adsabs/SciX_UAT_keywords huggingface dataset:
Statistical Study of the Kinetic Features of Supra-arcade Downflows Detected from Multiple Solar Flares

We have developed a tracking algorithm to determine the speeds of supra-arcade downflows (SADs) and set up a system to automatically track SADs and measure some interesting parameters. By conducting an analysis of six flares observed by the Atmospheric Imaging Assembly on the Solar Dynamics Observatory, we detect more smaller and slower SADs than prior work, due to the higher spatial resolution of our observational data. The inclusion of these events with smaller and slower SADs directly results in lower median velocities and widths than in prior work, but the fitted distributions and evolutions of the parameters still show good consistency with prior work. The observed distributions of the widths, speeds, and lifetimes of SADs are consistent with log-normal distributions, indicating that random and unstable processes are responsible for generating SADs during solar eruptions. Also, we find that the fastest SADs occur at approximately the middle of the height ranges. The number of SADs in each image versus time shows that there are "rest phases" of SADs, when few SADs are seen. These findings support the idea that SADs originate from a fluid instability. We compare our results with a numerical simulation that generates SADs using a mixture of the Rayleigh-Taylor instability and the Richtmyer-Meshkov instability, and find that the simulation generates quantities that are consistent with our observational results.

Keywords: solar flares; solar activity
The Nature of Blue Stars with Mid-infrared Excesses in the Large Magellanic Cloud

We present low-resolution optical spectra and classifications of 92 blue objects with mid-infrared excesses in the Large Magellanic Cloud. The majority of these objects were selected with the criteria of U - B < 0 and V < 17 from the potential young stellar object (YSO) candidates in Gruendl & Chu (GC09), which were identified based on Spitzer Infrared Array Camera and Multiband Imaging Photometer for Spitzer observations in conjunction with optical photometry from the Magellanic Clouds Photometric Survey. Many of the sample objects have ambiguous classifications. We examined the properties of these 92 objects using low-resolution optical spectra obtained with the SOAR 4.1 m Telescope at Cerro Pachon and the Blanco 4 m Telescope at Cerro Tololo Inter-American Observatory, supplemented by available photometric and imaging observations. We estimated the spectral types, temperatures, and luminosities of these objects from the optical to near-IR spectral energy distributions based on the photometric data, and further examined stellar absorption line features in the optical spectra to verify the spectral types. The interstellar/circumstellar environments, assessed from nebular line imaging observations and nebular lines detected in the stellar spectra, further helped constrain the nature of stars. Among these 92 objects, we confirm 42 stars as YSOs, and the remaining 50 objects as protoplanetary nebulae, post-AGB/RGB stars, blue evolved massive stars, stars with dust in vicinity, or uncertain classifications. Our results show that the photometric criteria in GC09 are generally effective in the initial selection of YSO candidates, and the low-resolution spectroscopy combined with environment assessment can be useful to better constrain the classifications and ameliorate most ambiguities.

Keywords: large magellanic cloud; young stellar objects; stellar classification
DESI-253.2534+26.8843: A New Einstein Cross Spectroscopically Confirmed with Very Large Telescope/MUSE and Modeled with GIGA-Lens

Gravitational lensing provides unique insights into astrophysics and cosmology, including the determination of galaxy mass profiles and constraining cosmological parameters. We present spectroscopic confirmation and lens modeling of the strong lensing system DESI-253.2534+26.8843, discovered in the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys data. This system consists of a massive elliptical galaxy surrounded by four blue images forming an Einstein Cross pattern. We obtained spectroscopic observations of this system using the Multi Unit Spectroscopic Explorer on ESO's Very Large Telescope and confirmed its lensing nature. The main lens, which is the elliptical galaxy, has a redshift of z L1 = 0.636 ± 0.001, while the spectra of the background source images are typical of a starburst galaxy and have a redshift of z s = 2.597 ± 0.001. Additionally, we identified a faint galaxy foreground of one of the lensed images, with a redshift of z L2 = 0.386. We employed the GIGA-Lens modeling code to characterize this system and determined the Einstein radius of the main lens to be ${\theta }_{{\rm{E}}}=2\buildrel{\prime\prime}\over{.} {520}_{-0.031}^{+0.032}$ , which corresponds to a velocity dispersion of σ = 379 ± 2 km s-1. Our study contributes to a growing catalog of this rare kind of strong lensing system and demonstrates the effectiveness of spectroscopic integral field unit observations and advanced modeling techniques in understanding the properties of these systems.

