adsabs/SciX_UAT_keywords huggingface dataset:
"Spiders" on the Moon: Morphological Evidence for Geologically Recent Regolith Drainage into Subsurface Voids

On the Moon, the surface morphology at the scale of meters and tens of meters is typically smooth and subdued due to regolith gardening. Sharp, "crisp," meter-scale morphologic features are observed only where the regolith is either thin or recently disturbed. Such crisp morphologies are typically created by geologically recent meteoritic impacts of different scales. The prominent exception is so-called irregular mare patches (IMPs), rare small features of debated origin. We report here on the discovery of previously unknown crisp immature morphological features (named "spiders" due to their central circular region and radiating "legs") not related to impacts and even more rare. The spiders are meters-deep depressions with near-radial chutes open toward the center which make an incipient dendritic pattern 50–80 m in diameter. All spiders found thus far occur in clusters in the same region in Mare Tranquillitatis in the immediate proximity to small IMPs. We interpret spiders as the result of an energetic granular flow of the regolith draining into shallow subsurface voids following the sudden collapse of the roofs of the voids. Regolith gardening destroys the spiders' legs rapidly, on a timescale of a million years. If the entrance into the subsurface void remains unclogged, a spider appears to evolve into a pit; otherwise it evolves into a gentle depression and finally disappears. Our interpretation of spiders provides a consistent explanation of all of their features, occurrence settings, and associations.

Keywords: lunar surface
Galaxies in the Zone of Avoidance with the Spitzer Space Telescope

In this work we describe 59 galaxies discovered in the talk-forum of the Milky Way Project (MWP) by volunteers of the project. The MWP is based on infrared images by the Spitzer Space Telescope and primarily probes the Zone of Avoidance for star-forming regions located in the Milky Way. The list of galaxies found here include 35 galaxies previously not described, a possible galaxy cluster and two galaxies that could be located in the Great Attractor Wall. The volunteers mainly discovered larger (d > 12″) galaxies with a higher star-forming rate and likely missed galaxies with low star-forming rate or small galaxies. This work demonstrates that many galaxies in the Zone of Avoidance remain undiscovered in the GLIMPSE data set.

Keywords: infrared galaxies; galaxy clusters; spiral galaxies; great attractor
On the Presence of a Universal Acceleration Scale in Elliptical Galaxies

Dark matter phenomena in rotationally supported galaxies exhibit a characteristic acceleration scale of g† ≈ 1.2 × 10-10 m s-2. Whether this acceleration is a manifestation of a universal scale, or merely an emergent property with an intrinsic scatter, has been debated in the literature. Here we investigate whether a universal acceleration scale exists in dispersion-supported galaxies using two uniform sets of integral field spectroscopy (IFS) data from SDSS-IV MaNGA and ATLAS3D. We apply the spherical Jeans equation to 15 MaNGA and 4 ATLAS3D slow-rotator E0 (i.e., nearly spherical) galaxies. Velocity dispersion profiles for these galaxies are well determined with observational errors under control. Bayesian inference indicates that all 19 galaxies are consistent with a universal acceleration of ${g}_{\dagger }={1.5}_{-0.6}^{+0.9}\times {10}^{-10}$ m s-2. Moreover, all 387 data points from the radial bins of the velocity dispersion profiles are consistent with a universal relation between the radial acceleration traced by dynamics and that predicted by the observed distribution of baryons. This universality remains if we include 12 additional non-E0 slow-rotator elliptical galaxies from ATLAS ${}^{3{\rm{D}}}$ . Finally, the universal acceleration from MaNGA and ATLAS3D is consistent with that for rotationally supported galaxies, so our results support the view that dark matter phenomenology in galaxies involves a universal acceleration scale.

