Sazuna/UAT_keywords huggingface dataset:
TOI-421 b: A Hot Sub-Neptune with a Haze-free, Low Mean Molecular Weight Atmosphere. Common features of sub-Neptune atmospheres observed to date include signatures of aerosols at moderate equilibrium temperatures (∼500–800 K) and a prevalence of high mean molecular weight atmospheres, perhaps indicating novel classes of planets such as water worlds. Here we present a 0.83–5 μm JWST transmission spectrum of the sub-Neptune TOI-421 b. This planet is unique among previously observed counterparts in its high equilibrium temperature (Teq ≈ 920 K) and its Sun-like host star. We find marked differences between the atmosphere of TOI-421 b and those of sub-Neptunes previously characterized with JWST, which all orbit late K and M stars. Specifically, water features in the NIRISS/SOSS bandpass indicate a low mean molecular weight atmosphere consistent with solar metallicity and no appreciable aerosol coverage. Hints of SO2 and CO (but not CO2 or CH4) also exist in our NIRSpec/G395M observations, but not at sufficient signal-to-noise ratio to draw firm conclusions. Our results support a picture in which sub-Neptunes hotter than ∼850 K do not form hydrocarbon hazes owing to a lack of methane to photolyze. TOI-421 b additionally fits the paradigm of the radius valley for planets orbiting FGK stars being sculpted by mass-loss processes, which would leave behind primordial atmospheres overlying rock/iron interiors. Further observations of TOI-421 b and similar hot sub-Neptunes will confirm whether haze-free atmospheres and low mean molecular weights are universal characteristics of such objects.

Keywords: Exoplanet atmospheric composition , Transmission spectroscopy, James Webb Space Telescope, Mini Neptunes, Exoplanet atmospheres
A Two-hump Spectrum in the Prompt Emission of GRB 240825A. An extra hard spectral component that extends to GeV energies, in additional to the typical sub-MeV Band component, appears in several gamma-ray bursts (GRBs) detected by the Fermi Large Area Telescope. Only in one case (i.e., GRB 090926A) is a spectral break feature at the high-energy end identified in the extra hard component, but the photon counts are not enough to distinguish between the cutoff model and the broken power-law model for the spectral break. In this work, we report the detection of an extra hard component showing the spectral break in GRB 240825A. We find that a broken power-law model fits the spectral data of the extra component better than a single power law with an exponential cutoff in the time-resolved spectrum for the second emission pulse, with a break at about 40 MeV. This spectral feature disfavors the gamma-ray opacity to pair creation as the origin of the spectral break, but points to an intrinsic peak for the extra component. The low ratio between the peak of the extra hard component and that of the Band component challenges the synchrotron self-Compton origin for the extra component. Alternative scenarios, such as the inverse Compton scattering of the photosphere emission, are discussed. In addition, we find a clear transition from the prompt emission to afterglow emission at GeV energies in GRB 240825A, manifested by a temporal steep decay and a unique spectral evolution.

Keywords: Gamma-ray bursts, High energy astrophysics
MeerKAT H I Observations of Low Surface Brightness/Ultradiffuse Galaxy Candidates Projected around Two Southern Loose Groups. A large catalog of low surface brightness galaxies (LSBGs) from the Dark Energy Survey showed significant clustering around nearby galaxy groups and clusters. Using the HIPASS survey, we tried to determine the redshift of a subsample of these LSBGs and determine whether they were members of the groups they were projected near, but this was hampered by HIPASS's high spectral rms. This Letter reports on MeerKAT H I observations to determine the redshifts of 52 LSBG candidates projected in the vicinity of two groups from our previous HIPASS study. The main goal is to investigate and ascertain whether these LSBGs are genuine group members. H I was detected with MeerKAT and redshifts were determined for only five of the 52 candidates within a velocity range of ±2500 km s−1 of their respective group velocities. All five H I detections were blue LSBGs, and two of them were confirmed to be ultradiffuse galaxies (UDGs). Both these UDGs were group members, while the other three detections were either foreground or background galaxies. In this Letter we explore scenarios that can explain the 90% nondetection. MeerKAT's excellent sensitivity allows us to conclude that the majority of the nondetected candidates, particularly the blue galaxies, are not group members but lie at higher redshifts. However, this still leaves the open question of why Tanoglidis LSBG candidates, in particular the red ones, appear to be clustered in projection around nearby groups.

