adsabs/SciX_UAT_keywords huggingface dataset:
Classification of Magnetohydrodynamic Simulations Using Wavelet Scattering Transforms

The complex interplay of magnetohydrodynamics, gravity, and supersonic turbulence in the interstellar medium (ISM) introduces a non-Gaussian structure that can complicate a comparison between theory and observation. In this paper, we show that the wavelet scattering transform (WST), in combination with linear discriminant analysis (LDA), is sensitive to non-Gaussian structure in 2D ISM dust maps. WST-LDA classifies magnetohydrodynamic (MHD) turbulence simulations with up to a 97% true positive rate in our testbed of 8 simulations with varying sonic and Alfvénic Mach numbers. We present a side-by-side comparison with two other methods for non-Gaussian characterization, the reduced wavelet scattering transform (RWST) and the three-point correlation function (3PCF). We also demonstrate the 3D-WST-LDA, and apply it to the classification of density fields in position-position-velocity (PPV) space, where density correlations can be studied using velocity coherence as a proxy. WST-LDA is robust to common observational artifacts, such as striping and missing data, while also being sensitive enough to extract the net magnetic field direction for sub-Alfvénic turbulent density fields. We include a brief analysis of the effect of point-spread functions and image pixelization on 2D-WST-LDA applied to density fields, which informs the future goal of applying WST-LDA to 2D or 3D all-sky dust maps to extract hydrodynamic parameters of interest.

Keywords: interstellar medium; magnetohydrodynamical simulations; non-gaussianity; convolutional neural networks; astronomy data analysis
Transient Jupiter Co-orbitals from Solar System Sources

We demonstrate dynamical pathways from main-belt asteroid and Centaur orbits to those in co-orbital motion with Jupiter, including the retrograde (inclination i > 90°) state. We estimate that, at any given time, there should be ∼1 km-scale or larger escaped asteroid in a transient direct (prograde) orbit with semimajor axis near that of Jupiter's (a ≃ aJ), with proportionally more smaller objects as determined by their size distribution. Most of these objects would be in the horseshoe dynamical state, and are hard to detect due to their moderate eccentricities (spending most of their time beyond 5 au) and longitudes relative to Jupiter being spread nearly all over the sky. We also show that ≈1% of the transient asteroid co-orbital population is on retrograde orbits with Jupiter. This population, like the recently identified asteroid (514107) 2015 BZ509, can spend millions of years with a ≃ aJ including tens or hundreds of thousands of years formally in the retrograde 1:-1 co-orbital resonance. Escaping near-Earth asteroids (NEAs) are thus likely the precursors of the handful of known high-inclination objects with a ≃ aJ. We compare the production of Jovian co-orbitals from escaping NEAs with those from incoming Centaurs. We find that temporary direct co-orbitals are likely dominated by Centaur capture, but we only find production of (temporary) retrograde Jovian co-orbitals (including very long-lived ones) from the NEA source. We postulate that the primordial elimination of the inner solar system's planetesimal population could provide a supply route for a metastable outer solar system reservoir for the high-inclination Centaurs.

Keywords: celestial mechanics; orbital resonances; inclination; orbital motion; asteroids; jupiter trojans
Functionality of Ice Line Latitudinal EBM Tenacity (FILLET). Protocol Version 1.0. A CUISINES Intercomparison Project

Energy balance models (EBMs) are 1D or 2D climate models that can provide insights into planetary atmospheres, particularly with regard to habitability. Because EBMs are far less computationally intensive than 3D general circulation models (GCMs), they can be run over large uncertain parameter spaces and can be used to explore long-period phenomena, like carbon and Milankovitch cycles. Because horizontal dimensions are incorporated in EBMs, they can explore processes that are beyond the reach of 1D radiative-convective models (RCMs). EBMs are, however, dependent on parameterizations and tunings to account for physical processes that are neglected. Thus, EBMs rely on observations and results from GCMs and RCMs. Different EBMs have included a wide range of parameterizations (for albedo, radiation, and heat diffusion) and additional physics, such as carbon cycling and ice sheets. This CUISINES exoplanet model intercomparison project (exoMIP) will compare various EBMs across a set of numerical experiments. The set of experiments will include Earth-like planets at different obliquities, parameter sweeps across obliquity, and variations in instellation and CO2 abundance, to produce hysteresis diagrams. We expect a range of different results due to the choices made in the various codes, highlighting which results are robust across models and which are dependent on parameterizations or other modeling choices. Additionally, the project will allow developers to identify model defects and determine which parameterizations are most useful or relevant to the problem of interest. Ultimately, this exoMIP will allow us to improve the consistency between EBMs and accelerate the process of discovering habitable exoplanets.

