adsabs/SciX_UAT_keywords huggingface dataset:
Temperature Structures of Embedded Disks: Young Disks in Taurus Are Warm

The chemical composition of gas and ice in disks around young stars sets the bulk composition of planets. In contrast to protoplanetary disks (Class II), young disks that are still embedded in their natal envelope (Class 0 and I) are predicted to be too warm for CO to freeze out, as has been confirmed observationally for L1527 IRS. To establish whether young disks are generally warmer than their more evolved counterparts, we observed five young (Class 0/I and I) disks in Taurus with the Atacama Large Millimeter/submillimeter Array, targeting C17O 2 - 1, H2CO ${3}_{\mathrm{1,2}}-{2}_{\mathrm{1,1}}$ , HDO ${3}_{\mathrm{1,2}}-{2}_{\mathrm{2,1}}$ , and CH3OH 5K - 4K transitions at 0"48 × 0"31 resolution. The different freeze-out temperatures of these species allow us to derive a global temperature structure. C17O and H2CO are detected in all disks, with no signs of CO freeze-out in the inner ∼100 au and a CO abundance close to ∼10-4. The H2CO emission originates in the surface layers of the two edge-on disks, as witnessed by the especially beautiful V-shaped emission pattern in IRAS 04302+2247. HDO and CH3OH are not detected, with column density upper limits more than 100 times lower than for hot cores. Young disks are thus found to be warmer than more evolved protoplanetary disks around solar analogs, with no CO freeze-out (or only in the outermost part of ≳100 au disks) or processing. However, they are not as warm as hot cores or disks around outbursting sources and therefore do not have a large gas-phase reservoir of complex molecules.

Keywords: protostars; protoplanetary disks; young stellar objects; astrochemistry
The Main Asteroid Belt: The Primary Source of Debris on Comet-like Orbits

Jupiter-family comets (JFCs) contribute a significant amount of debris to near-Earth space. However, telescopic observations of these objects seem to suggest that they have short physical lifetimes. If this is true, the material generated will also be short-lived, but fireball observation networks still detect material on cometary orbits. This study examines centimeter-to-meter-scale sporadic meteoroids detected by the Desert Fireball Network from 2014 to 2020 originating from JFC-like orbits. Analyzing each event's dynamic history and physical characteristics, we confidently determined whether they originated from the main asteroid belt or the trans-Neptunian region. Our results indicate that <4% of sporadic meteoroids on JFC-like orbits are genetically cometary. This observation is statistically significant and shows that cometary material is too friable to survive in near-Earth space. Even when considering shower contributions, meteoroids on JFC-like orbits are primarily from the main belt. Thus, the presence of genuine cometary meteorites in terrestrial collections is highly unlikely.

Keywords: meteoroids; solar system; meteorites; fireballs; meteors; short period comets; near-earth objects; small solar system bodies; meteorite composition; orbital evolution; dynamical evolution; orbits
Predicting the Solar Wind at the Parker Solar Probe Using an Empirically Driven MHD Model

Since its launch on 2018 August 12, Parker Solar Probe (PSP) has completed its first and second orbits around the Sun, having reached down to 35.7 solar radii at each perihelion. In anticipation of the exciting new data at such unprecedented distances, we have simulated the global 3D heliosphere using an MHD model coupled with a semi-empirical coronal model using the best available photospheric magnetograms as input. We compare our heliospheric MHD simulation results with in situ measurements along the PSP trajectory from its launch to the completion of the second orbit, with particular emphasis on the solar wind structure around the first two solar encounters. Furthermore, we show our model prediction for the third perihelion, which occurred on 2019 September 1. Comparison of the MHD results with PSP observations provides new insights into solar wind acceleration. Moreover, PSP observations reveal how accurately the Air Force Data Assimilative Photospheric flux Transport-Wang-Sheeley-Arge-based predictions work throughout the inner heliosphere.

