adsabs/SciX_UAT_keywords huggingface dataset:
Stable Coronal X-Ray Emission over 20 yr of XZ Tau

XZ Tau AB is a frequently observed binary young stellar object in the Taurus Molecular Cloud; XZ Tau B has been classified as an EXOr object. We present new Chandra/HETG-ACIS-S observations of XZ Tau AB, complemented with variability monitoring of the system with XMM-Newton, to constrain the variability of this system and identify high-resolution line diagnostics to better understand the underlying mechanisms that produce the X-rays. We observe two flares with XMM-Newton but find that outside of these flares the coronal X-ray spectrum of XZ Tau AB is consistent over 20 yr of observations. We compare the ensemble of XZ Tau AB X-ray observations over time with the scatter across stars observed in point-in-time observations of the Orion Nebula Cluster and find that both overlap in terms of plasma properties, i.e., some of the scatter observed in the X-ray properties of stellar ensembles stems from intrinsic source variability.

Keywords: young stellar objects; x-ray stars; flare stars
Upward Overshooting in Turbulent Compressible Convection. II. Simulations at Large Relative Stability Parameters

In this paper, we present the results of three-dimensional numerical simulation of upward overshooting in turbulent compressible convection at large relative stability parameter S. Similar to the previous simulations at small S, we find that the convectively stable zone can be partitioned into three layers: the thermal adjustment layer, the turbulent dissipation layer, and the thermal dissipation layer. Despite of this similarity, there exist significant differences in several aspects. First, for small S, the thermal structure is altered considerably near the interface between the convectively unstable and stable zones. For extremely large S, the thermal structure is only slightly changed. Second, the overshooting distance decreases at small S, but it can increase when S is large enough. Third, for small S, the fluid motions tend to be less active when S increases. However, the fluid motions can be more active when S is large enough. We find that the structure of counter cells has a significant impact on the penetration depth.

Keywords: stellar interiors; stellar cores; stellar convective zones; stellar structures; hydrodynamical simulations
Starspot Mapping with Adaptive Parallel Tempering. I. Implementation of Computational Code

Starspots are thought to be regions of locally strong magnetic fields, similar to sunspots, and they can generate photometric brightness modulations. To deduce stellar and spot properties, such as spot emergence and decay rates, we implement a computational code for starspot modeling. It is implemented with an adaptive parallel tempering algorithm and an importance sampling algorithm for parameter estimation and model selection in the Bayesian framework. For evaluating the performance of the code, we apply it to synthetic light curves produced with three spots. The light curves are specified in the spot parameters, such as the radii, intensities, latitudes, longitudes, and emergence/decay durations. The spots are circular with specified radii and intensities relative to the photosphere, and the stellar differential rotation coefficient is also included in the light curves. As a result, stellar and spot parameters are uniquely deduced, and the number of spots is correctly determined: the three-spot model is preferable because the model evidence is much greater than that of the two-spot model by orders of magnitude and more than that of the four-spot model by a more modest factor, whereas the light curves are produced to have two or one local minimum during one equatorial rotation period by adjusting the values of longitude. The spot emergence and decay rates can be estimated with error less than an order of magnitude, considering the difference of the number of spots.

Keywords: starspots; bayesian statistics; markov chain monte carlo; importance sampling; model selection; astrostatistics
Evaluating Low-frequency Pulsar Observations to Monitor Dispersion with the Giant Metrewave Radio Telescope

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) project has the primary goal of detecting and characterizing low-frequency gravitational waves through high-precision pulsar timing. The mitigation of interstellar effects is crucial to achieve the necessary precision for gravitational wave detection. Effects like dispersion and scattering are stronger at lower observing frequencies, with the variation of these quantities over week-month timescales requiring high-cadence multifrequency observations for pulsar timing projects. In this work, we utilize the dual-frequency observing capability of the Giant Metrewave Radio Telescope (GMRT) and evaluate the potential decrease in dispersion measure (DM) uncertainties when combined with existing pulsar timing array data. We present the timing analysis for four millisecond pulsars observed with the GMRT simultaneously at 322 and 607 MHz, and compare the DM measurements with those obtained through NANOGrav observations with the Green Bank Telescope and Arecibo Observatory at 1400-2300 MHz frequencies. Measured DM values with the GMRT and NANOGrav program show significant offsets for some pulsars, which could be caused by pulse profile evolution between the two frequency bands. In comparison to the predicted DM uncertainties when incorporating these low-frequency data into the NANOGrav data set, we find that higher-precision GMRT data is necessary to provide improved DM measurements. Through the detection and analysis of pulse profile baseline ripple in data on test pulsar B1929+10, we find that, while not important for these data, it may be relevant for other timing data sets. We discuss the possible advantages and challenges of incorporating GMRT data into NANOGrav and International Pulsar Timing Array data sets.

