adsabs/SciX_UAT_keywords huggingface dataset:
The Behavior of HgMn Stars in the Far-UV-Paper 26: HD 207857

The analysis of two spectra of the Mn P Ga spectroscopic binary HD 207857 recorded by the SWP camera on board the International Ultraviolet Explorer reveals one instance of variation of the far-ultraviolet flux of this star over a time interval of about 10 months. The orbital period of this spectroscopic binary being 338.2 days, the observed increases of flux below 1800 Å could be due to the presence of a hot companion.

Keywords: chemically peculiar stars; mercury-manganese stars
Flux Eruption Events Drive Angular Momentum Transport in Magnetically Arrested Accretion Flows

We evolve two high-resolution general relativistic magnetohydrodynamic simulations of advection-dominated accretion flows around nonspinning black holes (BHs), each over a duration ~3 × 105 GM BH/c 3. One model captures the evolution of a weakly magnetized (SANE) disk and the other that of a magnetically arrested disk (MAD). Magnetic flux eruptions in the MAD model push out gas from the disk and launch strong winds with outflow efficiencies at times reaching 10% of the incoming accretion power. Despite the substantial power in these winds, average mass outflow rates remain low out to a radius ~100GM BH/c 2, only reaching ~60%-80% of the horizon accretion rate. The average outward angular momentum transport is primarily radial in both modes of accretion, but with a clear distinction: magnetic flux eruption-driven disk winds cause a strong vertical flow of angular momentum in the MAD model, while for the SANE model, the magnetorotational instability (MRI) moves angular momentum mostly equatorially through the disk. Further, we find that the MAD state is highly transitory and nonaxisymmetric, with the accretion mode often changing to a SANE-like state following an eruption before reattaining magnetic flux saturation with time. The Reynolds stress changes directions during such transitions, with the MAD (SANE) state showing an inward (outward) stress, possibly pointing to intermittent MRI-driven accretion in MADs. Pinning down the nature of flux eruptions using next-generation telescopes will be crucial in understanding the flow of mass, magnetic flux, and angular momentum in sub-Eddington accreting BHs like M87* and Sagittarius A*.

Keywords: astrophysical black holes; magnetohydrodynamics; general relativity; accretion; magnetohydrodynamical simulations
A Sample of Massive Black Holes in Dwarf Galaxies Detected via [Fe X] Coronal Line Emission: Active Galactic Nuclei and/or Tidal Disruption Events

The massive black hole (BH) population in dwarf galaxies (M BH ≲ 105 M ⊙) can provide strong constraints on the origin of BH seeds. However, traditional optical searches for active galactic nuclei (AGNs) only reliably detect high-accretion, relatively high-mass BHs in dwarf galaxies with low amounts of star formation, leaving a large portion of the overall BH population in dwarf galaxies relatively unexplored. Here, we present a sample of 81 dwarf galaxies (M ⋆ ≤ 3 × 109 M ⊙) with detectable [Fe X]λ6374 coronal line emission indicative of accretion onto massive BHs, only two of which were previously identified as optical AGNs. We analyze optical spectroscopy from the Sloan Digital Sky Survey and find [Fe X]λ6374 luminosities in the range L [Fe x] ≈ 1036-1039 erg s-1, with a median value of 1.6 × 1038 erg s-1. The [Fe X] λ6374 luminosities are generally much too high to be produced by stellar sources, including luminous Type IIn supernovae (SNe). Moreover, based on known SNe rates, we expect at most eight Type IIn SNe in our sample. That said, the [Fe X]λ6374 luminosities are consistent with accretion onto massive BHs from AGNs or tidal disruption events (TDEs). We find additional indicators of BH accretion in some cases using other emission line diagnostics, optical variability, and X-ray and radio emission (or some combination of these). However, many of the galaxies in our sample only have evidence for a massive BH based on their [Fe X]λ6374 luminosities. This work highlights the power of coronal line emission to find BHs in dwarf galaxies missed by other selection techniques and to probe the BH population in bluer, lower-mass dwarf galaxies.

