adsabs/SciX_UAT_keywords huggingface dataset:
The Metallicities of Five Small High-velocity Clouds

High-velocity clouds (HVCs) are multiphase gas structures whose velocities (∣v LSR∣ ≥ 100 km s-1) are too high to be explained by Galactic disk rotation. While large HVCs are well characterized, compact and small HVCs (with H I angular sizes of a few degrees) are poorly understood. Possible origins for such small clouds include Milky Way (MW) halo gas or fragments of the Magellanic System, but neither their origin nor their connection to the MW halo has been confirmed. We use new Hubble Space Telescope/Cosmic Origins Spectrograph UV spectra and Green Bank Telescope H I spectra to measure the metallicities of five small HVCs in the southern Galactic sky projected near the Magellanic System. We build a set of distance-dependent Cloudy photoionization models for each cloud and calculate their ionization-corrected metallicities. All five small HVCs have oxygen metallicities ≤0.17 Z ⊙, indicating they do not originate in the disk of the MW. Two of the five have metallicities of 0.16-0.17 Z ⊙, similar to the Magellanic Stream, suggesting these clouds are fragments of the Magellanic System. The remaining three clouds have much lower metallicities of 0.02-0.04 Z ⊙. While the origin of these low-metallicity clouds is unclear, they could be gaseous minihalos or gas stripped from dwarf galaxies by ram pressure or tidal interactions. These results suggest that small HVCs do not all reside in the inner MW halo or the Magellanic System, but instead can trace more distant structures. *Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with program 15887.

Keywords: milky way galaxy; high-velocity clouds; chemical abundances; ultraviolet astronomy; local group; circumgalactic medium
The NuSTAR and Chandra View of CL 0217+70 and Its Tell-tale Radio Halo

Mergers of galaxy clusters are the most energetic events in the universe, driving shock and cold fronts, generating turbulence, and accelerating particles that create radio halos and relics. The galaxy cluster CL 0217+70 is a remarkable late-stage merger, with a double peripheral radio relic and a giant radio halo. Chandra detects surface brightness (SB) edges that correspond to radio features within the halo. In this work, we present a study of this cluster with Nuclear Spectroscopic Telescope Array and Chandra data using spectro-imaging methods. The global temperature is found to be kT = 9.1 keV. We set an upper limit for the inverse Compton (IC) flux of ~2.7 × 10-12 erg s-1 cm-2, and a lower limit to the magnetic field of 0.08 μG. Our local IC search revealed a possibility that IC emission may have a significant contribution at the outskirts of the radio halo emission and on/near shock regions within ~0.6 r 500 of clusters. We detected a "hot spot" feature in our temperature map coincident with an SB edge, but our investigation on its origin is inconclusive. If the "hot spot" is the downstream of a shock, we set a lower limit of kT > 21 keV to the plasma that corresponds to ${ \mathcal M }$ ~2. We found three shock fronts within 0.5 r 500. Multiple weak shocks within the cluster center hint at an ongoing merger activity and continued feeding of the giant radio halo. CL 0217+70 is the only example hosting these secondary shocks in multiple form.

Keywords: galaxy clusters; shocks; high energy astrophysics; intracluster medium; non-thermal radiation sources
A Radial-velocity Search for Binary RR Lyrae Variables

We report 272 radial velocities for 19 RR Lyrae variables. For most of the stars we have radial velocities for the complete pulsation cycle. These data are used to determine robust center-of-mass radial velocities that have been compared to values from the literature in a search for evidence of binary systems. Center-of-mass velocities were determined for each star using Fourier Series and template fits to the radial velocities. Our center-of-mass velocities have uncertainties from ±0.16 km s-1 to ±2.5 km s-1, with a mean uncertainty of ±0.92 km s-1. We combined our center-of-mass velocities with values from the literature to look for deviations from the mean center-of-mass velocity of each star. Fifteen RR Lyrae show no evidence of binary motion (BK And, CI And, Z CVn, DM Cyg, BK Dra, RR Gem, XX Hya, SZ Leo, BX Leo, TT Lyn, CN Lyr, TU Per, U Tri, RV UMa, and AV Vir). In most cases this conclusion is reached due to the sporadic sampling of the center-of-mass velocities over time. Three RR Lyrae show suspicious variation in the center-of-mass velocities that may indicate binary motion but do not prove it (SS Leo, ST Leo, and AO Peg). TU UMa was observed by us near a predicted periastron passage (at 0.14 in orbital phase) but the absence of additional center-of-mass velocities near periastron makes the binary detection, based on radial velocities alone, uncertain. Two stars in our sample show Hγ emission in phases 0.9-1.0: SS Leo and TU UMa.