Keywords: millisecond pulsars; radio interferometers
Synthetic Observations of the Infalling Rotating Envelope: Links between the Physical Structure and Observational Features

We performed synthetic observations of the Ulrich, Cassen, and Moosman (UCM) model to understand the relation between the physical structures of the infalling envelope around a protostar and their observational features in molecular lines, adopting L1527 as an example. We also compared the physical structure and synthetic position-velocity (P-V) diagrams of the UCM model and a simple ballistic (SB) model. There are multiple ways to compare synthetic data with observational data. We first calculated the correlation coefficient. The UCM model and the SB model show similarly good correlation with the observational data. While the correlation reflects the overall similarity between the cube datasets, we can alternatively compare specific local features, such as the centrifugal barrier in the SB model or the centrifugal radius in the UCM model. We evaluated systematic uncertainties in these methods. In the case of L1527, the stellar mass values estimated using these methods are all lower than the value derived from previous Keplerian analysis of the disk. This may indicate that the gas infall motion in the envelope is retarded by, e.g., magnetic fields. We also showed analytically that, in the UCM model, the spin-up feature of the P-V diagram is due to the infall velocity rather than the rotation. The line-of-sight velocity V is thus ∝x -0.5, where x is the offset. If the infall is retarded, rotational velocity should dominate so that V is proportional to x -1, as is often observed in the protostellar envelope.

Keywords: star formation; protostars; astrochemistry
Digitization, Measurement, and Analysis of a 1905 Barnard Atlas Photographic Plate

In the second paper in this series, we improve on our previous demonstration of the ability of a commercially available graphic arts scanner and cost-effective analysis tools to produce scientifically useful scans of astronomical photographic plates. We describe a method using freely available tools to extract magnitude measurements from the star images on sky-survey plates, such as are stored in observatory archives around the world. We detail the use of this method on one plate in particular, Plate 8 in E. E. Barnard's A Photographic Atlas of Selected Regions of the Milky Way, examine the effects of our scanning method on our magnitude measurements, discuss the difficulties encountered when measuring the magnitudes of stars in crowded fields, and present a case study of red supergiant stars appearing within the field. Our work results in a catalog of more than 66,000 measurements of stellar positions and magnitudes in the central 6.°8 × 6.°8 field of view.

Keywords: catalogs; surveys; star atlases; astrometry; variable stars; milky way galaxy; photographic photometry
Detailed Chemical Abundances of Star Clusters in the Large Magellanic Cloud

We derive the first detailed chemical abundances of three star clusters in the Large Magellanic Cloud (LMC), NGC 1831 (436 ± 22 Myr), NGC 1856 (350 ± 18 Myr), and [SL63]268 (1230 ± 62 Myr) using integrated-light spectroscopic observations obtained with the Magellan Echelle spectrograph on Magellan Baade telescope. We derive [Fe/H], [Mg/Fe], [Ti/Fe], [Ca/Fe], [Ni/Fe], [Mn/Fe], [Cr/Fe], and [Na/Fe] for the three clusters. Overall, our results match the LMC abundances obtained in the literature as well as those predicted by detailed chemical evolution models. For clusters NGC 1831 and NGC 1856, the [Mg/Fe] ratios appear to be slightly depleted compared with [Ca/Fe] and [Ti/Fe]. This could be hinting at the well-known Mg-Al abundance anti-correlation observed in several Milky Way globular clusters. We note, however, that higher signal-to-noise observations are needed to confirm such a scenario, particularly for NGC 1831. We also find a slightly enhanced integrated-light [Na/Fe] ratio for cluster [SL63]268 compared with those from the LMC field stars, possibly supporting a scenario of intracluster abundance variations. We stress that detailed abundance analysis of individual stars in these LMC clusters is required to confirm the presence or absence of multiple stellar populations.

