adsabs/SciX_UAT_keywords huggingface dataset:
Fine-grained Rim Formation via Kinetic Dust Aggregation in Shock Waves Around Evaporating Icy Planetesimals

Fine-grained rims (FGRs) are frequently found around chondrules in primitive chondrites. The remarkable feature of FGRs is their submicron-sized and nonporous nature. The typical thickness of FGRs around chondrules is 10-100 μm. Recently, a novel idea was proposed for the origin of FGRs: high-speed collisions between chondrules and fine dust grains called the kinetic dust aggregation process. Experimental studies revealed that (sub)micron-sized ceramic particles can stick to a ceramic substrate in a vacuum when the impact velocity is approximately in the range of 0.1-1 km s-1. In this study, we examine the possibility of FGR formation via kinetic dust aggregation in chondrule-forming shock waves. When shock waves are created by undifferentiated icy planetesimals, fine dust grains would be released from the planetary surface due to the evaporation of icy planetesimals. We consider the dynamics of chondrules behind the shock front and calculate the growth of FGRs via kinetic dust aggregation based on simple one-dimensional calculations. We found that nonporous FGRs with a thickness of 10-100 μm would be formed in shock waves around evaporating icy planetesimals.

Keywords: chondrules; chondrites; planetesimals; shocks
Effective Temperature Estimations from Line Depth Ratios in the H- and K-band Spectra of IGRINS

Determining accurate effective temperatures of stars buried in the dust-obscured Galactic regions is extremely difficult from photometry. Fortunately, high-resolution infrared spectroscopy is a powerful tool for determining the temperatures of stars with no dependence on interstellar extinction. It has long been known that the depth ratios of temperature-sensitive and relatively insensitive spectral lines are excellent temperature indices. In this work, we provide the first extensive line depth ratio (LDR) method application in the infrared region that encompasses both the H and K bands (1.48 μm - 2.48 μm). We applied the LDR method to high-resolution (R ≃ 45,000) H- and K-band spectra of 110 stars obtained with the Immersion Grating Infrared Spectrograph. Our sample contained stars with 3200 < T eff (K) < 5500, 0.20 ≤ log g < 4.6, and -1.5 < [M/H] < 0.5. The application of this method in the K band yielded 21 new LDR-T eff relations. We also report five new LDR-T eff relations found in the H-band region, augmenting the relations already published by other groups. The temperatures found from our calibrations provide reliable temperatures within ~70 K accuracy compared to spectral T eff values from the literature.

Keywords: stellar effective temperatures; stellar properties; stellar spectral lines; infrared spectroscopy
The Lyα Reference Sample. XIV. Lyα Imaging of 45 Low-redshift Star-forming Galaxies and Inferences on Global Emission

We present Lyα imaging of 45 low-redshift star-forming galaxies observed with the Hubble Space Telescope. The galaxies have been selected to have moderate to high star formation rates (SFRs) using far-ultraviolet (FUV) luminosity and Hα equivalent width criteria, but no constraints on Lyα luminosity. We employ a pixel stellar continuum fitting code to obtain accurate continuum-subtracted Lyα, Hα, and Hβ maps. We find that Lyα is less concentrated than FUV and optical line emission in almost all galaxies with significant Lyα emission. We present global measurements of Lyα and other quantities measured in apertures designed to capture all of the Lyα emission. We then show how the escape fraction of Lyα relates to a number of other measured quantities (mass, metallicity, star formation, ionization parameter, and extinction). We find that the escape fraction is strongly anticorrelated with nebular and stellar extinction, weakly anticorrelated with stellar mass, but no conclusive evidence for correlations with other quantities. We show that Lyα escape fractions are inconsistent with common dust extinction laws, and discuss how a combination of radiative transfer effects and clumpy dust models can help resolve the discrepancies. We present an SFR calibration based on Lyα luminosity, where the equivalent width of Lyα is used to correct for nonunity escape fraction, and show that this relation provides a reasonably accurate SFR estimate. We also show stacked growth curves of Lyα for the galaxies that can be used to find aperture loss fractions at a given physical radius.

