adsabs/SciX_UAT_keywords huggingface dataset:
Hot Subdwarf Atmospheric Parameters, Kinematics, and Origins Based on 1587 Hot Subdwarf Stars Observed in Gaia DR2 and LAMOST DR7

Based on the Gaia DR2 catalog of hot subdwarf star candidates, we identified 1587 hot subdwarf stars with spectra in LAMOST DR7. We present atmospheric parameters for these stars by fitting the LAMOST spectra with TLUSTY/SYNSPEC non-LTE synthetic spectra. Combining LAMOST radial velocities and Gaia Early Data Release 3 (EDR3) parallaxes and proper motions, we also present the Galactic space positions, velocity vectors, orbital parameters and the Galactic population memberships of the stars. With our He classification scheme, we identify four groups of He rich hot subdwarf stars in the Teff - log g and Teff - log(nHe/nH) diagrams. We find two extreme He-rich groups (eHe-1 and eHe-2) for stars with log(nHe/nH) ≥ 0 and two intermediate He-rich groups (iHe-1 and iHe-2) for stars with -1 ≤ log(nHe/nH) < 0. We also find that over half of the stars in Group eHe-1 are thick-disk stars, while over half of the stars in Group eHe-2 correspond to thin-disk stars. The disk population fractions of Group iHe-1 are between those of Group eHe-1 and eHe-2. Almost all of the stars in Group iHe-2 belong to the thin disk. These differences indicate that the four groups probably have very different origins. Comparisons between hot subdwarf stars in the halo and in the Galactic globular cluster ω Cen show that only He-deficient stars with -2.2 ≤ log(nHe/nH) < -1 have similar fractions. Hot subdwarfs with log(nHe/nH) ≥ 0 in ω Cen have no counterparts in the thick-disk and halo populations, but they appear in the thin disk.

Keywords: o subdwarf stars; stellar kinematics
The Supersonic Project: Star Formation in Early Star Clusters without Dark Matter

The formation mechanism of globular clusters (GCs) has long been debated by astronomers. It was recently proposed that supersonically induced gas objects (SIGOs)-which formed in the early Universe due to the supersonic relative motion of baryons and dark matter at recombination-could be the progenitors of early GCs. In order to become GCs, SIGOs must form stars relatively efficiently despite forming outside of dark matter halos. We investigate the potential for star formation in SIGOs using cosmological hydrodynamic simulations, including the aforementioned relative motions of baryons and dark matter, molecular hydrogen cooling in primordial gas clouds, and explicit star formation. We find that SIGOs do form stars and that the nascent star clusters formed through this process are accreted by dark matter halos on short timescales (~a few hundred megayears). Thus, SIGOs may be found as intact substructures within these halos, analogous to many present-day GCs. From this result, we conclude that SIGOs are capable of forming star clusters with similar properties to globular clusters in the early Universe, and we discuss their detectability by upcoming JWST surveys.

Keywords: globular star clusters; star formation; galactic and extragalactic astronomy; extragalactic astronomy
The Open Cluster Chemical Abundances and Mapping Survey. VI. Galactic Chemical Gradient Analysis from APOGEE DR17

The goal of the Open Cluster Chemical Abundances and Mapping (OCCAM) survey is to constrain key Galactic dynamic and chemical evolution parameters by the construction and analysis of a large, comprehensive, uniform data set of infrared spectra for stars in hundreds of open clusters. This sixth contribution from the OCCAM survey presents analysis of SDSS/APOGEE Data Release 17 (DR17) results for a sample of stars in 150 open clusters, 94 of which we designate to be "high-quality" based on the appearance of their color-magnitude diagram. We find the APOGEE DR17-derived [Fe/H] values to be in good agreement with those from previous high-resolution spectroscopic open cluster abundance studies. Using a subset of the high-quality sample, the Galactic abundance gradients were measured for 16 chemical elements, including [Fe/H], for both Galactocentric radius (R GC) and guiding center radius (R guide). We find an overall Galactic [Fe/H] versus R GC gradient of -0.073 ± 0.002 dex kpc-1 over the range of 6 > R GC < 11.5 kpc, and a similar gradient is found for [Fe/H] versus R guide. Significant Galactic abundance gradients are also noted for O, Mg, S, Ca, Mn, Na, Al, K, and Ce. Our large sample additionally allows us to explore the evolution of the gradients in four age bins for the remaining 15 elements.