Keywords: galaxy chemical evolution; star clusters; large magellanic cloud
ICM-SHOX. I. Methodology Overview and Discovery of a Gas–Dark Matter Velocity Decoupling in the MACS J0018.5+1626 Merger

Galaxy cluster mergers are rich sources of information to test cluster astrophysics and cosmology. However, cluster mergers produce complex projected signals that are difficult to interpret physically from individual observational probes. Multi-probe constraints on the gas and dark matter (DM) cluster components are necessary to infer merger parameters that are otherwise degenerate. We present Improved Constraints on Mergers with SZ, Hydrodynamical simulations, Optical, and X-ray (ICM-SHOX), a systematic framework to jointly infer multiple merger parameters quantitatively via a pipeline that directly compares a novel combination of multi-probe observables to mock observables derived from hydrodynamical simulations. We report a first application of the ICM-SHOX pipeline to MACS J0018.5+1626, wherein we systematically examine simulated snapshots characterized by a wide range of initial parameters to constrain the MACS J0018.5+1626 merger geometry. We constrain the epoch of MACS J0018.5+1626 to the range 0–60 Myr post-pericenter passage, and the viewing angle is inclined ≈27°–40° from the merger axis. We obtain constraints for the impact parameter (≲250 kpc), mass ratio (≈1.5–3.0), and initial relative velocity when the clusters are separated by 3 Mpc (≈1700–3000 km s‑1). The primary and secondary clusters initially (at 3 Mpc) have gas distributions that are moderately and strongly disturbed, respectively. We discover a velocity space decoupling of the DM and gas distributions in MACS J0018.5+1626, traced by cluster-member galaxy velocities and the kinematic Sunyaev–Zel'dovich effect, respectively. Our simulations indicate this decoupling is dependent on the different collisional properties of the two distributions for particular merger epochs, geometries, and viewing angles.

Keywords: galaxy clusters; intracluster medium; hydrodynamical simulations; observational cosmology; x-ray astronomy; sunyaev-zeldovich effect; galaxy spectroscopy; strong gravitational lensing
New Velocity Measurements of NGC 5128 Globular Clusters Out to 130 kpc: Outer Halo Kinematics, Substructure, and Dynamics

We present new radial velocity measurements from the Magellan and the Anglo-Australian Telescopes for 175 previously known and 121 newly confirmed globular clusters (GCs) around NGC 5128, the nearest accessible massive early-type galaxy at D = 3.8 Mpc. Remarkably, 28 of these newly confirmed GCs are at projected radii $\gt 50^{\prime} $ (≳54 kpc), extending to ~130 kpc, in the outer halo where few GCs had been confirmed in previous work. We identify several subsets of GCs that spatially trace halo substructures that are visible in red giant branch star maps of the galaxy. In some cases, these subsets of GCs are kinematically cold, and may be directly associated with and originate from these specific stellar substructures. From a combined kinematic sample of 645 GCs, we see evidence for coherent rotation at all radii, with a higher rotation amplitude for the metal-rich GC subpopulation. Using the tracer mass estimator, we measure a total enclosed mass of 2.5 ± 0.3 × 1012 M ⊙ within ~120 kpc, an estimate that will be sharpened with forthcoming dynamical modeling. The combined power of stellar mapping and GC kinematics makes NGC 5128 an ongoing keystone for understanding galaxy assembly at mass scales inaccessible in the Local Group. * This paper includes data gathered with the 6.5 m Magellan Telescope at Las Campanas Observatory, Chile.

