adsabs/SciX_UAT_keywords huggingface dataset:
Walk on the Low Side: LOFAR Explores the Low-frequency Radio Emission of GASP Jellyfish Galaxies

Jellyfish galaxies, characterized by long filaments of stripped interstellar medium extending from their disks, are the prime laboratories to study the outcomes of ram pressure stripping. At radio wavelengths, they often show unilateral emission extending beyond the stellar disk, and an excess of radio luminosity with respect to that expected from their current star formation rate. We present new 144 MHz images provided by the LOFAR Two-metre Sky Survey for a sample of six galaxies from the GASP survey. These galaxies are characterized by a high global luminosity at 144 MHz (6-27 × 1022 W Hz-1), in excess compared to their ongoing star formation rate. The comparison of radio and Hα images smoothed with a Gaussian beam corresponding to ~10 kpc reveals a sublinear spatial correlation between the two emissions with an average slope of k = 0.50. In their stellar disk we measure k = 0.77, which is close to the radio-to-star formation linear relation. We speculate that, as a consequence of the ram pressure, in these jellyfish galaxies cosmic ray transport is more efficient than in normal galaxies. Radio tails typically have higher radio-to-Hα ratios than the disks, thus we suggest that the radio emission is boosted by electrons stripped from the disks. In all galaxies, the star formation rate has decreased by a factor ≤10 within the last ~108 yr. The observed radio emission is consistent with the past star formation, so we propose that this recent decline may be the cause of their radio luminosity-to-star formation rate excess.

Keywords: radio astronomy; extragalactic radio sources; late-type galaxies
Long GMC Lifetimes: Using the Method and Data of Meidt et al. (2015) with a Correction

Meidt et al. derived short lifetimes of 20-30 Myr for giant molecular clouds (GMCs) in M51. Their novel approach utilizes a decline of the GMC population during their inter-arm passage from one spiral arm to the next. Using the inter-arm travel time ttravel as a fiducial clock, they converted the decline rate to GMC lifetimes. They implicitly adopted zero as the pattern speed of the spiral arms, resulting in a very short ttravel. However, it is well established that the spiral arm pattern in M51 is rotating, and that ttravel should be measured with respect to the rotating spiral pattern. Here we use the same method and data of Meidt et al. and re-evaluate GMC lifetimes by accounting for the pattern speed given by Meidt et al. This correction gives a longer ttravel, and consequently longer GMC lifetimes of 60-500 Myr.

Keywords: giant molecular clouds; galaxy dynamics
Locating Heating Channels of the Solar Corona in a Plage Region with the Aid of High-resolution 10830 Å Filtergrams

In this paper, with a set of high-resolution He I 10830 Å filtergrams, we select an area in a plage, very likely an extreme-ultraviolet (EUV) moss area, as an interface layer to follow the clues of coronal heating channels down to the photosphere. The filtergrams are obtained from the 1 m aperture New Vacuum Solar Telescope. We make a distinction between the darker and the brighter regions in the selected area and name the two regions enhanced absorption patches (EAPs) and low absorption patches (LAPs). With well-aligned, nearly simultaneous data from multiple channels of the Atmospheric Imaging Assembly and the continuum of the Helioseismic and Magnetic Imager on board Solar Dynamics Observatory, we compare the EUV/UV emissions, emission measure, mean temperature, and continuum intensity in the two kinds of regions. The following progress is made: (1) The mean EUV emissions over EAPs are mostly stronger than the corresponding emissions over LAPs except for the emission at 335 Å. The UV emissions at 1600 and 1700 Å fail to capture the difference between the two regions. (2) In the logarithmic temperature range of 5.6–6.2, EAPs have a higher EUV emission measure than LAPs, but they have a lower mean coronal temperature. (3) The mean continuum intensity over EAPs is lower. Based on the above progress, we suggest that the energy for coronal heating in the moss region can be traced down to some areas in intergranular lanes with enhanced density of both cool and hot material. The lower temperature over the EAPs is due to the greater fraction of cool material over there.