Keywords: black holes; active galaxies; active galactic nuclei; low-luminosity active galactic nuclei; dwarf galaxies; tidal disruption
Magnetic Switchbacks Heat the Solar Corona

Magnetic switchbacks are short magnetic field reversals ubiquitously observed in the solar wind. The origin of switchbacks remains an important open science question, because of switchbacks' possible role in the heating and acceleration of the solar wind. Here, we report observations of 501 robust switchbacks, using magnetic and plasma measurements from the first eight encounters by the Parker Solar Probe. More than 46% (6%) of switchbacks are rotational (tangential; TD) discontinuities (RD), defined as magnetic discontinuities with large (small) relative normal components of magnetic field and proton velocity. Magnetic reconnection in the solar atmosphere can be a source of the observed RD-type switchbacks. It is discovered that: (1) the RD-to-TD ratio exponentially decays with increasing heliocentric distance at rate 0.06 [R S -1], and (2) TD-type switchbacks contain 64% less magnetic energy than RD-type switchbacks, suggesting that RD-type switchbacks may relax into TD-type switchbacks. It is estimated that relaxing switchbacks generated via magnetic reconnection in the solar atmosphere can transfer an additional 16% of the total reconnected magnetic energy into heating and/or accelerating the solar corona, within 11.6 [R S] of the reconnection site, below the critical Alfvén surface.

Keywords: solar coronal heating; solar magnetic reconnection; solar wind; heliosphere
Boundary Layer Circumplanetary Accretion: How Fast Could an Unmagnetized Planet Spin Up through Its Disk?

Gas giant planets are expected to accrete most of their mass via a circumplanetary disk. If the planet is unmagnetized and initially slowly rotating, it will accrete gas via a radially narrow boundary layer and rapidly spin up. Radial broadening of the boundary layer as the planet spins up reduces the specific angular momentum of accreted gas, allowing the planet to find a terminal rotation rate short of the breakup rate. Here, we use axisymmetric viscous hydrodynamic simulations to quantify the terminal rotation rate of planets accreting from their circumplanetary disks. For an isothermal planet-disk system with a disk scale height h/r = 0.1 near the planetary surface, spin-up switches to spin-down at between 70% and 80% of the planet's breakup rate. In a qualitative difference from vertically averaged models-where spin-down can coexist with mass accretion-we observe decretion accompanying solutions where angular momentum is being lost. The critical spin rate depends upon the disk thickness near the planet. For a disk scale height of h/r = 0.15, the critical spin rate drops to between 60% and 70% of the planet's breakup rate. In the disk outside the boundary layer, we identify meridional circulation flows, which are unsteady and instantaneously asymmetric across the midplane. The simulated flows are strong enough to vertically redistribute solid material in early stage satellite formation. We discuss how exoplanetary rotation measurements, when combined with spectroscopic and variability studies of protoplanets with circumplanetary disks, could determine the role of magnetic and nonmagnetic processes in setting planet spins.

Keywords: hydrodynamical simulations; planet formation; accretion
The Coma Cluster at LOFAR Frequencies. II. The Halo, Relic, and a New Accretion Relic

We present LOw Frequency ARray observations of the Coma Cluster field at 144 MHz. The cluster hosts one of the most famous radio halos, a relic, and a low surface brightness bridge. We detect new features that allow us to make a step forward in the understanding of particle acceleration in clusters. The radio halo extends for more than 2 Mpc, which is the largest extent ever reported. To the northeast of the cluster, beyond the Coma virial radius, we discover an arc-like radio source that could trace particles accelerated by an accretion shock. To the west of the halo, coincident with a shock detected in the X-rays, we confirm the presence of a radio front, with different spectral properties with respect to the rest of the halo. We detect a radial steepening of the radio halo spectral index between 144 and 342 MHz, at ~30' from the cluster center, that may indicate a non-constant re-acceleration time throughout the volume. We also detect a mild steepening of the spectral index toward the cluster center. For the first time, a radial change in the slope of the radio-X-ray correlation is found, and we show that such a change could indicate an increasing fraction of cosmic-ray versus thermal energy density in the cluster outskirts. Finally, we investigate the origin of the emission between the relic and the source NGC 4789, and we argue that NGC 4789 could have crossed the shock originating the radio emission visible between its tail and the relic.

