adsabs/SciX_UAT_keywords huggingface dataset:
New CCD Astrometric Observations of Himalia Using Gaia DR2 in 2015-2021

The long arc and high-quality astrometric measurements of outer irregular satellites are prerequisites for improving their orbital theories and increasing the precision of their ephemerides. In order to obtain good astrometric positions of Himalia, the largest irregular satellite of Jupiter, we have processed and reduced 911 ground-based CCD frames obtained between 2015 and 2021 by three telescopes (including 1 and 2.4 m telescopes at Yunnan Observatory, and 0.8 m telescope at Purple Mountain Observatory) over 61 nights. Subtracting off the companion star of our target by constructing an effective point-spread function (ePSF) model in some CCD frames, the ePSF-subtracted technique is used to reduce the centering error. Some additional techniques are applied in data reduction to further improve positional accuracy and precision of Himalia. This includes geometric distortion correction, weighted polynomial plate models, and the precision-premium effect, since their relative positional measurements have better precision when two objects are very near (e.g., less than 60″). The star catalog Gaia DR2 is used for astrometric calibration, and theoretical positions of Himalia are retrieved from JPL Horizons ephemeris, including the satellite ephemeris Jup344 and the newest planetary ephemeris DE441. Our results show the mean (O - C)s (observed minus computed) of the positional residuals of Himalia are -0."004 and 0.″005 in R.A. and decl., respectively, and their corresponding standard deviations are about 0.″020 in each direction.

Keywords: observational astronomy; astronomical techniques
Degenerate Interpretations of O3 Spectral Features in Exoplanet Atmosphere Observations Due to Stellar UV Uncertainties: A 3D Case Study with TRAPPIST-1 e

TRAPPIST-1 e is a potentially habitable terrestrial exoplanet orbiting an ultracool M dwarf star and is a key target for observations with the James Webb Space Telescope. One-dimensional photochemical modeling of terrestrial planetary atmospheres has shown the importance of the incoming stellar UV flux in modulating the concentration of chemical species, such as O3 and H2O. In addition, three-dimensional (3D) modeling has demonstrated anisotropy in chemical abundances due to transport in tidally locked exoplanet simulations. We use the Whole Atmosphere Community Climate Model Version 6 (WACCM6), a 3D Earth system model, to investigate how uncertainties in the incident UV flux, combined with transport, affect observational predictions for TRAPPIST-1 e (assuming an initial Earth-like atmospheric composition). We use two semiempirical stellar spectra for TRAPPIST-1 from the literature. The UV flux ratio between them can be as large as a factor of 5000 in some wavelength bins. Consequently, the photochemically produced total O3 columns differ by a factor of 26. Spectral features of O3 in both transmission and emission spectra vary between these simulations (e.g., differences of 20 km in the transmission spectrum effective altitude for O3 at 0.6 μm). This leads to potential ambiguities when interpreting observations, including overlap with scenarios that assume alternative O2 concentrations. Hence, to achieve robust interpretations of terrestrial exoplanetary spectra, characterization of the UV spectra of their host stars is critical. In the absence of such stellar measurements, atmospheric context can still be gained from other spectral features (e.g., H2O), or by comparing direct imaging and transmission spectra in conjunction.

Keywords: exoplanet atmospheres; exoplanets; exoplanet atmospheric composition; transmission spectroscopy
HAWC+/SOFIA Polarimetry in L1688: Relative Orientation of Magnetic Field and Elongated Cloud Structure

