adsabs/SciX_UAT_keywords huggingface dataset:
Effect of Magnetic Diffusion in the Chromosphere on the Solar Wind

We investigate nonideal magnetohydrodynamical (MHD) effects in the chromosphere on the solar wind by performing MHD simulations for Alfvén-wave-driven winds, explicitly including ohmic and ambipolar diffusion. We find that MHD waves are significantly damped in the chromosphere by ambipolar diffusion so that the Alfvénic Poynting flux that reaches the corona is substantially reduced. As a result, the coronal temperature and the mass-loss rate of the solar wind are considerably reduced, compared with those obtained from an ideal MHD case, which is indicative of the great importance of the nonideal MHD effects in the solar atmosphere. However, the temperature and the mass-loss rate are recovered by a small increase in the convection-originated velocity perturbation at the photosphere because of the sensitive dependence of the ambipolar diffusion and reflection of Alfvén waves on the physical properties of the chromosphere. We also find that density perturbations in the corona are reduced by the ambipolar diffusion of Alfvén waves in the chromosphere because the nonlinear generation of compressible perturbations is suppressed.

Keywords: solar wind; stellar winds; magnetohydrodynamical simulations; alfven waves
BURSTT: Bustling Universe Radio Survey Telescope in Taiwan

Fast Radio Bursts (FRBs) are bright millisecond-duration radio transients that appear about 1000 times per day, all-sky, for a fluence threshold 5 Jy ms at 600 MHz. The FRB radio-emission physics and the compact objects involved in these events are subjects of intense and active debate. To better constrain source models, the Bustling Universe Radio Survey Telescope in Taiwan (BURSTT) is optimized to discover and localize a large sample of rare, high-fluence, and nearby FRBs. This population is the most amenable to multi-messenger and multi-wavelength follow-up, which allows a deeper understanding of source mechanisms. BURSTT will provide horizon-to-horizon sky coverage with a half power field-of-view (FoV) of ~104 deg2, a 400 MHz effective bandwidth between 300 and 800 MHz, and subarcsecond localization, which is made possible using outrigger stations that are hundreds to thousands of km from the main array. Initially, BURSTT will employ 256 antennas. After tests of various antenna designs and optimizing the system's performance, we plan to expand to 2048 antennas. We estimate that BURSTT-256 will detect and localize ~100 bright (≥100 Jy ms) FRBs per year. Another advantage of BURSTT's large FoV and continuous operation will be its greatly enhanced monitoring of FRBs for repetition. The current lack of sensitive all-sky observations likely means that many repeating FRBs are currently cataloged as single-event FRBs.

Keywords: radio transient sources; astronomical instrumentation; wide-field telescopes; very long baseline interferometry
Coronal Loop Heating by Nearly Incompressible Magnetohydrodynamic and Reduced Magnetohydrodynamic Turbulence Models

The transport of waves and turbulence beyond the photosphere is central to the coronal heating problem. Turbulence in the quiet solar corona has been modeled on the basis of the nearly incompressible magnetohydrodynamic (NI MHD) theory to describe the transport of low-frequency turbulence in open magnetic field regions. It describes the evolution of the coupled majority quasi-2D and minority slab component, driven by the magnetic carpet and advected by a subsonic, sub-Alfvénic flow from the lower corona. In this paper, we couple the NI MHD turbulence transport model with an MHD model of the solar corona to study the heating problem in a coronal loop. In a realistic benchmark coronal loop problem, we find that a loop can be heated to ~1.5 million K by transport and dissipation of MHD turbulence described by the NI MHD model. We also find that the majority 2D component is as important as the minority slab component in the heating of the coronal loop. We compare our coupled MHD/NI MHD model results with a reduced MHD (RMHD) model. An important distinction between these models is that RMHD solves for small-scale velocity and magnetic field fluctuations and obtains the actual viscous/resistive dissipation associated with their evolution whereas NI MHD evolves scalar moments of the fluctuating velocity and magnetic fields and approximates dissipation using an MHD turbulence phenomenology. Despite the basic differences between the models, their simulation results match remarkably well, yielding almost identical heating rates inside the corona.

