adsabs/SciX_UAT_keywords huggingface dataset:
Star-forming Sites IC 446 and IC 447: An Outcome of End-dominated Collapse of Monoceros R1 Filament

We present an analysis of multiwavelength observations of Monoceros R1 (Mon R1) complex (at d ∼760 pc). An elongated filament (length ∼14 pc, mass ∼1465 M⊙) is investigated in the complex, which is the most prominent structure in the Herschel column density map. An analysis of the FUGIN 12CO(1-0) and 13CO(1-0) line data confirms the existence of the filament traced in a velocity range of [-5, +1] km s-1. The filament is found to host two previously known sites IC 446 and IC 447 at its opposite ends. A massive young stellar object (YSO) is embedded in IC 446, while IC 447 contains several massive B-type stars. The Herschel temperature map reveals the extended warm dust emission (at Td ∼ 15-21 K) toward both the ends of the filament. The Spitzer ratio map of 4.5 μm/3.6 μm emission suggests the presence of photodissociation regions and signature of outflow activity toward IC 446 and IC 447. Based on the photometric analysis of point-like sources, clusters of YSOs are traced mainly toward the filament ends. The filament is found to be thermally supercritical showing its tendency of fragmentation, which is further confirmed by the detection of a periodic oscillatory pattern (having a period of ∼3-4 pc) in the velocity profile of 13CO. Our outcomes suggest that the fragments distributed toward the filament ends have rapidly collapsed, and had formed the known star-forming sites. Overall, the elongated filament in Mon R1 is a promising sample of the "end-dominated collapse" scenario, as discussed by Pon et al. (2011, 2012).

Keywords: h ii regions; star formation; pre-main sequence stars; interstellar medium
A Practical Guide to the Partition Function of Atoms and Ions

The partition function, U, the number of available states in an atom or molecules, is crucial for understanding the physical state of any astrophysical system in thermodynamic equilibrium. There are surprisingly few useful discussions of the partition function's numerical value. Textbooks often define U; some give tables of representative values, while others do a deep dive into the theory of dense plasma. Most say that it depends on temperature, atomic structure, density, and that it diverges, that is, it goes to infinity, at high temperatures, but few give practical examples. We aim to rectify this. We show that there are two limits, one- and two-electron (or closed-shell) systems like H or He, and species with a complicated electronic structure like C, N, O, and Fe. The high-temperature divergence does not occur for one- and two-electron systems in practical situations because, at high temperatures, species are collisionally ionized to higher-ionization stages and are not abundant. The partition function is then close to the statistical weight of the ground state. There is no such simplification for many-electron species. U is temperature sensitive across the range of temperatures where an ion is abundant but remains finite at even the highest practical temperatures. The actual value depends on highly uncertain truncation theories in high-density plasmas. We show that there are various theories for continuum lowering but that they are not in good agreement. This remains a long-standing unsolved problem.

Keywords: atomic physics; interstellar atomic gas; emission nebulae
VVV CL001: Likely the Most Metal-poor Surviving Globular Cluster in the Inner Galaxy

We present the first high-resolution abundance analysis of the globular cluster VVV CL001, which resides in a region dominated by high interstellar reddening toward the Galactic bulge. Using H-band spectra acquired by the Apache Point Observatory Galactic Evolution Experiment, we identified two potential members of the cluster, and estimated from their Fe I lines that the cluster has an average metallicity of [Fe/H] = -2.45 with an uncertainty due to systematics of 0.24 dex. We find that the light-(N), α-(O, Mg, Si), and Odd-Z (Al) elemental abundances of the stars in VVV CL001 follo w the same trend as other Galactic metal-poor globular clusters. This makes VVV CL001 possibly the most metal-poor globular cluster identified so far within the Sun's galactocentric distance and likely one of the most metal-deficient clusters in the Galaxy after ESO280-SC06. Applying statistical isochrone fitting, we derive self-consistent age, distance, and reddening values, yielding an estimated age of ${11.9}_{-4.05}^{+3.12}$ Gyr at a distance of ${8.22}_{-1.93}^{+1.84}$ kpc, revealing that VVV CL001 is also an old GC in the inner Galaxy. The Galactic orbit of VVV CL001 indicates that this cluster lies on a halo-like orbit that appears to be highly eccentric. Both chemistry and dynamics support the hypothesis that VVV CL001 could be an ancient fossil relic left behind by a massive merger event during the early evolution of the Galaxy, likely associated with either the Sequoia or the Gaia-Enceladus-Sausage structures.