We present a study of the relative orientation between the magnetic field and elongated cloud structures for the ρ Oph A and ρ Oph E regions in L1688 in the Ophiuchus molecular cloud. Combining inferred magnetic field orientation from HAWC+ 154 μm observations of polarized thermal emission with column density maps created using Herschel submillimeter observations, we find consistent perpendicular relative alignment at scales of 0.02 pc (33"6 at d ≈ 137 pc) using the histogram of relative orientations (HRO) technique. This supports the conclusions of previous work using Planck polarimetry and extends the results to higher column densities. Combining this HAWC+ HRO analysis with a new Planck HRO analysis of L1688, the transition from parallel to perpendicular alignment in L1688 is observed to occur at a molecular hydrogen column density of approximately 1021.7 cm-2. This value for the alignment transition column density agrees well with values found for nearby clouds via previous studies using only Planck observations. Using existing turbulent, magnetohydrodynamic simulations of molecular clouds formed by colliding flows as a model for L1688, we conclude that the molecular hydrogen volume density associated with this transition is approximately ~104 cm-3. We discuss the limitations of our analysis, including incomplete sampling of the dense regions in L1688 by HAWC+.

Keywords: star formation; interstellar medium; interstellar magnetic fields; molecular clouds; giant molecular clouds
Thiols in the Interstellar Medium: First Detection of HC(O)SH and Confirmation of C2H5SH

The chemical compounds carrying the thiol group (-SH) have been considered essential in recent prebiotic studies regarding the polymerization of amino acids. We have searched for this kind of compound toward the Galactic Center quiescent cloud G+0.693-0.027. We report the first detection in the interstellar space of the trans-isomer of monothioformic acid (t-HC(O)SH) with an abundance of ∼1 × 10-10. Additionally, we provide a solid confirmation of the gauche isomer of ethyl mercaptan (g-C2H5SH) with an abundance of ∼3 × 10-10, and we also detect methyl mercaptan (CH3SH) with an abundance of ∼5 × 10-9. Abundance ratios were calculated for the three SH-bearing species and their OH analogs, revealing similar trends between alcohols and thiols with increasing complexity. Possible chemical routes for the interstellar synthesis of t-HC(O)SH, CH3SH, and C2H5SH are discussed, as well as the relevance of these compounds in the synthesis of prebiotic proteins in the primitive Earth.

Keywords: astrochemistry; chemical abundances; interstellar molecules; galactic center
Observations of the Chromospheric Evershed Flow of Sunspot Penumbra with the Application of the Self-organizing Map Technique

The sunspot penumbra is usually observed in the photosphere and it is of particular interest for its magnetoconvection, which seems to transport the heat from the top of the convection zone into the solar atmosphere. It is well known that the penumbra magnetic field extends into the upper layers of the solar atmosphere forming the so-called superpenumbra. Thanks to the application of the Self-organizing Map technique to a spectral data set containing monochromatic images acquired along the Ca II 854.2 nm and Hα 656.28 nm lines, we were able to segment the penumbra and to measure the plasma velocity along the chromospheric portions of penumbral filaments. We found that the head, body, and tail of penumbral filaments show vertical flows compatible with the persistence of the Evershed flow. Instead, the inverse Evershed flow has been observed only in the outer portion of the superpenumbra. We found that two opposite Evershed regimes work next to each other, without overlapping, and both contribute to the downflow around sunspots. These results confirm the uncombed model of the sunspot penumbra and provide some hints that the downflow around sunspots may be ascribed to the magnetic field dragging the plasma down.

Keywords: solar chromosphere; sunspots; solar photosphere; magnetohydrodynamics
Modeling the Variable Polarization of ϵ Aurigae In and Out of Eclipse

The eclipsing binary ϵ Aur is unique in being a very-long-period binary involving an evolved, variable F star and a suspected B main-sequence star enshrouded in an opaque circumstellar disk. The geometrical arrangement is that the disk is viewed almost perfectly edge on, with the alignment leading to a partial eclipse of the F star. Despite a global observing campaign for the 2009-11 eclipse, there remain outstanding questions about the nature of the binary, its components, the disk, and the evolutionary state of the system. We analyze optical-band polarimetry in conjunction with broadband color variations to interpret brightness variations across the surface of the F star. We model this both during and after the 1982-84 eclipse for which an extensive and dense data set exists. We develop a model in terms of surface temperature variations characterized by a small global variation overlaid with a temperature variation described with low-order spherical harmonics. While not providing a detailed fit to the data set, our modeling captures the overall characterization of the color and polarimetric variability. In particular, we are able to recover the gross behavior of the polarimetric excursion in the Q-U plane as observed during eclipse of the F star when compared to posteclipse behavior.