Keywords: exoplanet atmospheres; exoplanet surfaces; planetary climates; computational methods; earth atmosphere
Interstellar Bow Shocks around Fast Stars Passing through the Local Interstellar Medium

Bow shocks are produced in the local interstellar medium by the passage of fast stars from the Galactic thin-disk and thick-disk populations with velocities V * = 40-80 km s-1. Stellar transits of local H I clouds occur every 3500-7000 yr on average and last between 104 and 105 yr. There could be 10-20 active bow shocks around low-mass stars inside clouds within 15 pc of the Sun. At local cloud distances of 3-10 pc, their turbulent wakes have transverse radial extents R wake ≈ 100-300 au, angular sizes 10″-100″, and Lyα surface brightnesses of 2-8 R in gas with total hydrogen density n H ≈ 0.1 cm-3 and V * = 40-80 km s-1. These transit wakes may cover an area fraction f A ≈ (R wake/R cl) ≈ 10-3 of local H I clouds and be detectable in IR (dust), UV (Lyα, two-photon), or nonthermal radio emission. Turbulent heating in these wakes could produce the observed elevated rotational populations of H2 (J ≥ 2) and influence the endothermic formation of CH+ in diffuse interstellar gas at T > 103 K.

Keywords: interstellar clouds; stellar bow shocks
Ejecta Blankets at Small Craters on the Moon

Impact-derived ejecta covers most of the lunar surface, originating from recent impacts through to the beginning of the geologic record. Despite how common ejecta is, accurate measurements of ejecta thickness are difficult to obtain, and existing estimates of ejecta thickness vary widely. This study uses excavation by meter-scale impacts on the fresh ejecta blankets of larger, kilometer-scale impacts to make point measurements of ejecta thickness. We estimate ejecta thickness at the rims of 73 lunar craters (0.1–4.8 km diameter) and create isopach maps of ejecta thickness for three craters. We derive an equation for ejecta thickness, $t=0.14\pm 0.062{R}^{(0.77\pm 0.080)}{\left(r/R\right)}^{(-B)}$ , where r is the horizontal distance from the center of the crater, R is the center-to-rim crater radius, and B describes the rate at which ejecta thickness decays with radial distance. Our average value for B (2.8 ± 0.1) is similar to previous work, though we observe that B can vary significantly within an ejecta blanket.

Keywords: the moon; lunar impacts; ejecta; impact phenomena
How to Identify Exoplanet Surfaces Using Atmospheric Trace Species in Hydrogen-dominated Atmospheres

Sub-Neptunes (Rp ~ 1.25-4 REarth) remain the most commonly detected exoplanets to date. However, it remains difficult for observations to tell whether these intermediate-sized exoplanets have surfaces and where their surfaces are located. Here we propose that the abundances of trace species in the visible atmospheres of these sub-Neptunes can be used as proxies for determining the existence of surfaces and approximate surface conditions. As an example, we used a state-of-the-art photochemical model to simulate the atmospheric evolution of K2-18b and investigate its final steady-state composition with surfaces located at different pressures levels (Psurf). We find that the surface location has a significant impact on the atmospheric abundances of trace species, making them deviate significantly from their thermochemical equilibrium and "no-surface" conditions. This result arises primarily because the pressure-temperature conditions at the surface determine whether photochemically produced species can be recycled back to their favored thermochemical equilibrium forms and transported back to the upper atmosphere. For an assumed H2-rich atmosphere for K2-18b, we identify seven chemical species that are most sensitive to the existence of surfaces: ammonia (NH3), methane (CH4), hydrogen cyanide (HCN), acetylene (C2H2), ethane (C2H6), carbon monoxide (CO), and carbon dioxide (CO2). The ratio between the observed and the no-surface abundances of these species can help distinguish the existence of a shallow surface (Psurf < 10 bar), an intermediate surface (10 bar < Psurf < 100 bar), and a deep surface (Psurf > 100 bar). This framework can be applied together with future observations to other sub-Neptunes of interest.