Keywords: globular star clusters; chemical abundances; red giant branch
Exploiting Network Topology for Accelerated Bayesian Inference of Grain Surface Reaction Networks

In the study of grain-surface chemistry in the interstellar medium, there exists much uncertainty regarding the reaction mechanisms with few constraints on the abundances of grain-surface molecules. Bayesian inference can be performed to determine the likely reaction rates. In this work, we consider methods for reducing the computational expense of performing Bayesian inference on a reaction network by looking at the geometry of the network. Two methods of exploiting the topology of the reaction network are presented. One involves reducing a reaction network to just the reaction chains with constraints on them. After this, new constraints are added to the reaction network and it is shown that one can separate this new reaction network into subnetworks. The fact that networks can be separated into subnetworks is particularly important for the reaction networks of interstellar complex-organic molecules, whose surface reaction networks may have hundreds of reactions. Both methods allow the maximum-posterior reaction rate to be recovered with minimal bias.

Keywords: astrostatistics strategies; astrochemistry; reaction rates; interstellar abundances; dark interstellar clouds
FAUST. II. Discovery of a Secondary Outflow in IRAS 15398-3359: Variability in Outflow Direction during the Earliest Stage of Star Formation?

We have observed the very low-mass Class 0 protostar IRAS 15398-3359 at scales ranging from 50 to 1800 au, as part of the Atacama Large Millimeter/Submillimeter Array Large Program FAUST. We uncover a linear feature, visible in H2CO, SO, and C18O line emission, which extends from the source in a direction almost perpendicular to the known active outflow. Molecular line emission from H2CO, SO, SiO, and CH3OH further reveals an arc-like structure connected to the outer end of the linear feature and separated from the protostar, IRAS 15398-3359, by 1200 au. The arc-like structure is blueshifted with respect to the systemic velocity. A velocity gradient of 1.2 km s-1 over 1200 au along the linear feature seen in the H2CO emission connects the protostar and the arc-like structure kinematically. SO, SiO, and CH3OH are known to trace shocks, and we interpret the arc-like structure as a relic shock region produced by an outflow previously launched by IRAS 15398-3359. The velocity gradient along the linear structure can be explained as relic outflow motion. The origins of the newly observed arc-like structure and extended linear feature are discussed in relation to turbulent motions within the protostellar core and episodic accretion events during the earliest stage of protostellar evolution.

Keywords: stellar jets; star formation; shocks; protostars; young stellar objects; interstellar medium; interstellar molecules; astrochemistry
Hybrid Very Long Baseline Interferometry Imaging and Modeling with THEMIS

Generating images from very long baseline interferometric observations poses a difficult, and generally not unique, inversion problem. This problem is simplified by the introduction of constraints, some generic (e.g., positivity of the intensity) and others motivated by physical considerations (e.g., smoothness, instrument resolution). It is further complicated by the need to simultaneously address instrumental systematic uncertainties and sparse coverage in the u-v plane. We report a new Bayesian image reconstruction technique in the parameter estimation framework THEMIS that has been developed for the Event Horizon Telescope. This has two key features: first, the full Bayesian treatment of the image reconstruction makes it possible to generate a full posterior for the images, permitting a rigorous and quantitative investigation into the statistical significance of image features. Second, it is possible to seamlessly incorporate directly modeled features simultaneously with image reconstruction. We demonstrate this second capability by incorporating a narrow, slashed ring in reconstructions of simulated M87 data in an attempt to detect and characterize the photon ring. We show that it is possible to obtain high-fidelity photon ring sizes, enabling mass measurements with accuracies of 2%-5% that are essentially insensitive to astrophysical uncertainties, and creating opportunities for precision tests of general relativity.

