adsabs/SciX_UAT_keywords huggingface dataset:
Observations and Modeling of the Onset of Fast Reconnection in the Solar Transition Region

Magnetic reconnection is a fundamental plasma process that plays a critical role not only in energy release in the solar atmosphere, but also in fusion, astrophysics, and other space plasma environments. One of the challenges in explaining solar observations in which reconnection is thought to play a critical role is to account for the transition of the dynamics from a slow quasi-continuous phase to a fast and impulsive energetic burst of much shorter duration. Despite the theoretical progress in identifying mechanisms that might lead to rapid onset, a lack of observations of this transition has left models poorly constrained. High-resolution spectroscopic observations from NASA's Interface Region Imaging Spectrograph now reveal tell-tale signatures of the abrupt transition of reconnection from a slow phase to a fast, impulsive phase during UV bursts or explosive events in the Sun's atmosphere. Our observations are consistent with numerical simulations of the plasmoid instability, and provide evidence for the onset of fast reconnection mediated by plasmoids and new opportunities for remote-sensing diagnostics of reconnection mechanisms on the Sun.

Keywords: solar transition region; solar chromosphere; solar magnetic reconnection; solar magnetic fields
Testing Models of Triggered Star Formation with Young Stellar Objects in Cepheus OB4

OB associations are home to newly formed massive stars, whose turbulent winds and ionizing flux create H II regions rich with star formation. Studying the distribution and abundance of young stellar objects (YSOs) in these ionized bubbles can provide essential insight into the physical processes that shape their formation, allowing us to test competing models of star formation. In this work, we examined one such OB association, Cepheus OB4 (Cep OB4)-a well-suited region for studying YSOs due to its Galactic location, proximity, and geometry. We created a photometric catalog from Spitzer/Infrared Array Camera (IRAC) mosaics in bands 1 (3.6 μm) and 2 (4.5 μm). We supplemented the catalog with photometry from the Wide-field Infrared Survey Explorer, the Two Micron All Sky Survey, IRAC bands 3 (5.8 μm) and 4 (8.0 μm), MIPS 24 μm, and MMIRS near-infrared data. We used color-color selections to identify 821 YSOs, which we classified using the IR slope of the YSOs' spectral energy distributions, finding 67 Class I, 103 flat spectrum, 569 Class II, and 82 Class III YSOs. We conducted a clustering analysis of the Cep OB4 YSOs and fit their spectral energy distributions. We found many young Class I objects distributed in the surrounding shell and pillars as well as a relative age gradient of unclustered sourcesin the bubble surrounding the OB association, with YSOs generally decreasing in age with distance from the central cluster. Both of these results indicate that the expansion of the H II region may have triggered star formation in CepOB4.

Keywords: young stellar objects; star formation; star forming regions; infrared excess; protoplanetary disks
The Hazy and Metal-rich Atmosphere of GJ 1214 b Constrained by Near- and Mid-infrared Transmission Spectroscopy

The near-infrared transmission spectrum of the warm sub-Neptune exoplanet GJ 1214 b has been observed to be flat and featureless, implying a high metallicity atmosphere with abundant aerosols. Recent JWST MIRI Low Resolution Spectrometer observations of a phase curve of GJ 1214 b showed that its transmission spectrum is flat out into the mid-infrared. In this paper, we use the combined near- and mid-infrared transmission spectrum of GJ 1214 b to constrain its atmospheric composition and aerosol properties. We generate a grid of photochemical haze models using an aerosol microphysics code for a number of background atmospheres spanning metallicities from 100 to 1000× solar, as well as a steam atmosphere scenario. The flatness of the combined data set largely rules out atmospheric metallicities ≤300× solar due to their large corresponding molecular feature amplitudes, preferring values ≥1000× solar and column haze production rates ≥10-10 g cm-2 s-1. The steam atmosphere scenario with similarly high haze production rates also exhibits sufficiently small molecular features to be consistent with the transmission spectrum. These compositions imply that atmospheric mean molecular weights ≥15 g mol-1 are needed to fit the data. Our results suggest that haze production is highly efficient on GJ 1214 b and could involve non-hydrocarbon, non-nitrogen haze precursors. Further characterization of GJ 1214 b's atmosphere would likely require multiple transits and eclipses using JWST across the near- and mid-infrared, potentially complemented by ground-based high-resolution transmission spectroscopy.

