adsabs/SciX_UAT_keywords huggingface dataset:
ΣSFR-M ∗ Diagram: A Valuable Galaxy Evolution Diagnostic to Complement (s)SFR-M ∗ Diagrams

The specific star formation rate (sSFR) is commonly used to describe the level of galaxy star formation (SF) and to select quenched galaxies. However, since it is a relative measure of the young-to-old population, an ambiguity in its interpretation may arise because a low sSFR can be due to either a substantial previous mass buildup or SF activity that is low. We show, using large samples spanning 0 < z < 2, that the normalization of the star formation rate (SFR) by the physical extent over which SF is taking place (i.e., the SFR surface density, ΣSFR) overcomes this ambiguity. ΣSFR has a strong physical basis, being tied to the molecular gas density and the effectiveness of stellar feedback, so we propose ΣSFR-M * as an important galaxy evolution diagram to complement (s)SFR-M * diagrams. Using the ΣSFR-M * diagram we confirm the Schiminovich et al. result that the level of SF along the main sequence today is only weakly mass-dependent-high-mass galaxies, despite their redder colors, are as active as blue, low-mass ones. At higher redshift, the slope of the "ΣSFR main sequence" steepens, signaling the epoch of bulge buildup in massive galaxies. We also find that ΣSFR based on the optical isophotal radius more cleanly selects both starbursting and spheroid-dominated (early-type) galaxies than the sSFR. One implication of our analysis is that the assessment of the inside-out versus outside-in quenching scenarios should consider both sSFR and ΣSFR radial profiles, because ample SF may be present in bulges with low sSFRs (red color).

Keywords: galaxy evolution; galaxy quenching; galaxy radii; galaxy colors
Magnetized Oscillatory Double-diffusive Convection

We study the properties of oscillatory double-diffusive convection (ODDC) in the presence of a uniform vertical background magnetic field. ODDC takes place in stellar regions that are unstable according to the Schwarzschild criterion and stable according to the Ledoux criterion (sometimes called semiconvective regions), which are often predicted to reside just outside the core of intermediate-mass main-sequence stars. Previous hydrodynamic studies of ODDC have shown that the basic instability saturates into a state of weak wave-like convection, but that a secondary instability can sometimes transform it into a state of layered convection, where layers then rapidly merge and grow until the entire region is fully convective. We find that magnetized ODDC has very similar properties overall, with some important quantitative differences. A linear stability analysis reveals that the fastest-growing modes are unaffected by the field, but that other modes are. Numerically, the magnetic field is seen to influence the saturation of the basic instability, overall reducing the turbulent fluxes of temperature and composition. This in turn affects layer formation, usually delaying it, and occasionally suppressing it entirely for sufficiently strong fields. Further work will be needed, however, to determine the field strength above which layer formation is actually suppressed in stars. Potential observational implications are briefly discussed.

Keywords: magnetohydrodynamics; astrophysical fluid dynamics; magnetohydrodynamical simulations
A New Grid of Model Atmospheres for Metal-poor Ultracool Brown Dwarfs

We present a new grid of model atmospheres and synthetic spectra down to extremely metal-poor ([M/H] ≥ -3.0), ultracool (Teff ≥ 900 K) brown dwarfs based on the PHOENIX code (Hauschildt et al. 1997). The grid is publicly available and continuously updated as new models are computed. The models are aimed at characterizing metal-poor halo brown dwarfs identified in surveys such as Backyard Worlds: Planet 9 (Kuchner et al. 2017; Schneider et al. 2020) and brown dwarfs in globular clusters (Dieball et al. 2019). Our setup includes the formation and depletion by gravitational settling of condensate clouds as defined for the BT-Settl grid of Allard et al. (2012), using the version 15.5 branch of PHOENIX, which was split off from the main branch (Barman et al. 2011; Husser et al. 2013). Our calculations employ the Allard & Homeier cloud formation model (Helling et al. 2008; Allard et al. 2012).

