Sazuna/UAT_keywords huggingface dataset:
Chronology of Episodic Accretion in Protostars—An ALMA Survey of the CO and H<SUB>2</SUB>O Snowlines. Episodic accretion has been used to explain the wide range of protostellar luminosities, but its origin and influence on the star-forming process are not yet fully understood. We present an ALMA survey of N2H+ (1-0) and HCO+ (3-2) toward 39 Class 0 and Class I sources in the Perseus molecular cloud. N2H+ and HCO+ are destroyed via gas-phase reactions with CO and H2O, respectively, thus tracing the CO and H2O snowline locations. A snowline location at a much larger radius than that expected from the current luminosity suggests that an accretion burst has occurred in the past that has shifted the snowline outward. We identified 18/18 Class 0 and 9/10 Class I post-burst sources from N2H+ and 7/17 Class 0 and 1/8 Class I post-burst sources from HCO+. The accretion luminosities during the past bursts are found to be ∼10-100 L ⊙. This result can be interpreted as either evolution of burst frequency or disk evolution. In the former case, assuming that refreeze-out timescales are 1000 yr for H2O and 10,000 yr for CO, we found that the intervals between bursts increase from 2400 yr in the Class 0 stage to 8000 yr in the Class I stage. This decrease in the burst frequency may reflect that fragmentation is more likely to occur at an earlier evolutionary stage when the young stellar object is more prone to instability.

Keywords: Protostars, Star formation, Interstellar medium, Astrochemistry
The NLTE Analyses of Carbon Emission Lines in the Atmospheres of O- and B-type Stars. We present a model atom for C I-C II-C III-C IV using the most up-to-date atomic data and evaluated the non-local thermodynamic equilibrium line formation in classical 1D atmospheric models of O-B-type stars. Our models predict the emission lines of C II 9903 Å and 18,535 Å to appear at effective temperature T eff ≥ 17,500 K, those of C II 6151 Å and 6461 Å to appear at T eff &gt; 25,000 K, and those of C III 5695, 6728-44 and 9701-17 Å to appear at T eff ≥ 35,000 K (log g = 4.0). Emission occurs in the lines of minority species, where the photoionization-recombination mechanism provides a depopulation of the lower levels to a greater extent than the upper levels. For C II 9903 and 18535 Å, the upper levels are mainly populated from the C III reservoir through the Rydberg states. For C III 5695 and 6728-44 Å, the lower levels are depopulated due to photon losses in UV transitions at 885, 1308, and 1426-28 Å, which become optically thin in the photosphere. We analyzed the lines of C I, C II, C III, and C IV for twenty-two O-B-type stars with temperature range between 15,800 ≤T eff ≤ 38,000 K. Abundances from emission lines of C I, C II, and C III are in agreement with those from absorption ones for most of the stars. We obtained log ɛ C = 8.36 ± 0.08 from twenty B-type stars, that is in line with the present-day Cosmic Abundance Standard. The obtained carbon abundances in 15 Mon and HD 42088 from emission and absorption lines are 8.27 ± 0.11 and 8.31 ± 0.11, respectively.

Keywords: Early-type stars, Chemical abundances, OB stars, Stellar photospheres, Stellar atmospheres
Probing the Environment of High-z Quasars Using the Proximity Effect in Projected Quasar Pairs. We have used spectra of 181 projected quasar pairs at separations ≤1.5‧ from the Sloan Digital Sky Survey Data Release 12 in the redshift range of 2.5-3.5 to probe the proximity regions of the foreground quasars. We study the proximity effect both in the longitudinal and in the transverse directions, by carrying out a comparison of the Lyα absorption lines originating from the vicinity of quasars to those originating from the general intergalactic medium at the same redshift. We found an enhancement in the transmitted flux within 4 Mpc to the quasar in the longitudinal direction. However, the trend is found to be reversed in the transverse direction. In the longitudinal direction, we derived an excess overdensity profile showing an excess up to r ≤ 5 Mpc after correcting for the quasar’s ionization, taking into account the effect of low spectral resolution. This excess overdensity profile matches with the average overdensity profile in the transverse direction without applying any correction for the effect of the quasar’s ionization. Among various possible interpretations, we found that the anisotropic obscuration of the quasar’s ionization seems to be the most probable explanation. This is also supported by the fact that all of our foreground quasars happen to be type 1 AGNs. Finally, we constrain the average quasar’s illumination along the transverse direction as compared to that along the longitudinal direction to be ≤27% (3σ confidence level).