Keywords: rr lyrae variable stars; binary stars
On the Rate of Crustal Failures in Young Magnetars

The activity of magnetars is powered by their intense and dynamic magnetic fields and has been proposed as the trigger to extragalactic fast radio bursts. Here we estimate the frequency of crustal failures in young magnetars, by computing the magnetic stresses in detailed magnetothermal simulations including Hall drift and ohmic dissipation. The initial internal topology at birth is poorly known but is likely to be much more complex than a dipole. Thus, we explore a wide range of initial configurations, finding that the expected rate of crustal failures varies by orders of magnitude depending on the initial magnetic configuration. Our results show that this rate scales with the crustal magnetic energy, rather than with the often used surface value of the dipolar component related to the spin-down torque. The estimated frequency of crustal failures for a given dipolar component can vary by orders of magnitude for different initial conditions, depending on how much magnetic energy is distributed in the crustal nondipolar components, likely dominant in newborn magnetars. The quantitative reliability of the expected event rate could be improved by a better treatment of the magnetic evolution in the core and the elastic/plastic crustal response, not included here. Regardless of that, our results are useful inputs in modeling the outburst rate of young Galactic magnetars, and their relation with the fast radio bursts in our and other galaxies.

Keywords: magnetars; neutron stars; magnetic fields; radio transient sources
Unified Simulations of Planetary Formation and Atmospheric Evolution. II. Rapid Disk Clearing by Photoevaporation Yields Low-mass Super-Earth Atmospheres

Super-Earths possess low-mass H2/He atmospheres (typically less than 10% by mass). However, the origins of super-Earth atmospheres have not yet been ascertained. We investigate the role of rapid disk clearing by photoevaporation during the formation of super-Earths and their atmospheres. We perform unified simulations of super-Earth formation and atmospheric evolution in evolving disks that consider both photoevaporative winds and magnetically driven disk winds. For the growth mode of planetary cores, we consider two cases in which planetary embryos grow with and without pebble accretion. Our main findings are summarized as follows. (i) The time span of atmospheric accretion is shortened by rapid disk dissipation due to photoevaporation, which prevents super-Earth cores from accreting massive atmospheres. (ii) Even if planetary cores grow rapidly by embryo accretion in the case without pebble accretion, the onset of runaway gas accretion is delayed because the isolation mass for embryo accretion is small. Together with rapid disk clearing, the accretion of massive atmospheres can be avoided. (iii) After rapid disk clearing, a number of high-eccentricity embryos can remain in outer orbits. Thereafter, such embryos may collide with the super-Earths, leading to efficient impact erosion of accreted atmospheres. Therefore, we find that super-Earths with low-mass H2/He atmospheres are naturally produced by N-body simulations that consider realistic disk evolution.

Keywords: exoplanet formation; exoplanet atmospheres; exoplanet dynamics; planet formation
Magnetic Helicity Signs and Flaring Propensity: Comparing the Force-free Parameter with the Helicity Signs of Hα Filaments and X-Ray Sigmoids

Sigmoids produce strong eruptive events. Earlier studies have shown that the ICME axial magnetic field Bz can be predicted with some credibility by observing the corresponding filament or the polarity inversion line in the region of eruption and deriving the magnetic field direction from that. Sigmoids are coronal structures often associated with filaments in the sigmoidal region. In this study, we compare filament chirality with sigmoid handedness to observe their correlation. Second, we perform nonlinear force-free approximations of the coronal magnetic connectivity using photospheric vector magnetograms underneath sigmoids to obtain a weighted-average value of the force-free parameter and to correlate it with filament chirality and the observed coronal sigmoid handedness. Importantly, we find that the sigmoids and their filament counterparts do not always have the same helicity signs. Production of eruptive events by regions that do not have the same signs of helicities is ∼3.5 times higher than when they do. A case study of magnetic energy/ helicity evolution in NOAA AR 12473 is also presented.

