adsabs/SciX_UAT_keywords huggingface dataset:
Coronal and Chromospheric Emission in A-type Stars

Cool stars on the main sequence generate X-rays from coronal activity, powered by a convective dynamo. With increasing temperature, the convective envelope becomes smaller and X-ray emission fainter. We present Chandra/HRC-I observations of four single stars with early A spectral types. Only the coolest star of this sample, τ 3 Eri (T eff ≈ 8, 000 K), is detected with $\mathrm{log}({L}_{X}/{L}_{\mathrm{bol}})=-7.6$ while the three hotter stars (T eff ≥ 8, 000 K), namely δ Leo, β Leo, and ι Cen, remain undetected with upper limits $\mathrm{log}({L}_{X}/{L}_{\mathrm{bol}})\lt -8.4$ . The drop in X-ray emission thus occurs in a narrow range of effective temperatures around ~8100 K and matches the drop of activity in the C III and O VI transition region lines.

Keywords: a stars; stellar atmospheres; stellar activity
Highly Energetic Electrons Accelerated in Strong Solar Flares as a Preferred Driver of Sunquakes

Sunquakes are enhanced seismic waves excited in some energetic solar flares. Up to now, their origin has still been controversial. In this Letter, we select and study 20 strong flares in Solar Cycle 24, whose impulse phase is fully captured by the Reuven Ramaty High Energy Solar Spectroscopic Imager. For 11 out of 12 sunquake-active flares in our sample, the hard X-ray emission shows a good temporal and spatial correlation with the white-light enhancement and the sunquake. Spectral analysis also reveals a harder photon spectrum that extends to several hundred keV, implying a considerable population of flare-accelerated nonthermal electrons at high energies. Quantitatively, the total energy of electrons above 300 keV in sunquake-active flares is systematically different from that in sunquake-quiet flares, while the difference is marginal for electrons above 50 keV. All these facts support highly energetic electrons as a preferred driver of the sunquakes. Such an electron-driven scenario can be reasonably accommodated in the framework of a recently proposed selection rule for sunquake generation. For the remaining one event, the sunquake epicenter is cospatial with a magnetic imprint, i.e., a permanent change of magnetic field on the photosphere. Quantitative calculation shows that the flare-induced downward Lorentz force can do enough work to power the sunquake, acting as a viable sunquake driver for this specific event.

Keywords: solar flares; solar flare spectra; solar particle emission; helioseismology; solar x-ray flares; solar white-light flares
Cloud Dissipation and Disk Wind in the Late Phase of Star Formation

We perform a long-term simulation of star and disk formation using three-dimensional nonideal magnetohydrodynamics. The simulation starts from a prestellar cloud and proceeds through the long-term evolution of the circumstellar disk until ∼1.5 × 105 yr after protostar formation. The disk has size ≲50 au and little substructure in the main accretion phase because of the action of magnetic braking and the magnetically driven outflow to remove angular momentum. The main accretion phase ends when the outflow breaks out of the cloud, causing the envelope mass to decrease rapidly. The outflow subsequently weakens as the mass accretion rate also weakens. While the envelope-to-disk accretion continues, the disk grows gradually and develops transient spiral structures, due to gravitational instability. When the envelope-to-disk accretion ends, the disk becomes stable and reaches a size ≳300 au. In addition, about 30% of the initial cloud mass has been ejected by the outflow. A significant finding of this work is that after the envelope dissipates a revitalization of the wind occurs, and there is mass ejection from the disk surface that lasts until the end of the simulation. This mass ejection (or disk wind) is generated because the magnetic pressure significantly dominates both the ram pressure and thermal pressure above and below the disk at this stage. Using the angular momentum flux and mass-loss rate estimated from the disk wind, the disk dissipation timescale is estimated to be ∼106 yr.