Keywords: solar coronal heating
Constraints on Fluctuating Star Formation Rates for Intermediate-mass Galaxies with Hα and UV Luminosities

We study the recent star formation histories (SFHs) of 575 intermediate-mass galaxies (IMGs, 109 ≤ M/M ⊙ ≤ 1010) in COSMOS at 0.3 < z < 0.4 by comparing their Hα and UV luminosities. These two measurements trace star formation rates (SFRs) on different timescales and together reveal fluctuations in recent activity. We compute L Hα from Magellan IMACS spectroscopy, while L UV is derived from rest-frame 2800 Å photometry. Dust corrections are applied to each band independently. We compare the deviation of L Hα and L UV from their respective star-forming sequences (i.e., ${\rm{\Delta }}\mathrm{log}{L}_{{\rm{H}}\alpha }$ and ${\rm{\Delta }}\mathrm{log}{L}_{\mathrm{UV}}$ ), and after accounting for observational uncertainties we find a small intrinsic scatter between the two quantities (σ δ ≲ 0.03 dex). This crucial observational constraint precludes strong fluctuations in the recent SFHs of IMGs: simple linear SFH models indicate that a population of IMGs would be limited to only factors of ≲2 change in SFR over 200 Myr and ≲30% on shorter timescales of 20 Myr. No single characteristic SFH for IMGs, such as an exponentially rising/falling burst, can reproduce the individual and joint distribution of ${\rm{\Delta }}\mathrm{log}{L}_{{\rm{H}}\alpha }$ and ${\rm{\Delta }}\mathrm{log}{L}_{\mathrm{UV}}$ . Instead, an ensemble of SFHs is preferred. Finally, we find that IMG SFHs predicted by recent hydrodynamic simulations, in which feedback drives rapid and strong SFR fluctuations, are inconsistent with our observations.

Keywords: galaxies; galaxy evolution; galaxy formation; galaxy spectroscopy; star formation
Climate Outcomes of Earth-similar Worlds as a Function of Obliquity and Rotation Rate

A set of simulations with a 3D global climate model are performed to investigate the roles of obliquity and rotation period in the habitability of Earthlike exoplanets. The simulations cover the obliquity-rotation parameter space, from 0° to 90° in obliquity and 1-128 days in rotation period. The simulated global mean temperatures are warmest at 45° obliquity with fast rotations, due to the modification of the greenhouse effect from the spatial redistribution of clouds and water vapor. The slow-moving insolation-cloud mechanism, previously found in simulations with slow rotations and zero obliquity, also produces a cooling trend from intermediate obliquity to high obliquity, with the coldest climate occurring at 90° obliquity for all rotation periods. At low obliquities and fast rotation, persistent snow and sea ice can form, producing cooler temperatures. A Climate Habitability metric is defined, based on temperature and precipitation, which compares well with observations when applied to a simulation using Earth's obliquity and rotation. Over a wider range of obliquity and rotation period, the Climate Habitability ranges from 10% to 70% of the terrestrial area. Overall, the simulated global mean surface temperature shows a much larger spread across the range of simulated rotation periods at 45° obliquity compared to 0° obliquity. Therefore, we conclude that 3D exoplanet simulations using intermediate obliquities (e.g., 45°) instead of 0° will reveal a wider range of possible climate conditions for specific orbital configurations. In addition, Earth's climate habitability can increase by 25% if the obliquity increases from 23.°5 to 45°.

Keywords: exoplanet atmospheric variability; planetary climates
The Behavior of Bp He-weak Stars in the Far-UV-Paper 5: HD 142301

The analysis of eigthteen spectra of the B8 He-weak Si star HD 142301 recorded by the SWP camera on board the International Ultraviolet Explorer reveals large variations of the far-ultraviolet spectral energy distribution over a time interval of 14 yr and 5 months. Variations are present shortwards of 1700 Å which can be partly attributed to variations of lines and continuum of Si II. Other lines due to C II and Si III also contribute to the opacity in this range too. The variations suggest that silicon is not distributed in a uniform manner over the surface and that the horizontal gradients of the silicon abundance are probably large.