Keywords: space weather; solar filaments
The Maximum Energy of Shock-accelerated Cosmic Rays

Identifying the accelerators of Galactic cosmic ray (CR) protons with energies up to a few PeV (1015 eV) remains a theoretical and observational challenge. Supernova remnants (SNRs) represent strong candidates because they provide sufficient energetics to reproduce the CR flux observed at Earth. However, it remains unclear whether they can accelerate particles to PeV energies, particularly after the very early stages of their evolution. This uncertainty has prompted searches for other source classes and necessitates comprehensive theoretical modeling of the maximum proton energy, ${E}_{\max }$ , accelerated by an arbitrary shock. While analytic estimates of ${E}_{\max }$ have been put forward in the literature, they do not fully account for the complex interplay between particle acceleration, magnetic field amplification, and shock evolution. This paper uses a multizone, semianalytic model of particle acceleration based on kinetic simulations to place constraints on ${E}_{\max }$ for a wide range of astrophysical shocks. In particular, we develop relationships between ${E}_{\max }$ , shock velocity, size, and ambient medium. We find that SNRs can only accelerate PeV particles under a select set of circumstances, namely, if the shock velocity exceeds ~104 km s-1 and escaping particles drive magnetic field amplification. However, older and slower SNRs may still produce observational signatures of PeV particles due to populations accelerated when the shock was younger. Our results serve as a reference for modelers seeking to quickly produce a self-consistent estimate of the maximum energy accelerated by an arbitrary astrophysical shock. 1 1 Presented as a thesis to the Department of Astronomy and Astrophysics, The University of Chicago, in partial fulfillment of the requirements for a Ph.D. degree.

Keywords: cosmic rays; galactic cosmic rays; supernova remnants; shocks; gamma-rays; neutrino astronomy; gamma-ray astronomy
Dynamical Relics of the Ancient Galactic Halo

We search for dynamical substructures in the LAMOST DR3 very metal-poor (VMP) star catalog. After cross-matching with Gaia DR2, there are ∼3300 VMP stars with available high-quality astrometric information that have halo-like kinematics. We apply a method based on the self-organizing map StarGO to find groups clustered in the 4D space of orbital energy and angular momentum. We identify 57 dynamically tagged groups (DTGs), which we label DTG-1 to DTG-57. Most of them belong to existing massive substructures in the nearby halo, such as the Gaia Sausage or Sequoia. The stream identified by Helmi et al. is recovered, but the two disjointed portions of the substructure appear to have distinct dynamical properties. The very retrograde substructure Rg5 found previously by Myeong et al. is also retrieved. We report six new DTGs with highly retrograde orbits, two with very prograde orbits, and 12 with polar orbits. By mapping other data sets (APOGEE halo stars, and catalogs of r-process-enhanced and carbon-enhanced metal-poor [CEMP] stars) onto the trained neuron map, we can associate stars with detailed chemical abundances with the DTGs and look for associations with chemically peculiar stars. The highly eccentric Gaia Sausage groups contain representatives of both debris from the satellite itself (which is α-poor) and the Splashed Disk, sent up into eccentric halo orbits from the encounter (and which is α-rich). The new prograde substructures also appear to be associated with the Splashed Disk. The DTGs belonging to the Gaia Sausage host two relatively metal-rich r-II stars and six CEMP stars in different subclasses, consistent with the idea that the Gaia Sausage progenitor is a massive dwarf galaxy. Rg5 is dynamically associated with two highly r-process-enhanced stars with [Fe/H] ∼ -3. This finding indicates that its progenitor might be an ultrafaint dwarf galaxy that has experienced r-process enrichment from neutron star mergers.

Keywords: milky way stellar halo; milky way formation; r-process
Unveiling the Origin of Peculiar Diffuse Radio Emission in A1351

Abell 1351 (A1351) is a massive merging cluster that hosts a giant radio halo and a bright radio edge blended in the halo. In this paper, we present the first ever spectral analysis of this cluster using GMRT 610 MHz and VLA 1.4 GHz archival data and discuss the radio edge property. Using Chandra data, we report the first tentative detection of shock front at the location of the edge in A1351 with discontinuities in both X-ray surface brightness and temperature. Our analysis strengthens the previous claim of the detected "edge" being a high luminosity radio relic. The radio relic has an integrated spectral index α = - 1.63 ± 0.33 and radio power P 1.4 GHz = 4.46 ±0.61 × 1024 W Hz-1 with a largest linear size of 570 kpc. The radio spectral index map shows steepening in the shock downstream region. Our analysis favors the scenario where the diffusive shock acceleration of particles is responsible for the origin of the radio relic in the presence of a strong magnetic field. We have also estimated the magnetic field at the relic location assuming equipartition condition.