Keywords: strong gravitational lensing
Deficit of Hot Dust in Low-redshift Active Galactic Nuclei

We assemble a broadband spectral energy distribution (SED) ranging from the optical to the mid-infrared of nearby active galactic nuclei (AGNs) at z < 0.4. The SED fitting analysis is performed using semiempirical templates derived from Palomar-Green quasars to classify the sample into normal, warm-dust-deficient (WDD), and hot-dust-deficient (HDD) AGNs. Kolmogorov-Smirnov tests reveal that HDD AGNs exhibit higher AGN luminosity on average than normal and WDD AGNs. The HDD fraction, on the other hand, is only weakly correlated with the black hole mass and inversely correlated with the Eddington ratio. By fixing the other parameters, we conclude that the HDD fraction is primarily connected with the AGN luminosity. This implies that there is a causal connection between the covering factor of the hot dust component and AGN luminosity, possibly due to the sublimation of the innermost dust or the thickening of the intervening gas in the broad-line region. Analysis of the outflow properties traced by the wing of [O III]λ5007 suggests that outflows may be related to the formation and maintenance of the hot dust component. Finally, we demonstrate through comparison with previous studies that the classification of HDD AGNs requires careful subtraction of the host galaxy light.

Keywords: quasars
SOAR/Goodman Spectroscopic Assessment of Candidate Counterparts of the LIGO/Virgo Event GW190814

On 2019 August 14 at 21:10:39 UTC, the LIGO/Virgo Collaboration (LVC) detected a possible neutron star-black hole merger (NSBH), the first ever identified. An extensive search for an optical counterpart of this event, designated GW190814, was undertaken using the Dark Energy Camera on the 4 m Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory. Target of Opportunity interrupts were issued on eight separate nights to observe 11 candidates using the 4.1 m Southern Astrophysical Research (SOAR) telescope's Goodman High Throughput Spectrograph in order to assess whether any of these transients was likely to be an optical counterpart of the possible NSBH merger. Here, we describe the process of observing with SOAR, the analysis of our spectra, our spectroscopic typing methodology, and our resultant conclusion that none of the candidates corresponded to the gravitational wave merger event but were all instead other transients. Finally, we describe the lessons learned from this effort. Application of these lessons will be critical for a successful community spectroscopic follow-up program for LVC observing run 4 (O4) and beyond. *Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, Inovações e Comunicações (MCTIC) do Brasil, the US National Science Foundations National Optical-Infrared Astronomy Research Laboratory (NOIRLab), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU).

Keywords: gravitational waves; spectroscopy; neutron stars; black holes
Steep Cosmic-Ray Spectra with Revised Diffusive Shock Acceleration

Galactic cosmic rays (CRs) are accelerated at the forward shocks of supernova remnants (SNRs) via diffusive shock acceleration (DSA), an efficient acceleration mechanism that predicts power-law energy distributions of CRs. However, observations of nonthermal SNR emission imply CR energy distributions that are generally steeper than E-2, the standard DSA prediction. Recent results from kinetic hybrid simulations suggest that such steep spectra may arise from the drift of magnetic structures with respect to the thermal plasma downstream of the shock. Using a semi-analytic model of nonlinear DSA, we investigate the implications that these results have on the phenomenology of a wide range of SNRs. By accounting for the motion of magnetic structures in the downstream, we produce CR energy distributions that are substantially steeper than E-2 and consistent with observations. Our formalism reproduces both modestly steep spectra of Galactic SNRs (∝E-2.2) and the very steep spectra of young radio supernovae (∝E-3).

