Sazuna/UAT_keywords huggingface dataset:
Decoding the Bifurcated Red Giant Branch as a Tracer of Multiple Stellar Populations in the Young Large Magellanic Cloud Cluster NGC 2173. Multiple stellar populations (MPs) representing star-to-star light-element abundance variations are common in nearly all ancient Galactic globular clusters (GCs). Here we provide the strongest evidence yet that the populous, ~1.7 Gyr old Large Magellanic Cloud cluster NGC 2173 also exhibits light-element abundance variations. Thus, our results suggest that NGC 2173 is the youngest cluster for which MPs have been confirmed to date. Our conclusion is based on the distinct bifurcation at the tip of its red giant branch in high-quality color-magnitude diagrams generated from Hubble Space Telescope imaging observations. Our results are further supported by a detailed analysis of "pseudo-UBI" maps, which reveal clear evidence of a bimodality in the cluster's red giant branch color distribution. Young clusters in the Magellanic Clouds can provide critical insights into galaxy evolution histories. Our discovery of MPs in NGC 2173 suggests that ancient Galactic GCs and young massive clusters might share a common formation process.

Keywords: Red giant branch, Stellar populations, Large Magellanic Cloud, Red giant stars, Hertzsprung Russell diagram, Star clusters
Rotational Modulation of Spectroscopic Zeeman Signatures in Low-mass Stars. Accurate tracers of the stellar magnetic field and rotation are cornerstones for the study of M dwarfs and for reliable detection and characterization of their exoplanetary companions. Such measurements are particularly challenging for old, slowly rotating, fully convective M dwarfs. To explore the use of new activity and rotation tracers, we examined multiyear near-infrared (NIR) spectroscopic monitoring of two such stars-GJ 699 (Barnard's Star) and Teegarden's Star-carried out with the Habitable-zone Planet Finder spectrograph. We detected periodic variations in absorption line widths across the stellar spectrum, with higher amplitudes toward longer wavelengths. We also detected similar variations in the strength and width of the 12435.67 Å neutral potassium (K I) line, a known tracer of the photospheric magnetic field. Attributing these variations to rotational modulation, we confirm the known 145 ± 15 day rotation period of GJ 699, and measure the rotation period of Teegarden's Star to be 99.6 ± 1.4 days. Based on simulations of the K I line and the wavelength dependence of the line-width signal, we argue that the observed signals are consistent with varying photospheric magnetic fields and the associated Zeeman effect. These results highlight the value of detailed line profile measurements in the NIR for diagnosing stellar magnetic field variability. Such measurements may be pivotal for disentangling activity and exoplanet-related signals in spectroscopic monitoring of old, low-mass stars.

Keywords: M dwarf stars, Stellar activity, Stellar rotation, Stellar spectral lines, Near infrared astronomy
H<SUB>2</SUB> Cooling and Gravitational Collapse of Supersonically Induced Gas Objects. We study the formation and gravitational collapse of supersonically induced gas objects (SIGOs) in the early universe. We run cosmological hydrodynamics simulations of SIGOs, including relative streaming motions between baryons and dark matter. Our simulations also follow nonequilibrium chemistry and molecular hydrogen cooling in primordial gas clouds. A number of SIGOs are formed in the run with fast-streaming motions of 2 times the rms of the cosmological velocity fluctuations. We identify a particular gas cloud that condensates by H2 cooling without being hosted by a dark matter halo. The SIGO remains outside the virial radius of its closest halo, and it becomes Jeans unstable when the central gas-particle density reaches ~100 cm-3 with a temperature of ~200 K. The corresponding Jeans mass is ~105 M ⊙, and thus the formation of primordial stars or a star cluster is expected in the SIGO.