Keywords: chemically peculiar stars; helium-weak stars
Ariel's Elastic Thicknesses and Heat Fluxes

The surface of Ariel displays regions that were resurfaced in the geologically recent past. Some of these regions include large chasmata that exhibit evidence for flexure. To estimate Ariel's heat fluxes, we analyzed flexure associated with the Pixie Group of chasmata, including Pixie, Kewpie, Brownie, Kra, Sylph, and an unnamed chasma, and the Kachina Group of chasmata, which includes Kachina Chasmata. We analyzed topography of these chasmata using digital elevation models developed for this work. Our results indicate that Ariel's elastic thicknesses range between 4.4 ± 0.7 km and 11.4 ± 1.4 km across the imaged surface. The younger Kachina Group has a relatively low elastic thickness of 4.4 ± 0.7 km compared to most chasmata in the older Pixie Group (4.1 ± 0.3 km to 11.4 ± 1.4 km). A pure H2O ice lithosphere would correspond to heat fluxes ranging from 17 to 46 mW m-2 for the Kachina Group and from 6 to 40 mW m-2 for the Pixie Group. Alternatively, if NH3 hydrates are present in Ariel's lithosphere, then the estimated heat fluxes are lower, ranging from 3 to 18 mW m-2 for the Kachina Group and from 1 to 16 mW m-2 for the Pixie Group. These results indicate that accounting for NH3 hydrates in the lithosphere substantially alters the resulting heat flux estimates, which could have important implications for understanding the lithospheric properties of other icy bodies where NH3-bearing species are expected to be present in their lithospheres. Our results are consistent with Ariel experiencing tidal heating generated from mean motion resonances with neighboring satellites in the past, in particular Titania and Miranda.

Keywords: uranian satellites; tectonics; natural satellites (solar system); natural satellite surfaces; planetary science
Astraea: Predicting Long Rotation Periods with 27 Day Light Curves

The rotation periods of planet-hosting stars can be used for modeling and mitigating the impact of magnetic activity in radial velocity measurements and can help constrain the high-energy flux environment and space weather of planetary systems. Millions of stars and thousands of planet hosts are observed with the Transiting Exoplanet Survey Satellite (TESS). However, most will only be observed for 27 contiguous days in a year, making it difficult to measure rotation periods with traditional methods. This is especially problematic for field M dwarfs, which are ideal candidates for exoplanet searches, but which tend to have periods in excess of the 27 day observing baseline. We present a new tool, Astraea, for predicting long rotation periods from short-duration light curves combined with stellar parameters from Gaia DR2. Using Astraea, we can predict the rotation periods from Kepler 4 yr light curves with 13% uncertainty overall (and a 9% uncertainty for periods >30 days). By training on 27 day Kepler light-curve segments, Astraea can predict rotation periods up to 150 days with 9% uncertainty (5% for periods >30 days). After training this tool on these 27 day Kepler light-curve segments, we applied Astraea to real TESS data. For the 195 stars that were observed by both Kepler and TESS, we were able to predict the rotation periods with 55% uncertainty despite the wild differences in systematics.

Keywords: stellar rotation; main sequence stars; random forests
Galaxy and Mass Assembly: A Comparison between Galaxy-Galaxy Lens Searches in KiDS/GAMA

