adsabs/SciX_UAT_keywords huggingface dataset:
Extratidal Stars and Chemical Abundance Properties of Two Metal-poor Globular Clusters M53 (NGC 5024) and NGC 5053

We search for extratidal stars around two metal-poor Galactic globular clusters, M53 and NGC 5053, using the near-infrared APOGEE spectra. Applying the t-distributed stochastic neighbor embedding (t-SNE) algorithm on the chemical abundances and radial velocities results in identification of two isolated stellar groups composed of cluster member stars in the t-SNE projection plane. With additional selection criteria of radial velocity, location in the color-magnitude diagram, and abundances from a manual chemical analysis, we find a total of 73 cluster member candidates; seven extratidal stars are found beyond the tidal radii of the two clusters. The extratidal stars around the clusters tend to be located along the leading direction of the cluster proper motion, and the individual proper motion of these stars also seems to be compatible to those of clusters. Interestingly, we find that one extratidal star of NGC 5053 is located on the southern outskirts of M53, which is part of common stellar envelope by the tidal interaction between two clusters. We discuss the nature of this star in the context of the tidal interaction between two clusters. We find apparent Mg-Al anticorrelations with a clear gap and spread (∼0.9 dex) in Al abundances for both clusters, and a light Si abundance spread (∼0.3 dex) for NGC 5053. Since all extratidal stars have Mg-enhanced and Al-depleted features, they could be first-generation stars of two globular clusters. Our results support that M53 and NGC 5053 originated in dwarf galaxies and are surrounded by extended stellar substructures of more numerous populations of clusters.

Keywords: globular star clusters; stellar abundances
FRagmentation and Evolution of Dense Cores Judged by ALMA (FREJA). I. Overview: Inner ∼1000 au Structures of Prestellar/Protostellar Cores in Taurus

We have performed survey-type observations in 1 mm continuum and molecular lines toward dense cores (32 prestellar + 7 protostellar) with an average density of ≳105 cm-3 in the Taurus molecular clouds using the Atacama Large Millimeter/submillimeter Array-Atacama Compact Array (ALMA-ACA) stand-alone mode with an angular resolution of 6"5 (∼900 au). The primary purpose of this study is to investigate the innermost part of dense cores with view to understanding the initial condition of star formation. In the protostellar cores, contributions from protostellar disks dominate the observed continuum flux with a range of 35%-90%, except for the very low-luminosity object. For the prestellar cores, we have successfully confirmed continuum emission from dense gas with a density of ≳3 × 105 cm-3 toward approximately one-third of the targets. Thanks to the lower spatial frequency coverage with the ACA 7 m array, the detection rate is significantly higher than that of the previous surveys, which have zero or one continuum-detected sources among a large number of starless samples using the ALMA Main Array. The statistical counting method tells us that the lifetime of prestellar cores until protostar formation therein approaches the freefall time as the density increases. Among the prestellar cores, at least two targets have possible internal substructures, which are detected in continuum emission with the size scale of ∼1000 au if we consider the molecular line (C18O and N2D+) distributions. These results suggest that small-scale fragmentation/coalescence processes occur in a region smaller than 0.1 pc, which may determine the final core mass associated with individual protostar formation before starting the dynamical collapse of the core with a central density of ∼(0.3-1) × 106 cm-3.

Keywords: star formation; protostars; molecular clouds; collapsing clouds
Photometric and Spectroscopic Study of the EXor-like Eruptive Young Star Gaia19fct

Gaia19fct is one of the Gaia-alerted eruptive young stars that has undergone several brightening events. We conducted monitoring observations using multifilter optical and near-infrared photometry, as well as near-infrared spectroscopy, to understand the physical properties of Gaia19fct and investigate whether it fits into the historically defined two classes. We present the analyses of light curves, color variations, spectral lines, and CO modeling. The light curves show at least five brightening events since 2015, and the multifilter color evolutions are mostly gray. The gray evolution indicates that bursts are triggered by mechanisms other than extinction. Our near-infrared spectra exhibit both absorption and emission lines and show time variability throughout our observations. We found lower rotational velocity and lower temperature from the near-infrared atomic absorption lines than from the optical lines, suggesting that Gaia19fct has a Keplerian rotating disk. The CO overtone features show a superposition of absorption and emission components, which is unlike other young stellar objects. We modeled the CO lines, and the result suggests that the emission and absorption components are formed in different regions. We found that although Gaia19fct exhibits characteristics of both types of eruptive young stars, FU Orionis-type objects and EX Lupi-type objects, it shows more similarity with EXors in general.