Keywords: magnetohydrodynamics; solar coronal loops; interplanetary turbulence
The Initial Magnetic Field Distribution in AB Stars

Stars are born with magnetic fields, but the distribution of their initial field strengths remains uncertain. We combine observations with theoretical models of magnetic field evolution to infer the initial distribution of magnetic fields for AB stars in the mass range of 1.6-3.4 M ⊙. We tested a variety of distributions with different shapes and found that a distribution with a mean of ~800 G and a full width of ~600 G is most consistent with the observed fraction of strongly magnetized stars as a function of mass. Our most-favored distribution is a Gaussian with a mean of μ = 770 G and standard deviation of σ = 146 G. Independent approaches to measure the typical field strength suggest values closer to 2-3 kG, a discrepancy that could suggest a mass-dependent and bimodal initial field distribution, or an alternative theoretical picture for the origin of these magnetic fields.

Keywords: stellar magnetic fields; stellar evolution; low mass stars
Metallicity and α-Element Abundance Gradients along the Sagittarius Stream as Seen by APOGEE

Using 3D positions and kinematics of stars relative to the Sagittarius (Sgr) orbital plane and angular momentum, we identify 166 Sgr stream members observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) that also have Gaia DR2 astrometry. This sample of 63/103 stars in the Sgr trailing/leading arm is combined with an APOGEE sample of 710 members of the Sgr dwarf spheroidal core (385 of them newly presented here) to establish differences of 0.6 dex in median metallicity and 0.1 dex in [α/Fe] between our Sgr core and dynamically older stream samples. Mild chemical gradients are found internally along each arm, but these steepen when anchored by core stars. With a model of Sgr tidal disruption providing estimated dynamical ages (I.e., stripping times) for each stream star, we find a mean metallicity gradient of 0.12 ± 0.03 dex Gyr-1 for stars stripped from Sgr over time. For the first time, an [α/Fe] gradient is also measured within the stream, at 0.02 ± 0.01 dex Gyr-1 using magnesium abundances and at 0.04 ± 0.01 dex Gyr-1 using silicon, which imply that the Sgr progenitor had significant radial abundance gradients. We discuss the magnitude of those inferred gradients and their implication for the nature of the Sgr progenitor within the context of the current family of Milky Way satellite galaxies, and we suggest that more sophisticated Sgr models are needed to properly interpret the growing chemodynamical detail we have on the Sgr system.

Keywords: sagittarius dwarf spheroidal galaxy; milky way stellar halo; chemical abundances; galaxy evolution; tidal tails; galaxy chemical evolution; galaxy abundances; stellar kinematics; stellar abundances; dwarf galaxies
Detecting Exoplanets Closer to Stars with Moderate Spectral Resolution Integral-field Spectroscopy

While radial velocity surveys have demonstrated that the population of gas giants peaks around 3 au, the most recent high-contrast imaging surveys have only been sensitive to planets beyond ~10 au. Sensitivity at small angular separations from stars is currently limited by the variability of the point-spread function. We demonstrate how moderate-resolution integral-field spectrographs can detect planets at smaller separations (≲ 0.3") by detecting the distinct spectral signature of planets compared to the host star. Using OSIRIS (R ≈ 4000) at the W.M. Keck Observatory, we present the results of a planet search via this methodology around 20 young targets in the Ophiuchus and Taurus star-forming regions. We show that OSIRIS can outperform high-contrast coronagraphic instruments equipped with extreme adaptive optics and non-redundant masking in the 0.05"-0.3" regime. As a proof of concept, we present the 34σ detection of a high-contrast M dwarf companion at ≈0.1" with flux ratio of ≈ 0.92% around the field F2 star HD 148352. We developed an open-source Python package, breads, for the analysis of moderate-resolution integral-field spectroscopy data in which the planet and the host star signal are jointly modeled. The diffracted starlight continuum is forward-modeled using a spline model, which removes the need for prior high-pass filtering or continuum normalization. The code allows for analytic marginalization of linear hyperparameters, which simplifies the posterior sampling of other parameters (e.g., radial velocity, effective temperature). This technique could prove very powerful when applied to integral-field spectrographs such as NIRSpec on the JWST and other upcoming first-light instruments on the future Extremely Large Telescopes.