Keywords: open star clusters; galactic abundances; milky way evolution; chemical abundances
Center-to-limb Variation of Transition-region Doppler Shifts in Active Regions

Studying Doppler shifts provides deep insights into the flow of mass and energy in the solar atmosphere. We perform a comprehensive measurement of Doppler shifts in the transition region and its center-to-limb variation (CLV) in the strong-field regions (∣ B ∣ ≥ 50 G) of 50 active regions (ARs), using the Si IV 1394 Å line recorded by the Interface Region Imaging Spectrometer. To locate the ARs and identify strong-field regions, we have used the magnetograms obtained by the Helioseismic and Magnetic Imager (HMI). We find that in strong-field regions, on average, all the ARs show a mean redshift ranging between 4 and 11 km s-1, which varies with ARs. These flows show a mild CLV, with sizable magnitudes at the limb and substantial scatter in the mid-longitude range. Our observations do not support the idea that redshifts in the lower transition region (T ≲ 0.1 MK) are produced by field-aligned downflows as a result of impulsive heating and they warrant an alternative interpretation, such as a downflow of type-II spicules in the presence of a chromospheric wall created by cooler type-I spicules.

Keywords: the sun; solar active regions; solar activity; sunspots; solar physics; solar photosphere; solar chromosphere; solar atmosphere; solar magnetic fields; solar spicules
Protostellar Cores in Sagittarius B2 N and M

We present 500 and 700 au resolution 1 and 3 mm Atacama Large Millimeter/submillimeter Array observations, respectively, of protostellar cores in protoclusters Sagittarius B2 (Sgr B2) North (N) and Main (M), parts of the most actively star-forming cloud in our Galaxy. Previous lower-resolution (5000 au) 3 mm observations of this region detected ~150 sources inferred to be young stellar objects (YSOs) with M > 8 M ⊙. With a 10-fold increase in resolution, we detect 371 sources at 3 mm and 218 sources in the smaller field of view at 1 mm. The sources seen at low resolution are observed to fragment into an average of two objects. About one-third of the observed sources fragment. Most of the sources we report are marginally resolved and are at least partially optically thick. We determine that the observed sources are most consistent with Stage 0/I YSOs, i.e., rotationally supported disks with an active protostar and an envelope, that are warmer than those observed in the solar neighborhood. We report source-counting-based inferred stellar mass and the star formation rate of the cloud: 2800 M ⊙ and 0.0038 M ⊙ yr-1 for Sgr B2 N and 6900 M ⊙ and 0.0093 M ⊙ yr-1 for Sgr B2 M, respectively.

Keywords: star formation; initial mass function; star forming regions; protostars; young stellar objects
Far-Infrared Line Diagnostics: Improving N/O Abundance Estimates for Dusty Galaxies

The nitrogen-to-oxygen (N/O) abundance ratio is an important diagnostic of galaxy evolution because the ratio is closely tied to the growth of metallicity and the star formation history in galaxies. Estimates for the N/O are traditionally made with optical lines that could suffer from extinction and excitation effects, so the N/O is arguably measured better through far-infrared (far-IR) fine-structure lines. Here we show that the [N III]57 μm/[O III]52 μm line ratio, denoted N3O3, is a physically robust probe of N/O. This parameter is insensitive to gas temperature and only weakly dependent on electron density. Although it has a dependence on the hardness of the ionizing radiation field, we show that it is well corrected when the [Ne III]15.5 μm/[Ne II]12.8 μm line ratio is included. We verify the method, and characterize its intrinsic uncertainties by comparing the results to photoionization models. We then apply our method to a sample of nearby galaxies using new observations obtained with SOFIA/FIFI-LS in combination with available Herschel/PACS data, and the results are compared with optical N/O estimates. We find evidence for a systematic offset between the far-IR and optically derived N/O. We argue that the likely reason is that our far-IR method is biased toward younger and denser H II regions, while the optical methods are biased toward older H II regions as well as diffuse ionized gas. This work provides a local template for studies of the abundance of interstellar medium in the early Universe.