Keywords: galactic cosmic rays; shocks; supernova remnants; cosmic rays; spectral index; high energy astrophysics
A Comprehensive Analysis of Repeating Fast Radio Bursts

Nearly 700 fast radio burst (FRB) sources have been detected so far, of which 29 are found to burst out repeatedly. Although a firm connection between at least some FRBs and magnetars has been established, the trigger mechanism and radiation process in these enigmatic phenomena are still highly controversial. In this study, we build a sample of 16 repeating FRBs from which at least five bursts have been detected, including the most active four repeaters of FRBs 20121102A, 20180916B, 20190520B, and 20201124A. Various key parameters of their bursts are collected from the literature, which include the arrival time, pulse width, dispersion measure (DM), Faraday rotation measure (RM), bandwidth, waiting time, peak flux, and fluence. The distribution and time evolution of these parameters are investigated. Potential correlations between various parameter pairs are also extensively explored. The behaviors of different repeaters are then compared. It is found that the DM of FRB 20121102A seems to increase continuously on a long timescale. While the DM of most repeaters varies in a narrow range of ±3 cm-3 pc, FRB 20190520B is found to have a large variation range of ±12 cm-3 pc. The RM evolves with time in a much more chaotic behavior in different repeaters. A linear correlation is found between the absolute mean RM and DMHost, which may provide a method to estimate the redshift of FRBs. Generally, the waiting time shows a similar bimodal distribution for the active repeating sources. The implications of these features to the underlying physics are discussed.

Keywords: radio transient sources; astrostatistics; high energy astrophysics; burst astrophysics; radio bursts; magnetars; pulsars; radio pulsars; neutron stars
HST Low-resolution Stellar Library

In order to provide fundamental stellar spectra that extend into the UV, Hubble Space Telescope's Space Telescope Imaging Spectrograph targeted 556 stars via proposals GO9088, GO9786, GO10222, and GO13776. Exposures through three low-resolution gratings provide wavelength coverage from 0.2 < λ < 1 μm at λ/Δλ ~ 1000. The UV grating (G230LB) scatters red light that results in unwanted signal, especially in cool stars. We applied scattered-light corrections and flux corrections arising from pointing errors relative to the center of the 0.″2 slit based on Worthey et al. We present 513 fully reduced stellar spectra, fluxed, dereddened, and cross correlated to zero velocity. Because of the broad spectral range, we can simultaneously study Hα and Mg II λ2800, indicators of chromospheric activity. Their behaviors are decoupled. Besides three cool dwarfs and one giant with mild flares in Hα, only Be stars show strong Hα emission. Mg2800 emission, however, strongly anticorrelates with temperature such that warm stars show absorption and stars cooler than 5000 K universally show chromospheric emission regardless of dwarf/giant status or metallicity. Transformed to Mg2800 flux emerging from the stellar surface, we find a correlation with temperature with approximately symmetric astrophysical scatter. Previous work had indicated a basal level with asymmetric scatter to strong values. The discrepancy is primarily due to our improved treatment of extinction. We confirm statistically significant time variability in Mg2800 strength for one star.

Keywords: hubble space telescope; stellar chromospheres
Errors When Constraining Hot Blackbody Parameters with Optical Photometry

Measuring blackbody parameters for objects hotter than a few 104 K with optical data alone is common in many astrophysical studies. However this process is prone to large errors because at those temperatures the optical bands are mostly sampling the Rayleigh-Jeans tail of the spectrum. Here we quantify these errors by simulating different blackbodies, sampling them in various bands with realistic measurement errors, and refitting them to blackbodies using two different methods and two different priors. We find that when using only optical data, log-uniform priors perform better than uniform priors. Still, measured temperatures of blackbodies above ~35,000 K can be wrong by ~10,000 K, and only lower limits can be obtained for temperatures of blackbodies hotter than ~50,000 K. Bolometric luminosities estimated from optical-only blackbody fits can be wrong by factors of 3-5. When adding space-based ultraviolet data, these errors shrink significantly. For when such data are not available, we provide plots and tables of the distributions of true temperatures that can result in various measured temperatures. It is important to take these distributions into account as systematic uncertainties when fitting hot blackbodies with optical data alone.