Keywords: direct imaging; exoplanet detection methods; high contrast techniques; high resolution spectroscopy
Runaway Stars as Possible Sources of the Elliptical Ring Structures in NGC 7538

Two large, ∼500 M⊙ elliptical ring structures have been identified in the high-mass star-forming region NGC 7538. The origin of these ring structures is unknown, so we investigate the possibility that a runaway O- or B-type star may have originated in or passed through the region and created either or both of the ring structures via stellar wind. In testing this hypothesis, we identify one candidate star, BD +61 2408, that may have formed the northern ring. It is a B3e star with a mass of ∼8 M⊙ and a surface temperature of ∼20,000 K. Its position, motion, timescale, and spectral type are all consistent with the star being a candidate for having formed one of the ring structures in NGC 7538.

Keywords: star formation
The MOSDEF Survey: The Variation of the Dust Attenuation Curve with Metallicity

We derive a UV-optical stellar dust attenuation curve of galaxies at z = 1.4-2.6 as a function of gas-phase metallicity. We use a sample of 218 star-forming galaxies, excluding those with very young or heavily obscured star formation, from the MOSFIRE Deep Evolution Field survey with Hα, Hβ, and [N II]λ 6585 spectroscopic measurements. We constrain the shape of the attenuation curve by comparing the average flux densities of galaxies sorted into bins of dust obscuration using Balmer decrements, i.e., Hα-to-Hβ luminosities. The average attenuation curve for the high-metallicity sample ( $12+\mathrm{log}({\rm{O}}/{\rm{H}})\gt 8.5$ , corresponding to ${M}_{* }\gtrsim {10}^{10.4}$ ${M}_{\odot }$ ) has a shallow slope, identical to that of the Calzetti local starburst curve, and a significant UV 2175 Å extinction bump that is ∼0.5× the strength of the Milky Way bump. On the other hand, the average attenuation curve of the low-metallicity sample ( $12+\mathrm{log}({\rm{O}}/{\rm{H}})\sim 8.2-8.5$ ) has a steeper slope similar to that of the SMC curve, only consistent with the Calzetti slope at the 3σ level. The UV bump is not detected in the low-metallicity curve, indicating the relative lack of the small dust grains causing the bump at low metallicities. Furthermore, we find that on average the nebular reddening (E(B - V)) is a factor of 2 times larger than that of the stellar continuum for galaxies with low metallicities, while the nebular and stellar reddening are similar for galaxies with higher metallicities. The latter is likely due to a high surface density of dusty clouds embedding the star-forming regions but also reddening the continuum in the high-metallicity galaxies.

Keywords: galaxy evolution; interstellar dust extinction; high-redshift galaxies; galaxy abundances; chemical abundances; galaxy properties; galaxies; interstellar dust; interstellar abundances
BICEP/Keck. XVII. Line-of-sight Distortion Analysis: Estimates of Gravitational Lensing, Anisotropic Cosmic Birefringence, Patchy Reionization, and Systematic Errors

We present estimates of line-of-sight distortion fields derived from the 95 and 150 GHz data taken by BICEP2, BICEP3, and the Keck Array up to the 2018 observing season, leading to cosmological constraints and a study of instrumental and astrophysical systematics. Cosmological constraints are derived from three of the distortion fields concerning gravitational lensing from large-scale structure, polarization rotation from magnetic fields or an axion-like field, and the screening effect of patchy reionization. We measure an amplitude of the lensing power spectrum ${A}_{L}^{\phi \phi }=0.95\pm 0.20$ . We constrain polarization rotation, expressed as the coupling constant of a Chern-Simons electromagnetic term g a γ ≤ 2.6 × 10-2/H I , where H I is the inflationary Hubble parameter, and an amplitude of primordial magnetic fields smoothed over 1 Mpc B 1Mpc ≤ 6.6 nG at 95 GHz. We constrain the rms of optical depth fluctuations in a simple "crinkly surface" model of patchy reionization, finding A τ < 0.19 (2σ) for the coherence scale of L c = 100. We show that all of the distortion fields of the 95 and 150 GHz polarization maps are consistent with simulations including lensed ΛCDM, dust, and noise, with no evidence for instrumental systematics. In some cases, the EB and TB quadratic estimators presented here are more sensitive than our previous map-based null tests at identifying and rejecting spurious B-modes that might arise from instrumental effects. Finally, we verify that the standard deprojection filtering in the BICEP/Keck data processing is effective at removing temperature to polarization leakage.