Keywords: galaxy evolution; chemical abundances; abundance ratios; far infrared astronomy
The MUSTANG Galactic Plane Survey (MGPS90) Pilot

We report the results of a pilot program for a Green Bank Telescope MUSTANG-2 Galactic Plane survey at 3 mm (90 GHz), MGPS90. The survey achieves a typical 1σ depth of 1-2 mJy beam-1 with a 9″ beam. We describe the survey parameters, quality assessment process, cataloging, and comparison with other data sets. We have identified 709 sources over seven observed fields selecting some of the most prominent millimeter-bright regions between 0 deg < ℓ < 50 deg (total area ≈7.5 deg 2). The majority of these sources have counterparts at other wavelengths. By applying flux selection criteria to these sources, we successfully recovered several known hypercompact H II (HCH II) regions but did not confirm any new ones. We identify 126 sources that have mm-wavelength counterparts but do not have cm-wavelength counterparts and are therefore candidate HCH II regions; of these, 10 are morphologically compact and are strong candidates for new HCH II regions. Given the limited number of candidates in the extended area in this survey compared to the relatively large numbers seen in protoclusters W51 and W49, it appears that most HCH II regions exist within dense protoclusters. Comparing the counts of HCH II to ultracompact H II (UCH II) regions, we infer the HCH II region lifetime is 16%-46% that of the UCH II region lifetime. We additionally separated the 3 mm emission into dust and free-free emission by comparing with archival 870 $\mu {\rm{m}}$ and 20 cm data. In the selected pilot fields, most (≳80%) of the 3 mm emission comes from plasma, either through free-free or synchrotron emission.

Keywords: surveys; star formation; h ii regions; dust continuum emission; interstellar dust
Continuous Jets and Backflow Models for the Formation of W50/SS 433 in Magnetohydrodynamics Simulations

The formation mechanism of the W50/SS 433 complex has long been a mystery. We propose a new scenario in which the SS 433 jets themselves form the W50/SS 433 system. We carry out magnetohydrodynamics simulations of the propagation of two side jets using the public code CANS+. As found in previous jet studies, when the propagating jet is lighter than the surrounding medium, the shocked plasma flows back from the jet tip to the core. We find that the morphology of light jets is spheroidal at early times; afterward, the shell and wings are developed by the broadening spherical cocoon. The morphology depends strongly on the density ratio of the injected jet to the surrounding medium. Meanwhile, the ratio of the lengths of the two side jets depends only on the density profile of the surrounding medium. We also find that most of the jet kinetic energy is dissipated at the oblique shock formed by the interaction between the backflow and beam flow, rather than at the jet terminal shock. The position of the oblique shock is spatially consistent with the X-ray and TeV gamma-ray hotspots of W50.

Keywords: magnetohydrodynamical simulations
An Explanation for the Overrepresentation of Tidal Disruption Events in Post-starburst Galaxies

Tidal disruption events (TDEs) provide a valuable probe for studying the dynamics of stars in the nuclear environments of galaxies. Recent observations show that TDEs are strongly overrepresented in post-starburst or "green valley" galaxies, although the underlying physical mechanism remains unclear. Considering the possible interaction between stars and active galactic nucleus (AGN) disks, the TDE rates can be greatly changed compared to those in quiescent galactic nuclei. In this work, we revisit TDE rates by incorporating an evolving AGN disk within the framework of "loss cone" theory. We numerically evolve the Fokker-Planck equations by considering star-disk interactions, in situ star formation in the unstable region of the outer AGN disk, and evolution of the accretion process of supermassive black holes. We find that the TDE rates are enhanced by about 2 orders of magnitude shortly after the AGN transitions into an inactive stage. During this phase, the accumulated stars rapidly scatter into the loss cone due to the disappearance of the inner standard thin disk. Our results provide an explanation for the overrepresentation of TDEs in post-starburst galaxies.

Keywords: tidal disruption; green valley galaxies; active galactic nuclei
TOI-3984 A b and TOI-5293 A b: Two Temperate Gas Giants Transiting Mid-M Dwarfs in Wide Binary Systems

We confirm the planetary nature of two gas giants discovered by TESS to transit M dwarfs with stellar companions at wide separations. TOI-3984 A (J = 11.93) is an M4 dwarf hosting a short-period (4.353326 ± 0.000005 days) gas giant (M p = 0.14 ± 0.03 M J and R p = 0.71 ± 0.02 R J) with a wide-separation white dwarf companion. TOI-5293 A (J = 12.47) is an M3 dwarf hosting a short-period (2.930289 ± 0.000004 days) gas giant (M p = 0.54 ± 0.07 M J and R p = 1.06 ± 0.04 R J) with a wide-separation M dwarf companion. We characterize both systems using a combination of ground- and space-based photometry, speckle imaging, and high-precision radial velocities from the Habitable-zone Planet Finder and NEID spectrographs. TOI-3984 A b (T eq = 563 ± 15 K and $\mathrm{TSM}={138}_{-27}^{+29}$ ) and TOI-5293 A b ( ${T}_{\mathrm{eq}}={675}_{-30}^{+42}$ K and TSM = 92 ± 14) are two of the coolest gas giants among the population of hot Jupiter-sized gas planets orbiting M dwarfs and are favorable targets for atmospheric characterization of temperate gas giants and 3D obliquity measurements to probe system architecture and migration scenarios.