Keywords: stellar atmospheres; brown dwarfs; stellar abundances
Harvesting BAT-GUANO with NITRATES (Non-Imaging Transient Reconstruction and Temporal Search): Detecting and Localizing the Faintest Gamma-Ray Bursts with a Likelihood Framework

The detection of the gravitational wave (GW) counterpart GRB 170817A, underluminous compared to the cosmological gamma-ray burst (GRB) population by a factor of 10,000, motivates significant effort in detecting and localizing a dim, nearby, and slightly off-axis population of short GRBs. Swift's Burst Alert Telescope (BAT) is one of the most sensitive GRB detectors in operation, and the only one that regularly localizes GRBs to arcminute precision, critical to rapid follow-up studies. However, the utility of BAT in targeted subthreshold searches had been historically curtailed by the unavailability of the necessary raw data for analysis. The new availability of time-tagged event data from the GUANO system motivates a renewed focus on developing sensitive targeted search analysis techniques to maximally exploit these data. While computationally cheap, we show that the typical coded-mask deconvolution imaging is limited in its sensitivity due to several factors. We formalize a maximum likelihood framework for the analysis of BAT data wherein signals are forward modeled through the full instrument response, and-coupled with the development of new response models-demonstrate its superior sensitivity to typical imaging via archival comparisons, injection campaigns, and a large number of low-latency GRB discoveries and confirmed arcminute localizations to date. We also demonstrate independent localization of some out-of-field-of-view GRBs for the first time. NITRATES's increased sensitivity boosts the discovery rate of GRB 170817A-like events in BAT by a factor of at least 3-4×, along with enabling joint analyses and searches with other GRB, GW, neutrino, and FRB instruments. We provide public access to the response functions and search pipeline code.

Keywords: gamma-ray bursts; space telescopes; gravitational wave sources
What Produces Dust Polarization in the HH 212 Protostellar Disk at 878 μm: Dust Self-scattering or Dichroic Extinction?

We report the results of new dust polarization of a nearly edge-on disk in the HH 212 protostellar system, obtained with the Atacama Large Millimeter/submillimeter Array at ∼0.035″ (14 au) resolution in a continuum at λ ∼ 878 μm. Dust polarization is detected within ∼44 au of the central source, where a rotationally supported disk has formed. The polarized emission forms V-shaped structures opening to the east, and probably to the west, arising from the disk surfaces and arm structures further away in the east and west, which could be due to potential spiral arms excited in the outer disk. The orientations of the polarization are mainly parallel to the minor axis of the disk, with some in the western part tilting slightly away from the minor axis to form a concave shape with respect to the center. This tilting of the orientations of the polarization is expected from dust self-scattering, e.g., by 50-75 μm grains in a young disk. The intensity and degree of the polarization both peak near the central source with a small dip at the central source and decrease toward the edges. These decreases in the intensity and degree of polarization are expected from dichroic extinction by grains aligned by poloidal fields, but may also be consistent with dust self-scattering if the grain size decreases toward the edges. It is possible that both mechanisms are needed to produce the observed dust polarization, suggesting the presence of both grain growth and poloidal fields in the disk.

Keywords: star formation; accretion; young stellar objects; polarimetry; magnetic fields
A Parametric Model for the Shapes of Black Hole Shadows in Non-Kerr Spacetimes

The Event Horizon Telescope (EHT) is taking the first images of black holes resolved at horizon scales to measure their shadows and probe accretion physics. A promising avenue for testing the hypothesis that astrophysical black holes are described by the Kerr solution to Einstein's equations is to compare the size and shape of the shadow a black hole casts on the surrounding emission to the predictions of the Kerr metric. We develop here an efficient parametric framework to perform this test. We carry out ray-tracing simulations for several parameterized non-Kerr metrics to create a large data set of non-Kerr shadows that probe the allowed parameter space for the free parameters of each metric. We then perform principal components analysis (PCA) on this set of shadows and show that only a small number of components are needed to accurately reconstruct all shadows within the set. We further show that the amplitude of the PCA components are smoothly related to the free parameters in the metrics and, therefore, that these PCA components can be fit to EHT observations in order to place constraints on the free parameters of these metrics that will help quantify any potential deviations from the Kerr solution.