Keywords: dark matter; non-standard theories of gravity; elliptical galaxies; modified newtonian dynamics
Observational Evidence for a Spin-up Line in the P- \dot{P} Diagram of Millisecond Pulsars

It is believed that millisecond pulsars attain their fast spins by accreting matter and angular momentum from companion stars. Theoretical modeling of the accretion process suggests a spin-up line in the period-period derivative (P- $\dot{P}$ ) diagram of millisecond pulsars, which plays an important role in population studies of radio millisecond pulsars and accreting neutron stars in X-ray binaries. Here we present observational evidence for such a spin-up line using a sample of 143 radio pulsars with P < 30 ms. We also find that PSRs J1823-3021A and J1824-2452A, located near the classic spin-up line, are consistent with the broad population of millisecond pulsars. Finally, we show that our approach of Bayesian inference can probe accretion physics, allowing constraints to be placed on the accretion rate and the disk-magnetosphere interaction.

Keywords: millisecond pulsars; neutron stars; stellar accretion
Silicate Grain Growth due to Ion Trapping in Oxygen-rich Supernova Remnants like Cassiopeia A

Core-collapse supernovae can condense large masses of dust post-explosion. However, sputtering and grain-grain collisions during the subsequent passage of the dust through the reverse shock can potentially destroy a significant fraction of the newly formed dust before it can reach the interstellar medium. Here we show that in oxygen-rich supernova remnants like Cassiopeia A, the penetration and trapping within silicate grains of the same impinging ions of oxygen, silicon, and magnesium that are responsible for grain surface sputtering can significantly reduce the net loss of grain material. We model conditions representative of dusty clumps (density contrast of χ = 100) passing through the reverse shock in the oxygen-rich Cassiopeia A remnant and find that, compared to cases where the effect is neglected as well as facilitating the formation of grains larger than those that had originally condensed, ion trapping increases the surviving masses of silicate dust by factors of up to two to four, depending on initial grain radii. For higher density contrasts (χ ≳ 180), we find that the effect of gas accretion on the surface of dust grains surpasses ion trapping, and the survival rate increases to ∼55% of the initial dust mass for χ = 256.

Keywords: astrophysical dust processes; interstellar dust processes; core-collapse supernovae; supernova remnants; hydrodynamics
Numerical Simulations of Tidal Deformation and Resulting Light Curves of Small Bodies: Material Constraints of 99942 Apophis and 1I/'Oumuamua

In this paper, we present an open-source software (Simulator of Asteroid Malformation Under Stress, SAMUS) that simulates constant-density, constant-viscosity liquid bodies subject to tidal forces for a range of assumed viscosities and sizes. This software solves the Navier-Stokes equations on a finite-element mesh, incorporating the centrifugal, Coriolis, self-gravitational, and tidal forces. The primary functionality is to simulate the deformation of minor bodies under the influence of tidal forces. It may therefore be used to constrain the composition and physical structure of bodies experiencing significant tidal forces, such as 99942 Apophis and 1I/'Oumuamua. We demonstrate that SAMUS will be useful to constrain the material properties of Apophis during its near-Earth flyby in 2029. Depending on the material properties, Apophis may experience an area change of up to 0.5%, with similar effects on the photometric brightness. We also apply SAMUS to constrain the material dynamic viscosity of 1I/'Oumuamua, the first interstellar object discovered traversing the inner solar system. 'Oumuamua experienced a close approach to the Sun at perihelion (q ≃ 0.25 au) during which there were significant tidal forces that may have caused deformation of the body. This deformation could have lead to observable changes in the photometric light curve based on the material properties. The application of SAMUS to produce synthetic observations which incorporate tidal deformation effects demonstrates that no deformation-an infinite dynamic viscosity-best reproduces the photometric data. While these results indicate that 'Oumuamua did not experience significant tidal deformation, a sophisticated model incorporating nonprincipal axis rotation is necessary to conclusively analyze both 'Oumuamua and Apophis.

Keywords: interstellar objects; comets; hydrodynamics
Turbulence and Diffusive Transport of Cosmic Rays in the Very Local Interstellar Medium