Keywords: exoplanet systems; extrasolar gaseous giant planets
First Observations of the Brown Dwarf HD 19467 B with JWST

We observed HD 19467 B with JWST's NIRCam in six filters spanning 2.5-4.6 μm with the long-wavelength bar coronagraph. The brown dwarf HD 19467 B was initially identified through a long-period trend in the radial velocity of the G3V star HD 19467. HD 19467 B was subsequently detected via coronagraphic imaging and spectroscopy, and characterized as a late-T type brown dwarf with an approximate temperature ~1000 K. We observed HD 19467 B as a part of the NIRCam GTO science program, demonstrating the first use of the NIRCam Long Wavelength Bar coronagraphic mask. The object was detected in all six filters (contrast levels of 2 × 10-4 to 2 × 10-5) at a separation of 1.″6 using angular differential imaging and synthetic reference differential imaging. Due to a guide star failure during the acquisition of a preselected reference star, no reference star data were available for post-processing. However, reference differential imaging was successfully applied using synthetic point-spread functions developed from contemporaneous maps of the telescope's optical configuration. Additional radial velocity data (from Keck/HIRES) are used to constrain the orbit of HD 19467 B. Photometric data from TESS are used to constrain the properties of the host star, particularly its age. NIRCam photometry, spectra, and photometry from the literature, and improved stellar parameters are used in conjunction with recent spectral and evolutionary substellar models to derive the physical properties of HD 19467 B. Using an age of 9.4 ± 0.9 Gyr inferred from spectroscopy, Gaia astrometry, and TESS asteroseismology, we obtain a model-derived mass of 62 ± 1 M J, which is consistent within 2σ with the dynamically derived mass of ${81}_{-12}^{+14}$ M J.

Keywords: brown dwarfs; coronagraphic imaging; high contrast techniques
A Highway for Atmospheric Ion Escape from Earth during the Impact of an Interplanetary Coronal Mass Ejection

A single satellite hardly measures the overall ion escape rate from a planet. Therefore, the question concerning the long-term atmospheric evolution of whether a planetary magnetic field protects its atmosphere or aggravates atmospheric loss remains unresolved. Here, combined data from multiple platforms including Cluster, DMSP, IMAGE, and Polar satellites and ground stations show that, when an interplanetary coronal mass ejection (ICME) arrived, an ion transport "highway" formed in the inner magnetosphere on the strongly magnetized Earth, along which the magnetospheric processes promoted quick and significant atmospheric ion escape. The escape rate merely through this highway probably reached 1027 s-1, far exceeding the current overall rates on those nonmagnetized planets, e.g., Mars and Venus. The relevant interplanetary conditions induced by the ICME were extreme by today's standards but may have been quite common in the ancient solar system. These present-day observations thus allow us to depict a fast but common ion escape scenario on the strongly magnetized planets in ancient times when the Sun was extremely active.

Keywords: space plasmas; earth atmosphere; plasma physics; planetary magnetospheres; earth ionosphere
Characterizing the γ-Ray Variability of Active Galactic Nuclei with the Stochastic Process Method

Gamma-ray astronomy in the time domain has been by now progressed further as the variabilities of active galactic nuclei (AGNs) on different timescales have been reported a lot. We study the γ-ray variabilities of 23 jetted AGNs by applying a stochastic process method to the ~12.7 yr long-term light curve (LC) obtained by the Fermi-Large Area Telescope (Fermi-LAT). In this method, the stochastically driven damped simple harmonic oscillator (SHO) and the damped random-walk (DRW) models are used to model the long-term LCs. Our results show that the long-term variabilities of 23 AGNs can be characterized well by both SHO and DRW models. However, the SHO model is restricted in the overdamped mode, and the parameters are poorly constrained. The SHO power spectral densities (PSDs) are the same as those of the typical DRW PSD. In the plot of the rest-frame timescale that corresponds to the broken frequency in the PSD versus black hole mass, the intrinsic, characteristic γ-ray timescales of 23 AGNs occupy almost the same space with the optical variability timescales obtained from the accretion disk emission. This suggests a connection between the jet and the accretion disk. As with the optical variability of the AGN accretion disk, the γ-ray timescale is also consistent with the thermal timescale caused by the thermal instability in the standard accretion disk of AGNs.