Keywords: black hole physics; black holes; principal component analysis; astronomical simulations; general relativity; spacetime metric; geodesics
UV and Hα HST Observations of Six GASP Jellyfish Galaxies

Star-forming, Hα-emitting clumps are found embedded in the gaseous tails of galaxies undergoing intense ram pressure stripping in galaxy clusters, so-called jellyfish galaxies. These clumps offer a unique opportunity to study star formation under extreme conditions, in the absence of an underlying disk and embedded within the hot intracluster medium. Yet, a comprehensive, high-spatial-resolution study of these systems is missing. We obtained UVIS/Hubble Space Telescope (HST) data to observe the first statistical sample of clumps in the tails and disks of six jellyfish galaxies from the GASP survey; we used a combination of broadband (UV to I) filters and a narrowband Hα filter. HST observations are needed to study the sizes, stellar masses, and ages of the clumps and their clustering hierarchy. These observations will be used to study the clump scaling relations and the universality of the star formation process, and to verify whether a disk is irrelevant, as hinted at by results from jellyfish galaxies. This paper presents the observations, data reduction strategy, and some general results based on the preliminary data analysis. The high spatial resolution of UVIS gives an unprecedentedly sharp view of the complex structure of the inner regions of the galaxies and of the substructures in the galaxy disks. We found clear signatures of stripping in regions very close in projection to the galactic disk. The star-forming regions in the stripped tails are extremely bright and compact and we did not detect a significant number of star-forming clumps in regions where MUSE did not detect any. The paper finally presents the development plan for the project.

Keywords: galaxy evolution; galaxy environments; ram pressure stripped tails; galaxy clusters; galactic and extragalactic astronomy
Reconstructing the Observed Ionizing Photon Production Efficiency at z ∼ 2 Using Stellar Population Models

The ionizing photon production efficiency, ξion, is a critical parameter that provides a number of physical constraints to the nature of the early universe, including the contribution of galaxies to the timely completion of the reionization of the universe. Here, we use KECK/MOSFIRE and ZFOURGE multiband photometric data to explore the ξion of a population of galaxies at z ∼ 2 with ${\mathrm{log}}_{10}({M}_{* }/{M}_{\odot })\sim 9.0\mbox{--}11.5$ . Our 130 Hα detections show a median ${\mathrm{log}}_{10}({\XI }_{\mathrm{ion}}[\mathrm{Hz}\,{\mathrm{erg}}^{-1}])$ of 24.8 ± 0.5 when dust corrected using a Calzetti et al. dust prescription. Our values are typical of mass/magnitude selected ξion values observed in the z ∼ 2 universe. Using BPASSv2.2.1 and Starburst99 stellar population models with simple parametric star formation histories (SFH), we find that even with models that account for effects of stellar evolution with binaries/stellar rotation, model galaxies at ${\mathrm{log}}_{10}({\XI }_{\mathrm{ion}}[\mathrm{Hz}\,{\mathrm{erg}}^{-1}])\lesssim 25.0$ have low Hα equivalent widths (EWs) and redder colors compared to our z ∼ 2 observed sample. We find that introducing starbursts to the SFHs resolve the tension with the models; however, due to the rapid time evolution of ξion, Hα EWs, and rest-frame optical colors, our Monte Carlo simulations of starbursts show that random distributions of starbursts in evolutionary time of galaxies are unlikely to explain the observed distribution. Thus, either our observed sample is specially selected based on their past SFH, or stellar models require additional mechanisms to reproduce the observed high UV luminosity of galaxies for a given production rate of hydrogen ionizing photons.

Keywords: galaxy evolution; emission line galaxies; galaxies; high-redshift galaxies
Temperature in Solar Sources of 3He-rich Solar Energetic Particles and Relation to Ion Abundances