Keywords: galaxy clusters; non-thermal radiation sources; extragalactic radio sources; intracluster medium
Black Hole Spin Measurements in LMC X-1 and Cyg X-1 Are Highly Model Dependent

The black hole spin parameter, a *, was measured to be close to its maximum value of 1 in many accreting X-ray binaries. In particular, a * ≳ 0.9 was found in a number of studies of LMC X-1. These measurements were claimed to take into account both statistical and systematic uncertainties. We perform new measurements using a recent simultaneous observation by NICER and NuSTAR, providing a data set of high quality. We use the disk continuum method together with improved models for coronal Comptonization. With the standard relativistic disk model and optically thin Comptonization, we obtain values of a * similar to those obtained before. We then consider modifications to the standard model. Using a color correction of 2, we find a * ≈ 0.64–0.84. We then consider disks with dissipation in surface layers. To account for that, we assume the standard disk is covered by a warm and optically thick Comptonizing layer. Our model with the lowest χ 2 then yields ${a}_{* }\approx {0.40}_{-0.32}^{+0.41}$ . In order to test the presence of such effects in other sources, we also study an X-ray observation of Cyg X-1 by Suzaku in the soft state. We confirm the previous findings of a * > 0.99 using the standard model, but then we find a weakly constrained ${a}_{* }\approx {0.82}_{-0.74}^{+0.16}$ when including an optically thick Comptonizing layer. We conclude that determinations of the spin using the continuum method can be highly sensitive to the assumptions about the disk structure.

Keywords: x-ray binary stars; x-ray sources; accretion; be stars; astrophysical black holes; rotating black holes
The APOGEE Data Release 16 Spectral Line List

The updated H-band spectral-line list (from λ15000-17000) adopted by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) for the SDSS-IV Data Release 16 (DR16) is presented in this work. The APOGEE line list is a combination of atomic and molecular lines, with data drawn from laboratory, theoretical, and astrophysical sources. Oscillator strengths and damping constants are adjusted using high signal-to-noise, high-resolution spectra of the Sun, and α Boo (Arcturus), as "standard stars." Updates to the DR16 line list, as compared to the previous DR14 version, include the addition of molecular H2O and FeH lines, as well as a much larger (by a factor of ∼4) atomic line list, including a significantly greater number of transitions with hyperfine splitting. More recent references and line lists for the crucial molecules, CO and OH, as well as for C2 and SiH, are also included. In contrast to DR14, DR16 contains measurable lines from the heavy neutron-capture elements cerium (as Ce II), neodymium (as Nd II), and ytterbium (as Yb II), as well as one line from rubidium (as Rb I), which may be detectable in a small fraction of APOGEE red giants.

Keywords: high resolution spectroscopy; atomic spectroscopy; molecular spectroscopy; spectroscopy
MUSE Observations of NGC330 in the Small Magellanic Cloud: Helium Abundance of Bright Main-sequence Stars

We present observations of the most bright main-sequence stars in the Small Magellanic Cloud stellar cluster NGC 330 obtained with the integral-field spectrograph, the Multi Unit Spectroscopic Explorer, at the Very Large Telescope. The use of this valuable instrument allows us to study both photometric and spectroscopic properties of stellar populations of this young star cluster. The photometric data provide us a precise color-magnitude diagram, which seems to support the presence of two stellar populations of ages of ≈18 Myr and ≈30 Myr assuming a metallicity of Z = 0.002. Thanks to the spectroscopic data, we derive a helium abundance of 10 main-sequence stars within the effective radius of Reff = 20″ of NGC 330, thus leading to an estimation of $ \langle \epsilon ({\rm{He}}) \rangle = 10.93 ± 0.05(1σ). The helium elemental abundances of stars likely belonging to the two possible stellar populations do not show differences or dichotomy within the uncertainties. Thus, our results suggest that the two stellar populations of NGC 330, if they exist, share similar original He abundances. If we consider stellar rotation velocity in our analysis, a coeval (30 Myr) stellar population, experiencing different values of rotation, cannot be excluded. In this case, the mean helium abundance $\langle \epsilon (\mathrm{He}){\rangle }_{\mathrm{rot}} obtained in our analysis is 11.00 ± 0.05 dex. We also verified that possible non-LTE (NLTE) effects cannot be identified with our analysis because of the spectral resolution and they are within our derived abundance He uncertainties. Moreover, the analysis of the He abundance as a function of the distance from the cluster center of the observed stars does not show any correlation.