Strong gravitational lenses are a rare and instructive type of astronomical object. Identification has long relied on serendipity, but different strategies—such as mixed spectroscopy of multiple galaxies along the line of sight, machine-learning algorithms, and citizen science—have been employed to identify these objects as new imaging surveys become available. We report on the comparison between spectroscopic, machine-learning, and citizen-science identification of galaxy-galaxy lens candidates from independently constructed lens catalogs in the common survey area of the equatorial fields of the Galaxy and Mass Assembly survey. In these, we have the opportunity to compare high completeness spectroscopic identifications against high-fidelity imaging from the Kilo Degree Survey used for both machine-learning and citizen-science lens searches. We find that the three methods—spectroscopy, machine learning, and citizen science—identify 47, 47, and 13 candidates, respectively, in the 180 square degrees surveyed. These identifications barely overlap, with only two identified by both citizen science and machine learning. We have traced this discrepancy to inherent differences in the selection functions of each of the three methods, either within their parent samples (i.e., citizen science focuses on low redshift) or inherent to the method (i.e., machine learning is limited by its training sample and prefers well-separated features, while spectroscopy requires sufficient flux from lensed features to lie within the fiber). These differences manifest as separate samples in estimated Einstein radius, lens stellar mass, and lens redshift. The combined sample implies a lens candidate sky density of ∼0.59 deg-2 and can inform the construction of a training set spanning a wider mass-redshift space. A combined approach and refinement of automated searches would result in a more complete sample of galaxy-galaxy lens candidates for future surveys.

Keywords: giant elliptical galaxies; strong gravitational lensing; redshift surveys; galaxy dark matter halos
Do Oceanic Convection and Clathrate Dissociation Drive Europa's Geysers?

Water vapor geysers on Europa have been inferred from observations made by the Galileo spacecraft, the Hubble Space Telescope, and the Keck Observatory. Unlike the water-rich geysers observed on Enceladus, Europa's geysers appear to be an intermittent phenomenon, and the dynamical mechanism permitting water to sporadically erupt through a kilometers-thick ice sheet is not well understood. Here we outline and explore the hypothesis that the Europan geysers are driven by CO2 gas released by dissociation and depressurization of CO2 clathrate hydrates initially sourced from the subsurface ocean. We show that CO2 hydrates can become buoyant to the upper ice-water interface under plausible oceanic conditions, namely, if the temperature or salinity conditions of a density-stratified two-layer water column evolve to permit the onset of convection that generates a single mixed layer. To quantitatively describe the eruptions once the CO2 has been released from the hydrate state, we extend a one-dimensional hydrodynamical model that draws from the literature on volcanic magma explosions on Earth. Our results indicate that for a sufficiently high concentration of exsolved CO2, these eruptions develop vertical velocities of ~700 m s-1. These high velocities permit the ejecta to reach heights of ~200 km above the Europan surface, thereby explaining the intermittent presence of water vapor at these high altitudes. Molecules ejected via this process will persist in the Europan atmosphere for a duration of about 10 minutes, limiting the timescale over which geyser activity above the Europan surface may be observable. Our proposed mechanism requires Europa's ice shell thickness to be d ≲ 10 km.

Keywords: europa; ocean planets; volcanism
Galactic Archaeology at High Redshift: Inferring the Nature of GRB Host Galaxies from Abundances

We identify the nature of high-redshift long gamma-ray burst (LGRB) host galaxies by comparing the observed abundance ratios in the interstellar medium with detailed chemical evolution models accounting for the presence of dust. We compare abundance data from LGRB afterglow spectra to abundance patterns as predicted by our models for different galaxy types. We analyze [X/Fe] abundance ratios (where X is C, N, O, Mg, Si, S, Ni, Zn) as functions of [Fe/H]. Different galaxies (irregulars, spirals, spheroids) are, in fact, characterized by different star formation histories, which produce different [X/Fe] versus [Fe/H] relations ("time-delay model"). This allows us to identify the star formation history of the host galaxies and to infer their age (I.e., the time elapsed from the beginning of star formation) at the time of the GRB events. Unlike previous works, we use newer models in which we adopt updated stellar yields and prescriptions for dust production, accretion, and destruction. We consider a sample of seven LGRB host galaxies. Our results suggest that two of them (GRB 050820, GRB 120815A) are star-forming spheroids, two (GRB 081008, GRB 161023A) are spirals, and three (GRB 090926A, GRB 050730, GRB 120327A) are irregulars. The inferred ages of the considered host galaxies span from 10 Myr to slightly more than 1 Gyr.