Keywords: astronomical methods; optical astronomy; ultraviolet astronomy
3D Morphology of Open Clusters in the Solar Neighborhood with Gaia EDR 3. II. Hierarchical Star Formation Revealed by Spatial and Kinematic Substructures

We identify members of 65 open clusters in the solar neighborhood using the machine-learning algorithm StarGO based on Gaia EDR3 data. After adding members of 20 clusters from previous studies we obtain 85 clusters, and study their morphology and kinematics. We classify the substructures outside the tidal radius into four categories: filamentary (f1) and fractal (f2) for clusters <100 Myr, and halo (h) and tidal tail (t) for clusters >100 Myr. The kinematical substructures of f1-type clusters are elongated; these resemble the disrupted cluster Group X. Kinematic tails are distinct in t-type clusters, especially Pleiades. We identify 29 hierarchical groups in four young regions (Alessi 20, IC 348, LP 2373, LP 2442); 10 among these are new. The hierarchical groups form filament networks. Two regions (Alessi 20, LP 2373) exhibit global orthogonal expansion (stellar motion perpendicular to the filament), which might cause complete dispersal. Infalling-like flows (stellar motion along the filament) are found in UBC 31 and related hierarchical groups in the IC 348 region. Stellar groups in the LP 2442 region (LP 2442 gp 1-5) are spatially well mixed but kinematically coherent. A merging process might be ongoing in the LP 2442 subgroups. For younger systems (≲30 Myr), the mean axis ratio, cluster mass, and half-mass-radius tend to increase with age values. These correlations between structural parameters may imply two dynamical processes occurring in the hierarchical formation scenario in young stellar groups: (1) filament dissolution and (2) subgroup mergers.

Keywords: open star clusters; young star clusters; stellar kinematics; stellar dynamics; astrostatistics
The Atmosphere of HD 149026b: Low Metal Enrichment and Weak Energy Transport

Recent JWST eclipse spectra of the high-density hot Saturn HD 149026b between 2.35 and 5.08 μm have allowed for in-depth study of its atmosphere. To understand its atmospheric properties, we have created a grid of 1D radiative-convective–thermochemical–equilibrium atmosphere models and spectra with PICASO 3.0. In agreement with previous work, we find that the presence of gaseous TiO creates a thermal inversion, which is inconsistent with the data. The presence of gaseous VO, however, which condenses at temperatures 200 K cooler, does not cause such inversions but alters the temperature–pressure profile of the atmosphere. We estimate an atmospheric metallicity of solar without VO and solar with VO, a factor of ∼10 times smaller than previous work from Bean et al., who relied on atmosphere retrievals. We attribute this significant difference in metallicity to a larger temperature gradient at low pressures in radiative equilibrium models. Such models with lower metallicities readily fit the strong CO2 feature at 4.3 μm. Our lower estimated metallicity makes HD 149026b more consistent with the mass–metallicity relationship for other giant planets. We find a C/O ratio of with and without VO. The best-fit heat redistribution factor without VO is 1.17, a very high value suggesting very little dayside energy transport and no energy transport to the nightside. The heat redistribution factor shrinks to a more plausible value of , with VO, which we regard as circumstantial evidence for the molecule in the atmosphere of HD 149026b.

Keywords: exoplanet atmospheres; atmospheric composition
Inferring the Interstellar Magnetic Field Direction from Energetic Neutral Atom Observations of the Heliotail

Determining the magnitude and direction of the interstellar magnetic field (B ISM) is a long-standing problem. To date, some methods to infer the direction and magnitude have utilized best-fit models to the positions of the termination shock and heliopause measured by Voyager 1 and 2. Other models use the circularity of the Interstellar Boundary Explorer (IBEX) ribbon assuming a secondary energetic neutral atom (ENA) mechanism. Previous studies have revealed that the B ISM organizes the orientation of the heliotail with respect to the solar meridian. Here we propose a new way to infer the direction of the B ISM based on ENA observations of the heliotail. IBEX observations of the heliotail have revealed high-latitude lobes of enhanced ENA flux at energies >2 keV. Analyses showed that the high-latitude lobes are nearly aligned with the solar meridian, while also exhibiting a rotation with solar cycle. We show, using steady-state solar wind conditions, that the inclination of the lobes reproduced with commonly used values for the angle (α BV ) between B ISM and the interstellar flow in the hydrogen deflection plane (40° < α BV < 60°) is inconsistent with the IBEX ENA observations. We report that 0° < α BV < 20° best replicates the heliotail lobe inclinations observed by IBEX. Additionally, our model results indicate that the variation of the solar magnetic field magnitude with solar cycle causes the longitudinal rotation of the lobes observed by IBEX by affecting the inclination of the lobes.