Keywords: fu orionis stars; young stellar objects; circumstellar disks; multi-color photometry; high resolution spectroscopy; light curves
A Rapidly Varying Red Supergiant X-Ray Binary in the Galactic Center

We analyzed multiwavelength observations of the previously identified Galactic center X-ray binary CXO 174528.79-290942.8 (XID 6592) and determine that the near-infrared counterpart is a red supergiant based on its spectrum and luminosity. Scutum X-1 is the only previously known X-ray binary with a red supergiant donor star and closely resembles XID 6592 in terms of X-ray luminosity (LX), absolute magnitude, and IR variability (LIR,var), supporting the conclusion that XID 6592 contains a red supergiant donor star. The XID 6592 infrared counterpart shows variability of ∼0.5 mag in the Wide-field Infrared Survey Explorer-1 band (3.4 μm) on timescales of a few hours. Other infrared data sets also show large-amplitude variability from this source at earlier epochs but do not show significant variability in recent data. We do not expect red supergiants to vary by ∼50% in luminosity over these short timescales, indicating that the variability should be powered by the compact object. However, the X-ray luminosity of this system is typically ∼1000× less than the variable luminosity in the infrared and falls below the Chandra detection limit. While X-ray reprocessing can produce large-amplitude fast infrared variability, it typically requires ${L}_{{\rm{X}}}\gt \,\gt $ LIR,var to do so, indicating that another process must be at work. We suggest that this system may be a supergiant fast X-ray transient (SFXT), and that a large (∼1038 ergs s-1), fast (102-4 s) X-ray flare could explain the rapid IR variability and lack of a long-lasting X-ray outburst detection. SFXTs are normally associated with blue supergiant companions, so if confirmed, XID 6592 would be the first red supergiant SFXT, as well as the second X-ray red supergiant binary.

Keywords: x-ray binary stars; late-type supergiant stars; infrared astronomy; infrared sources; x-ray astronomy; x-ray sources
Searches for Extremely Metal-poor Galaxies Using Arecibo Legacy Fast ALFA-Selected Dwarf Galaxies

We present a study of nearby dwarf galaxies selected from the Arecibo Legacy Fast ALFA (ALFALFA) blind H I survey. A primary goal of the project was to utilize a nonstandard selection method with the hope of detecting previously unrecognized extremely metal-poor (XMP) galaxies. The study was motivated by the recent discovery of two XMP galaxies, Leo P and Leoncino, which were both originally found via the ALFALFA survey. We have obtained narrowband Hα images for 42 dwarf systems, many of which are located in the Local Void in front of the Pisces-Perseus Supercluster. Spectra for 11 of the best candidates resulted in the determination of metal abundances for 10 of the systems. None were found to be extremely metal-poor, although one system (AGC 123350) was found to have an oxygen abundance of log(O/H)+12 = 7.46, or ~6% solar. One of the galaxies in our sample exhibits a high oxygen abundance for its luminosity, suggesting the possibility that it may have a tidal origin. * Based in part on observations obtained with the Hobby-Eberly Telescope, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen.