Keywords: solar wind; interplanetary physics; magnetohydrodynamical simulations; heliosphere
Higher-order Turbulence Statistics in the Sub-Alfvénic Solar Wind Observed by Parker Solar Probe

Parker Solar Probe has been the first spacecraft to enter the deep corona below the Alfvén critical point. Here we examine the higher-order statistical properties of magnetic-field fluctuations in the sub-Alfvénic solar wind and compare the results with the neighboring super-Alfvénic region. The intermittency and multifractal properties are analyzed by inspecting the probability density functions, the scale-dependent kurtosis, and fractal spectrum of magnetic-field fluctuations. It is found that the magnetic-field fluctuations present distinct intermittency and multifractal properties in the inertial range and the B R component reveals much higher intermittency and more asymmetrical multifractal spectrum than the other components. The non-Gaussian self-similarity of fluctuations of B R at larger scales has also been observed. Further comparative analysis shows that all the solar wind periods share nearly the same intermittency and multifractal features, and the only difference lies in that the fluctuations of the B T and B N components have slight variations.

Keywords: solar wind; interplanetary turbulence
Gas Sloshing and Cold Fronts in Pre-merging Galaxy Cluster A98

We present deep Chandra observations of the pre-merger galaxy cluster Abell 98 (A98). A98 is a complex merging system. While the northern (A98N) and central subclusters (A98S) are merging along the north-south direction, A98S is undergoing a separate late-stage merger, with two distinct X-ray cores. We report the detection of a gas sloshing spiral and two cold front edges in A98N. We find two more surface brightness edges along the east direction of the eastern core and west direction of the western core of A98S. By measuring the gas temperatures and densities across those edges, we confirm that the eastern edge appears to be a cold front while the western edge is a shock front with a Mach number ${ \mathcal M }$ ≈ 1.5. We detect a spiral structure and a "tail" of X-ray emission associated with the eastern core of A98S. Our measurement indicates that the tail is cooler than the surrounding gas at a 4.2σ level. This may suggest that the tail and the spiral structures are the results of ram-pressure stripping, as the eastern core orbits in the main cluster's gravitational potential.

Keywords: x-ray astronomy
Giant Metrewave Radio Telescope Detections of Two High-opacity HI 21 cm Absorbers at z ≈ 1.2

We report the discovery of two remarkable high-opacity HI 21 cm absorbers against low-luminosity active galactic nuclei (AGNs), at z = 1.2166 toward J0229+0044 and at z = 1.1630 toward J0229+0053. The absorbers were detected in an unbiased Giant Metrewave Radio Telescope survey for HI 21 cm absorption against radio sources in the DEEP2 survey fields, covering z ≈ 0.73-1.53, and including sources without known redshifts. The velocity-integrated HI 21 cm optical depths are (74.2 ± 7.8) km s-1 (J0229+0044) and (78.41 ± 0.81) km s-1 (J0229+0053), higher than that of any known redshifted HI 21 cm absorber at z > 0.12, and implying high HI column densities, >1022 cm-2. The emission redshift of J0229+0044 is consistent with the HI 21 cm absorption redshift, while the strength and velocity spread of the absorption against J0229+0053 suggest that it too arises from gas in the AGN environment: both absorbers are thus likely to be "associated" systems. The two AGNs have low rest-frame 1.4 GHz radio and 1215 Å ultraviolet luminosities (≲1026.1 W Hz-1 and ≲1021.7 W Hz-1, respectively), both significantly lower than the typical luminosities of AGNs against which HI 21 cm searches have hitherto been carried out at z ≳ 1. The paucity of HI 21 cm absorbers at z ≳ 1 may be due to a luminosity bias in high-z AGN samples that have been searched for HI 21 cm absorption, where the high AGN ultraviolet luminosity affects physical conditions in its environment, ionizing the neutral hydrogen.

Keywords: active galactic nuclei; radio spectroscopy; neutral hydrogen clouds
Stellar Multiplicity in an RMHD Simulation with Stellar Feedback

Unlike the Sun, most observed stars are part of a multiple star system, but the formation of such systems is still not well understood. To study this problem, we utilize a run from the STARFORGE simulation suite that simulates the evolution of star-forming Giant Molecular Clouds (GMCs). The run starts with a GMC of an initial mass of 2 × 104 M⊙ and takes gravity, magneto-hydrodynamics and non-isothermal thermodynamics into account. The simulation also includes stellar feedback in the forms of radiation and protostellar jets, where the radiation field is explicitly evolved. In this note, we discuss the orbital period, semimajor axis, multiplicity fraction, and multiplicity frequency distributions, and compare these with observations. We find good agreement with the observed multiplicity fractions but also find a deficit of mass ratios close to unity and close companions, which we attribute to the lack of resolved disks in our simulation.