Keywords: star formation; stellar jets; magnetohydrodynamical simulations; protostars; protoplanetary disks; circumstellar disks
Collective Neutrino Oscillations and Heavy-element Nucleosynthesis in Supernovae: Exploring Potential Effects of Many-body Neutrino Correlations

In high-energy astrophysical processes involving compact objects, such as core-collapse supernovae or binary neutron star mergers, neutrinos play an important role in the synthesis of nuclides. Neutrinos in these environments can experience collective flavor oscillations driven by neutrino–neutrino interactions, including coherent forward scattering and incoherent (collisional) effects. Recently, there has been interest in exploring potential novel behaviors in collective oscillations of neutrinos by going beyond the one-particle effective or "mean-field" treatments. Here, we seek to explore implications of collective neutrino oscillations, in the mean-field treatment and beyond, for the nucleosynthesis yields in supernova environments with different astrophysical conditions and neutrino inputs. We find that collective oscillations can impact the operation of the ν p-process and r-process nucleosynthesis in supernovae. The potential impact is particularly strong in high-entropy, proton-rich conditions, where we find that neutrino interactions can nudge an initial ν p-process neutron-rich, resulting in a unique combination of proton-rich low-mass nuclei as well as neutron-rich high-mass nuclei. We describe this neutrino-induced neutron-capture process as the "ν i-process." In addition, nontrivial quantum correlations among neutrinos, if present significantly, could lead to different nuclide yields compared to the corresponding mean-field oscillation treatments, by virtue of modifying the evolution of the relevant one-body neutrino observables.

Keywords: core-collapse supernovae; supernova neutrinos; neutrino oscillations; nucleosynthesis; r-process; p-process
The Intrinsic Shapes of Low Surface Brightness Galaxies (LSBGs): A Discriminant of LSBG Galaxy Formation Mechanisms

We use the low surface brightness galaxy (LSBG) samples created from the Hyper Suprime-Cam Subaru Strategic Program (781 galaxies), the Dark Energy Survey (20977 galaxies), and the Legacy Survey (selected via H I detection in the Arecibo Legacy Fast ALFA Survey, 188 galaxies) to infer the intrinsic shape distribution of the LSBG population. To take into account the effect of the surface brightness cuts employed when constructing LSBG samples, we simultaneously model both the projected ellipticity and the apparent surface brightness in our shape inference. We find that the LSBG samples are well characterized by oblate spheroids, with no significant difference between red and blue LSBGs. This inferred shape distribution is in good agreement with similar inferences made for ultra-diffuse cluster galaxy samples, indicating that environment does not play a key role in determining the intrinsic shape of LSBGs. We also find some evidence that LSBGs are more thickened than similarly massive high surface brightness dwarfs. We compare our results to intrinsic shape measures from contemporary cosmological simulations, and find that the observed LSBG intrinsic shapes place considerable constraints on the formation path of such galaxies. In particular, LSBG production via the migration of star formation to large radii produces intrinsic shapes in good agreement with our observational findings.

Keywords: low surface brightness galaxies; dwarf galaxies; observational astronomy; astronomical methods; galaxy structure
A Spatially Dependent Correction of Gaia EDR3 Parallax Zero-point Offset based on 0.3 million LAMOST DR8 Giant Stars

We have studied the zero-point offset of Gaia Early Data Release 3 (EDR3) parallaxes based on a sample of 0.3 million giant stars built from the LAMOST data with distance accuracy better than 8.5%. The official parallax zero-point corrections largely reduce the global offset in the Gaia EDR3 parallaxes: the global parallax offsets are -27.9 μas and -26.5 μas (before correction) and +2.6 μas and +2.9 μas (after correction) for the five- and six-parameter solutions, respectively. The bias of the raw parallax measurements is significantly dependent on the G magnitudes, spectral colors, and positions of stars. The official parallax zero-point corrections could reduce parallax bias patterns with G magnitudes, while could not fully account the patterns in the spaces of the spectral colors and positions. In the current paper, a spatially dependent parallax zero-point correction model for Gaia EDR3 five-parameter solution in the LAMOST footprint is first provided with the advantage of huge number of stars in our sample.