Keywords: dwarf irregular galaxies; metallicity; galaxy spectroscopy; galaxy chemical evolution; star forming regions; galaxy abundances; starburst galaxies; h i line emission; star formation
Detection of X-Ray Bursts in Astronomical Time Series: The Burst of GRO J1744-28 as an Example

To automatically, accurately, and quickly detect local changes in time-series data continuously emitted by X-ray sources, an autoencoder-based unsupervised learning anomaly detection method is proposed. Here, we consider the X-ray burst of GRO J1744-28 as our case study. First, we tested the proposed method using simulation data and a test set based on a phenomenologically motivated light-curve fitting of different burst types. Our method exhibited superior performance, achieving F-scores of 0.969 and 0.936 for the detection of small bursts with low peak count rates such as structured bursts and microbursts, respectively. Then, based on Rossi X-ray Timing Detector observation data for GRO J1744-28 during the outburst period, we identified low-amplitude bursts using the proposed method and analyzed the burst regularity of GRO J1744-28. Our approach does not require complex modeling and has powerful feature extraction and detection capabilities, which can be used to automatically and efficiently detect changes in a data stream.

Keywords: x-ray astronomy; astronomy data analysis; computational methods; time series analysis
Observability of Substructures in the Planet-forming Disk in the (Sub)centimeter Wavelength with SKA and ngVLA

Current imaging observations of protoplanetary disks using the Atacama Large Millimeter/submillimeter Array (ALMA) primarily focus on the submillimeter wavelength, leaving a gap in effective observational approaches for centimeter-sized dust, which is crucial to the issue of planet formation. The forthcoming Square Kilometre Array (SKA) and ngVLA may rectify this deficiency. In this paper, we employ multifluid hydrodynamic numerical simulations and radiative transfer calculations to investigate the potential of SKA1-Mid, ngVLA, and SKA2 for imaging protoplanetary disks at subcentimeter/centimeter wavelengths. We create mock images with ALMA/SKA/ngVLA at multiwavelengths based on the hydrodynamical simulation output and test different sensitivity and spatial resolutions. We discover that both SKA and ngVLA will serve as excellent supplements to the existing observational range of ALMA, and their high resolution enables them to image substructures in the disk's inner region (∼5 au from the stellar). Our results indicate that SKA and ngVLA can be utilized for more extended monitoring programs in the centimeter wave band. While in the subcentimeter range, ngVLA possesses the capability to produce high-fidelity images within shorter observation times (∼1 hr on source time) than previous research, holding potential for future survey observations. We also discuss for the first time the potential of SKA2 for observing protoplanetary disks at a 0.7 cm wavelength.

Keywords: protoplanetary disks; planet formation; hydrodynamics; astronomical simulations; astrophysical fluid dynamics
Tiger: Concept Study for a New Frontiers Enceladus Habitability Mission

Data returned from the Cassini-Huygens mission have strengthened Enceladus, a small icy moon of Saturn, as an important target in the search for life in our solar system. Information gathered from Cassini to support this includes the presence of a subsurface liquid water ocean, vapor plumes and ice grains emanating from its south polar region, and the detection of essential elements and organic material that could potentially support life. However, several outstanding questions remain regarding the connectivity of plume material to the ocean and the composition of the complex organic material. Herein we introduce Tiger, a mission concept developed during the 2020 Planetary Science Summer School at NASA's Jet Propulsion Laboratory. Tiger is a flyby mission that would help further constrain the habitability of Enceladus through two science objectives: (1) determine whether Enceladus's volatile inventory undergoes synthesis of complex organic species that are evidence for a habitable ocean, and (2) determine whether Enceladus's plume material is supplied directly from the ocean or if it interfaces with other reservoirs within the ice shell. To address the science goals in a total of eight flybys, Tiger would carry a four-instrument payload, including a mass spectrometer, a single-band ice-penetrating radar, an ultraviolet imaging spectrograph, and an imaging camera. We discuss Tiger's instrument and mission architecture, as well as the trades and challenges associated with a habitability-focused New Frontiers-class flyby mission to Enceladus.