Keywords: Lyman alpha forest, Quasars
Asymptotic Giant Branch Stars in the Nearby Dwarf Galaxy Leo P. We have conducted a highly sensitive census of the evolved-star population in the metal-poor dwarf galaxy Leo P and detected four asymptotic giant branch (AGB) star candidates. Leo P is one of the best examples of a nearby analog of high-redshift galaxies because of its primitive metal content (2% of the solar value), proximity, and isolated nature, ensuring a less complicated history. Using medium-band optical photometry from the Hubble Space Telescope (HST), we have classified the AGB candidates by their chemical type. We have identified one oxygen-rich source which appears to be dusty in both the HST and Spitzer observations. Its brightness, however, suggests it may be a planetary nebula or post-AGB object. We have also identified three carbon-rich candidates, one of which may be dusty. Follow-up observations are needed to confirm the nature of these sources and to study the composition of any dust that they produce. If dust is confirmed, these stars would likely be among the most metal-poor examples of dust-producing stars known and will provide valuable insight into our understanding of dust formation at high redshift. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Re- search in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program HST-GO-14845.

Keywords: Late stellar evolution, Dwarf galaxies, Carbon stars, Stellar mass loss, Asymptotic giant branch stars, Mira variable stars, Extreme carbon stars, Circumstellar dust, Metallicity
Radiative Association of Atomic and Ionic Carbon. We present calculated cross sections and rate coefficients for the formation of the dicarbon cation ({{{C}}}2+) by the radiative association process in collisions of a {{C}}{(}3P) atom and a {{{C}}}+{(}2{P}o) ion. Molecular structure calculations for a number of low-lying doublet and quartet states of {{{C}}}2+ are used to obtain the potential energy surfaces and transition dipole moments coupling the states of interest, substantially increasing the available molecular data for {{{C}}}2+. Using a quantum-mechanical method, we explore a number of allowed transitions and determine those contributing to the radiative association process. The calculations extend the available data for this process down to the temperature of 100 K, where the rate coefficient is found to be about 2× {10}-18 {cm}}3 {{{s}}}-1. We provide analytical fits suitable for incorporation into astrochemical reaction databases.

Keywords: Molecular physics, Laboratory astrophysics
SDSS-IV MaNGA: Evidence for Enriched Accretion onto Satellite Galaxies in Dense Environments. We investigate the environmental dependence of the local gas-phase metallicity in a sample of star-forming galaxies from the MaNGA survey. Satellite galaxies with stellar masses in the range 9&lt; {log}({M}* /{M}⊙ )&lt; 10 are found to be ∼0.05 dex higher in metallicity than centrals of similar stellar mass. Within the low-mass satellite population, we find that the interstellar medium (ISM) metallicity depends most strongly on the stellar mass of the galaxy that is central to the halo, though there is no obvious difference in the metallicity gradients. At fixed total stellar mass, the satellites of high-mass (M * &gt; 1010.5 M ⊙) centrals are ∼0.1 dex more metal-rich than the satellites of low-mass (M * &lt; 1010 M ⊙) centrals, controlling for local stellar mass surface density and gas fraction. Fitting a gas regulator model to the spaxel data, we are able to account for variations in the local gas fraction, stellar mass surface density, and local escape velocity-dependent outflows. We find that the best explanation for the metallicity differences is the variation in the average metallicity of accreted gas between different environments that depends on the stellar mass of the dominant galaxies in each halo. This is interpreted as evidence for the exchange of enriched gas between galaxies in dense environments that is predicted by recent simulations.