Keywords: Dwarf galaxies, Galaxy groups, H I line emission
Coincident Multimessenger Bursts from Eccentric Supermassive Binary Black Holes. Supermassive binary black holes are a key target for the future Laser Interferometer Space Antenna and excellent multimessenger sources across the electromagnetic (EM) spectrum. However, unique features of their EM emission that are needed to distinguish them from single supermassive black holes are still being established. Here, we conduct the first magnetohydrodynamic simulation of disk accretion onto equal-mass, nonspinning, eccentric binary black holes in full general relativity, incorporating synchrotron radiation transport through the dual jet in postprocessing. Focusing on a binary in the strong-field dynamical spacetime regime with eccentricity e = 0.3 as a point of principle, we show that the total accretion rate exhibits periodicity on the binary orbital period. We also show, for the first time, that this periodicity is reflected in the jet Poynting luminosity and the optically thin synchrotron emission from the jet base. Furthermore, we find a distinct EM signature for eccentric binaries: they spend more time in a low emission state (at apocenter) and less in a high state (at pericenter). Additionally, we find that the eccentric binary quasiperiodic gravitational-wave (GW) bursts are coincident with the bursts in Poynting luminosity and synchrotron emission. Finally, we discuss how multimessenger EM and GW observations of these systems can help probe plasma physics in their jet.

Keywords: Accretion, Jets, Black holes, High energy astrophysics, General relativity, Gravitational waves, Magnetohydrodynamical simulations
Unveiling a Young Thick Disk in the Milky Way. The thickness of a galaxy's disk provides a valuable probe of its formation and evolution history. Observations of the Milky Way and local galaxies have revealed an ubiquitous disk structure with two distinctive components: an old thick disk and a relatively young thin disk. The formation of this dual-disk structure and the mechanisms that develop the thickness of the disk are still unclear. Whether the disk thickness is inherited from the birth environment or is established through secular dynamical heating after formation is under debate. In this work, we identify a relatively young (∼6.6 billion yr old), geometric thick disk in the Milky Way, with a scale height of 0.64 kpc at the solar circle. This young, thick component exhibits comparable thickness and flaring strength to the canonical old thick disk but is more radially extended and systematically younger. We also identify thin disk components that formed before and after this young thick disk. Detailed analysis of the solar vicinity structure suggests that the young thick disk marks the onset of a new phase of upside-down disk formation. These findings strongly discount the role of secular dynamical heating and support a turbulent, bursty birth environment as the primary mechanism behind thick disk formation. The existence of two thick disk components suggests that the Milky Way has undergone at least two episodes of turbulent and bursty star formation, likely triggered by galaxy mergers.

Keywords: Galaxy stellar disks, Stellar populations, Galaxy structure, Chemical abundances, Galaxy evolution, Milky Way Galaxy
First JWST/NIRSpec Spectroscopy of O Stars in the Small Magellanic Cloud. Determining how much mass is removed by stellar winds is crucial to understanding massive star evolution and feedback. However, traditional spectroscopic diagnostics in the UV and optical are not sensitive enough to characterize weak stellar winds of OB stars in low-metallicity environments. A new tool to access weak stellar winds is provided by spectroscopy in the infrared (IR). Stellar atmosphere models indicate that the hydrogen Brα line at λ 4.05 μm is a useful mass-loss rate indicator, particularly at low metallicity. The unprecedented capabilities of the NIRSpec spectrograph on board the James Webb Space Telescope allow us to measure this line in spectra of massive stars in other galaxies. In this work, we present the first NIRSpec spectra of O-type stars in the Small Magellanic Cloud (SMC), which has a metallicity of only 20% solar. Our sample consists of 13 stars with spectral types ranging from O2 to O9.5, including supergiants, giants, and dwarfs. The stars belong to NGC 346, the most massive young cluster in the SMC. We describe the observing strategy and data reduction, highlighting the treatment of the nebular background emission. The spectra cover the 2.8─5.1 μm wavelength range, and we detect the Brα line in emission in each of our sample stars. Using a combination of spectral and photometric data ranging from the UV to the IR, we improve the measurements of stellar luminosity and reddening. A first qualitative comparison of the observed Brα line with stellar atmosphere models shows its potential as a wind diagnostic for weak-winded stars.