Keywords: exoplanet atmospheres; exoplanet atmospheric composition; exoplanet surfaces; extrasolar gaseous planets; extrasolar rocky planets
Detection of Faint Sources by the UltraViolet Imaging Telescope Onboard AstroSat Using Poisson Distribution of Background

We present an improved approach for constructing the UV source catalogs using observations from the UltraViolet Imaging Telescope (UVIT) onboard AstroSat, by considering the Poisson distribution of the UV background. The method is tested extensively using fields that are not crowded, the Small Magellanic Cloud and M31 (Field 13). The results are compared with previous studies that used UVIT observations. This approach is successful in detecting fainter sources and produces a large number of new sources (∼15% –92% more). Most of the newly discovered UV sources fall in the faint end of the source distribution (m ≳ 22). The counterparts at other wavelengths are identified for most sources. This approach is more efficient for source detection and provides an opportunity to explore new classes of UV sources.

Keywords: ultraviolet astronomy; celestial objects catalogs; poisson distribution; small magellanic cloud; andromeda galaxy
MAXI J0637-430: A Possible Candidate for Bulk Motion Comptonization?

The transient Galactic black hole candidate MAXI J0637-430 went through an outburst in 2019-2020 for the very first time. This outburst was active for almost 6 months from 2019 November to 2020 May. We study the spectral properties of this source during that outburst using archival data from NICER/XTI, Swift/XRT, and NuSTAR/FPM satellites/instruments. We have analyzed the source during six epochs on which simultaneous NICER-NuSTAR and Swift/XRT-NuSTAR data were available. Using both phenomenological and physical model fitting approaches, we analyzed the spectral data in the broad 0.7-70 keV energy band. We first used a combination of disk blackbody with power-law, disk blackbody with broken power-law, and disk blackbody with power-law and bulk motion Comptonization (BMC) models. For a better understanding of the accretion picture, e.g., understanding how the accretion rates change with the changing size of the perceived Compton cloud, we used the two-component advective flow (TCAF) model with the broken power-law, and TCAF with the combined power-law, BMC models. For the last three epochs, the diskbb+power-law and TCAF models were able to spectrally fit the data for acceptable χ 2/degrees of freedom (dof). However, for the first three epochs, we needed an additional component to fit spectra for acceptable χ 2/dof. From our analysis, we reported on the possible presence of another component during these first three epochs when the source was in the high soft state. This additional component in this state is best described by the BMC phenomenon. From the TCAF model fitting, we estimated the average mass of the source as ${8.1}_{-2.7}^{+1.3}\,{M}_{\odot }$ .

Keywords: x-ray binary stars; black holes; stellar accretion disks; shocks; compact radiation sources
The Utility of Infrasound in Global Monitoring of Extraterrestrial Impacts: A Case Study of the 2008 July 23 Tajikistan Bolide

Among the various observational techniques used for the detection of large bolides on a global scale is a low-frequency sound known as infrasound. Infrasound, which is also one of the four sensing modalities used by the International Monitoring System, offers continuous global monitoring and can be leveraged for planetary defense. Infrasonic records can provide an additional dimension for event characterization and a distinct perspective that might not be available through any other observational method. This paper describes the infrasonic detection and characterization of the bolide that disintegrated over Tajikistan on 2008 July 23. This event was detected by two infrasound stations at distances of 1530 and 2130 km. Propagation paths to one of the stations were not predicted by the model, despite being clearly detected. The presence of the signal is attributed to the acoustic energy being trapped in a weak but leaky stratospheric AtmoSOFAR channel. The infrasound signal analysis indicates that the shock originated at the point of the main breakup at an altitude of 35 km. The primary mode of the shock production of the signal detected at the two stations was a spherical blast resulting from the main gross fragmentation episode. The energy estimate, based on the signal period, is 0.17–0.51 kt of TNT equivalent, suggesting a mass of 6.6–23.5 tons. The corresponding object radius, assuming the chondritic origin, was 0.78–1.18 m.