Keywords: multiple stars; binary stars; star formation; computational astronomy
Tidally Tilted Pulsations in HD 265435, a Subdwarf B Star with a Close White Dwarf Companion

Tidally tilted pulsators (TTPs) are an intriguing new class of oscillating stars in binary systems; in such stars, the pulsation axis coincides with the line of apsides, or tidal axis, of the binary. All three TTPs discovered so far have been δ Scuti stars. In this Letter, we report the first conclusive discovery of tidally tilted pulsations in a subdwarf B (sdB) star. HD 265435 is an sdB-white dwarf binary with a 1.65 hr period that has been identified and characterized as the nearest potential Type Ia supernova progenitor. Using TESS 20 s cadence data from Sectors 44 and 45, we show that the pulsation axis of the sdB star has been tidally tilted into the orbital plane and aligned with the tidal axis of the binary. We identify 31 independent pulsation frequencies, 27 of which have between 1 and 7 sidebands separated by the orbital frequency (ν orb) or multiples thereof. Using the observed amplitude and phase variability due to tidal tilting, we assign ℓ and m values to most of the observed oscillation modes and use these mode identifications to generate preliminary asteroseismic constraints. Our work significantly expands our understanding of TTPs, as we now know that (i) they can be found in stars other than δ Scuti pulsators, especially highly evolved stars that have lost their H-rich envelopes, and (ii) tidally tilted pulsations can be used to probe the interiors of stars in very tight binaries.

Keywords: asteroseismology; b subdwarf stars; pulsation modes; tidal distortion
Discovery of Extended Tidal Tails around the Globular Cluster Palomar 13

We use photometry from the DECam Legacy Survey to detect candidate tidal tails extending ∼5° on either side of the Palomar 13 globular cluster. The tails are aligned with the proper motion of Palomar 13 and are consistent with its old, metal-poor stellar population. We identify three RR Lyrae stars (RRLs) that are plausibly associated with the tails, in addition to four previously known in the cluster. From these RRLs, we find that the mean distance to the cluster and tails is 23.6 ± 0.2 kpc and estimate the total (initial) luminosity of the cluster to be ${L}_{V}={5.1}_{-3.4}^{+9.7}\times {10}^{3}\,{L}_{\odot }$ , consistent with previous claims that its initial luminosity was higher than its current luminosity. Combined with previously determined proper motion and radial velocity measurements of the cluster, we find that Palomar 13 is on a highly eccentric orbit (e ∼ 0.8) with a pericenter of ∼9 kpc and an apocenter of ∼69 kpc, and a recent pericentric passage of the cluster ∼75 Myr ago. We note a prominent linear structure in the interstellar dust map that runs parallel to the candidate tidal features, but conclude that reddening due to dust is unlikely to account for the structure that we observe. If confirmed, the Palomar 13 stellar stream would be one of very few streams with a known progenitor system, making it uniquely powerful for studying the disruption of globular clusters, the formation of the stellar halo, and the distribution of matter within our Galaxy.

Keywords: stellar structures; milky way stellar halo; milky way dynamics
NICER Observations of Thermonuclear Bursts from 4U 1728-34: Detection of Oscillations prior to the Onset of Two Bursts

We present temporal and time-resolved spectral analyses of all the thermonuclear X-ray bursts observed from the neutron star low-mass X-ray binary 4U 1728-34 with NICER from 2017 June to 2019 September. In total, we detected 11 X-ray bursts from the source and performed time-resolved spectroscopy. Unlike some of the earlier results for other bursting sources from NICER, our spectral results indicate that the use of a scaling factor for the persistent emission is not statistically necessary. This is primarily a result of the strong interstellar absorption in the line of sight toward 4U 1728-34, which causes the count rates to be significantly lower at low energies. We also searched for burst oscillations and detected modulations in six different bursts at around the previously known burst oscillation frequency of 363 Hz. Finally, we report the detection of oscillations prior to two bursts at 356 and 359 Hz, respectively. This is the first time in the literature where burst oscillations are detected before the rapid rise in X-ray flux, from any known burster. These oscillations disappear as soon as the burst starts to rise and occur at a somewhat lower frequency than the oscillations we detect during the bursts.