Keywords: Cosmology, Early universe, Hydrodynamical simulations, Population III stars, High-redshift galaxy clusters
Luminosities and Masses of Single Galactic Post-asymptotic Giant Branch Stars with Distances from Gaia EDR3: The Revelation of an s-process Diversity. Post-asymptotic giant branch (AGB) stars are exquisite probes of AGB nucleosynthesis. However, the previous lack of accurate distances jeopardized comparison with theoretical AGB models. The Gaia Early Data Release 3 (Gaia EDR3) has now allowed for a breakthrough in this research landscape. In this study, we focus on a sample of single Galactic post-AGBs for which chemical abundance studies were completed. We combined photometry with geometric distances to carry out a spectral energy distribution (SED) analysis and derive accurate luminosities. We subsequently determined their positions on the Hertzsprung-Russell (HR) diagram and compared this with theoretical post-AGB evolutionary tracks. While most objects are in the post-AGB phase of evolution, we found a subset of low-luminosity objects that are likely to be in the post-horizontal branch phase of evolution, similar to AGB-manqué objects found in globular clusters. Additionally, we also investigated the observed bimodality in the s-process enrichment of Galactic post-AGB single stars of similar T eff and metallicities. This bimodality was expected to be a direct consequence of luminosity with the s-process rich objects having evolved further on the AGB. However, we find that the two populations, the s-process enriched and non-enriched, have similar luminosities (and hence initial masses), revealing an intriguing chemical diversity. For a given initial mass and metallicity, AGB nucleosynthesis appears inhomogeneous and sensitive to other factors, which could be mass loss, along with convective and non-convective mixing mechanisms. Modeling individual objects in detail will be needed to investigate which parameters and processes dominate the photospheric chemical enrichment in these stars.

Keywords: Post-asymptotic giant branch stars, Stellar luminosities, Asymptotic giant branch, Nucleosynthesis, Stellar properties, Dredge-up, Late stellar evolution, Chemical abundances, Galaxy abundances, Spectral energy distribution, Photometry, S-process
The Rotation of Magnetic Flux Ropes Formed during Solar Eruption. Eruptions of solar filaments often show rotational motion about their rising direction, but the mechanism governing such rotation, and how the rotation is related to the initial morphology of the preeruptive filament (and cospatial sigmoid), filament chirality, and magnetic helicity, remains elusive. The conventional view of rotation as a result of a magnetic flux rope (MFR) undergoing ideal kink instability still has difficulty explaining these relationships. Here we propose an alternative explanation for the rotation during eruptions by analyzing a magnetohydrodynamic simulation in which magnetic reconnection initiates an eruption from a sheared arcade configuration, and an MFR is formed during eruption via reconnection. The simulation reproduces a reverse-S-shaped MFR with dextral chirality, and the axis of this MFR rotates counterclockwise while rising, which compares favorably with a typical filament eruption observed from dual viewing angles. By calculating the twist and writhe numbers of the modeled MFR during its eruption, we found that, accompanied by the rotation, the nonlocal writhe of the MFR's axis decreases while the twist of its surrounding field lines increases, and this is distinct from kink instability, which converts magnetic twist into the writhe of the MFR axis.

Keywords: Solar coronal mass ejections, Solar filaments, Solar filament eruptions, Solar prominences
Core Electron Heating by Triggered Ion Acoustic Waves in the Solar Wind. Perihelion passes on Parker Solar Probe orbits 6-9 have been studied to show that solar wind core electrons emerged from 15 solar radii with a temperature of 55 ± 5 eV, independent of the solar wind speed, which varied from 300 to 800 km s-1. After leaving 15 solar radii and in the absence of triggered ion acoustic waves at greater distances, the core electron temperature varied with radial distance, R, in solar radii, as 1900R -4/3 eV because of cooling produced by the adiabatic expansion. The coefficient, 1900, reproduces the minimum core electron perpendicular temperature observed during the 25 days of observation. In the presence of triggered ion acoustic waves, the core electrons were isotropically heated as much as a factor of two above the minimum temperature, 1900R -4/3 eV. Triggered ion acoustic waves were the only waves observed in coincidence with the core electron heating. They are the dominant wave mode at frequencies greater than 100 Hz at solar distances between 15 and 30 solar radii.

