adsabs/SciX_UAT_keywords huggingface dataset:
Rovibrationally Resolved Photodissociation of AlH via Excited Electronic States

Photodissociation processes are of great importance for modeling interstellar chemistry since it is a key destruction pathway for small molecules. Here, we present a detailed ab initio study of AlH photodissociation. Potential energy curves and transition dipole moments for AlH are obtained by using the internally contracted multireference configuration interaction method and the Davidson correction (icMRCI+Q), as well as the aug-cc-pV6Z basis set. Except for the X1Σ+, A1Π, and C1Σ+ states, five higher excited 31Σ+, 21Π, 31Π, 41Σ+, and 41Π states are considered in order to investigate the photodissociation pathways in the vacuum ultraviolet region. State-resolved cross sections of transitions from all the rovibrational levels of the X1Σ+ state to seven singlet excited states are computed for photon wavelengths ranging from 500 Å to the threshold. Photodissociation cross sections in local thermal equilibrium (LTE) are obtained at temperatures from 500 to 10,000 K. Applications of the LTE cross sections to compute photodissociation rates in the standard interstellar radiation field and blackbody radiation field are briefly discussed.

Keywords: astrochemistry; molecule destruction; interstellar molecules; radiative processes
Models of the Mass-ejection Histories of Pre-planetary Nebulae. IV. Magnetized Winds and the Origins of Jets, Bullets, and FLIERs

This paper explores the influences and consequences of toroidal magnetic fields in shaping the visible lobes of pre-planetary nebulae (prePNe). To probe these influences we start with the steady, diverging, and field-free wind model of our previous papers and add weak to moderate toroidal fields to the winds in order to generate arrays of outcomes after 500 yr, after which the structures grow almost homologously. As expected, toroidal fields in the stellar winds invariably form very thin and dense axial features whose structure is best described as a thin cold jet with an ultra-dense and neutral leading knot, or bullet, at its tip. The speed of the leading knot depends only on the density contrast (the ratio of injected to ambient gas densities at the nozzle) and wind injection speed, but not on the field strength or opening angle. The lobes formed by the ram pressure of the winds take a variety of forms and sizes that depend primarily on the geometric structure of the injected gas and the density contrast. About 20% of the Hubble Space Telescope images of prePNe show unique signatures of shaping by toroidal fields. Pairs of low-ionization knots seen along the major axis of fully ionized PNe, often called FLIERs, are easily explained as the very dense, cold, and neutral remnants of magnetically formed knots.

Keywords: protoplanetary nebulae; post-asymptotic giant branch; stellar winds; jets
Breaking the Degeneracy in Magnetic Cataclysmic Variable X-Ray Spectral Modeling Using X-Ray Light Curves

We present an analysis of mock X-ray spectra and light curves of magnetic cataclysmic variables using an upgraded version of the 3D CYCLOPS code. This 3D representation of the accretion flow allows us to properly model total and partial occultation of the postshock region by the white dwarf as well as the modulation of the X-ray light curves due to the phase-dependent extinction of the preshock region. We carried out detailed postshock region modeling in a four-dimensional parameter space by varying the white dwarf mass and magnetic field strength as well as the magnetosphere radius and the specific accretion rate. To calculate the postshock region temperature and density profiles, we assumed equipartition between ions and electrons; took into account the white dwarf gravitational potential, the finite size of the magnetosphere, and a dipole-like magnetic field geometry; and considered cooling by both bremsstrahlung and cyclotron radiative processes. By investigating the impact of the parameters on the resulting X-ray continuum spectra, we show that there is an inevitable degeneracy in the four-dimensional parameter space investigated here, which compromises X-ray continuum spectral fitting strategies and can lead to incorrect parameter estimates. However, the inclusion of X-ray light curves in different energy ranges can break this degeneracy, and it therefore remains, in principle, possible to use X-ray data to derive fundamental parameters of magnetic cataclysmic variables, which represents an essential step toward understanding their formation and evolution.

Keywords: cataclysmic variable stars; magnetic variable stars
A New Definition of Exoplanet Habitability: Introducing the Photosynthetic Habitable Zone

It may be possible to detect biosignatures of photosynthesis in an exoplanet's atmosphere. However, such a detection would likely require a dedicated study, occupying a large amount of telescope time. It is therefore prudent, while searching for signs of life that we may recognize, to pick the best target possible. In this work, we present a new region, the photosynthetic habitable zone (PHZ)-the distance from a star where both liquid water and oxygenic photosynthesis can occur. It is therefore the region where detectable biosignatures of oxygenic photosynthesis are most likely to occur. Our analysis indicates that in the most ideal conditions for life and no atmospheric effects, the PHZ is almost as broad as the habitable zone. On the other hand, if conditions for life are anything less than excellent and atmospheric effects are even moderate, the PHZ is concentrated at larger separations around more massive stars. Such cases are also not tidally locked to their host star, which could result in planetary rotation periods similar to the Earth's. We identify five planets, Kepler-452 b, Kepler-1638 b, Kepler-1544 b, Kepler-62 e, and Kepler-62 f, that are consistently in the PHZ for a variety of conditions, and we predict their day lengths to be between 9 and 11 hr. We conclude that the parameter space in which we should search for signs of life is much narrower than the standard habitable zone.