Keywords: habitable planets; saturnian satellites; ocean planets; astrobiology; interdisciplinary astronomy
Constraining the Internal Structures of Venus and Mars from the Gravity Response to Atmospheric Loading

The gravity fields of celestial bodies that possess an atmosphere are periodically perturbed by the redistribution of fluid mass associated with atmospheric dynamics. A component of this perturbation is due to the gravitational response of the body to the deformation of its surface induced by the atmospheric pressure loading. The magnitude of this effect depends on the relation between the loading and the response in terms of geopotential variations measured by the load Love numbers. In this work, we simulate and analyze the gravity field generated by the atmospheres of Venus and Mars by accounting for different models of their internal structure. By precisely characterizing the phenomena that drive the mass transportation in the atmosphere through general circulation models, we determine the effect of the interior structure on the response to the atmospheric loading. An accurate estimation of the time-varying gravity field, which measures the atmospheric contribution, may provide significant constraints on the interior structure through the measurement of the load Love numbers. A combined determination of tidal and load Love numbers would enhance our knowledge of the interior of planetary bodies, providing further geophysical constraints in the inversion of internal structure models.

Keywords: planetary interior; planetary atmospheres; atmospheric tides; tides; gravitational fields
Determining the Variations of Ca-K Index and Features Using Century-long Equal-contrast Images from Kodaikanal Observatory

In an earlier analysis of Ca-K spectroheliograms obtained at the Kodaikanal Observatory, the "Good" images were used to investigate variations in the chromosphere. However, the contrast of the images varied on a day-to-day basis. We have developed a new methodology to generate images to form a uniform time series. We adjusted each image's contrast until the FWHM of the normalized intensity distribution attained a value between 0.10 and 0.11. This "equal-contrast technique" is expected to compensate for the change of emulsion, development, and contrast of the images due to centering of the Ca-K line on the exit slit and sky transparency. In addition, this procedure will correct variations in density-to-intensity conversion for different images. We find that the correlation between sunspot and Ca-K line data improves to a large extent. For example, the correlation coefficient (CC) between monthly averaged sunspots and Ca-K plage areas for the equal-contrast data reaches 0.9 compared to 0.75 for the "Good" data with unequal contrast. The CC for equal-contrast images improves to ∼0.78 from ∼0.46 for the "Okay" data with unequal contrast. Even the CC between the plage area and the daily sunspot number is 0.85 for 100 years of data. This methodology also permits us, for the first time, to study the variations in enhanced, active, and quiet networks with time with high accuracy over about a century. Further, this procedure can be used to combine data from different observatories to make a long time series.

Keywords: solar cycle; active sun; quiet sun
The Secondary Maximum of T CrB Caused by Irradiation of the Red Giant by a Cooling White Dwarf

Both the 1866 and 1946 outbursts of the recurrent symbiotic nova T CrB have displayed a mysterious secondary maximum peaking in brightness ~5 months past the primary one. Common to all previous modeling attempts was the rejection of plain irradiation of the red giant (RG), on the basis that the secondary maximum of T CrB would have been out of phase with the transit at superior conjunction of the RG. Implicit to this line of reasoning is the assumption of a constant temperature for the white dwarf (WD) irradiating the RG. I show by radiative modeling that irradiation of the RG by a cooling WD nicely reproduces the photometric evolution of the secondary maximum, both in terms of brightness and color, removes the phasing offset, and provides a straightforward explanation that will be easy to test at the next and imminent outburst.

Keywords: recurrent novae; symbiotic binary stars; symbiotic novae; white dwarf stars; red giant stars
Gravity Investigation to Characterize Enceladus's Ocean and Interior

A key objective for the future exploration of the icy moon Enceladus is the characterization of the habitable conditions in its internal ocean. Radio science instrumentation on board a spacecraft in a low-altitude orbit about Enceladus would enable gravity measurements that are fundamental to providing constraints on its internal structure. We present here the concept of operations and expected results of the gravity investigation for a New Frontiers–class mission. Numerical simulations are carried out to determine the gravity field in spherical harmonics to degree and order 30 and the Love number k 2 and its phase. By combining Enceladus's shape measured by Cassini and the geophysical constraints obtained through the processing of the simulated radio science data, a Bayesian inference network is used for the interior model inversion. Our results indicate that the gravity investigation would enable tight constraints on core radius and density, ocean depth and density, and ice shell rigidity. By assuming a high core rigidity and a preliminary modeling of dissipation in the ice shell, our interior model inversion also yields information on the ice shell viscosity. Further data on the hydrosphere properties might be gathered through optical navigation data by accurately measuring Enceladus's orientation model.

