highlighted by the temporal antecedence of the former relative to the latter and by convergent cross mapping, namely a statistical test for detecting causality. It would thus be expected that higher levels of temporal variability in the planetary and stellar magnetic fields may be correlated to a higher heat transfer efficiency achieved in the interior of these celestial bodies.

Keywords: magnetic fields; stellar magnetic fields; geomagnetic fields; magnetohydrodynamics; stellar convective zones; m dwarf stars; low mass stars
Kinematics and Collimation of the Two-sided Jets in NGC 4261: VLBI Study on Subparsec Scales

We report multifrequency VLBI studies of the subparsec scale structure of the two-sided jets in the nearby radio galaxy NGC 4261. Our analyses include new observations using the source frequency phase referencing technique with the Very Long Baseline Array at 44 and 88 GHz, as well as archival data at 15 and 43 GHz. Our results show an extended double-sided structure at 43/44 GHz and provide a clear image of the nuclear region at 88 GHz, showing a core size of ~0.09 mas and a brightness temperature of ~1.3 × 109 K. Proper motions are measured for the first time in the two-sided jets, with apparent speeds ranging from 0.31 ± 0.14 c to 0.59 ± 0.40 c in the approaching jet and 0.32 ± 0.14 c in the receding jet. The jet-to-counterjet brightness ratio allows us to constrain the viewing angle to between ~54° and 84° and the intrinsic speed to between ~0.30 c and 0.55 c. We confirm the parabolic shape of the upstream jet on both sides of the central engine, with a power-law index of 0.56 ± 0.07. Notably, the jet collimation is found to be already completed at subparsec scales, with a transition location of about 0.61 pc, which is significantly smaller than the Bondi radius of 99.2 pc. This behavior can be interpreted as the initial confinement of the jet by external pressure from either the geometrically thick, optically thin advection-dominated accretion flows or the disk wind launched from it. Alternatively, the shape transition may also be explained by the internal flow transition from a magnetically dominated to a particle-dominated regime.

Keywords: submillimeter astronomy; high angular resolution; very long baseline interferometry; low-luminosity active galactic nuclei