Keywords: Stellar luminosities, O stars, Infrared spectroscopy, Young massive clusters, Small Magellanic Cloud, Stellar winds, James Webb Space Telescope, Stellar atmospheres
Transfer of Entropy between the Magnetic Field and Solar Energetic Particles during an Interplanetary Coronal Mass Ejection. The thermodynamics of solar wind bulk plasma have been routinely measured and quantified, unlike those of solar energetic particles (SEPs), whose thermodynamic properties have remained elusive until recently. The thermodynamic kappa (κEP) that parameterizes the statistical distribution of SEP kinetic energy contains information regarding the population's level of correlation and effective degrees of freedom (deff). At the same time, the intermittent kappa (κΔB) that parameterizes the statistical distribution of magnetic field increments contains information about the correlation and deff involved in magnetic field fluctuations. Correlations between particles can be affected by magnetic field fluctuations, leading to a relationship between κEP and κΔB. In this Letter, we examine the relationship of deff and entropy between energetic particles and the magnetic field via the spatial variation of their corresponding parameter kappa values. We compare directly the values of κEP and κΔB using Parker Solar Probe IS⊙IS and FIELDS measurements during a SEP event associated with an interplanetary coronal mass ejection (ICME). Remarkably, we find that κEP and κΔB are anticorrelated via a linear relationship throughout the passing of the ICME, indicating a proportional exchange of deff from the magnetic field to energetic particles, i.e., κΔB ∼ (‑0.15 ± 0.03)κEP, interpreted as an effective coupling ratio. This finding is crucial for improving our understanding of ICMEs and suggests that they help to produce an environment that enables the transfer of entropy from the magnetic field to energetic particles due to changes in intermittency of the magnetic field.

Keywords: Solar energetic particles, Solar magnetic fields, Solar wind, Solar coronal mass ejections
JWST's Sharper View of EX Lup: Cold Water from Ice Sublimation during Accretion Outbursts. The unstable accretion phases during pre-main-sequence evolution of T Tauri stars produce variable irradiation and heating of planet-forming regions. A strong accretion outburst was observed with Spitzer-InfraRed Spectrograph in 2008 in EX Lup, the prototype of EXor variables, and found to increase the mid-infrared water and OH emission and decrease organic emission, suggesting large chemical changes. We present here two JWST-MIRI epochs of quiescent EX Lup in 2022 and 2023 obtained over a decade after the 2008 outburst and several months after a moderate burst in 2022. With JWST's sharper spectral view, we can now analyze water emission as a function of temperature in the two MIRI epochs and, approximately, also in the previous Spitzer epochs. This new analysis shows a strong cold water vapor "burst" in low-energy lines during the 2008 outburst, which we consider clear evidence for enhanced ice sublimation due to a recession of the snowline, as found in protostellar envelopes. JWST shows that EX Lup still has an unusually strong emission from cold water in comparison to other T Tauri disks, suggesting &gt;10 yr long freeze-out timescales in the inner disk surface. EX Lup demonstrates that outbursts can significantly change the observed organic-to-water ratios and increase the cold water reservoir, providing chemical signatures to study the recent accretion history of disks. This study provides an unprecedented demonstration of the chemical evolution triggered by accretion outbursts in the class II phase and of the high potential of time-domain experiments to reveal processes that may have fundamental implications on planet-forming bodies near the snowline.