Keywords: stellar parallax; stellar distance; parallax
Seeing the Bigger Picture: Rosetta Mission Amateur Observing Campaign and Lessons for the Future

Amateur astronomers can make useful contributions to the study of comets. They add temporal coverage and multiscale observations that can aid the study of fast-changing and large-scale comet features. We document and review the amateur observing campaign set up to complement the Rosetta space mission, including the data submitted to date, and consider the campaign's effectiveness in the light of experience from previous amateur comet campaigns. We report the results of surveys of campaign participants, the amateur astronomy community, and schools who participated in a comet 46P observing campaign. We draw lessons for future campaigns, which include the need for clarity of objectives; recognizing the wider impact campaigns can have on increasing science capital; clear, consistent, timely, and tailored guidance; easy upload procedures with built-in quality control; and regular communication, feedback, and recognition.

Keywords: comets; short period comets; amateur astronomy; amateur astronomers; astrophotography; remote telescope astrophotography
KIC 12602250: A Low-amplitude Double-mode Delta Scuti Star with Amplitude Modulation

We report a detailed light-curve analysis of the Kepler target Kepler Input Catalog (KIC) 12602250. The results show that KIC 12602250 is a low-amplitude radial double-mode δ-Scuti star with amplitude modulation. The Fourier analysis of the long cadence data (i.e., Q0-Q17, spanning 1471 days) reveals that the variations of the light curve are dominated by the strongest mode with frequency F0 = 11.6141 d-1, suggesting that KIC 12602250 is a δ-Scuti star. The other independent mode F1 = 14.9741 d-1 is newly detected. The amplitude of the light variations of KIC 12602250 is ~0.06 mag, which indicates that this is a low-amplitude δ-Scuti star; but the ratio of F0/F1 is estimated as 0.7756, which is typical of HADS, and a slow amplitude growth is detected in F1 and f3, which could be due to stellar evolution, suggesting that KIC 12602250 could be a post-main-sequence δ Scuti that is crossing the instability strip for the first time.

Keywords: delta scuti variable stars; multi-periodic variable stars; stellar evolution
Unveiling the Physical Conditions in NGC 6910

Deep and wide-field optical photometric observations along with multiwavelength archival data sets have been employed to study the physical properties of the cluster NGC 6910. The study also examines the impact of massive stars on their environment. The age, distance, and reddening of the cluster are estimated to be ∼4.5 Myr, 1.72 ± 0.08 kpc, and $E{(B-V)}_{\min }=0.95$ mag, respectively. The mass function slope (Γ = -0.74 ± 0.15) in the cluster region is found to be flatter than the Salpeter value (-1.35), indicating the presence of an excess number of massive stars. The cluster also shows mass segregation toward the central region due to their formation processes. The distribution of warm dust emission is investigated toward the central region of the cluster, showing the signature of the impact of massive stars within the cluster region. Radio continuum clumps powered by massive B-type stars (age range ∼0.07-0.12 Myr) are traced that are located away from the center of the stellar cluster NGC 6910 (age ∼4.5 Myr). Based on the values of different pressure components exerted by massive stars, the photoionized gas associated with the cluster is found to be the dominant feedback mechanism in the cluster. Overall, the massive stars in the cluster might have triggered the birth of young, massive B-type stars in the cluster. This argument is supported with evidence of the observed age gradient between the cluster and the powering sources of the radio clumps.

Keywords: open star clusters; star formation; initial mass function; luminosity function
Successful Recovery of an Observed Meteorite Fall Using Drones and Machine Learning

We report the first-time recovery of a fresh meteorite fall using a drone and a machine-learning algorithm. The fireball was observed on 2021 April 1 over Western Australia by the Desert Fireball Network, for which a fall area was calculated for the predicted surviving mass. A search team arrived on-site and surveyed 5.1 km2 area over a 4 day period. A convolutional neural network, trained on previously recovered meteorites with fusion crusts, processed the images on our field computer after each flight. Meteorite candidates identified by the algorithm were sorted by team members using two user interfaces to eliminate false positives. Surviving candidates were revisited with a smaller drone, and imaged in higher resolution, before being eliminated or finally being visited in person. The 70 g meteorite was recovered within 50 m of the calculated fall line, demonstrating the effectiveness of this methodology, which will facilitate the efficient collection of many more observed meteorite falls.