Keywords: eclipsing binary stars; f supergiant stars; polarimetry; stellar atmospheres; stellar pulsations; starlight polarization
Some New Results and Perspectives Regarding the Kuiper Belt Object Arrokoth's Remarkable, Bright Neck

One of the most striking and curious features of the small Kuiper Belt Object (KBO), Arrokoth, explored by New Horizons is the bright, annular neck it exhibits at the junction between its two lobes. Here we summarize past reported findings regarding the properties of this feature and then report new results regarding its dimensions, reflectivity and color, shape profile, and lack of identifiable craters. We conclude by enumerating possible origin scenarios for this unusual feature. New results include a new estimated measurement of the observed neck area of 8 ± 1.5 km2, a total neck surface area of 32 km2, a 12.5:1 ratio of circumference to height, a normal reflectance histogram of the observed neck, and the fact that no significant (i.e., >2σ) neck color units were identified, meaning the neck's color is generally spatially uniform at the 1.5 km pixel-1 scale of the best color images. Although several origin hypotheses for the bright material in the neck are briefly discussed, none can be conclusively demonstrated to be the actual origin mechanism at this time; some future tests are identified.

Keywords: kuiper belt; classical kuiper belt objects; trans-neptunian objects; small solar system bodies; flyby missions; comet surfaces
The Long Filament of PSR J2030+4415

New X-ray and optical observations shed light on the remarkable X-ray filament of the gamma-ray pulsar PSR J2030+4415. Images of the associated Hα bow shock's evolution over the past decade compared with its velocity structure provide an improved kinematic distance of ~0.5 kpc. These velocities also imply that the pulsar spin axis lies ~15° from the proper motion axis, which is close to the plane of the sky. The multi-bubble shock structure indicates that the bow shock standoff was compressed to a small value ~20-30 yr ago when the pulsar broke through the bow shock to its present bubble. This compression allowed multi-TeV pulsar e ± to escape to the external interstellar medium (ISM), lighting up an external magnetic field structure as the filament. The narrow filament indicates excellent initial confinement and the full 15' (2.2 pc = 7 lt-yr) projected length of the filament indicates rapid e ± propagation to its end. Spectral variation along the filament suggests that the injected particle energy evolved during the breakthrough event.

Keywords: pulsars; nebulae; x-ray astronomy
CLEAR: Spatially Resolved Emission Lines and Active Galactic Nuclei at 0.6 < z < 1.3

We investigate spatially resolved emission-line ratios in a sample of 219 galaxies (0.6 < z < 1.3) detected using the G102 grism on the Hubble Space Telescope Wide Field Camera 3 taken as part of the CANDELS Lyα Emission at Reionization survey to measure ionization profiles and search for low-luminosity active galactic nuclei (AGN). We analyze [O III] and Hβ emission-line maps, enabling us to spatially resolve the [O III]/Hβ emission-line ratio across the galaxies in the sample. We compare the [O III]/Hβ ratio in galaxy centers and outer annular regions to measure ionization differences and investigate the potential of sources with nuclear ionization to host AGN. We investigate some of the individual galaxies that are candidates to host strong nuclear ionization and find that they often have low stellar mass and are undetected in X-rays, as expected for low-luminosity AGN in low-mass galaxies. We do not find evidence for a significant population of off-nuclear AGN or other clumps of off-nuclear ionization. We model the observed distribution of [O III]/Hβ spatial profiles and find that most galaxies are consistent with a small or zero difference between their nuclear and off-nuclear line ratios, but 6%-16% of galaxies in the sample are likely to host nuclear [O III]/Hβ that is ~0.5 dex higher than in their outer regions. This study is limited by large uncertainties in most of the measured [O III]/Hβ spatial profiles; therefore, deeper data, e.g., from deeper HST/WFC3 programs or from JWST/NIRISS, are needed to more reliably measure the spatially resolved emission-line conditions of individual high-redshift galaxies.