Keywords: heliosphere; interstellar magnetic fields; interstellar medium; heliosheath; solar system; heliopause; termination shock; magnetohydrodynamics; magnetohydrodynamical simulations; the sun
R-process-rich Stellar Streams in the Milky Way

We present high-resolution Magellan/MIKE spectra of 22 bright (9 < V < 13.5) metal-poor stars (-3.18 < [Fe/H] < -1.37) in three different stellar streams, the Helmi debris stream, the Helmi trail stream, and the ω Centauri progenitor stream. We augment our Helmi debris sample with results for 10 stars by Roederer et al. for a total of 32 stars. Detailed chemical abundances of light elements as well as heavy neutron-capture elements have been determined for our 22 stars. All three streams contain carbon-enhanced stars. For 13 stars, neutron-capture element lines were detectable, and they all show signatures in agreement with the scaled solar r-process pattern, albeit with a large spread of -0.5 < [Eu/Fe] < +1.3. Eight of these stars show an additional small s-process contribution superposed onto their r-process pattern. This could be discerned because of the relatively high signal-to-noise ratio of the spectra given that the stars are close by in the halo. Our results suggest that the progenitors of these streams experienced one or more r-process events early on, such as a neutron star merger or another prolific r-process source. This widely enriched these host systems before their accretion by the Milky Way. The small s-process contribution suggests the presence of asymptotic giant branch stars and associated local (inhomogeneous) enrichment as part of the ongoing chemical evolution by low-mass stars. Stars in stellar streams may thus be a promising avenue for studying the detailed history of large dwarf galaxies and their role in halo assembly with easily accessible targets for high-quality spectra of many stars. * This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

Keywords: halo stars; milky way stellar halo; population ii stars; dwarf galaxies
A Diamond Ring in X-Ray: Investigating the Diffuse Emission of Supernova Remnant G28.8+1.5

Supernova remnants are products of supernova explosions, which are usually observed in the X-ray band. However, the diffuse emission from the galactic supernova remnant G28.8+1.5 has been overlooked following initial searches for pulsars and pulsar wind nebulae. This paper presents background-subtracted imaging and spatial-resolved spectral analyses using archival XMM-Newton data to reveal unprecedented details of the diffuse emission, providing updated estimates of distance and age. For the first time, an iron overabundance is revealed and the progenitor is suggested to be 9–15 M ⊙. These findings refine our understanding of G28.8+1.5 and contribute to a broader comprehension of the Galactic supernova remnant family.

Keywords: supernova remnants; x-ray astronomy
On the Nature of Organic Dust in Novae

Recent astronomical observations and planetary missions have found that complex organics are prevalent throughout the universe, from the solar system to distant galaxies. However, the detailed chemical composition and the synthesis pathway of these organics are still unclear. Circumstellar envelopes represent excellent laboratories to study the abiological synthesis of extraterrestrial organics. Novae, having very short dynamical lifetimes, can put severe constraints on the chemical pathway of organic synthesis. Here, we report a laboratory simulation of carbonaceous dust with inclusion of Nitrogen in the form of Quenched Nitrogen-included Carbonaceous Composite (QNCC). QNCC is produced by the quenched condensation of plasma gas generated from the nitrogen gas, and aromatic and/or aliphatic hydrocarbon solids by applying microwave discharge (2.45 GHz, 300 W). We have shown that the spectra of QNCC have a close resemblance to the observed infrared spectra of novae. The results of the infrared and X-ray analyses suggest that the nitrogen inclusion in the form of amine plays an important role in the origin of the broad 8 μm feature of dusty novae. We conclude that QNCC is at present the best laboratory analog of organic dust formed in the circumstellar medium of dusty classical novae, which carries the unidentified infrared bands in novae via thermal emission process.

Keywords: carbonaceous grains; interstellar dust; classical novae