Keywords: meteorites; fireballs; convolutional neural networks
The Hα Emission Line Variations of HR 6819

The star system HR 6819 was recently proposed by Rivinius et al. as the site of the nearest example of a stellar-mass black hole. Their spectra show evidence of two components: a B3 III star in a 40 day orbit and a stationary B-emission line star. Based upon the orbital mass function and the lack of evidence of a spectral component with reflex orbital motion, Rivinius et al. suggested that HR 6819 is a triple system consisting of an inner B3 III star plus black hole binary orbited by a distant Be star. Here we present an alternative model based upon an examination of the Hα emission line in their spectra. We show that the emission line displays the small reflex motion expected for the companion in the inner orbit and shows variations in profile shape related to orbital phase. This indicates that HR 6819 is a binary system consisting of a massive Be star and a low-mass companion that is the stripped down remnant of a former mass donor star in a mass transfer binary.

Keywords: binary stars; white dwarf stars; circumstellar disks
Electronic Spectroscopy of Anthracene Cations and Protonated Anthracene in the Search for Carriers of Diffuse Interstellar Bands

The helium-tagging technique was employed to record absorption spectra of cold anthracene cations and protonated anthracene. The evaluation of the spectra of the chromophore with a different number of attached He atoms allows getting the precise band positions of the molecular ions in the gas phase. The positions of the two most intense bands of anthracene, suitable for astrophysical detection, were found to be λmax = 3478.9 ± 1.8 Å and λmax = 7068.9 ± 5.7 Å. A considerable shift of the red band position compared to a previous measurement was attributed to a temperature effect. No coincidence of the absorption bands in astrophysical observational spectra was found. This allows estimating the upper limit for the abundance of anthracene cations per H nuclei <10-9 along the HD 183143 line of sight. We discuss possible reasons for such a low abundance of this molecular ion.

Keywords: polycyclic aromatic hydrocarbons; interstellar molecules; astrochemistry; molecular spectroscopy; diffuse interstellar bands
The Solar Neighborhood L: Spectroscopic Discovery of K Dwarfs Younger Than 1 Gyr and New Binaries within 30 pc

As part of a comprehensive effort to characterize the nearest stars, the CHIRON echelle spectrograph on the CTIO/SMARTS 1.5 m telescope is being used to acquire high-resolution (R = 80,000) spectra of K dwarfs within 50 pc. This paper provides spectral details about 35 K dwarfs from five benchmark sets with estimated ages spanning 20 Myr-5.7 Gyr. Four spectral age and activity indicators are tested, three of which aligned with the estimated ages of the benchmark groups-the Na I doublet (5889.95 and 5895.92 Å), the Hα line (6562.8 Å), and the Li I resonance line (6707.8 Å). The benchmark stars are then used to evaluate seven field K dwarfs exhibiting variable radial velocities for which initial CHIRON data did not show obvious companions. Two of these stars are estimated to be younger than 700 Myr, while one exhibits stellar activity unusual for older K-dwarf field stars and is possibly young. The four remaining stars turn out to be spectroscopic binaries, two of which are being reported here for the first time with orbital periods found using CHIRON data. Spectral analysis of the combined sample of 42 benchmark and variable radial velocity stars indicates temperatures ranging from 3900 to 5300 K and metallicities from -0.4 < [Fe/H] < +0.2. We also determine $\mathrm{log}g=4.5\mbox{--}4.7$ for main-sequence K dwarfs. Ultimately, this study will target several thousand of the nearest K dwarfs and provide results that will serve present and future studies of stellar astrophysics and exoplanet habitability.

Keywords: k dwarf stars; late-type dwarf stars; solar neighborhood; spectroscopy; stellar activity; stellar ages; stellar associations; stellar properties
Sample-based Spectral Mapping around Landing Sites on the Moon-Lunar Timescale Part 1

Studying the geology of a terrestrial body and its evolution requires being able to set up a chronology. Crater statistics is a powerful tool to do so. However, calibration of this method, which is derived on the Moon, requires properly defined geological units, large enough for reliable crater statistics, and dated samples representative of these geological units. Yet, little effort has been put into choosing a representative set of lunar mission units and samples based on spectral information. Therefore, accuracy and precision of so-called cratering-chronology models remain limited. Here, we analyzed near-infrared orbital data to suggest a geological unit as a reference unit for Apollo 11, 12, 14-17 and Luna 16, 20, and 24 landing sites. We used near-infrared spectral information from orbital data and laboratory measurements to identify the set of samples that is the most representative of the landing-site reference units. We now provide spectrally constrained reference units, for which crater frequencies can be derived, and our sample recommendation will assign the reference age for updating the currently existing cratering-chronology models. We highlight that determining a reference geological unit in the highlands of the Moon is challenging because of the very nature of the terrain, while it is more straightforward for mare units. However, ejecta presence on these mare units comprises solvable challenges.