Keywords: galaxies; active galaxies; emission line galaxies
Detecting Exomoons from Radial Velocity Measurements of Self-luminous Planets: Application to Observations of HR 7672 B and Future Prospects

The detection of satellites around extrasolar planets, so called exomoons, remains a largely unexplored territory. In this work, we study the potential of detecting these elusive objects from radial velocity monitoring of self-luminous, directly imaged planets. This technique is now possible thanks to the development of dedicated instruments combining the power of high-resolution spectroscopy and high-contrast imaging. First, we demonstrate a sensitivity to satellites with a mass ratio of 1%-4% at separations similar to the Galilean moons from observations of a brown-dwarf companion (HR 7672 B; K mag = 13; 0.″7 separation) with the Keck Planet Imager and Characterizer (R ~ 35,000 in the K band) at the W. M. Keck Observatory. Current instrumentation is therefore already sensitive to large unresolved satellites that could be forming from gravitational instability akin to binary star formation. Using end-to-end simulations, we then estimate that future instruments such as the Multi-Object Diffraction-limited High-resolution Infrared Spectrograph, planned for the Thirty Meter Telescope, should be sensitive to satellites with mass ratios of ~10-4. Such small moons would likely form in a circumplanetary disk similar to the Jovian satellites in the solar system. Looking for the Rossiter-McLaughlin effect could also be an interesting pathway to detecting the smallest moons on short orbital periods. Future exomoon discoveries will allow precise mass measurements of the substellar companions that they orbit and provide key insight into the formation of exoplanets. They would also help constrain the population of habitable Earth-sized moons orbiting gas giants in the habitable zone of their stars.

Keywords: natural satellites (extrasolar); direct imaging; radial velocity; exoplanet detection methods
X-Ray Spectral Characterization of the Young Cygnus OB2 Population

We analyze the X-ray spectra of the ~8000 sources detected in the Cygnus OB2 Chandra Legacy Survey (this focus issue), with the goals of characterizing the coronal plasma of the young low-mass stars in the region and estimating their intrinsic X-ray luminosities. We adopt two different strategies for X-ray sources for which more or less than 20 photons were detected. For the brighter sample we fit the spectra with absorbed isothermal models. In order to limit uncertainties, for most of the fainter Cygnus OB2 members in this sample we constrain the spectral parameters to characteristic ranges defined from the brightest stars. For X-ray sources with <20 net photons we adopt a conversion factor from detected photon flux to intrinsic flux. This was defined, building on the results for the previous sample, as a function of the 20% quantile of the detected photon energy distributions, which we prove to also correlate well with extinction. We then use the X-ray extinction from the spectral fits to constrain the ratio between optical and X-ray extinction toward Cyg OB2, finding it consistent with standard "Galactic" values, when properly accounting for systematics. Finally, we exploit the large number of sources to constrain the average coronal abundances of several elements, through two different ensemble analyses of the X-ray spectra of low-mass Cyg OB2 members. We find the pattern of abundances to be largely consistent with that derived for the young stellar coronae in the Orion Nebula Cluster.

Keywords: stellar activity; stellar coronae; pre-main sequence stars; t tauri stars; stellar abundances; x-ray stars
Implications of Symmetry and Pressure in Friedmann Cosmology. III. Point Sources of Dark Energy that Tend toward Uniformity

We show that point sources of dark energy can explain accelerated late-time expansion and, simultaneously, satisfy observational constraints on massive compact objects. Population III stellar collapse into GEneric Objects of Dark Energy (GEODEs) between 8 ≲ z ≲ 20 mimics the observed ΩΛ within the typical concordance cosmology. We determine the appropriate dynamical model of aggregate GEODE flow within covariant linear perturbation theory. We find that all continuum fluid properties, at large scales, are determined by the internal properties and spin of individual GEODEs. For large spin, the spatial distribution of GEODEs becomes uniform on scales ≲200 Mpc. The power spectrum of cold dark matter is essentially unaltered. A Population III GEODE scenario provides an observationally consistent physical origin for accelerated late-time expansion while imposing no new constraints on structure formation.