Keywords: emission line galaxies; galaxies; lyman-alpha galaxies; starburst galaxies; astronomical methods; observational astronomy
Observation of Magnetic Reconnection in a Region of Strong Turbulence

We examine a rare and interesting observation of magnetic reconnection embedded in a large-scale region of strong turbulence in which magnetic field annihilation is energizing ions and electrons. The magnetic reconnection event is in Earth's magnetotail and is associated with enhanced energetic particle fluxes indicating local particle acceleration. Despite substantial electric and magnetic field fluctuations throughout the surrounding, large-scale region, the ongoing magnetic reconnection has many similar properties to laminar, 2D magnetic reconnection including Hall electric fields, Hall magnetic fields, a thin electron current sheet, and ion and electron jets. Notably, the electron jet emerging from the electron diffusion region (EDR) appears to transport sufficient off-diagonal momentum to infer that off-diagonal electron stress can support the reconnection electric field in the EDR even in a turbulent environment. Although the electron jet appears to be briefly (~1 s) deflected or possibly interrupted by an electromagnetic disturbance, the reconnection appears to otherwise continue for a long period (~30 minutes) as evidenced by a persistent ion jet. This particular finding implies that the fundamental electron-scale processes inside of the EDR in turbulent magnetic reconnection are not necessarily distinct from those in laminar magnetic reconnection. These observations provide direct confirmation that magnetic reconnection can not only be responsible for but also can continue in regions of large-scale turbulence. Because the electric and magnetic fields of strong turbulence are linked to particle acceleration, it follows that particle acceleration also can continue as a consequence of turbulent magnetic reconnection.

Keywords: space plasmas; planetary magnetospheres; magnetic fields; plasma physics; plasma astrophysics
Variability of the Incompressible Energy Cascade Rate in Solar Wind Turbulence around Mars

We present a statistical analysis on the variability of the incompressible energy cascade rate in the solar wind around Mars, making use of an exact relation for fully developed turbulence and more than five years of Mars Atmosphere and Volatile EvolutioN (MAVEN) observations. Using magnetic field and plasma data, we compute the energy cascade rate at the magnetohydrodynamics (MHD) scales in the pristine solar wind. From our statistical results, we conclude that the incompressible energy cascade rate decreases as the Martian heliocentric distance increases, for each of the three explored Martian years. Moreover, we suggest that the presence of proton cyclotron waves, associated with the extended Martian hydrogen exosphere, do not have a significant effect on the nonlinear cascade of energy at the MHD scales.

Keywords: interplanetary turbulence; solar wind; plasma physics
Velocity-inverted Three-dimensional Distribution of the Gas Clouds in the Type 2 AGN NGC 1068

Spatially resolved velocity maps at high resolutions of 1-10 pc are becoming available for many nearby active galactic nuclei (AGNs) in both optical/infrared atomic emission lines and submillimeter molecular lines. For the former, it is known that a linear relationship appears to exist between the velocity of the ionized gas clouds and the distance from the nucleus in the inner ∼100 pc region, where these clouds are outflowing. Here we demonstrate that, in such a case, we can actually derive the three-dimensional (3D) geometrical distribution of the clouds directly from the velocity map. Revisiting such a velocity map taken by the Hubble Space Telescope for the prototypical Type 2 AGN NGC 1068, we implement the visualization of the 3D distribution derived from the map, and show that this inner narrow-line region has indeed a hollow-cone structure, consistent with previous modeling results. Quite possibly, this is the outer extended part of the polar elongated dusty material seen in the recent mid-infrared interferometry at parsec scale. Conversely, the latter small-scale geometry is inferred to have a hollow-cone outflowing structure as the inward extension of the derived 3D distribution above. The AGN obscuring "torus" is argued to be the inner optically thick part of this hollow-cone outflow, and its shadowed side would probably be associated with the molecular outflow seen in certain submillimeter lines. We discuss the nature of the linear velocity field, which could be from an episodic acceleration that occurred ∼105 yr ago.