Keywords: Molecular spectroscopy, Young stellar objects, Pre-main sequence stars, Star formation, Stellar flares, Protoplanetary disks, Circumstellar disks, Time domain astronomy, Exoplanet formation, Time series analysis
The Ejection of Transient Jets in Swift J1727.8‑1613 Revealed by Time-dependent Visibility Modeling. High angular resolution radio observations of relativistic jets are necessary to understand the causal connection between accretion and jet ejection in low-mass X-ray binaries. Images from these observations can be difficult to reconstruct due to the rapid intra-observational motion and variability of transient jets. We have developed a time-dependent visibility model fitting and self-calibration procedure and applied it to a single 4 hr VLBA observation of the low-mass X-ray binary Swift J1727.8‑1613 during the bright flaring period of its 2023 outburst. This allowed us to detect and model a slightly resolved self-absorbed compact core, as well as three downstream transient jet knots. We were able to precisely measure the proper motion and flux density variability of these three jet knots, as well as (for the first time) their intra-observational expansion. Using simultaneous multifrequency data, we were also able to measure the spectral index of the furthest downstream jet knot, and the core, as well as the frequency-dependent core shift between 2.3 and 8.3 GHz. Using these measurements, we inferred the ejection dates of the three jet knots, including one to within ±40 minutes, which is one of the most precise ever measured. The ejection of the transient jet knots coincided with a bright X-ray flare and a drastic change in the X-ray spectral and timing properties as seen by HXMT, which is the clearest association ever seen between the launching of transient relativistic jets in an X-ray binary and a sudden change in the X-ray properties of the accretion inflow.

Keywords: Radio jets, Very long baseline interferometry, Relativistic jets, Stellar mass black holes, Transient sources, Low-mass x-ray binary stars
"CY1" Chondrites Produced by Impact Dehydration of the CI Chondrite Parent Body. The recently proposed Yamato-type (CY) chondrites share significant similarities with CI chondrites and Ryugu. We present major and trace elemental, Re–Os, and mass-independent Ti, Cr, and Fe isotope data for seven CY chondrites. The elemental data along with isotopic compositions reveal two distinct lithologies, here designated as CY1 and CY2, potentially originating from two different parent bodies. Although sharing similarities with CM chondrites, CY2 chondrites have distinct Cr isotope compositions, arguing against a close genetic relationship. The CY1 lithology exhibits elemental abundances similar to CI chondrites/Ryugu as well as Fe, Ti, and Cr isotope compositions that closely overlap with those of CI chondrites/Ryugu. This suggests that CI chondrites, CY1 chondrites, and Ryugu accreted in the same region of the solar system and may even originate from the same parent body. In fact, we find that the reduced water content and certain volatile element abundances alongside increased sulfide content and mass-dependent O isotope enrichments observed in CY1 compared to CI chondrites could be attributed to an impact-induced heating event on the CI parent body. This impact likely disrupted the CI parent body, resulting in the ejection of both CI and CY1 lithologies. Furthermore, given that there are presently only five known CI meteorite specimens, the close chemical composition between CY1 and CI chondrites substantially expands the data set for comparisons and referrals to the bulk solar system composition for nonvolatile elements. Finally, we propose that the "CY1" chondrites could be called "CI1T," while the designation "CY" chondrites could be restricted to "CY2" samples.

Keywords: Impact phenomena, Solar system formation, Cosmochemistry, Asteroids, Nucleosynthesis, Carbonaceous chondrites, Chemical abundances, Isotope shifts
Little Red Dots Are Tidal Disruption Events in Runaway-collapsing Clusters. I hypothesize a physical explanation for the "little red dots" (LRDs) discovered by the James Webb Space Telescope (JWST). The first star formation in the Universe occurs in dense clusters, some of which may undergo runaway collapse and form an intermediate mass black hole. This process would appear as a very dense stellar system, with recurring tidal disruption events (TDEs) as stellar material is accreted by the black hole. Such a system would be compact, UV-emitting, and exhibit broad Hα emission. If runaway collapse is the primary mechanism for forming massive black hole seeds, this process could be fairly common and explain the large volume densities of LRDs. In order to match the predicted number density of runaway collapse clusters, the tidal disruption rate must be on the order of 10‑4 per year. A top-heavy stellar initial mass function may be required to match observations without exceeding the predicted ΛCDM mass function. The TDE LRD hypothesis can be verified with follow-up JWST observations looking for TDE-like variability.