Keywords: infrared spectroscopy; the moon; craters
Pebble Trapping in Vortices: Three-dimensional Simulations

Disk vortices have been heralded as promising routes for planet formation due to their ability to trap significant amounts of pebbles. While the gas motions and trapping properties of two-dimensional vortices have been studied in enough detail in the literature, pebble trapping in three dimensions has received less attention, due to the higher computational demand. Here we use the Pencil Code to study 3D vortices generated by convective overstability and the trapping of solids within them. The gas is unstratified whereas the pebbles settle to the midplane due to vertical gravity. We find that for pebbles of normalized friction times of $\mathrm{St}=0.05$ and $\mathrm{St}=1$ , and dust-to-gas ratio $\varepsilon =0.01$ , the vortex column in the midplane is strongly perturbed. Yet when the initial dust-to-gas ratio is decreased the vortices remain stable and function as efficient pebble traps. Streaming instability is triggered even for the lowest dust-to-gas ratio ( ${\varepsilon }_{0}={10}^{-4}$ ) and smallest pebble sizes ( $\mathrm{St}=0.05$ ) we assumed, showing a path for planetesimal formation in vortex cores from even extremely subsolar metallicity. To estimate if the reached overdensities can be held together solely by their own gravity we estimate the Roche density at different radii. Depending on disk model and radial location of the pebble clump we do reach concentrations higher than the Roche density. We infer that if self-gravity was included for the pebbles then gravitational collapse would likely occur.

Keywords: protoplanetary disks; planet formation; planetary system formation; solar system formation; solar nebulae; circumstellar dust; circumstellar matter; circumstellar disks
Effects of the Librationally Induced Flow in Mercury's Fluid Core with an Outer Stably Stratified Layer

Observational constraints on Mercury's thermal evolution and magnetic field indicate that the top part of the fluid core is stably stratified. Here we compute how a stable layer affects the core flow in response to Mercury's main 88 day longitudinal libration, assuming various degrees of stratification, and study whether the core flow can modify the libration amplitude through viscous and electromagnetic torques acting on the core-mantle boundary (CMB). We show that the core flow strongly depends on the strength of the stratification near the CMB but that the influence of core motions on libration is negligible with or without a stably stratified layer. A stably stratified layer at the top of the core can, however, prevent resonant behavior with gravito-inertial modes by impeding radial motions and promote a strong horizontal flow near the CMB. The librationally driven flow is likely turbulent and might produce a nonaxisymmetric induced magnetic field with a strength of the order of 1% of Mercury's dipolar field.

Keywords: mercury (planet); libration; astrophysical fluid dynamics; internal waves; magnetic fields
Toward Assembling a Comprehensive Database of Substellar Accretion Rates

The relationship between mass and mass accretion rate in young objects is a powerful tool for interrogating formation and evolution. We analyze a comprehensive database of published accretion diagnostics for ~400 objects (0.005-2 M⊙), including 64 brown dwarfs. To eliminate systematics between studies as the source of the four orders of magnitude of observed scatter, we uniformly re-estimate masses and accretion rates from direct observables (e.g., spectral types, line luminosities). In particular, we adopt a single grid of evolutionary models to estimate masses, a single family of scaling relations to estimate accretion luminosity, and update line luminosities using Gaia DR2 distances. We find that this unification produces minimal effects on the observed relation, suggesting systematic and methodological variation is not responsible for the scatter. This paves the way for investigation of physical sources of scatter, including: age variation, intrinsic variability, formation mechanism, and accretion paradigm.

Keywords: pre-main sequence stars; low mass stars; brown dwarfs; accretion; stellar accretion; stellar accretion disks