Keywords: stellar abundances; small magellanic cloud; star clusters
Following the Lithium: Tracing Li-bearing Molecules across Age, Mass, and Gravity in Brown Dwarfs

Lithium is an important element for the understanding of ultracool dwarfs because it is lost to fusion at masses above ~68 MJ. Hence, the presence of atomic Li has served as an indicator of the nearby H-burning boundary at about 75 MJ between brown dwarfs and very low mass stars. Historically, the "lithium test," a search for the presence of the Li line at 670.8 nm, has been a marker if an object has a substellar mass. While the Li test could, in principle, be used to distinguish masses of later-type L-T dwarfs, Li is predominantly no longer found as an atomic gas but rather a molecular species such as LiH, LiF, LiOH, and LiCl in cooler atmospheres. The L- and T-type dwarfs are quite faint at 670 nm and thus challenging targets for high-resolution spectroscopy. But only recently have experimental molecular line lists become available for the molecular Li species, allowing molecular Li mass discrimination. Here we generated the latest opacity of these Li-bearing molecules and performed a thermochemical equilibrium atmospheric composition calculation of their abundances. Finally, we computed thermal emission spectra for a series of radiative-convective equilibrium models of cloudy and cloudless brown dwarf atmospheres (with Teff = 500-2400 K and $\mathrm{log}g=4.0\mbox{--}5.0$ ) to understand where the presence of atmospheric lithium-bearing species is most easily detected as a function of brown dwarf mass and age. After atomic Li, the best spectral signatures were found to be LiF at 10.5-12.5 μm and LiCl at 14.5-18.5 μm. Also, LiH shows a narrow feature at ~9.38 μm.

Keywords: brown dwarfs; l dwarfs; t dwarfs; t subdwarfs; spectral line lists
Variability in Short Gamma-Ray Bursts: Gravitationally Unstable Tidal Tails

Short gamma-ray bursts (GRBs) are thought to result from the merger of two neutron stars (NSs) or an NS and a stellar mass black hole (BH). The final stages of the merger are generally accompanied by the production of one or more tidal "tails" of ejecta, which fall back onto the remnant-disk system at late times. Using the results of a linear stability analysis, we show that if the material comprising these tails is modeled as adiabatic and the effective adiabatic index satisfies γ ≥ 5/3, then the tails are gravitationally unstable and collapse to form small-scale knots. We analytically estimate the properties of these knots, including their spacing along the tidal tail, the total number produced, and their effect on the mass return rate to the merger remnant. We perform hydrodynamical simulations of the disruption of a polytropic (with the polytropic and adiabatic indices γ equal), γ = 2 NS, by a BH and find agreement between the predictions of the linear stability analysis and the distribution of knots that collapse out of the instability. The return of these knots to the BH induces variability in the fallback rate, which can manifest as variability in the light curve of the GRB and—depending on how rapidly the instability operates—the prompt emission. The late-time variability induced by the return of these knots is also consistent with the extended emission observed in some GRBs.

Keywords: gamma-ray bursts; analytical mathematics; hydrodynamics; hydrodynamical simulations; accretion
Photon Rings and Shadow Size for General Axisymmetric and Stationary Integrable Spacetimes

There are now multiple direct probes of the region near black hole horizons, including direct imaging with the Event Horizon Telescope (EHT). As a result, it is now of considerable interest to identify what aspects of the underlying spacetime are constrained by these observations. For this purpose, we present a new formulation of an existing broad class of integrable, axisymmetric, stationary spinning black hole spacetimes, specified by four free radial functions, that makes manifest which functions are responsible for setting the location and morphology of the event horizon and ergosphere. We explore the size of the black hole shadow and higher-order photon rings for polar observers, approximately appropriate for the EHT observations of M87*, finding analogous expressions to those for general spherical spacetimes. Of particular interest, we find that these are independent of the properties of the ergosphere, but do directly probe the free function that defines the event horizon. Based on those expressions, we extend the nonperturbative, nonparametric characterization of the gravitational implications of various near-horizon measurements to spinning spacetimes. Finally, we demonstrate this characterization for a handful of explicit alternative spacetimes.