Keywords: neutron stars; x-ray bursts; low-mass x-ray binary stars
The Fate of the Merger Remnant in GW170817 and Its Imprint on the Jet Structure

The first neutron star binary merger detected in gravitational waves, GW170817, and the subsequent detection of its emission across the electromagnetic spectrum showed that these systems are viable progenitors of short γ-ray bursts (sGRB). The afterglow signal of GW170817 has been found to be consistent with a structured GRB jet seen off-axis, requiring significant amounts of relativistic material at large angles. This trait can be attributed to the interaction of the relativistic jet with the external wind medium. Here we perform numerical simulations of relativistic jets interacting with realistic wind environments in order to explore how the properties of the wind and central engine affect the structure of successful jets. We find that the angular energy distribution of the jet depends primarily on the ratio between the lifetime of the jet and the time it takes the merger remnant to collapse. We make use of these simulations to constrain the time it took for the merger remnant in GW170817 to collapse into a black hole based on the angular structure of the jet as inferred from afterglow observations. We conclude that the lifetime of the merger remnant in GW170817 was ≈1-1.7 s, which, after collapse, triggered the formation of the jet.

Keywords: gamma-ray bursts; jets
The 2018 Outburst of MAXI J1820+070 as Seen by Insight-HXMT

We present an analysis of the whole 2018 outburst of the black hole X-ray binary MAXI J1820+070 with Insight-HXMT data. We focus our study on the temporal evolution of the parameters of the source. We employ two different models to fit the disk's thermal spectrum: the Newtonian model diskbb and the relativistic model nkbb. These two models provide different pictures of the source in the soft state. With diskbb, we find that the inner edge of the disk is close to the innermost stable circular orbit of a fast-rotating black hole and the corona changes geometry from the hard to the soft state. With nkbb, we find that the disk is truncated in the soft state and that the coronal geometry does not change significantly during the whole outburst. However, the model with nkbb can predict an untruncated disk around a fast-rotating black hole if we assume that the disk inclination angle is around 30° (instead of ∼60°, which is the inclination angle of the jet and is usually adopted as the disk inclination angle in the literature) and we employ a high-density reflection model. In such a case, we measure a high value of the black hole spin parameter with observations in the soft state, in agreement with the high spin value found from the analysis of the reflection features and in disagreement with the low spin value found by previous continuum-fitting method measurements with the disk inclination angle set to the value of the jet inclination angle.

Keywords: low-mass x-ray binary stars; x-ray astronomy; accretion; astrophysical black holes; stellar mass black holes
Obliquity of an Earth-like Planet from Frequency Modulation of Its Direct-imaged Lightcurve: Mock Analysis from General Circulation Model Simulation

Direct-imaging techniques of exoplanets have made significant progress recently and will eventually enable monitoring of photometric and spectroscopic signals of Earth-like habitable planets. The presence of clouds, however, would remain as one of the most uncertain components in deciphering such direct-imaged signals of planets. We attempt to examine how the planetary obliquity produces different cloud patterns by performing a series of general circulation model simulation runs using a set of parameters relevant for our Earth. Then we use the simulated photometric lightcurves to compute their frequency modulation that is due to the planetary spin-orbit coupling over an entire orbital period, and we attempt to see to what extent one can estimate the obliquity of an Earth twin. We find that it is possible to estimate the obliquity of an Earth twin within the uncertainty of several degrees with a dedicated 4 m space telescope at 10 pc away from the system if the stellar flux is completely blocked. While our conclusion is based on several idealized assumptions, a frequency modulation of a directly imaged Earth-like planet offers a unique methodology to determine its obliquity.

Keywords: exoplanet atmospheres; exoplanet surface composition; hydrodynamics; radiative transfer; oblique rotators; biosignatures; direct imaging; stellar photometry; earth atmosphere; atmospheric circulation
HAZMAT. IX. An Analysis of the UV and X-Ray Evolution of Low-mass Stars in the Era of Gaia

Low-mass stars (≤1 M ⊙) are some of the best candidates for hosting planets with detectable life because of these stars' long lifetimes and relative ratios of planet to star mass and radius. An important aspect of these stars to consider is the amount of ultraviolet (UV) and X-ray radiation incident on planets in the habitable zones due to the ability of UV and X-ray radiation to alter the chemistry and evolution of planetary atmospheres. In this work, we build on the results of the HAZMAT I and HAZMAT III M-star studies to determine the intrinsic UV and X-ray flux evolution with age for M stars using Gaia parallactic distances. We then compare these results to the intrinsic fluxes of K stars adapted from HAZMAT V. We find that although the intrinsic M-star UV flux is 10-100 times lower than that of K stars, the UV fluxes in their respective habitable zone are similar. However, the habitable zone X-ray flux evolutions are slightly more distinguishable with a factor of 3-15 times larger X-ray flux for late M stars than for K stars. These results suggest that there may not be a K-dwarf advantage compared to M stars in the UV, but one may still exist in the X-ray.