We study the transport of fast charged particles, such as galactic cosmic rays, in the very local interstellar medium (VLISM), which is currently being explored by the two Voyager space probes. Guided by the observations of magnetic fluctuations, the paper develops a simple theoretical framework for computing scattering rates and spatial diffusion coefficients that can be used to model cosmic-ray transport in the VLISM. The local interstellar magnetic turbulence is represented as a superposition of (a) Alfvénic, (b) transverse 2D, and (c) longitudinal components obeying distinctive geometry rules in the plasma frame. The model is based on the weakly nonlinear formalism where particle trajectory's deviation from the unperturbed helix is caused primarily by guiding-center diffusion across the mean magnetic field. The transverse component plays the dominant role in perpendicular diffusion, while the longitudinal component has only a minor effect. Pitch-angle scattering is extremely weak in the VLISM, so that cosmic-ray transport can be considered essentially scatter-free on heliospheric scales. We test our theoretical model with the help of particle orbit simulations to find good agreement for perpendicular diffusion. We also find that cosmic rays disperse faster than in a conventional random walk (diffusive) process if the turbulence power spectrum contains fluctuations whose wavelength is larger than the size of the heliosphere.

Keywords: heliosphere; galactic cosmic rays; interstellar magnetic fields
Relationships between Photospheric Vertical Electric Currents and Hard X-Ray Sources in Solar Flares: Statistical Study

There are still debates over whether particle acceleration in solar flares may occur due to interruption of electric currents flowing along magnetic loops. To contribute to this debate, we performed the first statistical study of relationships between flare hard X-ray (HXR; 50-100 keV) sources observed by the Ramaty High-Energy Solar Spectroscopic Imager and photospheric vertical electric currents (PVECs, jr) calculated using vector magnetograms obtained with the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We analyzed a sample of 48 flares from the C3.0 to X3.1 class and were observed in the central part of the solar disk by both instruments in 2010-2015. We found that ≈70% of all HXR sources overlapped with islands or ribbons of enhanced (| {j}r| > {10}4 statampere cm-2) PVECs. However, less than ≈40% of the HXR sources overlapped with PVEC maxima, with an accuracy of ±3″. In more than half of the flares, there were HXR sources outside of regions of enhanced PVECs. We found no correlation between the intensity of the HXR sources and PVEC density or total PVEC under them. No systematic dissipation of PVECs under the HXR sources was found during the flares. Collectively, the results do not support the current-interruption flare models. However, the results indicate the importance of the presence of longitudinal currents in flare regions. To understand their specific role in the processes of energy release, plasma heating, and acceleration of particles requires further investigation.

Keywords: solar flares; solar x-ray flares; solar active region magnetic fields; solar photosphere; solar chromosphere
Dynamical Modeling of the C IV Broad Line Region of the z = 2.805 Multiply Imaged Quasar SDSS J2222+2745

We present the first ever models of a broad line region (BLR) at the peak of active galactic nucleus (AGN) activity using the multiply imaged z = 2.805 quasar SDSS J2222+2745. The modeled data consist of monthly spectra covering the broad C IV emission line over a 5.3 yr baseline. The models suggest a thick disk BLR that is inclined by ~40° to the observer's line of sight and with an emissivity weighted median radius of ${r}_{\mathrm{median}}={33.0}_{-2.1}^{+2.4}$ light days. The kinematics are dominated by near-circular Keplerian motion with the remainder inflowing. The rest-frame lag one would measure from the models is ${\tau }_{\mathrm{median}}={36.4}_{-1.8}^{+1.8}$ days, which is consistent with measurements based on cross-correlation. We show a possible geometry and transfer function based on the model fits and find that the model-produced velocity-resolved lags are consistent with those from cross-correlation. We measure a black hole mass of ${\mathrm{log}}_{10}({M}_{\mathrm{BH}}/{M}_{\odot })={8.31}_{-0.06}^{+0.07}$ , which requires a scale factor of ${\mathrm{log}}_{10}({f}_{\mathrm{mean},\sigma })={0.20}_{-0.07}^{+0.09}$ . This is the most precise MBH measurement for any AGN at cosmological distances and it demonstrates that the precision required for BH-host coevolution studies is attainable.