Keywords: bolides; fireballs; planetary atmospheres; impact phenomena; shocks; planetary bow shocks; asteroids; near-earth objects
OSSOS XXV: Large Populations and Scattering-Sticking in the Distant Trans-Neptunian Resonances

There have been 77 TNOs discovered to be librating in the distant trans-Neptunian resonances (beyond the 2:1 resonance, at semimajor axes greater than 47.7 au) in four well-characterized surveys: the Outer Solar System Origins Survey (OSSOS) and three similar prior surveys. Here, we use the OSSOS Survey Simulator to measure their intrinsic orbital distributions using an empirical parameterized model. Because many of the resonances had only one or very few detections, j:k resonant objects were grouped by k in order to have a better basis for comparison between models and reality. We also use the Survey Simulator to constrain their absolute populations, finding that they are much larger than predicted by any published Neptune migration model to date; we also find population ratios that are inconsistent with published models, presenting a challenge for future Kuiper Belt emplacement models. The estimated population ratios between these resonances are largely consistent with scattering-sticking predictions, though further discoveries of resonant TNOs with high-precision orbits will be needed to determine whether scattering-sticking can explain the entire distant resonant population or not.

Keywords: resonant kuiper belt objects
The Far UV Variability of τ Her (HD 147394) Recorded by IUE

Evidence is presented that the far-ultraviolet flux of τ Her recorded by the International Ultraviolet Explorer is variable in a series of six spectra taken consecutively over 4 hr and 45 minutes The centroids of the resonance lines of O I, C II and Si II are blueshifted in several spectra and resume their original positions over a timescale of 3 hr 5 minutes. The FUV flux also varies over the same timescale. The far-ultraviolet variability is discussed in light of the recent TESS lightcurve observed for τ Her.

Keywords: b stars
A Magnetic White Dwarf Accretion Model for the Anomalous X-Ray Pulsar 4U 0142+61

The quiescent emission of the anomalous X-ray pulsar (AXP) 4U 0142+61 extends over a broad range of energy, from radio up to hard X-rays. In particular, this object is unique among soft gamma-ray repeaters (SGRs) and AXPs in presenting simultaneously mid-infrared emission and pulsed optical emission. In spite of the many propositions to explain this wide range of emission, it still lacks one that reproduces all of the observations. Filling this gap, we present a model to reproduce the quiescent spectral energy distribution of 4U 0142+61 from mid-infrared up to hard X-rays using plausible physical components and parameters. We propose that the persistent emission comes from a magnetic accreting white dwarf (WD) surrounded by a debris disk. This model assumes that (I) the hard X-rays are due to the bremsstrahlung emission from the postshock region of the accretion column, (II) the soft X-rays are originated by hot spots on the WD surface, and (III) the optical and infrared emissions are caused by an optically thick dusty disk, the WD photosphere, and the tail of the postshock region emission. In this scenario, the fitted model parameters indicate that 4U 0142+61 harbors a fast-rotator magnetic near-Chandrasekhar WD, which is very hot and hence young. Such a WD can be the recent outcome of a merger of two less massive WDs. In this case, 4U 0142+61 can evolve into a supernova Ia and hence give hints of the origin of these important astrophysical events. Additionally, we also present a new estimate of 4U 0142+61's distance, ${3.78}_{-0.18}^{+0.12}$ kpc, based on the measured hydrogen column density and new interstellar extinction 3D maps.