3He-rich solar energetic particles (SEPs) are believed to be accelerated in solar flares or jets by a mechanism that depends on the ion charge-to-mass (Q/M) ratio. It implies that the flare plasma characteristics (e.g., temperature) may be effective in determining the elemental abundances of 3He-rich SEPs. This study examines the relation between the suprathermal (≲0.2 MeV nucleon-1) abundances of the He-Fe ions measured on the Advanced Composition Explorer and temperature in the solar sources for 24 3He-rich SEP events in the period 2010-2015. The differential emission measure technique is applied to derive the temperature of the source regions from the extreme ultraviolet imaging observations on the Solar Dynamics Observatory. The obtained temperature distribution peaks at 2.0-2.5 MK that is surprisingly consistent with earlier findings based on in situ elemental abundance or charge state measurements. We have found a significant anticorrelation between 3He/4He ratio and solar source temperature with a coefficient -0.6. It is most likely caused by non-charge-stripping processes, as both isotopes would be fully ionized in the inferred temperature range. This study shows that the elemental ratios 4He/O, N/O, Ne/O, Si/O, S/O, Ca/O, Fe/O generally behave with temperature as expected from abundance enhancement calculations at ionization equilibrium. The C and Mg, the two species with small changes in the Q/M ratio in the obtained temperature range, show no such behavior with temperature and could be influenced by similar processes as for the 3He/4He ratio.

Keywords: solar extreme ultraviolet emission; solar flares; solar abundances; solar energetic particles
The Carnegie Supernova Project-I: Correlation between Type Ia Supernovae and Their Host Galaxies from Optical to Near-infrared Bands

We present optical and near-infrared (NIR) (ugriYJH) photometry of host galaxies of Type Ia supernovae (SN Ia) observed by the Carnegie Supernova Project-I. We determine host galaxy stellar masses and, for the first time, study their correlation with SN Ia standardized luminosity across optical and NIR (uBgVriYJH) bands. In the individual bands, we find that SNe Ia are more luminous in more massive hosts with luminosity offsets ranging between -0.07 ± 0.03 and -0.15 ± 0.04 mag after light-curve standardization. The slope of the SN Ia Hubble residual-host mass relation is negative across all uBgVriYJH bands with values ranging between -0.029 ± 0.029 and -0.093 ± 0.031 mag dex-1—implying that SNe Ia in more massive galaxies are brighter than expected. The near-constant observed correlations across optical and NIR bands indicate that dust may not play a significant role in the observed luminosity offset-host mass correlation. We measure projected separations between SNe Ia and their host centers, and find that SNe Ia that explode beyond a projected 10 kpc have a 50%- 60% reduction of the dispersion in Hubble residuals across all bands—making them a more uniform subset of SNe Ia. Dust in host galaxies, peculiar velocities of nearby SN Ia, or a combination of both may drive this result as the color excesses of SNe Ia beyond 10 kpc are found to be generally lower than those interior, but there is also a diminishing trend of the dispersion as we exclude nearby events. We do not find that SN Ia average luminosity varies significantly when they are grouped in various host morphological types. Host galaxy data from this work will be useful, in conjunction with future high-redshift samples, in constraining cosmological parameters. * This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

Keywords: type ia supernovae; galaxies; observational cosmology; galaxy photometry
Homologous Microflares with Mass Ejection and Plasma Heating on the Quiet Sun

Study of microflares on the quiet Sun is extremely important in learning the physics of both solar flare and atmospheric heating. Here, for the first time, we report the detailed observations of two homologous microflares from Atmospheric Imager Assembly (AIA) images and Heliospheric Magnetic Imager magnetograms in a very quiet region. The two microflares are observed clearly in the extreme ultraviolet and faintly in the ultraviolet in AIA images. They have an area of 59 Mm2 and 46 Mm2, and a duration of 25 minutes and 22 minutes. The magnetic cancellation continuously takes place during the microflares, and the magnetic flux decrease is apparent, with a value of 4 1018 Mx. The obvious mass ejections are observed during the microflares, and dimming occurs during and after the microflares. The velocity of mass ejection is up to 160 km s1. For the two microflares, the time of peak intensity in 30.4 nm precedes that of the coronal peak emission, up to 2.43.3 minutes. Their thermal energy is estimated to be 1.3 1027 erg and 2.5 1026 erg, respectively, which heats the corona up to 5.8 MK and 2.8 MK. There are many similarities of microflares on the quiet Sun to major flares in the active regions.