Keywords: Solar wind, Solar corona
Periodic Oscillations in LASCO Coronal Mass Ejection Speeds: Space Seismology. Coronal mass ejections (CMEs) are energetic eruptions of organized magnetic structures from the Sun. Therefore, the reconnection of the magnetic field during ejection can excite periodic speed oscillations of CMEs. A previous study showed that speed oscillations are frequently associated with CME propagation. The Solar and Heliospheric Observatory mission's white-light coronagraphs have observed about 30,000 CMEs from 1996 January to the end of 2019 December. This period of time covers two solar cycles (23 and 24). In the present study, the basic attributes of speed oscillations during this period of time were analyzed. We showed that the oscillation parameters (period and amplitude) significantly depend not only on the phase of a given solar cycle but also on the intensity of individual cycles as well. This reveals that the basic attributes of speed oscillation are closely related to the physical conditions prevailing inside the CMEs as well as in the interplanetary medium in which they propagate. Using this approximation, we estimated that, on average, the CME internal magnetic field varies from 18 up to 25 mG between minimum and maximum solar activity. The obtained results show that a detailed analysis of speed oscillations can be a very efficient tool for studying not only the physical properties of the ejections themselves but also the condition of the interplanetary medium in which they expand. This creates completely new perspectives for studying the physical parameters of CMEs shortly after their eruption in the Sun's environment (space seismology).

Keywords: Solar activity, The Sun, Solar physics
Evidence for X-Ray Emission in Excess to the Jet-afterglow Decay 3.5 yr after the Binary Neutron Star Merger GW 170817: A New Emission Component. For the first ~3 yrs after the binary neutron star merger event GW 170817, the radio and X-ray radiation has been dominated by emission from a structured relativistic off-axis jet propagating into a low-density medium with n &lt; 0.01 cm-3. We report on observational evidence for an excess of X-ray emission at δt &gt; 900 days after the merger. With L  x  ≈ 5 × 1038 erg s-1 at 1234 days, the recently detected X-ray emission represents a ≥3.2σ (Gaussian equivalent) deviation from the universal post-jet-break model that best fits the multiwavelength afterglow at earlier times. In the context of JetFit afterglow models, current data represent a departure with statistical significance ≥3.1σ, depending on the fireball collimation, with the most realistic models showing excesses at the level of ≥3.7σ. A lack of detectable 3 GHz radio emission suggests a harder broadband spectrum than the jet afterglow. These properties are consistent with the emergence of a new emission component such as synchrotron radiation from a mildly relativistic shock generated by the expanding merger ejecta, i.e., a kilonova afterglow. In this context, we present a set of ab initio numerical relativity binary neutron star (BNS) merger simulations that show that an X-ray excess supports the presence of a high-velocity tail in the merger ejecta, and argues against the prompt collapse of the merger remnant into a black hole. Radiation from accretion processes on the compact-object remnant represents a viable alternative. Neither a kilonova afterglow nor accretion-powered emission have been observed before, as detections of BNS mergers at this phase of evolution are unprecedented.

Keywords: Neutron stars, Gamma-ray bursts, Gravitational wave sources, X-ray transient sources, Radio transient sources
The Combined Effects of Two-body Relaxation Processes and the Eccentric Kozai-Lidov Mechanism on the Extreme-mass-ratio Inspirals Rate. Gravitational wave (GW) emissions from extreme-mass-ratio inspirals (EMRIs) are promising sources for low-frequency GW detectors. They result from a compact object, such as a stellar-mass black hole (BH), captured by a supermassive BH (SMBH). Several physical processes have been proposed to form EMRIs. In particular, weak two-body interactions over a long timescale (i.e., relaxation processes) have been proposed as a likely mechanism to drive the BH orbit to high eccentricity. Consequently, it is captured by the SMBH and becomes an EMRI. Here we demonstrate that EMRIs are naturally formed in SMBH binaries. Gravitational perturbations from an SMBH companion, known as the eccentric Kozai-Lidov (EKL) mechanism, combined with relaxation processes, yield a significantly more enhanced rate than any of these processes operating alone. Because EKL is sensitive to the orbital configuration, two-body relaxation can alter the orbital parameters, rendering the system in a more EKL-favorable regime. As SMBH binaries are expected to be prevalent in the universe, this process predicts a substantially high EMRI rate.