Keywords: active galactic nuclei; supermassive black holes; seyfert galaxies; interferometry
Global Spiral Density Wave Modes in Protoplanetary Disks: Morphology of Spiral Arms

We analyze two-armed global spiral density wave modes generated by gravitational instability in razor-thin, nonviscous, self-gravitating protoplanetary disks to understand the dependence of spiral arm morphology (pitch angle α and amplitude) on various disk conditions. The morphologies of the resulting spiral density wave modes closely resemble observations. Their pitch angles and pattern speeds are insensitive to the boundary conditions adopted. Gaussian disks exhibit more tightly wound spirals (smaller pitch angle) than power-law disks under the same conditions. We find that at a fixed disk-to-star mass ratio (Md/M*), the pitch angle increases with average Toomre's stability parameter ( $\overline{Q}$ ) or average disk aspect ratio ( $\overline{h}$ ). For a given $\overline{Q}$ , density wave modes with higher Md/M* have larger pitch angles, while the behavior reverses for a given $\overline{h}$ . The interdependence between pitch angle and disk properties can be roughly approximated by $\alpha \propto {c}_{s}^{2}/{M}_{d}$ , where cs is the sound speed. Our gravitational instability-excited spiral density waves can be distinguished from planet-launched spirals: (1) massive cool disks have spiral pitch angle falling with radius, while low-mass hot disks have spiral pitch angle rising with radius; (2) the profile of spiral amplitude presents several dips and bumps. We propose that gravitational instability-excited density waves can serve as an alternative scenario to explain the observed spiral arms in self-gravitating protoplanetary disks.

Keywords: protoplanetary disks; spiral arms; spiral pitch angle; gravitational instability; density wave model
Identifying Acoustic Wave Sources on the Sun. I. Two-dimensional Waves in a Simulated Photosphere

The solar acoustic oscillations are likely stochastically excited by convective dynamics in the solar photosphere, though few direct observations of individual source events have been made and their detailed characteristics are still unknown. Wave source identification requires measurements that can reliably discriminate the local wave signal from the background convective motions and resonant modal power. This is quite challenging as these noise contributions have amplitudes several orders of magnitude greater than the sources and the propagating wave fields they induce. In this paper, we employ a high-temporal-frequency filter to identify sites of acoustic emission in a radiative magnetohydrodynamic simulation. The properties of the filter were determined from a convolutional neural network trained to identify the two-dimensional acoustic Green's function response of the atmosphere, but once defined, it can be directly applied to an image time series to extract the signal of local wave excitation, bypassing the need for the original neural network. Using the filter developed, we have uncovered previously unknown properties of the acoustic emission process. In the simulation, acoustic events are found to be clustered at mesogranular scales, with peak emission quite deep, about 500 km below the photosphere, and sites of very strong emission can result from the interaction of two supersonic downflows that merge at that depth. We suggest that the method developed, when applied to high-resolution high-cadence observations, such as those forthcoming with the Daniel K. Inouye Solar Telescope, will have important applications in chromospheric wave studies and may lead to new investigations in high-resolution local helioseismology.

Keywords: the sun; solar physics; solar photosphere; helioseismology; solar granulation; solar oscillations
Observing the Inner Shadow of a Black Hole: A Direct View of the Event Horizon