Keywords: pulsars; optical pulsars; white dwarf stars; chandrasekhar limit; debris disks; stellar accretion disks; soft gamma-ray repeaters; magnetars; magnetic stars; magnetic fields
Implications of Narrow Spectra of Fast Radio Bursts

Fast radio bursts (FRBs) are millisecond-duration radio transients with extremely high brightness temperatures at cosmological distances, and the physical origin and the radiation mechanism of FRBs are still unknown. The observed spectral bandwidth of some FRBs appeared narrow compared with their peak frequencies, which could be used to constrain the radiation mechanism and the astrophysical environment of FRBs. In this work, we investigate some of the possible physical origins of the narrow spectra from the perspectives of intrinsic radiation mechanisms, coherent processes, radiative transfers, and interference processes. We find that: (1) If the observed narrow spectra of FRBs are attributed to the intrinsic radiation mechanism by a single charged particle, the particle's deflection angle should be much smaller than the radiation beaming angle. (2) Coherent process can cause narrow spectra. For the bunching mechanism, the narrow spectra might arise from the radiating bunches with a quasiperiodic distribution. For the maser mechanism, the negative absorption process can naturally cause a narrow spectrum. (3) Most absorption and scattering processes seem not to significantly change the observed spectra based on the current observation of some FRB repeaters. (4) Scintillation and plasma lensing in the FRB source environment can modulate the spectra, leading to the narrow spectra and the burst-to-burst variation of spectra. A planet-like object can generate the spectral modulation via gravitational lensing at the gigahertz band, but the observed burst-to-burst variation of the spectra does not support this scenario.

Keywords: compact radiation sources; radio transient sources; radio bursts; radiative processes; neutron stars
A Likely Magnetic Activity Cycle for the Exoplanet Host M Dwarf GJ 3512

Current radial velocity data from specialized instruments contain a large amount of information that may pass unnoticed if their analysis is not accurate. The joint use of Bayesian inference tools and frequency analysis has been shown as effective in revealing exoplanets but they have been used less frequently to investigate stellar activity. We intend to use radial velocity data of the exoplanet host star GJ 3512 to investigate its magnetic activity. Our study includes the analysis of the photometric data available. The main objectives of our work are to constrain the orbital parameters of the exoplanets in the system, to determine the current level of activity of the star and to derive an activity cycle length for it. An adaptive importance sampling method was used to determine the parameters of the exoplanets orbit. Generalized Lomb-Scargle periodograms were constructed with both radial velocity curve and photometric data. A careful analysis of the harmonic frequencies was conducted in each periodogram. Our fit to multiple Keplerian orbits constrained the orbital parameters of two giant gas planets orbiting the star GJ 3512. The host star showed an increase of its magnetic activity during the last observing campaign. The accurate fit of the radial velocity curve data to the multi-Keplerian orbit permitted to reveal the star rotation in the residuals of the best fit and estimate an activity cycle length of ∼14 yr.

Keywords: stellar activity; m dwarf stars; exoplanet systems; starspots; computational methods; late-type dwarf stars; bayesian statistics; importance sampling
Igniting Weak Interactions in Neutron Star Postmerger Accretion Disks

The merger of two neutron stars or a neutron star and a black hole typically results in the formation of a postmerger accretion disk. Outflows from disks may dominate the overall ejecta from mergers and be a major source of r-process nuclei in our universe. We explore the parameter space of such disks and their outflows and r-process yields by performing 3D general-relativistic magnetohydrodynamic simulations with weak interactions and approximate neutrino transport. We discuss the mapping between the initial binary parameters and the parameter space of the resulting disks, chiefly characterized by their initial accretion rate. We demonstrate the existence of an ignition threshold for weak interactions at around ~10-3 M⊙ s-1 for typical parameters by means of analytic calculations and numerical simulations. We find a degenerate, self-regulated, neutrino-cooled regime above the threshold and an advection-dominated regime below the threshold. Excess heating in the absence of neutrino cooling below the threshold leads to ≳60% of the initial disk mass being ejected in outflows, with typical velocities of ~(0.1-0.2)c, compared to ≲40% at ~(0.1-0.15)c above the threshold. While disks below the threshold show suppressed production of light r-process elements, disks above the threshold can produce the entire range of r-process elements, in good agreement with the observed solar system abundances. Disks below the ignition threshold may produce an overabundance of actinides seen in actinide-boost stars. As gravitational-wave detectors start to sample the neutron star merger parameter space, different disk realizations may be observable via their associated kilonova emission.

Keywords: accretion; magnetohydrodynamics; gravitation; nuclear abundances; nucleosynthesis; neutron stars; black holes