Keywords: Galaxy environments, Intergalactic gas, Scaling relations, Galaxy evolution, Galaxy accretion, Spectroscopy, Intergalactic abundances, Metallicity, Galactic abundances
A Statistical Study of Solar Filament Eruptions that Form High-speed Coronal Mass Ejections. Coronal mass ejections (CMEs) play a decisive role in driving space weather, especially the fast ones (e.g., with speeds above 800 km s-1). Understanding the trigger mechanisms of fast CMEs can help us gain important information in forecasting them. The filament eruptions accompanied with CMEs provide a good tracer in studying the early evolution of CMEs. Here we surveyed 66 filament-accompanied fast CMEs to analyze the correlation between the trigger mechanisms, namely either magnetic reconnection or ideal MHD process, associated flares, and CME speeds. Based on the data gathered from SDO, GONG, and STEREO, we find that (1) active region (AR) filament and intermediate filament (IF) eruptions show a higher probability for producing fast CMEs than quiet Sun (QS) filaments, while the probability of polar crown (PC) filament eruptions is zero in our statistic; (2) AR filament eruptions that produce fast CMEs are more likely to be triggered by magnetic reconnection, while QS filaments and IFs are more likely to be triggered by an ideal MHD process; (3) for AR filaments and IFs, it seems that the specific trigger mechanism does not have a significant influence on the resulting CME speeds, while for the QS filaments, the ideal MHD mechanism can more likely generate a faster CME; (4) comparing with previous statistical studies, the onset heights of filament eruptions and the decay indexes of the overlying field show some differences: for AR filaments and IFs, the decay indexes are larger and much closer to the theoretical threshold, while for QS filaments, the onset heights are higher than those obtained in previous results.

Keywords: Solar flares, Solar coronal mass ejections, Solar filament eruptions
Flaring Activity of Proxima Centauri from TESS Observations: Quasiperiodic Oscillations during Flare Decay and Inferences on the Habitability of Proxima b. We analyze the light curve of the M5.5 dwarf Proxima Centauri obtained by the Transiting Exoplanet Survey Satellite (TESS) in Sectors 11 and 12. In the ≈50 day long light curve we identified and analyzed 72 flare events. The flare rate was 1.49 events per day; in total, 7.2% of the observing time was classified as flaring. The estimated flare energies were on the order of 1030-1032 erg in the TESS passband (≈4.8× higher in bolometric energies, but on the same order of magnitude). Most of the eruptions appeared in groups. Two events showed quasiperiodic oscillations during their decay phase with a timescale of a few hours, which could be caused by quasiperiodic motions of the emitting plasma or oscillatory reconnection. From the cumulative flare frequency distribution we estimate that superflares with energy output of 1033 erg are expected to occur three times per year, while magnitude larger events (with 1034 erg) can occur every second year. This reduces the chances of habitability of Proxima Cen b, although earlier numerical models did not rule out the existence of liquid water on the planetary surface. We did not find any obvious signs of planetary transit in the light curve.

Keywords: Late-type stars, Planet hosting stars, Stellar activity, M stars, Low mass stars, Optical flares, Stellar flares, Habitable zone, Habitable planets, Stellar atmospheres
HaloSat: A CubeSat to Study the Hot Galactic Halo. HaloSat is a small satellite (CubeSat) designed to map soft X-ray oxygen line emission across the sky in order to constrain the mass and spatial distribution of hot gas in the Milky Way. The goal of HaloSat is to help determine if hot gas gravitationally bound to individual galaxies makes a significant contribution to the cosmological baryon budget. HaloSat was deployed from the International Space Station in 2018 July and began routine science operations in 2018 October. We describe the goals and design of the mission, the on-orbit performance of the science instrument, and initial observations.

Keywords: Space vehicle instruments, Circumgalactic medium, X-ray surveys, X-ray observatories, Hot ionized medium, Diffuse x-ray background
The Soft X-Ray Counterpart of Hanny’s Voorwerp Near IC 2497. We report the detection in Chandra Advanced CCD Imaging Spectrometer archival data of an elongated soft (&lt;3 keV) X-ray feature to the south of the Compton-thick active galactic nucleus (AGN) galaxy IC 2497, coincident with the emission-line feature known as Hanny’s Voorwerp. The data are consistent with the spatial correspondence between X-ray, optical emission-line, and radio features detected in nearby obscured AGNs (e.g., ESO 428-G014). The X-ray luminosity of the (0.3-3.0 keV) soft feature is ∼1.2 × 1040 erg s-1. We infer an [O III]/soft-X-ray ratio in the range of ∼200, consistent with the highest values measured in some of the clouds of NGC 4151. Overall, given the uncertainties, Hanny’s Voorwerp appears to be a feature consistent with the ionization cone emission of nearby AGNs. We estimate an X-ray recombination time of ∼2 × 107 yr, longer than the [O III] recombination time (∼8000 yr). This suggests that extended soft X-ray components may be a better diagnostic of overall long-term activity, while detection of an [O III] HV would point to a time-limited activity burst.