Keywords: Supermassive black holes, Galactic center, Galaxy structure, Galaxy nuclei, Gravitational wave astronomy, Gravitational waves, Astrophysical black holes
Revisiting the Rates and Demographics of Tidal Disruption Events: Effects of the Disk Formation Efficiency. Tidal disruption events (TDEs) are valuable probes of the demographics of supermassive black holes as well as the dynamics and population of stars in the centers of galaxies. In this Letter, we focus on studying how debris disk formation and circularization processes can impact the possibility of observing prompt flares in TDEs. First, we investigate how the efficiency of disk formation is determined by the key parameters, namely, the black hole mass M BH, the stellar mass m ⋆, and the orbital penetration parameter β that quantifies how close the disrupted star would orbit around the black hole. Then we calculate the intrinsic differential TDE rate as a function of these three parameters. Combining these two results, we find that the rates of TDEs with prompt disk formation are significantly suppressed around lighter black holes, which provides a plausible explanation for why the observed TDE host black hole mass distribution peaks between 106 and 107 M ⊙. Therefore, the consideration of disk formation efficiency is crucial for recovering the intrinsic black hole demographics from TDEs. Furthermore, we find that the efficiency of the disk formation process also impacts the distributions of both stellar orbital penetration parameter and stellar mass observed in TDEs.

Keywords: Accretion, Black hole physics, Supermassive black holes, Relativistic mechanics, High energy astrophysics, Stellar kinematics, Tidal disruption, Galaxy accretion disks, Stellar dynamics, Galaxy nuclei
An XMM-Newton EPIC X-Ray View of the Symbiotic Star R Aquarii. We present the analysis of archival XMM-Newton European Photon Imaging Camera (EPIC) X-ray observations of the symbiotic star R Aquarii. We used the Extended Source Analysis Software package to disclose diffuse soft X-ray emission extending up to 2.'2 (≍0.27 pc) from this binary system. The depth of these XMM-Newton EPIC observations reveals in unprecedented detail the spatial distribution of this diffuse emission, with a bipolar morphology spatially correlated with the optical nebula. The extended X-ray emission shares the same dominant soft X-ray-emitting temperature as the clumps in the jet-like feature resolved by Chandra in the vicinity of the binary system. The harder component in the jet might suggest that the gas cools down; however, the possible presence of nonthermal emission produced by the presence of a magnetic field collimating the mass ejection cannot be discarded. We propose that the ongoing precessing jet creates bipolar cavities filled with X-ray-emitting hot gas that feeds the more extended X-ray bubble as they get disrupted. These EPIC observations demonstrate that the jet feedback mechanism produced by an accreting disk around an evolved, low-mass star can blow hot bubbles, similar to those produced by jets arising from the nuclei of active galaxies.

Keywords: Symbiotic binary stars, Stellar jets, X-ray sources, Stellar winds
Dust Resurgence in Protoplanetary Disks Due to Planetesimal-Planet Interactions. Observational data on the dust content of circumstellar disks show that the median dust content in disks around pre-main-sequence stars in nearby star-forming regions seems to increase from ~1 to ~2 Myr and then decline with time. This behavior challenges the models where the small dust grains steadily decline by accumulating into larger bodies and drifting inwards on a short timescale (≤1 Myr). In this Letter we explore the possibility to reconcile this discrepancy in the framework of a model where the early formation of planets dynamically stirs the nearby planetesimals and causes high-energy impacts between them, resulting in the production of second-generation dust. We show that the observed dust evolution can be naturally explained by this process within a suite of representative disk-planet architectures.