Keywords: globular star clusters; radial velocity; galaxy stellar halos; galaxy masses; catalogs
The Binary Mass Ratio in the Black Hole Transient MAXI J1820+070

We present intermediate-resolution spectroscopy of the optical counterpart to the black hole X-ray transient MAXI J1820+070 (=ASASSN-18ey) obtained with the OSIRIS spectrograph on the 10.4 m Gran Telescopio Canarias. The observations were performed with the source close to the quiescent state and before the onset of renewed activity in 2019 August. We make use of these data and K-type dwarf templates taken with the same instrumental configuration to measure the projected rotational velocity of the donor star. We find Vrot sin I = 84 ± 5 km s-1 (1σ), which implies a donor to the black hole mass ratio $q={M}_{2}/{M}_{1}=0.072\pm 0.012$ for the case of a tidally locked and Roche-lobe filling donor star. The derived dynamical masses for the stellar components are ${M}_{1}=(5.95\pm 0.22){\sin }^{-3}I\,{M}_{\odot }$ and ${M}_{2}=(0.43\pm 0.08){\sin }^{-3}I\,{M}_{\odot }$ . The use of q, combined with estimates of the accretion disk size at the time of the optical spectroscopy, allows us to revise our previous orbital inclination constraints to 66° < I < 81°. These values lead to 95% confidence level limits on the masses of 5.73 < M1(M⊙) < 8.34 and 0.28 < M2(M⊙) < 0.77. Adopting instead the 63° ± 3° orientation angle of the radio jet as the binary inclination leads to ${M}_{1}={8.48}_{-0.72}^{+0.79}{M}_{\odot }$ and ${M}_{2}={0.61}_{-0.12}^{+0.13}\,{M}_{\odot }$ (1σ).

Keywords: stellar mass black holes; low-mass x-ray binary stars
What Do Halo CMEs Tell Us about Solar Cycle 25?

It is known that the weak state of the heliosphere due to diminished solar activity in cycle 24 backreacted on coronal mass ejections (CMEs) to make them appear wider for a given speed. One of the consequences of the weak state of the heliosphere is that more CMEs appear as halo CMEs (HCMEs), and halos are formed at shorter heliocentric distances. Current predictions for the strength of solar cycle (SC) 25 range from half to twice the strength of SC 24. We compare the HCME occurrence rate and other properties during the rise phase of cycles 23, 24, and 25 to weigh in on the strength of SC 25. We find that HCME and solar wind properties in SC 25 are intermediate between SCs 23 and 24, but closer to SC 24. The HCME occurrence rate, normalized to the sunspot number, is higher in SCs 24 and 25 than in SC 23. The solar wind total pressure in SC 25 is ~35% smaller than that in SC 23. Furthermore, the occurrence rates of high-energy solar energetic particle events and intense geomagnetic storms are well below the corresponding values in SC 23, but similar to those in SC 24. We conclude that cycle 25 is likely to be similar to or slightly stronger than cycle 24, in agreement with polar-field precursor methods for cycle 25 prediction.

Keywords: solar coronal mass ejections
VVV Survey Microlensing: Catalog of Best and Forsaken Events

We search for microlensing events in the zero-latitude area of the Galactic Bulge using the VVV Survey near-IR data. We have discovered a total sample of N = 630 events within an area covering 20.68 deg2 between the years 2010 and 2015. In this paper, we describe the search and present the data for the final sample, including near-IR magnitudes, colors, and proper motions, as well as the standard microlensing parameters. We use the near-IR color-magnitude and color-color diagrams to select NRC = 290 events with red-clump sources to analyze the extinction properties of the sample in the central region of the Galactic plane. The timescale distribution and its dependence in the longitude axis is presented. The mean timescale decreases as we approach the Galactic minor axis (b = 0°). Finally, we give examples of special microlensing events, such as binaries, short-timescale events, and events with a strong parallax effect.

Keywords: galaxy bulges; galactic bulge; galaxy structure; gravitational microlensing; microlensing event rate
Analytic Light Curves of Dense CSM Shock Breakout and Cooling