Keywords: strong gravitational lensing; active galactic nuclei; supermassive black holes; quasars; astronomy data modeling
The Stellar Merger Scenario for Black Holes in the Pair-instability Gap

The recent detection of GW190521 stimulated ideas on how to populate the predicted black hole (BH) pair-instability (PI) mass gap. One proposal is the dynamical merger of two stars below the PI regime forming a star with a small core and an oversized envelope. We outline the main challenges this scenario faces to form one BH in the gap. In particular, the core needs to avoid growing during the merger, and the merger product needs to retain enough mass, including in the subsequent evolution, and at core collapse (CC). We explore this scenario with detailed stellar evolution calculations, starting with ad hoc initial conditions enforcing no core growth during the merger. We find that these massive merger products are likely to be helium-rich and spend most of their remaining lifetime within regions of instabilities in the Herzsprung-Russell diagram, such as luminous blue variable eruptions. An energetic estimate of the amount of mass loss neglecting the back reaction of the star suggests that the total amount of mass that can be removed at low metallicity is ≲1 M⊙. This is small enough that at CC our models are retaining sufficient mass to form BHs in the PI gap similar to the recent ones detected by LIGO/Virgo. However, mass loss at the time of merger, the resulting core structure, and the mass loss at CC still need to be quantified for these models to confirm the viability of this scenario.

Keywords: massive stars; black holes; luminous blue variable stars; stellar mergers
Hidden Companions to Intermediate-mass Stars. XIV. Discovery of a 1.8 M ⊙, 0.3 au Companion to Ascella = Zeta Sagittarii A

Zeta Sagittarii is a nearby intermediate mass 13.2 au visual binary. Here we report on a VLTI/GRAVITY interferometric observation that revealed that ζ Sgr is actually a rather compact triple system: the visual primary Ascella (A) is a close equal mass ρ = 10.9 mas ↔ 0.3 au binary. Through isochrone fitting we find masses M Aa = 1.83M ⊙, M Ab = 1.81M ⊙ and M B = 2.03M ⊙. The hierarchy of the triple is low enough to allow for possibly interesting dynamical effects and its ultimate outcome will be a double or triple white dwarf system. We estimate that about 1 Myr ago ζ Sgr passed within a minimum distance of the Sun of 11 pc, rather than 3 pc as previously reported. *Based on observations collected at the European Southern Observatory, Chile, Program ID 111.24UP.001.

Keywords: multiple stars; a stars; optical interferometry
When Do Stalled Stars Resume Spinning Down? Advancing Gyrochronology with Ruprecht 147

Recent measurements of rotation periods ( ${P}_{\mathrm{rot}}$ ) in the benchmark open clusters Praesepe (670 Myr), NGC 6811 (1 Gyr), and NGC 752 (1.4 Gyr) demonstrate that, after converging onto a tight sequence of slowly rotating stars in mass-period space, stars temporarily stop spinning down. These data also show that the duration of this epoch of stalled spin-down increases toward lower masses. To determine when stalled stars resume spinning down, we use data from the K2 mission and the Palomar Transient Factory to measure ${P}_{\mathrm{rot}}$ for 58 dwarf members of the 2.7 Gyr old cluster Ruprecht 147, 39 of which satisfy our criteria designed to remove short-period or near-equal-mass binaries. Combined with the Kepler ${P}_{\mathrm{rot}}$ data for the approximately coeval cluster NGC 6819 (30 stars with M⋆ > 0.85 ${M}_{\odot }$ ), our new measurements more than double the number of ≈2.5 Gyr benchmark rotators and extend this sample down to ≈0.55 ${M}_{\odot }$ . The slowly rotating sequence for this joint sample appears relatively flat (22 ± 2 days) compared to sequences for younger clusters. This sequence also intersects the Kepler intermediate-period gap, demonstrating that this gap was not created by a lull in star formation. We calculate the time at which stars resume spinning down and find that 0.55 ${M}_{\odot }$ stars remain stalled for at least 1.3 Gyr. To accurately age-date low-mass stars in the field, gyrochronology formulae must be modified to account for this stalling timescale. Empirically tuning a core-envelope coupling model with open cluster data can account for most of the apparent stalling effect. However, alternative explanations, e.g., a temporary reduction in the magnetic braking torque, cannot yet be ruled out.