Keywords: black holes; rotating black holes; general relativity; spacetime metric; non-standard theories of gravity
Correlations between Ca II H and K Emission and the Gaia M Dwarf Gap

The Gaia M dwarf gap, also known as the Jao Gap, is a novel feature discovered in the Gaia Data Release 2 G versus BP-RP color–magnitude diagram. This gap represents a 17% decrease in stellar density in a thin magnitude band around the convective transition mass (∼0.35 M ⊙) on the main sequence. Previous work has demonstrated a paucity of Hα emission coincident with the G magnitude of the Jao Gap in the solar neighborhood. The exact mechanism that results in this paucity is as of yet unknown; however, the authors of the originating paper suggest that it may be the result of complex variations to a star's magnetic topology driven by the Jao Gap's characteristic formation and breakdown of stars' radiative transition zones. We present a follow-up investigating another widely used magnetic activity metric, Calcium II H and K emission. Ca II H and K activity appears to share a similar anomalous behavior as Hα does near the Jao Gap magnitude. We observe an increase in star-to-star variation of magnetic activity near the Jao Gap. We present a toy model of a star's magnetic field evolution, which demonstrates that this increase may be due to stochastic disruptions to the magnetic field originating from the periodic-mixing events characteristic of the convective kissing instabilities that drive the formation of the Jao Gap.

Keywords: stellar evolution; stellar evolutionary models
Variations of Peak He/H Ratios in Solar Energetic (E > 4 MeV) Particle Events and Comparisons with Solar Wind He/H Ratios

We determine solar energetic particle (SEP) event He/H peak intensity ratios AHe observed in the four energy channels of the SOHO EPHIN detector covering 4-53 MeV nuc-1. Those SEP AHe values range over two orders of magnitude for 43 large western hemisphere SEP events through the period 1997-2017. AHe of each SEP event are compared with average solar wind (SW) AHe values measured for 8 hr after the SEP event onsets with the solar wind experiment Faraday Cup instrument on Wind. AHe in the 4-8 and 8-21 MeV nuc-1 range are significantly correlated with SW AHe (CC = 0.45 and 0.41), but less so in the 21-41 and 41-53 MeV nuc-1 range, where uncertainties of the AHe values are higher. Median SEP AHe values decline slightly with increasing energy and are ≤0.5 their associated SW values. Both median SEP and SW AHe decline by a factor of ∼2 from the 27 events of cycle 23 to the 16 of cycle 24. Those results suggest a connection between the SW and the seed population of SEP events. An unexpected result is that SEP AHe increases with event peak intensities in our 43 events as He peak intensities increase faster than those of H in larger events.

Keywords: solar wind; solar energetic particles
Post-Newtonian Orbital Effects Induced by the Mass Quadrupole and Spin Octupole Moments of an Axisymmetric Body

The post-Newtonian orbital effects induced by the mass quadrupole and spin octupole moments of an isolated, oblate spheroid of constant density that is rigidly and uniformly rotating on the motion of a test particle are analytically worked out for an arbitrary orbital configuration and without any preferred orientation of the body's spin axis. The resulting expressions are specialized to the cases of (a) equatorial and (b) polar orbits. The opportunity offered by a hypothetical new spacecraft moving around Jupiter along a Juno-like highly elliptical, polar orbit to measure them is preliminarily studied. Although more difficult to be practically implemented, also the case of a less elliptical orbit is considered since it yields much larger figures for the relativistic effects of interest. The possibility of using the S-stars orbiting the supermassive black hole in Sgr A* at the Galactic Center as probes to potentially constrain some parameters of the predicted extended mass distribution surrounding the hole by means of the aforementioned orbital effects is briefly examined.

Keywords: general relativity; celestial mechanics; planetary probes