Keywords: galaxy clusters; extragalactic radio sources; intracluster medium; radio continuum emission
The Morphology-Density Relationship in 1 < z < 2 Clusters

The morphology-density relationship states that dense cosmic environments such as galaxy clusters have an overabundance of quiescent elliptical galaxies, but it is unclear at which redshift this relationship is first established. We study the morphology of four clusters with 1.2 < z < 1.8 using Hubble Space Telescope imaging and the morphology computation code STATMORPH. By comparing the median morphology of cluster galaxies to CANDELS field galaxies using Monte Carlo analysis, we find that two out of four clusters (at z = 1.19 and 1.75) have an established morphology-density relationship with more than 3σ significance. Approximately 50% of the galaxies in these clusters are bulge-dominated, compared to ∼30% in the field, and they are significantly more compact. This result is more significant for low-mass galaxies with $\mathrm{log}M/{M}_{\odot }\lessapprox 10.5$ , showing that low-mass galaxies are affected the most in clusters. We also find an intriguing system of two z ≈ 1.45 clusters at a unusually small 2D separation of 3' and 3D separation of ≈73 Mpc that exhibit no morphology-density relationship but have enhanced merger signatures. We conclude that the environmental mechanism responsible for the morphology-density relationship is (1) already active as early as z = 1.75; (2) forms compact, bulge-dominated galaxies; and (3) affects primarily low-mass galaxies. However, there is a significant degree of intracluster variance that may depend on the larger cosmological environment in which the cluster is embedded.

Keywords: galaxy evolution; high-redshift galaxy clusters; galaxy classification systems; galaxies
Water UV-shielding in the Terrestrial Planet-forming Zone: Implications for Oxygen-18 Isotope Anomalies in {{H}}_{2}(18}{O) Infrared Emission and Meteorites

An understanding of abundance and distribution of water vapor in the innermost region of protoplanetary disks is key to understanding the origin of habitable worlds and planetary systems. Past observations have shown H2O to be abundant and a major carrier of elemental oxygen in disk surface layers that lie within the inner few astronomical units of the disk. The combination of high abundance and strong radiative transitions leads to emission lines that are optically thick across the infrared spectral range. Its rarer isotopologue ${{\rm{H}}}_{2}^{18}{\rm{O}}$ traces deeper into this layer and will trace the full content of the planet-forming zone. In this work, we explore the relative distribution of ${{\rm{H}}}_{2}^{16}{\rm{O}}$ and ${{\rm{H}}}_{2}^{18}{\rm{O}}$ within a model that includes water self-shielding from the destructive effects of ultraviolet radiation. In this Letter we show that there is an enhancement in the relative ${{\rm{H}}}_{2}^{18}{\rm{O}}$ abundance high up in the warm molecular layer within 0.1-10 au due to self-shielding of CO, C18O, and H2O. Most transitions of ${{\rm{H}}}_{2}^{18}{\rm{O}}$ that can be observed with JWST will partially emit from this layer, making it essential to take into account how H2O self-shielding may effect the H2O to ${{\rm{H}}}_{2}^{18}{\rm{O}}$ ratio. Additionally, this reservoir of ${{\rm{H}}}_{2}^{18}{\rm{O}}$ -enriched gas in combination with the vertical "cold finger" effect might provide a natural mechanism to account for oxygen isotopic anomalies found in meteoritic material in the solar system.