Keywords: open star clusters; stellar evolution; stellar ages; stellar rotation
Inflated Eccentric Migration of Evolving Gas Giants II - Numerical Methodology and Basic Concepts

Hot and warm Jupiters (HJs&WJs) are gas-giant planets orbiting their host stars at short orbital periods, posing a challenge to their efficient in situ formation. Therefore, most HJs&WJs are thought to have migrated from an initially farther-out birth location. Current migration models, i.e., disk migration (gas-dissipation driven) and eccentric migration (tidal evolution driven), fail to produce the occurrence rate and orbital properties of HJs&WJs. Here we study the role of thermal evolution and its coupling to tidal evolution. We use AMUSE, a numerical environment, and MESA, planetary evolution modeling, to model in detail the coupled internal and orbital evolution of gas giants during their eccentric migration. In a companion paper, we use a simple semianalytic model, validated by our numerical model, and run a population-synthesis study. We consider the initially inflated radii of gas giants (expected following their formation), as well study the effects of the potentially slowed contraction and even reinflation of gas giants (due to tidal and radiative heating) on the eccentric migration. Tidal forces that drive eccentric migration are highly sensitive to the planetary structure and radius. Consequently, we find that this form of inflated eccentric migration operates on significantly (up to an order of magnitude) shorter timescales than previously studied eccentric-migration models. Therefore, inflated eccentric migration gives rise to the more rapid formation of HJs&WJs, higher occurrence rates of WJs, and higher rates of tidal disruptions, compared with previous eccentric-migration models that consider constant ~Jupiter radii for HJ and WJ progenitors. Coupled thermal-dynamical evolution of eccentric gas giants can therefore play a key role in their evolution.

Keywords: exoplanet formation; hot jupiters; exoplanet migration; exoplanet evolution
Impacts of Bar-driven Shear and Shocks on Star Formation

Bars drive gas inflow. As the gas flows inward, shocks and shear occur along the bar dust lanes. Such shocks and shear can affect the star formation (SF) and change the gas properties. For four barred galaxies, we present Hα velocity gradient maps that highlight bar-driven shocks and shear using data from the PHANGS-MUSE and PHANGS-ALMA surveys, which allow us to study bar kinematics in unprecedented detail. Velocity gradients are enhanced along the bar dust lanes, where shocks and shear are shown to occur in numerical simulations. Velocity gradient maps also efficiently pick up H II regions that are expanding or moving relative to the surroundings. We put pseudo-slits on the regions where velocity gradients are enhanced and find that Hα and CO velocities jump up to ∼170 km s‑1, even after removing the effects of circular motions due to the galaxy rotation. Enhanced velocity gradients either coincide with the peak of CO intensity along the bar dust lanes or are slightly offset from CO intensity peaks, depending on the objects. Using the Baldwin–Philips–Terlevich BPT diagnostic, we identify the source of ionization on each spaxel and find that SF is inhibited in the high-velocity gradient regions of the bar, and the majority of those regions are classified as a low-ionization nuclear emission-line region (LINER) or composite. This implies that SF is inhibited where bar-driven shear and shocks are strong. Our results are consistent with the results from the numerical simulations that show SF is inhibited in the bar where the shear force is strong.

Keywords: barred spiral galaxies; spiral galaxies; galaxy structure; star formation; galaxy kinematics
Implications of Abiotic Oxygen Buildup for Earth-like Complex Life

One of the chief paradoxes of molecular oxygen (O2) is that it is an essential requirement for multicellular eukaryotes on Earth while simultaneously posing a threat to their survival via the formation of reactive oxygen species. In this paper, the constraints imposed by O2 on Earth-like complex life are invoked to explore whether worlds with abiotic O2 inventories can harbor such organisms. By taking the major O2 sources and sinks of Earth-like planets into account using a simple model, it is suggested that worlds that receive time-averaged X-ray and extreme ultraviolet fluxes that are ≳10 times higher than Earth might not be capable of hosting complex lifeforms because the photolysis of molecules such as water may lead to significant O2 buildup. Methods for testing this hypothesis by searching for anticorrelations between biosignatures and indicators of abiotic O2 atmospheres are described. In the event, however, that life successfully adapts to high-oxygen environments, these worlds could permit the evolution of large and complex organisms.

Keywords: astrobiology; biosignatures; exoplanet atmospheres; exoplanet atmospheric composition; exoplanet evolution; exoplanet surface characteristics; exoplanets; stellar evolution; stellar physics