Keywords: enceladus; markov chain monte carlo; gravitational fields; tides
Analysis of Red Supergiants in VDBH 222

Recent surveys uncovered new young massive clusters (YMCs) that host dozens of red supergiants (RSGs) in the inner Milky Way. These clusters are ideal for studying the most recent and violent star formation events in the inner Galaxy. However, due to the high extinction that affects the Galactic plane, they need to be studied through infrared (IR) spectroscopy. IR spectra of RSGs have proven to be powerful tools for obtaining chemical abundances. We present the first [Fe/H] measurement (-0.07 ± 0.02) for the YMC VdBH 222 through analysis of its RSGs using Very Large Telescope/X-shooter spectra. We find no evidence for multiple stellar populations in this YMC, contrary to what is routinely observed in older massive clusters.

Keywords: star clusters; red supergiant stars; chemical abundances
Hours-long Near-UV/Optical Emission from Mildly Relativistic Outflows in Black Hole-Neutron Star Mergers

The ongoing LIGO-Virgo-KAGRA observing run O4 provides an opportunity to discover new multimessenger events, including binary neutron star (BNS) mergers such as GW170817 and the highly anticipated first detection of a multimessenger black hole-neutron star (BH-NS) merger. While BNS mergers were predicted to exhibit early optical emission from mildly relativistic outflows, it has remained uncertain whether the BH-NS merger ejecta provides the conditions for similar signals to emerge. We present the first modeling of early near-ultraviolet/optical emission from mildly relativistic outflows in BH-NS mergers. Adopting optimal binary properties, a mass ratio of q = 2, and a rapidly rotating BH, we utilize numerical relativity and general relativistic magnetohydrodynamic (GRMHD) simulations to follow the binary's evolution from premerger to homologous expansion. We use an M1 neutrino transport GRMHD simulation to self-consistently estimate the opacity distribution in the outflows and find a bright near-ultraviolet/optical signal that emerges due to jet-powered cocoon cooling emission, outshining the kilonova emission at early time. The signal peaks at an absolute magnitude of ~-15 a few hours after the merger, longer than previous estimates, which did not consider the first principles-based jet launching. By late 2024, the Rubin Observatory will have the capability to track the entire signal evolution or detect its peak up to distances of ≳1 Gpc. In 2026, ULTRASAT will conduct all-sky surveys within minutes, detecting some of these events within ~200 Mpc. The BH-NS mergers with higher mass ratios or lower BH spins would produce shorter and fainter signals.

Keywords: compact binary stars; compact objects; astrophysical black holes; relativistic jets; jets; gamma-ray bursts; near ultraviolet astronomy; ultraviolet astronomy; optical astronomy; transient sources; ultraviolet transient sources; hydrodynamical simulations
Why Do Chromospheric Oscillations in Sunspot Umbrae Appear to Propagate Downward?

Umbral oscillations constitute the most noticeable chromospheric feature of sunspot umbrae-large-amplitude oscillations of intensity (umbral flashes, if very strong) and line-of-sight velocity, with periods of about 3 minutes. These umbral oscillations are usually interpreted as acoustic waves propagating upward under the effect of gravity. However, there have been observational reports that intensity peaks tend to occur in downflowing phases of umbral oscillations, and this appears to be more compatible with downward propagation. We investigate whether this intensity-velocity correlation occurs persistently or not, by determining the vertical flux of the wave energy, based on Hα line measurements of the temperature and velocity. As a result, we find that the wave flux is persistently negative in sunspot umbrae, confirming the discrepancy specified above. We attribute this discrepancy to the nonzero fluctuation of net radiative heating. We find that when this effect is taken into account in the energy equation, the pressure is peaked during upflowing phases, being compatible with the notion of upward propagation. We conclude that temperature (and intensity) peaks occur during downflowing phases, not because of downward propagation, but because of radiative heat transport.

Keywords: sunspots; magnetohydrodynamics; solar chromosphere