Keywords: Intermediate-mass black holes, Galaxy formation, Tidal disruption, Active galactic nuclei, High-redshift galaxies
Star-crossed Clusters: Asteroseismic Ages for Individual Stars Are in Tension with the Ages of Their Host Clusters. A meta-analysis of seismic ages determined for individual stars in the well-studied open and globular clusters NGC 6819, NGC 6791, M67, M4, M19, M80, and M9 reveals both high variance across measurements and a possible discrepancy with independent, isochrone-based age determinations for the clusters in which these stars reside. The scatter among asteroseismic ages for individual stars in any one of these clusters far surpasses both the absolute age uncertainty computed for reference cluster M92 (5.4%) and the model-to-model systematic uncertainties in isochrones (roughly 10%). This suggests that either binary processes are significantly altering the masses of stars in these clusters, or some additional corrections, perhaps as a function of mass, metallicity, or surface gravity, are required to bring the asteroseismic age scale into concordance with ages inferred from isochrone or similar model fitting.

Keywords: Globular star clusters, Open star clusters, Asteroseismology
The Ophiuchus DIsk Survey Employing ALMA (ODISEA): A Unified Evolutionary Sequence of Planet-driven Substructures Explaining the Diversity of Disk Morphologies. Understanding the origin of substructures in protoplanetary disks and their connection to planet formation is currently one of the main challenges in astrophysics. While some disks appear smooth, most exhibit diverse substructures such as gaps, rings, or inner cavities, with varying brightness and depth. As part of the Ophiuchus DIsk Survey Employing ALMA, we previously proposed an evolutionary sequence to unify this diversity, driven by the formation of giant planets through core accretion and subsequent planet–disk interactions. By combining the disk evolution and planet formation code PLANETALP with the radiative transfer code RADMC-3D, we have now reproduced the key aspects of the proposed evolutionary sequence. Starting with a smooth disk (like, e.g., WLY 2-63), we modeled the evolution of a fiducial disk with a 1 MJup planet at 57 au. Within a few hundred orbits, a narrow gap forms, resembling ISO-Oph 17. By ∼0.1 Myr, the gap widens, and dust accumulates at the cavity edge, producing a structure similar to Elias 2-24. At ∼0.4 Myr, the disk evolves further into a morphology akin to DoAr 44, characterized by a smaller inner disk and a brighter inner rim. By ∼1 Myr, the system transitions to a single narrow ring resembling RXJ1633.9–2442. This line of work strongly supports the planetary origin of substructures and enables the possibility of identifying a population of planets that is currently beyond the reach of more direct detection techniques.

Keywords: Planetary system evolution, Pre-main sequence stars, Protoplanetary disks, Exoplanets
ALMASOP: Detection of Turbulence-induced Mass Assembly Shocks in Starless Cores. Star formation is a series of mass assembly processes and starless cores; those cold and dense condensations in molecular clouds play a pivotal role as initial seeds of stars. With only a limited sample of known starless cores, however, the origin and growth of such stellar precursors had not been well characterized previously. Meanwhile, the recent discovery of CH3OH emission, which is generally associated with the desorbed icy mantle in warm regions, particularly at the periphery of starless cores, also remains puzzling. We present sensitive Atacama Large Millimeter/submillimeter Array (Band 3) observations (at 3 mm) toward a sample of newly identified starless cores in the Orion molecular cloud. The spatially resolved images distinctly indicate that the observed CH3OH and N2H+ emission associated with these cores are morphologically anticorrelated and kinematically offset from each other. We postulate that the CH3OH emission highlights the desorption of icy mantle by shocks resulting from gas piling onto dense cores in the filaments traced by N2H+. Our magnetohydrodynamic simulations of star formation in turbulent clouds combined with radiative transfer calculations and imaging simulations successfully reproduced the observed signatures and reaffirmed the above scenario at work. Our result serves as an intriguing and exemplary illustration, a snapshot in time, of the dynamic star-forming processes in turbulent clouds. The results offer compelling insights into the mechanisms governing the growth of starless cores and the presence of gas-phase complex organic molecules associated with these cores.

Keywords: Magnetohydrodynamics, Star formation, Low mass stars, Shocks, Radio astronomy, Complex organic molecules