Keywords: exoplanet atmospheres
Sun-as-a-star Spectral Irradiance Observations of Transiting Active Regions

Major solar flares are prone to occur in active-region (AR) atmospheres associated with large, complex, dynamically evolving sunspots. This points to the importance of monitoring the evolution of starspots, not only in visible but also in ultraviolet (UV) and X-rays, in understanding the origin and occurrence of stellar flares. To this end, we perform spectral irradiance analysis on different types of transiting solar ARs by using a variety of full-disk synoptic observations. The target events are an isolated sunspot, spotless plage, and emerging flux in prolonged quiet-Sun conditions selected from the past decade. We find that the visible continuum and total solar irradiance become darkened when the spot is at the central meridian, whereas it is bright near the solar limb; UV bands sensitive to the chromosphere correlate well with the variation of total unsigned magnetic flux in the photosphere; amplitudes of extreme ultraviolet (EUV) and soft X-ray increase with the characteristic temperature, whose light curves are flat-topped due to their sensitivity to the optically thin corona; the transiting spotless plage does not show the darkening in the visible irradiance, while the emerging flux produces an asymmetry in all light curves about the central meridian. The multiwavelength Sun-as-a-star study described here indicates that the time lags between the coronal and photospheric light curves have the potential to probe the extent of coronal magnetic fields above the starspots. In addition, EUV wavelengths that are sensitive to temperatures just below 1 MK sometimes show antiphased variations, which may be used for diagnosing plasmas around starspots.

Keywords: solar spectral irradiance; sunspots; solar active regions; solar flares; solar analogs; starspots; stellar flares; time domain astronomy
HaloSat Observations of Heliospheric Solar Wind Charge Exchange

X-ray emission from solar wind charge exchange (SWCX) produced in interplanetary space contaminates every astrophysical observation, regardless of the line of sight. Unfortunately, the primary SWCX emission lines also happen to be important diagnostics of astrophysical plasmas. Models of SWCX emission are limited by two main uncertainties: the local solar wind fluxes along the line of sight and the charge exchange cross sections. The He cone, a localized density enhancement of helium neutrals, is the only heliospheric SWCX emission feature that is small enough and bright enough to be observationally isolated from the X-ray background and the broader SWCX emission. HaloSat, an X-ray CubeSat mission, has recently completed two series of specialized observations, near and far from the ecliptic plane, during two Earth transits of the He cone. These observations were used to test the predictions of an SWCX emission model against the emission observed at low ecliptic latitude, where the solar wind data are monitored, and at high ecliptic latitude, where the solar wind data are extrapolated. The measured SWCX emission for the set of observations near the ecliptic plane was consistent with the line intensities predicted by the model but underpredicted for the set of observations at high ecliptic latitude near the south ecliptic pole. Additionally, high-temperature Galactic halo emission components are reported for both spectral sets.

Keywords: diffuse x-ray background; solar wind; charge exchange ionization; milky way dark matter halo
Characterizing 3D Magnetic Structures in Sunspot Light Bridges

Light bridges (LBs) are narrow structures dividing sunspot umbra, and their role in active region evolution is yet to be explored. We investigated the magnetic structure of the two LBs: a narrow LB (with width ~810 km) and a considerably wider LB (2475 km) in the active region NOAA 12371. We employed: (1) the high-spatial-resolution spectropolarimetric data obtained by the Near InfraRed Imaging Spectropolarimeter (NIRIS) of the 1.6 m Goode Solar Telescope (GST) for studying the magnetic structure at the photosphere, and (2) the nonlinear force-free field (NLFFF) models, extrapolated from both the photospheric magnetogram from GST/NIRIS and from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, for studying the three-dimensional (3D) magnetic structure on a larger scale. Our observations reveal the presence of a field-free (or, more precisely, weak-field) region and the different velocity structures inside the two LBs. Analysis of the 3D NLFFF model shows a low-lying magnetic canopy as well as the enhanced current system above the LBs. The substantial difference between the LBs and the umbrae is found in the overall magnetic topology in that the field lines emanating from the two LBs are more twisted than that from the neighboring umbrae.