Dense circumstellar material (CSM) is thought to play an important role in observed luminous optical transients: if such CSM is shocked, e.g., by ejecta expelled from the progenitor during core-collapse, then radiation produced by the shock-heated CSM can power bright UV/optical emission. If the initial CSM has an "outer edge" where most of the mass is contained and at which the optical depth is large, then shock breakout-when photons are first able to escape the shocked CSM-occurs near it. The rather thin shell of shocked CSM subsequently expands, and in the ensuing cooling-envelope phase, radiative and adiabatic losses compete to expend the CSM thermal energy. Here we derive an analytic solution to the bolometric light curve produced by such shocked CSM. For the first time, we provide an analytic solution to the cooling-envelope phase that is applicable starting from shock breakout and until the expanding CSM becomes optically thin. In particular, we account for the planar CSM geometry that is relevant at early times and properly treat radiative losses within this planar phase. We show that these effects can dramatically impact the resulting light curves, particularly if the CSM optical depth is only marginally larger than c/v sh (where v sh is the shock velocity). This has important implications for interpreting observed fast optical transients, which have previously been modeled using either computationally expensive numerical simulations or more simplified models that do not properly capture the early light-curve evolution.

Keywords: ultraviolet transient sources; transient sources; time domain astronomy
Limits on the OH Molecule in the Smith High-velocity Cloud

We have used the Green Bank Telescope to search for the OH molecule at several locations in the Smith Cloud, one of the most prominent of the high-velocity clouds surrounding the Milky Way. Five positions with high H I column density were selected as targets for individual pointings, along with a square degree around a molecular cloud detected with the Planck telescope near the tip of the Smith Cloud. Gas in the Galactic disk with similar values of N HI has detectable OH emission. Although we found OH at velocities consistent with the foreground Aquila molecular cloud, nothing was found at the velocity of the Smith Cloud to an rms level of 0.7 mK (T b ) in a 1 km s‑1 channel. The three positions that give the strictest limits on OH are analyzed in detail. Their combined data imply a 5σ limit on N(H2)/N HI ≤ 0.03, scaled by a factor dependent on the OH excitation temperature and background continuum T ex/(T ex ‑ T bg). There is no evidence for far-infrared emission from dust within the Smith Cloud. These results are consistent with expectations for a low-metallicity diffuse cloud exposed to the radiation field of the Galactic halo, rather than a product of a galactic fountain.

Keywords: high-velocity clouds; hydroxyl sources; molecular gas
Photometric Metallicity and Distance for the Two RR Lyrae in Segue II and Ursa Major II Dwarf Galaxies Based on Multiband Light Curves

Multiband light curves of two RR Lyrae variables in Segue II and Ursa Major II ultrafaint dwarf galaxies were collected from near simultaneous observations using the Lulin One-meter Telescope in Vgri bands. Together with Gaia G-band light curves, we determined photometric metallicities using empirical relations involving pulsation period and Fourier parameter as dependent parameters. We demonstrated that the RR Lyrae photometric metallicity can be determined accurately when these empirical relations were employed at multiple wavelengths, which can potentially improve the distance determination based on RR Lyrae stars. The photometric metallicities based on our approach were found to be -2.27 ± 0.13 dex and -1.87 ± 0.16 dex for the RR Lyrae in Segue II and Ursa Major II UFD, respectively, with corresponding distance moduli of 17.69 ± 0.15 mag and 17.58 ± 0.15 mag, in agreement with previous literature determinations. This approach of photometric metallicity of RR Lyrae based on multiband optical light curves will be particularly relevant for distance measurements in the era of the Vera C Rubin's Legacy Survey of Space and Time.

Keywords: stellar distance; metallicity; dwarf galaxies; rr lyrae variable stars
Prospects for Life on Temperate Planets around Brown Dwarfs

There is growing evidence that brown dwarfs may be comparable to main-sequence stars in terms of their abundance. In this paper, we explore the prospects for the existence of life on Earth-like planets around brown dwarfs. We consider the following factors: (I) the length of time that planets can exist in the temporally shifting habitable zone, (II) the minimum photon fluxes necessary for oxygenic photosynthesis, and (III) the lower limits on the fluxes of ultraviolet radiation to drive prebiotic reactions ostensibly necessary for the origin of life. By taking these effects into consideration, we find that it is unlikely for brown dwarfs with masses ≲30 MJ to host habitable planets over geologically significant timescales. We also briefly discuss some of the major biosignatures that might arise on these planets, assess the likelihood of their detection, and highlight some avenues for further study.

Keywords: astrobiology; biosignatures; brown dwarfs; habitable planets; habitable zone; exoplanet evolution