Simulated images of a black hole surrounded by optically thin emission typically display two main features: a central brightness depression and a narrow "photon ring" consisting of strongly lensed images superimposed over the direct emission. The photon ring closely tracks a theoretical curve on the image plane corresponding to light rays that asymptote to bound photon orbits. The size and shape of this critical curve are purely governed by the Kerr geometry; in contrast, the size, shape, and depth of the observed brightness depression depend on the details of the emission region. For instance, images of spherical accretion models display a distinctive dark region-the "black hole shadow"-that completely fills the photon ring. By contrast, in models of equatorial disks extending to the event horizon, the darkest region in the image is restricted to a much smaller area-an inner shadow-whose edge lies near the direct lensed image of the equatorial horizon. Using both general relativistic MHD simulations and semi-analytic models, we demonstrate that the photon ring and inner shadow may be simultaneously visible in submillimeter images of M87*, where magnetically arrested disk simulations predict that the emission arises in a thin region near the equatorial plane. We show that the relative size, shape, and centroid of the photon ring and inner shadow can be used to estimate the black hole mass and spin, breaking degeneracies in measurements of these quantities from the photon ring alone. Both features may be accessible to direct observation via high-dynamic-range images with a next-generation Event Horizon Telescope.

Keywords: black holes; astrophysical black holes; black hole physics; event horizons; kerr black holes; general relativity; accretion; jets; gravitational lensing; magnetohydrodynamical simulations; plasma astrophysics; radiative transfer
Galactic Kites

I show that interstellar films of material thinner than a micron, drift away from the Galactic plane as a result of stellar radiation pressure. Such films, whether produced naturally by dust coagulation in proto-planetary disks or artificially by technological civilizations, would accumulate over the age of the Milky Way and hover above the Galactic disk at a scale-height set gravitationally by the dark matter halo. Limits on scattered starlight imply that this population carries a fraction below 2 × 10-3 of the interstellar medium mass.

Keywords: interstellar dynamics
Validation of Stereophotoclinometric Shape Models of Asteroid (101955) Bennu during the OSIRIS-REx Mission

NASA's OSIRIS-REx mission to asteroid (101955) Bennu relied on the production of real-time shape models for both spacecraft navigation and scientific analysis. The primary method of constructing shape models during the early phases of the mission was image-based stereophotoclinometry (SPC). The SPC shape models were used for operational planning, navigation, sample site selection, and initial scientific investigations. To this end, detailed analyses of the quality of each shape model and a thorough documentation of all sources of error were vital to ensure proper considerations of the limitations of each model. In this paper, we present methods used during the OSIRIS-REx mission to validate the SPC shape models and construct the associated quality reports. Although developed for the OSIRIS-REx mission, these validation techniques can be applied to SPC-derived shape models of other planetary bodies.

Keywords: asteroids; small solar system bodies; near-earth objects
Large Population of ALMA Galaxies at z > 6 with Very High [O III] 88 μm to [C II] 158 μm Flux Ratios: Evidence of Extremely High Ionization Parameter or PDR Deficit?

We present our new Atacama Large Millimeter/Submillimeter Array (ALMA) observations targeting [O III]88 μm, [C II]158 μm, [N II]122 μm, and dust-continuum emission for three Lyman break galaxies at z = 6.0293-6.2037, identified in the Subaru/Hyper Suprime-Cam survey. We clearly detect [O III] and [C II] lines from all of the galaxies at 4.3-11.8σ levels, and identify multi-band dust-continuum emission in two of the three galaxies, allowing us to estimate infrared luminosities and dust temperatures simultaneously. In conjunction with previous ALMA observations for six galaxies at z > 6, we confirm that all the nine z = 6-9 galaxies have high [O III]/[C II] ratios of ${L}_{[{\rm{O}}{\rm{III}}]}/{L}_{[{\rm{C}}{\rm{II}}]}\sim 3\mbox{--}20$, ∼10 times higher than z ∼ 0 galaxies. We also find a positive correlation between the [O III]/[C II] ratio and the Lyα equivalent width (EW) at the ∼90% significance level. We carefully investigate physical origins of the high [O III]/[C II] ratios at z = 6-9 using Cloudy, and find that high density of the interstellar medium, low C/O abundance ratio, and the cosmic microwave background attenuation are responsible to only a part of the z = 6-9 galaxies. Instead, the observed high [O III]/[C II] ratios are explained by 10-100 times higher ionization parameters or low photodissociation region (PDR) covering fractions of 0%-10%, both of which are consistent with our [N II] observations. The latter scenario can be reproduced with a density-bounded nebula with PDR deficit, which would enhance the Lyα, Lyman continuum, and ${{\rm{C}}}^{+}$ ionizing photons escape from galaxies, consistent with the [O III]/[C II]-Lyα EW correlation we find.