Keywords: sunspots; solar active region magnetic fields
Using Statistical Emulation and Knowledge of Grain-surface Diffusion for Bayesian Inference of Reaction Rate Parameters: An Application to a Glycine Network

There exists much uncertainty surrounding interstellar grain-surface chemistry. One of the major reaction mechanisms is grain-surface diffusion for which the binding energy parameter for each species needs to be known. However, these values vary significantly across the literature which can lead to debate as to whether or not a particular reaction takes place via diffusion. In this work we employ Bayesian inference to use available ice abundances to estimate the reaction rates of the reactions in a chemical network that produces glycine. Using this we estimate the binding energy of a variety of important species in the network, by assuming that the reactions take place via diffusion. We use our understanding of the diffusion mechanism to reduce the dimensionality of the inference problem from 49 to 14, by demonstrating that reactions can be separated into classes. This dimensionality reduction makes the problem computationally feasible. A neural network statistical emulator is used to also help accelerate the Bayesian inference process substantially. The binding energies of most of the diffusive species of interest are found to match some of the disparate literature values, with the exceptions of atomic and diatomic hydrogen. The discrepancies between these two species are related to the limitations of the physical and chemical models. However, the use of a dummy reaction of the form H + X $\longrightarrow $ HX is found to somewhat reduce the discrepancy with the binding energy of atomic hydrogen. Using the inferred binding energies in the full gas-grain version of UCLCHEM results in almost all the molecular abundances being recovered.

Keywords: astrostatistics strategies; bayesian statistics; reaction rates; astrochemistry; dark interstellar clouds; interstellar abundances
Revision of the GeV γ-Ray Emission in the Region of HESS J1813-178 with Fermi-LAT

HESS J1813-178 is one of the brightest and most compact TeV γ-ray sources, and whether its γ-ray emission is associated with supernova remnant (SNR), pulsar wind nebula (PWN), or young stellar cluster (YSC) is still under debate. By analyzing the GeV γ-ray data in the field of HESS J1813-178 using 14 yr of PASS 8 data recorded by the Fermi Large Area Telescope (Fermi-LAT), we report the discovery of three different sources with different spectra in this region. The hard source with a PL spectral index of 2.11 ± 0.08 has a small size extension, which is spatially and spectrally coincident with the TeV γ-ray emission from HESS J1813-178. CO observations display the dense molecular clouds surrounding HESS J1813-178 in the velocity range of 45–60 km s‑1. The possible origins of the γ-ray emission from HESS J1813-178 are discussed, including SNR G12.820.02, the PWN driven by the energetic X-ray pulsar PSR J1813-1749 and YSC Cl 1813-178. However, none of them can be ruled out clearly. Note that the maximum energy of protons in the hadronic model should exceed a few hundred TeV, which makes HESS J1813-178 a promising PeVatron. A detailed LHAASO data analysis about the morphology and spectrum would be helpful to investigate the origin of the γ-ray emission in this region and test its PeVatron nature.

Keywords: gamma-ray sources; supernova remnants; pulsar wind nebulae; star clusters; molecular clouds
NuSTAR Perspective on High-redshift MeV Blazars

With bolometric luminosities exceeding 1048 erg s-1, powerful jets, and supermassive black holes at their center, MeV blazars are some of the most extreme sources in the universe. Recently, the Fermi-Large Area Telescope detected five new γ-ray emitting MeV blazars beyond redshift z = 3.1. With the goal of precisely characterizing the jet properties of these extreme sources, we started a multiwavelength campaign to follow them up with joint Nuclear Spectroscopic Telescope Array, Swift, and the Southeastern Association for Research in Astronomy's optical telescopes. We observe six high-redshift quasars, four of them belonging to the new γ-ray emitting MeV blazars. Thorough X-ray analysis reveals spectral flattening at soft X-ray for three of these objects. The source NVSS J151002+570243 also shows a peculiar rehardening of the X-ray spectrum at energies E > 6 keV. Adopting a one-zone leptonic emission model, this combination of hard X-rays and γ-rays enables us to determine the location of the Inverse Compton peak and to accurately constrain the jet characteristics. In the context of the jet-accretion disk connection, we find that all six sources have jet powers exceeding accretion disk luminosity, seemingly validating this positive correlation even beyond z > 3. Our six sources are found to have ${10}^{9}\,{M}_{\odot }$ black holes, further raising the space density of supermassive black holes in the redshift bin z = [3, 4].