Keywords: observational cosmology; cosmic microwave background radiation; weak gravitational lensing; primordial magnetic fields; reionization
MOSEL Survey: JWST Reveals Major Mergers/strong Interactions Drive the Extreme Emission Lines in the Early Universe

Extreme emission line galaxies (EELGs), where nebular emissions contribute 30%-40% of the flux in certain photometric bands, are ubiquitous in the early Universe (z > 6). We utilize deep NIRCam imaging from the JWST Advanced Deep Extragalactic Survey (JADES) to investigate the properties of companion galaxies (projected distance <40 kpc, ∣dv∣ < 10,000 km s-1) around EELGs at z ~ 3. Tests with TNG100 simulation reveal that nearly all galaxies at z = 3 will merge with at least one companion galaxy selected using similar parameters by z = 0. The median mass ratio of the most massive companion and the total mass ratio of all companions around EELGs is more than 10 times higher the control sample. Even after comparing with a stellar mass and stellar mass plus specific star formation rate (SFR)-matched control sample, EELGs have 3 to 5 times higher mass ratios than the brightest companion and total mass ratio of all companions. Our measurements suggest that EELGs are more likely to be experiencing strong interactions or undergoing major mergers irrespective of their stellar mass or specific SFRs. We suspect that gas cooling induced by strong interactions and/or major mergers could be triggering the extreme emission lines, and the increased merger rate might be responsible for the overabundance of EELGs at z > 6.

Keywords: emission line galaxies; galaxy evolution; interacting galaxies; galaxy mergers; high-redshift galaxies
EIGER. V. Characterizing the Host Galaxies of Luminous Quasars at z ≳ 6

We report JWST/NIRCam measurements of quasar host galaxy emissions and supermassive black hole (SMBH) masses for six quasars at 5.9 < z < 7.1 in the Emission-line galaxies and Intergalactic Gas in the Epoch of Reionization (EIGER) project. We obtain deep NIRCam imaging in the F115W, F200W, and F356W bands, as well as F356W grism spectroscopy of the quasars. We use bright unsaturated stars to construct models of the point-spread functions (PSFs) and estimate the errors of these PSFs. We then measure or constrain the fluxes and morphology of the quasar host galaxies by fitting the quasar images as a point source plus an exponential disk. We successfully detect the host galaxies of three quasars, which have host-to-quasar-flux ratios of ∼1%–5%. Spectral energy distribution fitting suggests that these quasar host galaxies have stellar masses of M * ≳ 1010 M ⊙. For quasars with host galaxy nondetections, we estimate the upper limits of their stellar masses. We use the grism spectra to measure the Hβ line profile and the continuum luminosity, then estimate the SMBH masses for the quasars. Our results indicate that the positive relation between SMBH masses and host galaxy stellar masses already exists at redshift z ≳ 6. The quasars in our sample show a high BH-to-stellar-mass ratio of M BH/M * ∼ 0.15, which is about ∼2 dex higher than local relations. We find that selection effects only contribute partially to the high M BH/M * ratios of high-redshift quasars. This result hints at a possible redshift evolution of the M BH–M * relation.

Keywords: quasars; supermassive black holes
ATOCA: an Algorithm to Treat Order Contamination. Application to the NIRISS SOSS Mode

After a successful launch, the James Webb Space Telescope is preparing to undertake one of its principal mission objectives, the characterization of the atmospheres of exoplanets. The Single Object Slitless Spectroscopy (SOSS) mode of the Near Infrared Imager and Slitless Spectrograph (NIRISS) is the only observing mode that has been specifically designed for this objective. It features a wide simultaneous spectral range (0.6-2.8 μm) through two spectral diffraction orders. However, due to mechanical constraints, these two orders overlap slightly over a short range, potentially introducing a "contamination" signal in the extracted spectrum. We show that for a typical box extraction, this contaminating signal amounts to 1% or less over the 1.6-2.8 μm range (order 1), and up to 1% over the 0.85-0.95 μm range (order 2). For observations of exoplanet atmospheres (transits, eclipses or phase curves) where only temporal variations in flux matter, the contamination signal typically biases the results by order of 1% of the planetary atmosphere spectral features strength. To address this problem, we developed the Algorithm to Treat Order ContAmination (ATOCA). By constructing a linear model of each pixel on the detector, treating the underlying incident spectrum as a free variable, ATOCA is able to perform a simultaneous extraction of both orders. We show that, given appropriate estimates of the spatial trace profiles, the throughputs, the wavelength solutions, as well as the spectral resolution kernels for each order, it is possible to obtain an extracted spectrum accurate to within 10 ppm over the full spectral range.