Keywords: black hole physics; astronomy data modeling; computational astronomy; submillimeter astronomy; long baseline interferometry; general relativity
A Catalog of Faculae, Prominences, and Filaments for the Period 1929-1944 from the Astronomical Observatory of the University of Coimbra

The Astronomical Observatory of the University of Coimbra (Portugal) published a catalog with solar observations such as sunspots, faculae, prominences, and filaments for the period 1929-1944. In previous works, a machine-readable version on sunspot observations made in Coimbra was published. Here we extend that work and present a digital version of the facula, prominence, and filament observations made in that observatory. We have applied a quality control to the catalog, obtaining that the percentage of problematic or suspicious data found is lower than 1% of the total number of observations. In addition, we show an analysis of this catalog, as well as some comparisons between solar indices calculated from Coimbra data and those from other sources. Historical observations of faculae, prominences, and filaments are not as common as sunspot records, and in addition, few historical series of these solar features are available in digital version. For that reason, the catalog of solar observations published by the Coimbra Observatory is of enormous value. The recovery, publication, and availability of this catalog provide the scientific community with a valuable data set of solar characteristics that will help us to study in more detail the past solar magnetic field and long-term solar activity.

Keywords: astronomy databases; catalogs; sunspot cycle; sunspot groups
Compact and Variable Radio Emission from an Active Galaxy with Supersoft X-Ray Emission

RX J1301.9+2747 is a unique active galaxy with a supersoft X-ray spectrum that lacks significant emission at energies above 2 keV. In addition, it is one of few galaxies displaying quasiperiodic X-ray eruptions that recur on a timescale of 13-20 ks. We present multiepoch radio observations of RX J1301.9+2747 using GMRT, Very Large Array (VLA), and Very Long Baseline Array (VLBA). The VLBA imaging at 1.6 GHz reveals a compact radio emission unresolved at a scale of <0.7 pc, with a brightness temperature of T b > 5 × 107 K. The radio emission is variable by more than a factor of 2.5 over a few days, based on the data taken from VLA monitoring campaigns. The short-term radio variability suggests that the radio emitting region has a size as small as 8 × 10-4 pc, resulting in an even higher brightness temperature of T b ~ 1012 K. A similar limit on the source size can be obtained if the observed flux variability is not intrinsic and caused by the interstellar scintillation effect. The overall radio spectrum is steep with a time-averaged spectral index α = -0.78 ± 0.03 between 0.89 and 14 GHz. These observational properties rule out a thermal or star formation origin of the radio emission, and appear to be consistent with the scenario of episodic jet ejections driven by a magnetohydrodynamic process. Simultaneous radio and X-ray monitoring observations down to a cadence of hours are required to test whether the compact and variable radio emission is correlated with the quasiperiodic X-ray eruptions.

Keywords: active galactic nuclei; accretion; radio jets; relativistic jets
NGC 6822 as a Probe of Dwarf Galactic Evolution

NGC 6822 is the closest isolated dwarf irregular galaxy to the Milky Way. Its proximity and stellar mass ( ${10}^{8}{M}_{\odot }$ , large for a dwarf galaxy) allow for a detailed study of its kinematic properties. The red giant branch (RGB) stars at the galaxy's center are particularly interesting because they are aligned on an axis perpendicular to the galaxy's more extended H I disk. We detected a velocity gradient among the RGB population using spectra from Keck/Deep Imaging Multi-Object Spectrograph (DEIMOS). This rotation is aligned with the H I disk, but the sense of rotation is about the major axis of the central RGB population. We measured the rotation velocity (v) and velocity dispersion (σ) of the RGB population in five metallicity bins. We found an increase of rotation support (v/σ) with increasing metallicity, driven primarily by decreasing dispersion. We also deduced an increasing radial distance for lower metallicity stars at -0.5 kpc dex-1 by relating the observed stellar kinematics to position via NGC 6822's H I velocity curve. While the inverted metallicity gradient-like distribution could be interpreted as evidence for an outside-in formation scenario, it may instead indicate that stellar feedback disturbed a centrally star-forming galaxy over time. * 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: dwarf irregular galaxies; stellar kinematics
The Active Galactic Nuclei in the Hobby-Eberly Telescope Dark Energy Experiment Survey (HETDEX). III. A Red Quasar with Extremely High Equivalent Widths Showing Powerful Outflows