Keywords: active galactic nuclei; blazars; high energy astrophysics; jets
Three Years of ACB Phase Function Observations from the Mars Science Laboratory: Interannual and Diurnal Variability and Constraints on Ice Crystal Habit

We examine 3 yr of phase-function observations of water-ice clouds taken during the Aphelion Cloud Belt season by the Mars Science Laboratory (MSL). We derive lower-bound single-scattering phase functions for Mars years (MYs) 34, 35, and 36, over a range of scattering angles from 45° to 155°, expanding on the MY 34 phase function previously derived from MSL observations using the same method. We also modify the procedure used for MY 34 to make use of cloud opacity values derived from other MSL observations, often taken in conjunction with the phase-function observations. From these, we see little variability, both interannually and diurnally in the phase function at Gale Crater. We use our derived phase functions to attempt to constrain a dominant ice-crystal geometry by fitting a two-term Henyey–Greenstein function. In comparing to HG functions of Martian dust and modeled water-ice crystals, we see agreement especially with droxtal water-ice crystals, dust at Gale crater, and irregular volcanic glasses. This could be indicative of crystals composed of some irregular shape.

Keywords: mars; water vapor; planetary atmospheres; atmospheric clouds
Exploring Whether Super-puffs can be Explained as Ringed Exoplanets

An intriguing, growing class of planets are the "super-puffs," objects with exceptionally large radii for their masses and thus correspondingly low densities (≲0.3 g cm-3). Here we consider whether they could have large inferred radii because they are in fact ringed. This would naturally explain why super-puffs have thus far only shown featureless transit spectra. We find that this hypothesis can work in some cases but not all. The close proximity of the super-puffs to their parent stars necessitates rings with a rocky rather than icy composition. This limits the radius of the rings, and makes it challenging to explain the large size of Kepler 51b, 51c, 51d, and 79d unless the rings are composed of porous material. Furthermore, the short tidal locking timescales for Kepler 18d, 223d, and 223e mean that these planets may be spinning too slowly, resulting in a small oblateness and rings that are warped by their parent star. Kepler 87c and 177c have the best chance of being explained by rings. Using transit simulations, we show that testing this hypothesis requires photometry with a precision of somewhere between ∼10 ppm and ∼50 ppm, which roughly scales with the ratio of the planet and star's radii. We conclude with a note about the recently discovered super-puff HIP 41378f.

Keywords: occultation; exoplanets; exoplanet rings; exoplanet detection methods
Broad-velocity-width Molecular Features in the Galactic Plane

We performed a systematic search for broad-velocity-width molecular features (BVFs) in the disk part of our Galaxy by using the CO J = 1-0 survey data obtained with the Nobeyama Radio Observatory 45 m telescope. From this search, 58 BVFs were identified. In comparisons with the infrared and radio continuum images, 36 BVFs appeared to have both infrared and radio continuum counterparts, and 15 of them are described as molecular outflows from young stellar objects in the literature. In addition, 21 BVFs have infrared counterparts only, and eight of them are described as molecular outflows in the literature. One BVF (CO 16.134-0.553) does not have any luminous counterpart in the other wavelengths, which suggests that it may be an analog of high-velocity compact clouds in the Galactic center.

Keywords: milky way disk; interstellar clouds; interstellar molecules
Analytical Solution of the Tidal Evolution of Eccentricity and Semimajor Axis for Close-in Planets

Tides can have significant effects on the orbital evolution of close-in planets. The evolution of eccentricity and semimajor axis is usually obtained by numerical integration of a set of coupled differential equations. We present here closed-form solutions to the evolution equations and derive from them an expression of the circularization time.