Keywords: stellar evolution; low mass stars; ultraviolet astronomy; x-ray astronomy
Star Formation in Isolated Dwarf Galaxies Hosting Tidal Debris: Extending the Dwarf-Dwarf Merger Sequence

Like massive galaxies, dwarf galaxies are expected to undergo major mergers with other dwarfs. However, the end state of these mergers and the role that merging plays in regulating dwarf star formation are uncertain. Using imaging from the Hyper Suprime-Cam Subaru Strategic Program, we construct a sample of dwarf-dwarf mergers and examine the star formation and host properties of the merging systems. These galaxies are selected via an automated detection algorithm from a sample of 6875 spectroscopically selected isolated dwarf galaxies at z < 0.12 and $\mathrm{log}({M}_{\star }/{M}_{\odot })\lt 9.6$ from the Galaxy and Mass Assembly and Sloan Digital Sky Survey spectroscopic campaigns. We find a total tidal feature detection fraction of 3.29% (6.1% when considering only galaxies at z < 0.05). The tidal feature detection fraction rises strongly as a function of star formation activity; 15%-20% of galaxies with extremely high Hα equivalent width (Hα EW > 250 Å) show signs of tidal debris. Galaxies that host tidal debris are also systematically bluer than the average galaxy at fixed stellar mass. These findings extend the observed dwarf-dwarf merger sequence with a significant sample of dwarf galaxies, indicating that star formation triggered in mergers between dwarf galaxies continues after coalescence.

Keywords: dwarf galaxies; blue compact dwarf galaxies; galaxy mergers; observational astronomy; star formation; galaxy evolution
Spectropolarimetry of the Thermonuclear Supernova SN 2021rhu: High Calcium Polarization 79 Days after Peak Luminosity

We report spectropolarimetric observations of the Type Ia supernova (SN) SN 2021rhu at four epochs: -7, +0, +36, and +79 days relative to its B-band maximum luminosity. A wavelength-dependent continuum polarization peaking at 3890 ± 93 Å and reaching a level of ${p}_{\max }\,=\,1.78 \% \pm 0.02$ % was found. The peak of the polarization curve is bluer than is typical in the Milky Way, indicating a larger proportion of small dust grains along the sight line to the SN. After removing the interstellar polarization, we found a pronounced increase of the polarization in the Ca II near-infrared triplet, from ~0.3% at day -7 to ~2.5% at day +79. No temporal evolution in high-resolution flux spectra across the Na I D and Ca II H and K features was seen from days +39 to +74, indicating that the late-time increase in polarization is intrinsic to the SN as opposed to being caused by scattering of SN photons in circumstellar or interstellar matter. We suggest that an explanation for the late-time rise of the Ca II near-infrared triplet polarization may be the alignment of calcium atoms in a weak magnetic field through optical excitation/pumping by anisotropic radiation from the SN.

Keywords: supernovae; spectropolarimetry
13 yr of P Cygni Spectropolarimetry: Investigating Mass Loss through Hα, Periodicity, and Ellipticity

We report on over 13 yr of optical and near-ultraviolet spectropolarimetric observations of the famous luminous blue variable (LBV) P Cygni. In the lives of the most massive stars, LBVs are a critical transitional phase and achieve the largest mass-loss rates of any group of stars. Using spectropolarimetry, we are able to learn about the geometry of the near-circumstellar environment surrounding P Cygni and gain insights into LBV mass loss. Using data from the HPOL and WUPPE spectropolarimeters, we estimate the interstellar polarization contribution to P Cygni's spectropolarimetric signal, analyze the variability of the polarization across the Hα emission line, search for periodic signals in the data, and introduce a statistical method to search for preferred position angles in deviations from spherical symmetry that is novel to astronomy. Our data are consistent with previous findings, showing free electron scattering off of clumps uniformly distributed around the star. This is complicated, however, by structure in the percent polarization of the Hα line and a series of previously undetected periodicities.

Keywords: luminous blue variable stars; polarimetry; spectropolarimetry; stellar phenomena; circumstellar matter; time series analysis