We report an active galactic nucleus (AGN) with an extremely high equivalent width (EW), EWLyα+N V,rest ≳921 Å, in the rest frame, at z ~ 2.24 in the Hobby-Eberly Telescope Dark Energy Experiment Survey (HETDEX), as a representative case of the high-EW AGN population. The continuum level is a nondetection in the HETDEX spectrum; thus the measured EW is a lower limit. The source is detected with significant emission lines (>7σ) at Lyα + N V λ1241, C IV λ1549, and a moderate emission line (~4σ) at He II λ1640 within the wavelength coverage of HETDEX (3500-5500 Å). The r-band magnitude is 24.57 from the Hyper Suprime-Cam-HETDEX joint survey with a detection limit of r = 25.12 at 5σ. The Lyα emission line spans a clearly resolved region of ~10″ (85 kpc) in diameter. The Lyα line profile is strongly double peaked. The spectral decomposed blue gas and red gas Lyα emission are separated by ~1.″2 (10.1 kpc) with a line-of-sight velocity offset of ~1100 km s-1. This source is probably an obscured AGN with powerful winds.

Keywords: active galaxies; low-luminosity active galactic nuclei; agn host galaxies
Detecting and Locating Electromagnetic Counterparts to Gravitational Wave Sources Using Galactic Dust Scattering Halos

We discuss a technique for detecting and locating rapid transient electromagnetic counterparts to gravitational wave sources that affords a reprieve of several hours after the gravitational wave event. The technique relies on detecting a scattering halo produced if X-rays emitted at the gravitational wave event scatter off Galactic dust clouds. The travel-time delay of these scattered photons makes them detectable up to several hours after the prompt event; the location of the gravitational wave source will be at the geometric center of the halo, which can be determined with precision sufficient to allow the host galaxy to be identified.

Keywords: x-ray astronomy; gravitational waves; black holes
Alfvén Wave Connection between the Chromosphere and the Corona of the Sun: An Analytical Study

Alfvén waves are closely relevant to the three outstanding problems in the solar corona: coronal heating, solar wind acceleration, and the fractionization of low first ionization potential (FIP) elements. There has been increasing observational evidence for the Alfvén waves, not only in the corona, but also in the chromosphere. Here we investigate the Alfvén wave connection between the chromosphere and the corona based on the analytical solution of Alfvén waves in a layer where Alfvén speed varies along magnetic field lines with a constant gradient. The wave transmission of the layer is determined by two parameters: the Alfvénic cutoff frequency and the dimensionless thickness of the layer. It is shown that the ponderomotive acceleration originating from Alfvén waves is always directed upward in the solar atmosphere with the peak occurring in the chromosphere-corona transition region in association with downward low-frequency waves. We also find that some velocity amplitudes observed in the chromosphere of quiet regions and all the velocity amplitudes observed in active regions fall short of the theoretical estimates obtained with the assumption that the Alfvén waves generated below the chromosphere transport upward the energy required for the corona. We suggest considering the possibility that the Alfvén waves responsible for the coronal heating and the FIP fractionization originate from above the chromosphere.