Keywords: active galactic nuclei; gamma-rays; jets; light curves; time series analysis
Changing-look Active Galactic Nuclei Behavior Induced by Disk-captured Tidal Disruption Events

Recent observations of changing-look active galactic nuclei (AGNs) hint at a frequency of accretion activity not fully explained by tidal disruption events (TDEs) stemming from relaxation processes in nuclear star clusters (NSCs), traditionally estimated to occur at rates of 10‑4–10‑5 yr‑1 per galaxy. In this Letter, we propose an enhanced TDE rate through the AGN disk capture process, presenting a viable explanation for the frequent transitions observed in changing-look AGNs. Specifically, we investigate the interaction between the accretion disk and retrograde stars within NSCs, resulting in the rapid occurrence of TDEs within a condensed time frame. Through detailed calculations, we derive the time-dependent TDE rates for both relaxation-induced TDE and disk-captured TDE. Our analysis reveals that TDEs triggered by the disk capture process can notably amplify the TDE rate by several orders of magnitude during the AGN phase. This mechanism offers a potential explanation for the enhanced high-energy variability characteristic of changing-look AGNs.

Keywords: agn host galaxies; active galactic nuclei; black hole physics; galaxy nuclei; tidal disruption; x-ray transient sources
The GMRT High-resolution Southern Sky Survey for Pulsars and Transients. VII. Timing of the Spider Millisecond Pulsar PSR J1242–4712, a Bridge between Redback and Black Widow Pulsars

We present a timing solution for the 5.31 ms spider millisecond pulsar (MSP) J1242‑4712, discovered with the GMRT. PSR J1242‑4712 orbits a companion of minimum mass 0.08 M ⊙ with an orbital period of 7.7 hr and occupies a relatively unexplored region in the orbital period versus companion mass space. We did not detect gamma-ray pulsations for this MSP, and also could not identify the optical counterpart for PSR J1242–4712 in available optical/near-infrared data. The profile of J1242‑4712 evolves with frequency, showing a clear single component at lower frequencies and a three-component profile at 650 MHz. PSR J1242‑4712 eclipses for a very short duration near superior conjunction (orbital phase ∼ 0.23‑0.25) below 360 MHz. Moreover, significant dispersion measure delays and errors in the pulse times of arrivals are observed near inferior conjunction (orbital phase ∼ 0.7), along with an observed eclipse in one epoch at 650 MHz. Observed eclipses and significant orbital period variability suggest that PSR J1242‑4712 is possibly not a helium star‑white dwarf binary, but has a semi- or nondegenerate companion, indicating that this is a "spider" MSP lying in a region between typical black widows and redbacks. This system may represent a distinct category of spider MSPs, displaying characteristics that bridge the gap between known black widow and redback MSPs.

Keywords: binary pulsars; pulsars; millisecond pulsars; pulsar timing method; radio pulsars
Detection of the Actinide Th in an r-process-enhanced Star with Accretion Origin

The thorium and six second-peak r-process element (56 ≤ Z ≤ 72) abundances are determined for the α-poor star LAMOST J1124+4535 based on a high-resolution spectrum obtained with the High Dispersion Spectrograph on the Subaru Telescope. The age of J1124+4535 is 11.3 ± 4.4 Gyr using thorium and other r-process element abundances. J1124+4535 is confirmed to be a Galactic halo metal-poor ([Fe/H] = ‑1.27 ± 0.1) star with extreme r-process element overabundance ([Eu/Fe] = 1.13 ± 0.08) and α element deficiency ([Mg/Fe] = ‑0.31 ± 0.09) by the LAMOST-Subaru project. Along with the subsolar α-to-iron ratios (e.g., [Mg/Fe], [Si/Fe], [Ca/Fe]), the relatively low abundances of Na, Cr, Ni, and Zn in J1124+4535 show a significant departure from the general trends of the Galactic halo but are in good agreement with those of dwarf galaxies. The chemical abundances and kinematics of J1124+4535 suggest it was formed in the late stage of star formation in a dwarf galaxy that has been disrupted by the Milky Way. The star formation of its progenitor dwarf galaxy lasted more than 2 Gyr and has been affected by a rare r-process event before the occurrence of accretion event.

Keywords: milky way stellar halo; stellar abundances; galaxy mergers