Keywords: Planetary-disk interactions, Circumstellar disks, Planetary system formation, Collision physics, Protoplanetary disks, Planetesimals, Planetary migration
Compact-object Formation, Retention, and Growth through Accretion onto Gas-embedded White Dwarfs/Neutron Stars in Gas-enriched Globular Clusters. Observations of pulsars in globular clusters (GCs) give evidence that more than &gt;10%-20% of neutron stars (NSs) that ever formed in GCs were retained there. However, the velocity distribution of field pulsars peaks at 5-10 times the escape velocities of GCs. Consequently, only a small fraction of GC NSs should have been retained, which is potentially difficult to explain even accounting for low-velocity NSs formed through electron-capture supernovae (SNe). Thus, too few low-velocity NSs should have been retained, giving rise to the NS retention problem in GCs. Here we suggest a novel solution, in which the progenitors of most GC NSs were ONe white dwarfs (WDs) that accreted ambient intracluster gas and formed low-velocity NSs through accretion-induced collapse (AIC). The existence of an early gas-enriched environment in GCs is supported by observations of multiple stellar populations in GCs. It is thought that 10-100s of megayears after the formation of the first generation of stars, and after ONe WDs were already formed, GCs were replenished with gas, which formed a second generation of stars. Accretion of such replenished gas onto the ONe WDs catalyzed the AIC processes. The number of AIC-formed NSs is then sufficient to explain the large number of NSs retained in GCs. Similar processes might also drive CO WDs to produce Type Ia SNe or to merge and form NSs and similarly drive NSs to AIC and mergers producing BHs. Moreover, the wide variety of gas-catalyzed binary mergers and explosive transients suggested to occur in the gas-rich environments of an active galactic nucleus disk could similarly, and even more efficiently, occur in second-generation gas in GCs.

Keywords: Globular star clusters, Neutron stars, White dwarf stars, Bondi accretion, Young massive clusters, Stellar mass black holes, Novae
H I Gas Playing Hide-and-seek around a Powerful FRI-type Quasar at z   2.1. We present optical spectroscopic and milliarcsecond-scale radio continuum observations of the quasar M1540-1453 (z em = 2.104 ± 0.002) that show associated H I 21 cm absorption at z abs = 2.1139. At subkiloparsec scales, the powerful radio source with 1.4 GHz luminosity of 5.9 × 1027 W Hz-1 shows Fanaroff-Riley class I morphology caused by the interaction with dense gas within 70 pc of the active galactic nucleus (AGN). Interestingly, while there are indications for the presence of absorption from low-ionization species like Fe II, Si II, and Si III in the optical spectrum, the expected strong damped Lyα absorption is not detected at the redshift of the H I 21 cm absorber. In comparison to typical high-z quasars, the Lyα emission line is much narrower. The "ghostly" nature of the H I Lyα absorber partially covering the broad-line region of extent 0.05 pc and the detection of widespread H I 21 cm absorption covering the diffuse radio source (extent &gt;425 pc) imply the presence of a large clumpy H I halo, which may have been blown by the jet-interstellar medium (ISM) interaction. Further observations are needed to confirm the ghostly nature of the Lyα absorber and obtain a better understanding of the role played by the jet-ISM interaction in shaping the radio morphology of this powerful AGN. The study showcases how joint radio and optical analysis can shed light on the gaseous environment and origin of radio morphology in AGNs at high redshifts, when these are still the assembly sites of giant galaxies.

Keywords: Radio loud quasars, Active galactic nuclei, Radio jets, Quasar absorption line spectroscopy
Stellar Revival and Repeated Flares in Deeply Plunging Tidal Disruption Events. Tidal disruption events with tidal radius r t and pericenter distance r p are characterized by the quantity β = r t/r p, and "deep encounters" have β ≫ 1. It has been assumed that there is a critical β ≡ β c ~ 1 that differentiates between partial and full disruption: for β &lt; β c a fraction of the star survives the tidal interaction with the black hole, while for β &gt; β c the star is completely destroyed, and hence all deep encounters should be full. Here we show that this assumption is incorrect by providing an example of a β = 16 encounter between a γ = 5/3, solar-like polytrope and a 106 M ⊙ black hole-for which previous investigations have found β c ≃ 0.9-that results in the reformation of a stellar core post-disruption that comprises approximately 25% of the original stellar mass. We propose that the core reforms under self-gravity, which remains important because of the compression of the gas both near pericenter, where the compression occurs out of the orbital plane, and substantially after pericenter, where compression is within the plane. We find that the core forms on a bound orbit about the black hole, and we discuss the corresponding implications of our findings in the context of recently observed, repeating nuclear transients.

Keywords: Black hole physics, Hydrodynamical simulations, Supermassive black holes, Tidal disruption, Astrophysical black holes