Keywords: magnetohydrodynamics; alfven waves; solar chromosphere; solar atmosphere
Rapid Generation of Kilonova Light Curves Using Conditional Variational Autoencoder

The discovery of the optical counterpart, along with the gravitational waves (GWs) from GW170817, of the first binary neutron star merger has opened up a new era for multimessenger astrophysics. Combining the GW data with the optical counterpart, also known as AT 2017gfo and classified as a kilonova, has revealed the nature of compact binary merging systems by extracting enriched information about the total binary mass, the mass ratio, the system geometry, and the equation of state. Even though the detection of kilonovae has brought about a revolution in the domain of multimessenger astronomy, there has been only one kilonova from a GW-detected binary neutron star merger event confirmed so far, and this limits the exact understanding of the origin and propagation of the kilonova. Here, we use a conditional variational autoencoder (CVAE) trained on light-curve data from two kilonova models having different temporal lengths, and consequently, generate kilonova light curves rapidly based on physical parameters of our choice with good accuracy. Once the CVAE is trained, the timescale for light-curve generation is of the order of a few milliseconds, which is a speedup of the generation of light curves by 1000 times as compared to the simulation. The mean squared error between the generated and original light curves is typically 0.015 with a maximum of 0.08 for each set of considered physical parameters, while having a maximum of ≈0.6 error across the whole parameter space. Hence, implementing this technique provides fast and reliably accurate results.

Keywords: neural networks; neutron stars; compact objects; light curves
Discovery of a Shock-compressed Magnetic Field in the Northwestern Rim of the Young Supernova Remnant RX J1713.7–3946 with X-Ray Polarimetry

Supernova remnants (SNRs) provide insights into cosmic-ray acceleration and magnetic field dynamics at shock fronts. Recent X-ray polarimetric measurements by the Imaging X-ray Polarimetry Explorer (IXPE) have revealed radial magnetic fields near particle acceleration sites in young SNRs, including Cassiopeia A, Tycho, and SN 1006. We present here the spatially resolved IXPE X-ray polarimetric observation of the northwestern rim of SNR RX J1713.7–3946. For the first time, our analysis shows that the magnetic field in the particle acceleration sites of this SNR is oriented tangentially with respect to the shock front. Because of the lack of precise Faraday rotation measurements in the radio band, this was not possible before. The average measured polarization degree (PD) of the synchrotron emission is 12.5% ± 3.3%, lower than the one measured by IXPE in SN 1006, comparable to the Tycho one, but notably higher than the one in Cassiopeia A. On subparsec scales, localized patches within RX J1713.7–3946 display a PD of up to 41.5% ± 9.5%. These results are compatible with a shock-compressed magnetic field. However, in order to explain the observed PD, either the presence of a radial net magnetic field upstream of the shock or partial reisotropization of the turbulence downstream by radial magnetohydrodynamical instabilities can be invoked. From comparison of PD and magnetic field distribution with γ-rays and 12CO data, our results provide new inputs in favor of a leptonic origin of the γ-ray emission.

Keywords: x-ray astronomy; polarimetry; supernova remnants
Eruptivity in Solar Flares: The Challenges of Magnetic Flux Ropes

Two new schemes for identifying field lines involved in eruptions, the r-scheme and q-scheme, are proposed to analyze the eruptive and confined nature of solar flares, as extensions to the original rm scheme proposed in Lin et al. Motivated by three solar flares originating from NOAA Active Region 12192 that are misclassified by rm, we introduce refinements to the r-scheme employing the "magnetic twist flux" to approximate the force balance acting on a magnetic flux rope (MFR); in the q-scheme, the reconnected field is represented by those field lines that anchor in the flare ribbons. Based on data obtained by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager, the coronal magnetic field for 51 flares larger than M5.0 class, from 29 distinct active regions, is constructed using a nonlinear force-free field extrapolation model. Statistical analysis based on linear discriminant function analysis is then performed, revealing that despite both schemes providing moderately successful classifications for the 51 flares, the coronal mass ejection-eruptivity classification for the three target events can only be improved with the q-scheme. We find that the highly twisted field lines and the flare-ribbon field lines have equal average force-free constant α, but all of the flare-ribbon-related field lines are shorter than 150 Mm in length. The findings lead us to conclude that it is challenging to distinguish the MFR from the ambient magnetic field using any quantity based on common magnetic nonpotentiality measures.

Keywords: solar magnetic fields; solar physics; solar active regions; solar coronal mass ejections