Keywords: high-redshift galaxies; galaxy evolution; interstellar abundances; interstellar dust; gamma-ray bursts
On the Role of Dynamical Cooling in the Dynamics of Circumbinary Disks

Hydrodynamical interactions between binaries and circumbinary disks (CBDs) play an important role in a variety of astrophysical systems, from young stellar binaries to supermassive black hole binaries. Previous simulations of CBDs have mostly employed locally isothermal equations of state. We carry out 2D viscous hydrodynamic simulations of CBDs around equal-mass, circular binaries, treating the gas thermodynamics by thermal relaxation toward equilibrium temperature (the constant-β cooling ansatz, where β is the cooling time in units of the local Keplerian time). As an initial study, we use the grid-based code Athena++ on a polar grid, covering an extended disk outside the binary co-orbital region. We find that with a longer cooling time, the accretion variability is gradually suppressed, and the morphology of the CBD becomes more symmetric. The disk also shows evidence of hysteresis behavior depending on the initial conditions. Gas cooling also affects the rate of angular momentum transfer between the binary and the CBD, where given our adopted disk thickness and viscosity (H/r ~ 0.1 and α ~ 0.1), the binary orbit expands while undergoing accretion for most β values between 0 and 4.0 except over a narrow range of intermediate β values. The validity of using a polar grid excising the central domain is also discussed.

Keywords: accretion; hydrodynamics; hydrodynamical simulations; black hole physics; circumstellar disks; binary stars
Climate Change and Astronomy: A Look at Long-term Trends on Maunakea

Maunakea is one of the world's primary sites for astronomical observing, with multiple telescopes operating over submillimeter to optical wavelengths. With its summit higher than 4200 m above sea level, Maunakea is an ideal location for astronomy, with a historically dry, stable climate and minimal turbulence above the summit. Under a changing climate, however, we ask how the (above-)summit conditions may have evolved in recent decades since the site was first selected as an observatory location and how future-proof the site might be to continued change. We use data from a range of sources, including in situ meteorological observations, radiosonde profiles, and numerical reanalyses to construct a climatology at Maunakea over the previous 40 yr. We are interested in both the meteorological conditions (e.g., wind speed and humidity) and the image quality (e.g., seeing). We find that meteorological conditions were, in general, relatively stable over the period with few statistically significant trends and with quasi-cyclical interannual variability in astronomically significant parameters such as temperature and precipitable water vapor. We do, however, find that maximum wind speeds have increased over the past decades, with observed wind speeds above 15 m s-1 increasing in frequency by 1%-2%, which may have a significant impact on ground-layer turbulence. Further, we note that while the conditions themselves are not necessarily changing significantly, the combination of conditions that lead to dome closures (i.e., freezing conditions, increased summit wind speeds, and/or high humidities) are worsening to the point that the number of closure conditions have more than doubled in the last 20 yr. Importantly, we find that the Fried parameter has not changed in the last 40 yr, suggesting there has not been an increase in optical turbulence strength above the summit. Ultimately, more data and data sources-including profiling instruments-are needed at the site to ensure continued monitoring into the future and to detect changes in the summit climate.

Keywords: astronomical instrumentation; observatories; earth atmosphere
Stochastic Fragmentation in Protoplanetary Disks under External Irradiation

It has been shown that disk fragmentation within several tens of au from the star is very difficult when the gravitational instability and cooling time criterion are considered. However, in the stochastic fragmentation scenario, things may be different. We investigate stochastic fragmentation in protoplanetary disks. In the stochastic fragmentation model, we consider the effects of the external irradiation on the fragmentation process. For the stochastic fragmentation, owing to the probability of generating bound fragments at longer cooling times relative to the critical value and the short collapsing times in the external irradiation region, the inner boundary of the fragmentation region extends inward to 19 au. We find that the required self-gravitating lifetime for fragmentation in the external irradiation region is far less than the self-gravitating lifetime of the disk. Therefore, once a radius is in the external irradiation region, the fragmentation probability increases quickly to 1 after gravitational instability sets in, and fragmentation occurs within a short period of time. We also find that the self-gravitating lifetime required for fragmentation decreases significantly as the radius increases in the external irradiation region.

Keywords: accretion; stellar accretion disks; gravitational instability