Keywords: Active galactic nuclei, X-ray active galactic nuclei, High energy astrophysics
Discovery and Identification of MAXI J1621-501 as a Type I X-Ray Burster with a Super-orbital Period. MAXI J1621-501 is the first Swift/XRT Deep Galactic Plane Survey transient that was followed up with a multitude of space missions (NuSTAR, Swift, Chandra, NICER, INTEGRAL, and MAXI) and ground-based observatories (Gemini, IRSF, and ATCA). The source was discovered with MAXI on 2017 October 19 as a new, unidentified transient. Further observations with NuSTAR revealed two Type I X-ray bursts, identifying MAXI J1621-501 as a low mass x-ray binary with a neutron star primary. Overall, 24 Type I bursts were detected from the source during a 15 month period. At energies below 10 keV, the source spectrum was best fit with three components: an absorbed blackbody with kT = 2.3 keV, a cutoff power law with index Γ = 0.7, and an emission line centered on 6.3 keV. Timing analysis of the X-ray persistent emission and burst data has not revealed coherent pulsations from the source or an orbital period. We identified, however, a super-orbital period ∼78 days in the source X-ray light curve. This period agrees very well with the theoretically predicted radiative precession period of ∼82 days. Thus, MAXI J1621-501 joins a small group of sources characterized with super-orbital periods.

Keywords: Low-mass x-ray binary stars, X-ray transient sources, X-ray bursters
The X-Ray Halo Scaling Relations of Supermassive Black Holes. We carry out a comprehensive Bayesian correlation analysis between hot halos and direct masses of supermassive black holes (SMBHs), by retrieving the X-ray plasma properties (temperature, luminosity, density, pressure, and masses) over galactic to cluster scales for 85 diverse systems. We find new key scalings, with the tightest relation being {M}\bullet -{\text{}}{T}{{x}}, followed by {M}\bullet -{\text{}}{L}{{x}}. The tighter scatter (down to 0.2 dex) and stronger correlation coefficient of all the X-ray halo scalings compared with the optical counterparts (as the {M}\bullet -{σ }{{e}}) suggest that plasma halos play a more central role than stars in tracing and growing SMBHs (especially those that are ultramassive). Moreover, {M}\bullet  correlates better with the gas mass than dark matter mass. We show the important role of the environment, morphology, and relic galaxies/coronae, as well as the main departures from virialization/self-similarity via the optical/X-ray fundamental planes. We test the three major channels for SMBH growth: hot/Bondi-like models have inconsistent anticorrelation with X-ray halos and too low feeding; cosmological simulations find SMBH mergers as subdominant over most of cosmic time and too rare to induce a central-limit-theorem effect; the scalings are consistent with chaotic cold accretion, the rain of matter condensing out of the turbulent X-ray halos that sustains a long-term self-regulated feedback loop. The new correlations are major observational constraints for models of SMBH feeding/feedback in galaxies, groups, and clusters (e.g., to test cosmological hydrodynamical simulations), and enable the study of SMBHs not only through X-rays, but also via the Sunyaev-Zel’dovich effect (Compton parameter), lensing (total masses), and cosmology (gas fractions).

Keywords: Lenticular galaxies, Supermassive black holes, X-ray astronomy, Optical observation, Late-type galaxies, Hydrodynamical simulations, Interstellar medium, Circumgalactic medium, Brightest cluster galaxies, Intracluster medium, Elliptical galaxies
A VLA Polarimetric Study of the Galactic Center Radio Arc: Characterizing Polarization, Rotation Measure, and Magnetic Field Properties. The Radio Arc is one of the brightest systems of nonthermal filaments (NTFs) in the Galactic Center, located near several prominent H II regions (Sickle and Pistol) and the Quintuplet stellar cluster. We present observations of the Arc NTFs using the S, C, and X bands of the Very Large Array interferometer. Our images of total intensity reveal large-scale helical features that surround the Arc NTFs, very narrow subfilamentation, and compact sources along the NTFs. The distribution of polarized intensity is confined to a relatively small area along the NTFs. There are elongated polarized structures that appear to lack total intensity counterparts. We detect a range of rotation measure values from -1000 to -5800 rad m-2, likely caused by external Faraday rotation along the line of sight. After correcting for Faraday rotation, the intrinsic magnetic field orientation is found to generally trace the extent of the NTFs. However, the intrinsic magnetic field in several regions of the Arc NTFs shows an ordered pattern that is rotated with respect to the extent of the NTFs. We suggest this changing pattern may be caused by an additional magnetized source along the line of sight, so that we observe two field systems superposed in our observations. We suggest that the large-scale helical segments near the Radio Arc could be components of such a source causing these changes in the intrinsic magnetic field, and some variations in the polarization and rotation measure values along the NTFs.