Keywords: exoplanet dynamics; exoplanet tides
SupSpec: A Python Package for Measuring the Doppler Shifts of Supernova Spectral Lines

Here we present an open-source Python package, SupSpec, which aids in finding the velocity structure of spectral features throughout supernova (SN) ejecta. This code is designed to require minimal user interaction and can ingest an arbitrarily large number of spectra. By fitting a Gaussian distribution to the spectral sequence of a single SN over time, it finds the "velocity structure"—the evolution of the velocity of a specific absorption feature over time. SupSpec is available on GitHub and on Zenodo.

Keywords: markov chain monte carlo; spectroscopy; time domain astronomy
HCN J = 4-3, HNC J = 1-0, H13CN J = 1-0, and HC3N J = 10-9 Maps of Galactic Center Region. II. Physical Properties of Dense-gas Clumps and Probability of Star Formation

We report a statistical analysis exploring the origin of the overall low star formation efficiency (SFE) of the Galactic central molecular zone (CMZ) and the SFE diversity among the CMZ clouds using a wide-field HCN J = 4-3 map, whose optically thin critical density ( $\sim {10}^{7}\,{\mathrm{cm}}^{-3}$ ) is the highest among the tracers ever used in CMZ surveys. Logistic regression is performed to empirically formulate star formation probability of 195 HCN clumps, 13 of which contain star formation signatures. The explanatory parameters in the best-fit model are reduced into the virial parameter ${\alpha }_{\mathrm{vir}}$ without significant contribution from other parameters, whereas the performance of the model without ${\alpha }_{\mathrm{vir}}$ is no better than that using randomly generated data. The threshold ${\alpha }_{\mathrm{vir}}$ is 6, which translates into a volume density ( ${n}_{{{\rm{H}}}_{2}}$ ) of ${10}^{4.6}\,{\mathrm{cm}}^{-3}$ with the ${n}_{{{\rm{H}}}_{2}}$ - ${\alpha }_{\mathrm{vir}}$ correlation. The scarcity of the low- ${\alpha }_{\mathrm{vir}}$ clumps, whose fraction to all HCN clumps is 0.1, can be considered as one of the immediate causes of the suppressed SFE. No correlation between the clump size or mass and star formation probability is found, implying that HCN J = 4-3 does not immediately trace the mass of star-forming gas above a threshold density. Meanwhile, star-forming and non-star-forming clouds are degenerate in the physical parameters of the CS $J$ = 1-0 clouds, highlighting the efficacy of the HCN $J$ = 4-3 line to probe star-forming regions in the CMZ. The timescale of the high- ${\alpha }_{\mathrm{vir}}$ to low- ${\alpha }_{\mathrm{vir}}$ transition is $\lesssim 2\,\mathrm{Myr}$ , which is consistent with the tidal compression and X1/X2 orbit transition models but possibly does not fit the cloud-cloud collision picture.

Keywords: star formation; galactic center; molecular clouds
Large-amplitude, Wideband, Doppler-shifted, Ion Acoustic Waves Observed on the Parker Solar Probe

Electric field spectra measured on the Parker Solar Probe typically contain upwards of 1000 large-amplitude (∼15 mV m-1), wideband (∼100-15,000 Hz), few-second-duration, electric field waveforms per day. The satellite also collected about 85 three-second bursts of electric field waveforms per day at a data rate of ∼150,000 samples per second. Eight such bursts caught these waves, all of which were located in switchbacks of the magnetic field. A wave burst on 2019 September 7, when the spacecraft was at an altitude of 55 solar radii, is described. It contained Doppler-shifted ion acoustic waves that propagated in the direction opposite to the local magnetic field at all rest-frame frequencies from 60 Hz to nearly the proton plasma frequency of 2200 Hz, while no other wave modes were present. The eight bursts all contained ion acoustic waves whose individual net potentials were ≤1 V. A second burst, analyzed in conjunction with ion plasma measurements, showed that the ion acoustic waves were associated with broadened, plateaued, ion spectra containing unheated ions. Because the ion acoustic waves had phase velocities that varied from 140 to 90 km s-1, ions interacting with these waves via the Landau resonance had a chance to diffuse in parallel velocity space from 90 to 140 km s-1 in the plasma rest frame. The most likely generation mechanism of the ion acoustic waves is the ion-ion acoustic instability, while the electron-ion instability is a less likely candidate.

Keywords: solar wind