Keywords: spectrophotometry; space telescopes; exoplanet atmospheres; astronomy data reduction; transmission spectroscopy; astronomy data modeling
Multiwavelength Modeling the SED of Luminous Supersoft X-Ray Sources in Large Magellanic Cloud and Small Magellanic Cloud

Classical supersoft X-ray sources (SSSs) are understood as close binary systems in which a massive white dwarf accretes from its companion at rates sustaining steady hydrogen burning on its surface generating bolometric luminosities of 1036-2 × 1038 erg s-1. Here, we perform for the first time the global supersoft X-rays to near-infrared (NIR) spectral energy distribution (SED) for the brightest SSSs in the Large Magellanic Cloud and Small Magellanic Cloud. We test a model in which the ultraviolet-NIR is dominated by emission from a compact (unresolved) circumstellar nebula represented by the ionized gas outflowing from the SSS. The SED models correspond to luminosities of SSSs of a few times 1038-1039 erg s-1, radiating at blackbody temperatures of ≈3 × 105 K, and indicate a nebular continuum, whose emission measure of ≳2 × 1060 cm-3 corresponds to a wind mass loss at rates ≳2 × 10-6 M ⊙ yr-1. Such extreme parameters suggest that the brightest SSSs could be unidentified optical novae in a post-nova SSS state sustained at a high long-lasting luminosity by resumed accretion, possibly at super-Eddington rates. New observations and theoretical multiwavelength modeling of the global SED of SSSs are needed to reliably determine their parameters, and thus understand their proper stage in stellar evolution.

Keywords: fundamental parameters of stars; x-ray sources
The Impact of an Active Galactic Nucleus on Polycyclic Aromatic Hydrocarbon Emission in Galaxies: The Case of Ring Galaxy NGC 4138

We present a focused study of radially resolved varying PAH emission in the low-luminosity active galactic nucleus (AGN)-host NGC 4138 using deep Spitzer/infrared spectrograph spectral maps. Using new model PAH spectra, we investigate whether these variations could be associated with changes to the PAH grain size distribution due to photodestruction by the AGN. Separately, we model the effects of the varying radiation field within NGC 4138, and we use this model to predict the corresponding changes in the PAH emission spectrum. We find that PAH band ratios are strongly variable with radius in this galaxy with short-to-long wavelength band ratios peaking in the starburst ring. The changing mix of starlight appears to have a considerable effect on the trends in these band ratios, and our radiation model predicts the shapes of these trends. However, the amplitude of observed variation is ∼2.5 × larger than predicted for some ratios. A cutoff of small grains in the PAH size distribution, as has been suggested for AGN, together with changes in PAH ionization fraction could explain the behavior of the shorter bands, but this model fails to reproduce longer band behaviors. Additionally, we find that short-to-long wavelength PAH band ratios increase slightly within ∼270 pc of the center, suggesting that the AGN may directly influence PAH emission there.

Keywords: polycyclic aromatic hydrocarbons; agn host galaxies; interstellar medium; low-luminosity active galactic nuclei; infrared astronomy
Black Hole Accretion with Saturated Magnetic Pressure and Disk Wind

We construct an analytical black hole accretion disk model that incorporates both magnetic pressure and disk wind, which are found to be important from numerical simulations. A saturated magnetic pressure that relates the Alfvén velocity with local Keplerian velocity and gas sound speed is assumed in addition to radiation and gas pressures. The mass accretion rate is assumed to have a power-law form in response to mass loss in the wind. We find three sets of self-consistent solutions that are thermally stable and satisfy the model assumptions. At high accretion rates, the disk is geometrically and optically thick, resembling the slim disk solution. At relatively low accretion rates, our model predicts an accretion flow consisting of a geometrically thin and optically thick outer disk (similar to the standard disk), and a geometrically thick and optically thin inner disk (similar to the advection-dominated accretion flow, or ADAF). Thus, this is a natural solution for a truncated disk connected with an inner ADAF, which has been proposed to explain some observations. The magnetic pressure plays a more important role than the outflow in shaping the disk structure. The observed disk luminosity tends to saturate around 8 times the Eddington limit, suggesting that supercritical accretion onto black holes can be used for a black hole mass estimate, or a standard candle with known black hole masses.

Keywords: accretion; high energy astrophysics; black holes; hydrodynamics