Keywords: cosmological perturbation theory; dark energy; population iii stars; relativistic cosmology
Stellar Rotation in the K2 Sample: Evidence for Modified Spin-down

We analyze light curves of 284,834 unique K2 targets using a Gaussian process model with a quasi-periodic kernel function. By cross-matching K2 stars to observations from Gaia Data Release 2, we have identified 69,627 likely main-sequence stars. From these we select a subsample of 8977 stars on the main sequence with highly precise rotation period measurements. With this sample we recover the gap in the rotation period-color diagram first reported by McQuillan et al. While the gap was tentatively detected in Reinhold & Hekker, this work represents the first robust detection of the gap in K2 data for field stars. This is significant because K2 observed along many lines of sight at wide angular separation, in contrast to Kepler's single line of sight. Together with recent results for rotation in open clusters, we interpret this gap as evidence for a departure from the t-1/2 Skumanich spin-down law, rather than an indication of a bimodal star formation history. We provide maximum likelihood estimates and uncertainties for all parameters of the quasi-periodic light-curve model for each of the 284,834 stars in our sample.

Keywords: stellar properties; stellar rotation; k dwarf stars; g dwarf stars; gaussian processes regression; star clusters; stellar astronomy; stellar ages
Quantification of the Cross-helicity Turbulent Cascade in Compressible MHD Simulations

In plasma turbulence, energy and cross helicity are transferred across scales at a constant rate as a consequence of nonlinear interactions. In incompressible magnetohydrodynamics (MHD), the energy cascade rate of both quantities can be computed by means of the temporal evolution of second-order structure functions, known as Karman-Howarth-Monin (KHM) equations. In the present work, we derive the KHM equation to compute the energy cascade rate of cross helicity in compressible MHD. Using three-dimensional direct numerical simulations, we validate the equation and use it to measure the cross-helicity turbulence properties. Our results show a slower development of the cross-helicity cascade with respect to the energy one and the presence of inverse cascades of energy and cross helicity at large scales when in the presence of a strong mean field. We propose the relation of these phenomena with the longer duration of geomagnetic storms after the arrival of solar winds with large cross helicity and the observation of patchy inertial ranges displaying positive and negative cascade rates for certain solar wind intervals.

Keywords: interplanetary turbulence; solar wind; magnetohydrodynamical simulations; heliosphere
ELUCID. VIII. Simulating the Coma Galaxy Cluster to Calibrate Model and Understand Feedback

We conducted an investigation of the Coma cluster of galaxies by running a series of constrained hydrodynamic simulations with GIZMO-SIMBA and GADGET-3 based on initial conditions reconstructed from the SDSS survey volume in the ELUCID project. We compared simulation predictions and observations for galaxies, intracluster medium (ICM) and intergalactic medium (IGM) in and around the Coma cluster to constrain galaxy formation physics. Our results demonstrate that this type of constrained investigation allows us to probe in more detail the implemented physical processes, because the comparison between simulations and observations is free of cosmic variance and hence can be conducted in a "one-to-one" manner. We found that an increase in the earlier star formation rate and the supernova feedback of the original GIZMO-SIMBA model is needed to match observational data on stellar, interstellar medium, and ICM metallicity. The simulations without active galactic nucleus (AGN) feedback can well reproduce the observational ICM electron density, temperature, and entropy profiles, ICM substructures, and the IGM temperature–density relation, while the ones with AGN feedback usually fail. However, one requires something like AGN feedback to reproduce a sufficiently large population of quiescent galaxies, particularly in low-density regions. The constrained simulations of the Coma cluster thus provide a test bed to understand processes that drive galaxy formation and evolution.

Keywords: hydrodynamical simulations; galaxy formation; coma cluster; intracluster medium; intergalactic medium; metallicity; large-scale structure of the universe