Keywords: solar magnetic fields; solar activity; solar ultraviolet emission
Evidence for Cosmic Acceleration Is Robust to Observed Correlations between Type Ia Supernova Luminosity and Stellar Age

Type Ia supernovae (SNe Ia) are powerful standardizable candles for constraining cosmological models and provided the first evidence of the accelerated expansion of the universe. Their precision derives from empirical correlations, now measured from >1000 SNe Ia, between their luminosities, light-curve shapes, colors, and most recently with the stellar mass of their host galaxy. As mass correlates with other galaxy properties, alternative parameters have been investigated to improve SN Ia standardization though none have been shown to significantly alter the determination of cosmological parameters. We re-examine a recent claim, based on 34 SN Ia in nearby passive host galaxies, of a 0.05 mag Gyr-1 dependence of standardized SN Ia luminosity on host age, which, if extrapolated to higher redshifts, would be a bias up to 0.25 mag, challenging the inference of dark energy. We reanalyze this sample of hosts using both the original method and a Bayesian hierarchical model and find after a fuller accounting of the uncertainties the significance of a dependence on age to be ≤2σ and ∼1σ after the removal of a single poorly sampled SN Ia. To test the claim that a trend seen in old stellar populations can be applied to younger ages, we extend our analysis to a larger sample that includes young hosts. We find the residual dependence of host age (after all standardization typically employed for cosmological measurements) to be consistent with zero for 254 SNe Ia from the Pantheon sample, ruling out the large but low significance trend seen in passive hosts.

Keywords: cosmological constant; dark energy; type ia supernovae; observational cosmology; distance indicators
The Gamma-Ray Emission from the Supernova Remnant RX J1713.7-3946 Interacting with Two-phase Medium

We study the origin of gamma-rays from the supernova remnant (SNR) RX J1713.7-3946. Using an analytical model, we calculate the distribution of cosmic rays (CRs) around the SNRs. Motivated by the results of previous studies, we assume that the SNR is interacting with two-phase interstellar medium (ISM), where dense clumps are surrounded by tenuous interclump medium. We also assume that only higher-energy protons (≳TeV) can penetrate the dense clumps. We find that π 0-decay gamma-rays produced by protons reproduce the observed gamma-ray spectrum peaked at ~TeV. On the other hand, it has recently been indicated that the observed ISM column density (N p ), the X-ray surface brightness (I X ), and the gamma-ray surface brightness (I g ) at grid points across the SNR form a plane in the three-dimensional (3D) space of (N p , I X , I g ). We find that the planar configuration is naturally reproduced if the ISM or the CR electron-to-proton ratio is not spherically uniform. We show that the shift of the observed data in the 3D space could be used to identify which of the quantities, the ISM density, the CR electron-to-proton ratio, or the magnetic field, varies in the azimuthal direction of the SNR.

Keywords: supernova remnants; interstellar medium; cosmic ray sources; gamma-ray sources; x-ray sources
Unraveling the Eclipse Mechanism of a Binary Millisecond Pulsar Using Broadband Radio Spectra

The frequency dependent eclipses of the radio emission from millisecond pulsars in compact binary systems provide an opportunity to understand the eclipse mechanism and to determine the nature of the eclipsing medium. We combine multifrequency observations from the upgraded Giant Metrewave Radio Telescope and model the broadband radio spectrum in the optically thick to thin transition regime to constrain the eclipse mechanism. The best-fit model to the eclipse phase spectra favors synchrotron absorption by relativistic electrons. We are able to strongly constrain the frequency of onset of the eclipse to 345 ± 5 MHz, which is an order of magnitude more precise than previous estimates. The dependence on the magnetic field strength of synchrotron absorption allowed us to estimate the magnetic field strength of the eclipse medium to be ~13 G, which is very similar to the values obtained by considering a pressure balance between the incident pulsar wind and the stellar wind of the companion. Applying this method to other millisecond binary pulsars will enable us to determine if the eclipse mechanisms are all the same and also estimate the wind and magnetic field properties of the companion stars. The method could also be applied to other systems where pulsars interact with companion winds in binary systems and in all cases it will lead to a better understanding of the evolutionary processes.

Keywords: binary pulsars; interacting binary stars; close binary stars; radio pulsars; radio spectroscopy; millisecond pulsars; eclipses; eclipsing binary stars