Keywords: galaxy evolution
Nucleosynthesis of an 11.8 M⊙ Supernova with 3D Simulation of the Inner Ejecta: Overall Yields and Implications for Short-lived Radionuclides in the Early Solar System

Based on a 3D supernova simulation of an 11.8 M⊙ progenitor model with an initial solar composition, we study the nucleosynthesis using tracers covering the innermost 0.1 M⊙ of the ejecta. These ejecta are mostly proton-rich and contribute significant amounts of 45Sc and 64Zn. The production of heavier isotopes is sensitive to the electron fraction and hence the neutrino emission from the proto-neutron star. The yields of these isotopes are rather uncertain due to the approximate neutrino transport used in the simulation. In order to obtain the total yields for the whole supernova, we combine the results from the tracers with those for the outer layers from a suitable 1D model. Using the yields of short-lived radionuclides (SLRs), we explore the possibility that an 11.8 M⊙ supernova might have triggered the formation of the solar system and provided some of the SLRs measured in meteorites. In particular, we discuss two new scenarios that can account for at least the data on 41Ca, 53Mn, and 60Fe without exceeding those on the other SLRs.

Keywords: explosive nucleosynthesis; nucleosynthesis; supernova dynamics; core-collapse supernovae; supernova neutrinos; isotopic abundances; nuclear abundances; abundance ratios; chemical abundances
Flare Hunting in Hot Subdwarf and White Dwarf Stars from Cycles 1–5 of TESS Photometry

Stellar flares are critical phenomena on stellar surfaces, which are closely tied to stellar magnetism. While extensively studied in main-sequence (MS) stars, their occurrence in evolved compact stars, specifically hot subdwarfs and white dwarfs (WDs), remains scarcely explored. Based on Cycles 1–5 of TESS photometry, we conducted a pioneering survey of flare events in ∼12,000 compact stars, corresponding to ∼38,000 light curves with a 2 minute cadence. Through dedicated techniques for detrending light curves, identifying preliminary flare candidates, and validating them via machine learning, we established a catalog of 1016 flares from 193 compact stars, including 182 from 58 sdB/sdO stars and 834 from 135 WDs, respectively. However, all flaring compact stars showed signs of contamination from nearby objects or companion stars, preventing sole attribution of the detected flares. For WDs, it is highly probable that the flares originated from their cool MS companions. In contrast, the higher luminosities of sdB/sdO stars diminish companion contributions, suggesting that detected flares originated from sdB/sdO stars themselves or through close magnetic interactions with companions. Focusing on a refined sample of 23 flares from 13 sdB/sdO stars, we found their flare frequency distributions were slightly divergent from those of cool MS stars; instead, they resemble those of hot B/A-type MS stars having radiative envelopes. This similarity implies that the flares on sdB/sdO stars, if these flares did originate from them, may share underlying mechanisms with hot MS stars, which warrants further investigation.

Keywords: white dwarf stars; b subdwarf stars; stellar flares; photometry; random forests
Detection and Removal of Periodic Noise in Kepler/K2 Photometry with Principal Component Analysis

We present a novel method for detrending systematic noise from time series data using Principal Component Analysis (PCA) in Fast Fourier Transforms. This method is demonstrated on time series data obtained from the inaugural campaign of the Kepler K2 mission, as well as three objects of interest from Campaign 4. Unlike previous detrending techniques that utilize PCA, this method performs the detrending in Fourier space rather than temporal space. The advantage of performing the analysis in frequency space is that the technique is sensitive purely to the periodicity of the unwanted signal and not to its morphological characteristics. This method could improve measurements of low signal-to-noise photometric features by reducing systematics. We also discuss challenges and limitations associated with this technique.

Keywords: principal component analysis