Keywords: protoplanetary disks; astrochemistry
A Comparison of the Stellar, CO, and Dust-continuum Emission from Three Star-forming HUDF Galaxies at z ∼ 2

We compare the extent of the dust, molecular gas, and stars in three star-forming galaxies, at z = 1.4, 1.6, and 2.7, selected from the Hubble Ultra Deep Field based on their bright carbon monoxide (CO) and dust-continuum emission as well as their large rest-frame optical sizes. The galaxies have high stellar masses, ${M}_{* }\gt {10}^{11}{M}_{\odot }$ , and reside on, or slightly below, the main sequence of star-forming galaxies at their respective redshifts. We probe the dust and molecular gas using subarcsecond Atacama Large Millimeter/submillimeter Array observations of the 1.3 mm continuum and CO line emission, respectively, and probe the stellar distribution using Hubble Space Telescope observations at 1.6 μm. We find that for all three galaxies the CO emission appears ≳30% more compact than the stellar emission. For the z = 1.4 and 2.7 galaxies, the dust emission is also more compact, by ≳50%, than the stellar emission, whereas for the z = 1.6 galaxy, the dust and stellar emission have similar spatial extents. This similar spatial extent is consistent with observations of local disk galaxies. However, most high-redshift observations show more compact dust emission, likely because of the ubiquity of central starbursts at high redshift and the limited sensitivity of many of these observations. Using the CO emission line, we also investigate the kinematics of the cold interstellar medium in the galaxies, and find that all three have kinematics consistent with a rotation-dominated disk.

Keywords: interstellar medium; dust continuum emission; galaxy evolution; molecular gas
V505 Cyg: A Mass-transferring Marginal Contact Binary

High-precision photometric data of V505 Cyg from TESS and one spectrum observed by us are presented in this work, and the stellar atmospheric parameters of the primary component were derived via spectral fitting. Applying the Wilson-Devinney code, the photometric elements were computed for the first time. The results show V505 Cyg is a near contact binary with the primary filling its Roche Lobe and the secondary a little under-filling, along with the temperature difference of about ΔT = 1900 K between the two components. The eclipse is total and lasts for about 90 minutes, which is about 0.1 phase, so the photometric results are reliable. This system belongs to near-contact binaries with the light curves enhanced around the left shoulder of the secondary minimum, which can be explained by a hot spot on the secondary components due to mass transfer, via a stream from the primary components hitting the facing surface of the secondary components. Meanwhile, we conducted an orbital period investigation of it in detail. The orbital period study based on all times of minimum including 230 new ones determined by us reveals a secular period decreasing at the rate of dP/dt = -2.31 × 10-7 d · yr-1. The decrease of the orbital period can be the result of mass transfer between the components, agreeing well with the configuration and the asymmetric light curves of V505 Cyg. V505 Cyg is therefore another rare example of mass transferring marginal contact binary lying on the rapid evolutionary stage predicted by the Thermal Relaxation Oscillation theory. With the orbital period decrease, V505 Cyg will evolve into an overcontact binary.

Keywords: close binary stars; eclipsing binary stars; binary stars
Magnetic Field Signatures of Intermediate-sized Impact Craters on Mars

Magnetic field signatures over impact craters provide constraints for the history of the Martian dynamo. Due to limitations of the spatial resolution of magnetic field models, previous studies primarily focused on large impact craters (mostly ≥ 500 km in diameter). To fill the impact crater age gaps of previous studies, we investigate the magnetic field signature of 23 intermediate-sized craters (150-500 km in diameter) on Mars using both MAVEN data and a magnetic field model. Ten impact craters located in the South Province, the unmagnetized primordial crust, exhibit no or weak magnetic field signatures. The other 13 impact craters produce stronger magnetic anomalies, with the ratio of the averaged magnetic field inside and outside the craters (B in/B out) ranging from 0.4 to 1.2. The B in/B out values exhibit correlation coefficients of -0.54, -0.57, and -0.69 with the diameters of craters, calculated from the MAVEN data, the crustal field model at the surface, and 150 km altitude, respectively. A B in/B out larger than 1.0 usually appears in craters with smaller diameters, which is also demonstrated by the forward modeling in this study. Furthermore, the results of the forward modeling indicate that the craters of stronger magnetizations show a larger B in/B out. According to this, the Martian dynamo can be associated with the magnetization of craters of different ages, and the characteristic time of the dynamo can be limited. Our study supports the hypothesis that the Martian dynamo weakened or ceased at ~4.0 Ga and a late dynamo was perhaps active at ~3.7 Ga.

Keywords: mars; impact phenomena; planetary dynamics; magnetic fields