Keywords: Galaxy magnetic fields, Galactic center, Very Large Array, Milky Way magnetic fields, Non-thermal radiation sources, Interstellar synchrotron emission, Radio continuum emission, Radio astronomy
Redefining the Torus: A Unifying View of AGNs in the Infrared and Submillimeter. The advent of high-angular-resolution IR and submillimeter interferometry allows for spatially resolved observations of the parsec-scale environment of active galactic nuclei (AGNs), commonly referred to as the “torus.” While molecular lines show the presence of large, massive disks, the IR observations appear to be dominated by a strong polar component that has been interpreted as a dusty wind. This paper aims at using characteristics shared by AGNs in each of the wavebands and a set of simple physical principles to form a unifying view of these seemingly contradictory observations: dusty molecular gas flows in from galactic scales of ∼100 pc to the subparsec environment via a disk with small to moderate scale height. The hot, inner part of the disk puffs up due to IR radiation pressure and unbinds a large amount of the inflowing gas from the black hole’s gravitational potential, providing the conditions to launch a wind driven by the radiation pressure from the AGN. The dusty wind feeds back mass into the galaxy at a rate of the order of ∼0.1-100 M ⊙ yr-1, depending on the AGN luminosity and Eddington ratio. Angle-dependent obscuration as required by AGN unification is provided by a combination of disk, wind, and wind-launching region.

Keywords: Optical interferometry, Quasars, Active galaxies, Infrared astronomy, Active galactic nuclei, Submillimeter astronomy
Gas and Dust Temperature in Prestellar Cores Revisited: New Limits on Cosmic-Ray Ionization Rate. We develop a self-consistent model for the equilibrium gas temperature and size-dependent dust temperature in cold, dense, prestellar cores, assuming an arbitrary power-law size distribution of dust grains. Compact analytical expressions applicable to a broad range of physical parameters are derived and compared with predictions of the commonly used standard model. It is suggested that combining the theoretical results with observations should allow us to constrain the degree of dust evolution and the cosmic-ray ionization rate in dense cores, and to help with discriminating between different regimes of cosmic-ray transport in molecular clouds. In particular, assuming a canonical MRN distribution of grain sizes, our theory demonstrates that the gas-temperature measurements in the prestellar core L1544 are consistent with an ionization rate as high as ∼10-16 s-1, an order of magnitude higher than previously thought.

Keywords: Astrophysical dust processes, Dark interstellar clouds, Cosmic rays
Predicting the Observational Signature of Migrating Neptune-sized Planets in Low-viscosity Disks. The migration of planetary cores embedded in a protoplanetary disk is an important mechanism within planet-formation theory, relevant for the architecture of planetary systems. Consequently, planet migration is actively discussed, yet often results of independent theoretical or numerical studies are unconstrained due to the lack of observational diagnostics designed in light of planet migration. In this work we follow the idea of inferring the migration behavior of embedded planets by means of the characteristic radial structures that they imprint in the disk’s dust density distribution. We run hydrodynamical multifluid simulations of gas and several dust species in a locally isothermal α-disk in the low-viscosity regime (α = 10-5) and investigate the obtained dust structures. In this framework, a planet of roughly Neptune mass can create three (or more) rings in which dust accumulates. We find that the relative spacing of these rings depends on the planet’s migration speed and direction. By performing subsequent radiative transfer calculations and image synthesis we show that—always under the condition of a near-inviscid disk—different migration scenarios are, in principle, distinguishable by long-baseline, state-of-the-art Atacama Large Millimeter/submillimeter Array observations.

Keywords: Hydrodynamical simulations, Dust continuum emission, Protoplanetary disks, Radiative transfer simulations, Planet formation