Keywords: astrobiology; exoplanets
Rapid Black Hole Spin-down by Thick Magnetically Arrested Disks

Black hole (BH) spin can play an important role in galaxy evolution by controlling the amount of energy and momentum ejected from near the BH into the surroundings. We focus on radiatively inefficient and geometrically thick magnetically arrested disks (MADs) that can launch strong BH-powered jets. With an appropriately chosen adiabatic index, these systems can describe either the low-luminosity or highly super-Eddington BH accretion regimes. Using a suite of 3D general relativistic magnetohydrodynamic simulations, we find that for any initial spin, an MAD rapidly spins down the BH to the equilibrium spin of 0 < a eq ≲ 0.1, very low compared to a eq = 1 for the standard thin luminous (Novikov-Thorne) disks. This implies that rapidly accreting (super-Eddington) BHs fed by MADs tend to lose most of their rotational energy to magnetized relativistic outflows. In an MAD, a BH only needs to accrete 20% of its own mass to spin down from a = 1-0.2. We construct a semi-analytic model of BH spin evolution in MADs by taking into account the torques on the BH due to both the hydrodynamic disk and electromagnetic jet components, and find that the low value of a eq is due to both the jets slowing down the BH rotation and the disk losing a large fraction of its angular momentum to outflows. Our results have crucial implications for how BH spins evolve in active galaxies and other systems such as collapsars, where the BH spin-down timescale can be short enough to significantly affect the evolution of gamma-ray emitting BH-powered jets.

Keywords: accretion; black holes; magnetohydrodynamics; jets
The Highly Self-absorbed Blazar PKS 1351-018

PKS 1351-018 at a redshift of z = 3.71 is one of the most luminous, steady synchrotron sources with a luminosity >1047 erg s-1. The synchrotron luminosity does not seem to have varied by more than ~25% over 35 yr. In order to appreciate this remarkable behavior, if it were at z = 0.5, it would have a flux density at 15 GHz in a range of 110-137 Jy over 11 yr. In spite of this steady behavior, two strong γ-ray flares ≲1049 erg s-1 were detected in 2011 and 2016. There is a blazar-like behavior coexisting with the steady behavior. This study is aimed at elucidating the dual nature of this source. We find that the radio source is extremely compact with a bright core and a steep spectrum secondary component, 12 mas away, that appears to be constant in position and flux density in six epochs from 1995-2018. We estimate that a jet with a time averaged power of (5.2 ± 3.2) × 1045 erg s-1 terminates in this lobe, which is advancing ≳0.9c at a deprojected distance of 1-3 kpc from the central engine. This is the rare case of a young (~6000 yr), very powerful radio source that is viewed a few degrees from the jet axis. We find evidence of a high velocity (4000 km s-1), high ionization wind emanating from a luminous quasar. The young radio jet appears to experience modest bending as it navigates through the intense quasar environment.

Keywords: radio loud quasars
Detecting Long-period Variability in the SDSS Stripe 82 Standards Catalog

We report the results of a search for long-period (100 < P < 600 days) periodic variability in the SDSS Stripe 82 standards catalog. The SDSS coverage of Stripe 82 enables such a search because there are on average 20 observations per band in ugriz bands for about one million sources, collected over about 6 yr, with a faint limit of r ~ 22 mag and precisely calibrated 1%-2% photometry. We calculated the periods of variable source candidates in this sample using the Lomb-Scargle periodogram and considered the three highest periodogram peaks in each of the gri filters as relevant. Only those sources with gri periods consistent within 0.1% were later studied. We use the Kuiper statistic to ensure uniform distribution of data points in phased light curves. We present five sources with the spectra consistent with quasar spectra and plausible periodic variability. This SDSS-based search bodes well for future sensitive large-area surveys, such as the Rubin Observatory Legacy Survey of Space and Time, which, due to its larger sky coverage (about a factor of 60) and improved sensitivity (~2 mag), will be more powerful for finding such sources.

Keywords: astronomy data analysis; period determination; time series analysis; quasars; gravitational wave sources
Validation of the EDGES Low-band Antenna Beam Model

The response of the antenna is a source of uncertainty in measurements with the Experiment to Detect the Global Epoch of Reionization Signature (EDGES). We aim to validate the electromagnetic beam model of the low-band (50-100 MHz) dipole antenna with comparisons between models and against data. We find that simulations of a simplified model of the antenna over an infinite perfectly conducting ground plane are, with one exception, robust to changes in the numerical electromagnetic solver code or algorithm. For simulations of the antenna with the actual finite ground plane and realistic soil properties, we find that two out of three numerical solvers agree well. Applying our analysis pipeline to a simulated drift-scan observation from an early EDGES low-band instrument that had a 10 m × 10 m ground plane, we find residual levels after fitting and removing a five-term foreground model from the simulated data binned in local sidereal time (LST) average about 250 mK with ±40 mK variation between numerical solvers. A similar analysis of the primary 30 m × 30 m sawtooth ground plane reduced the LST-averaged residuals to about 90 mK with ±10 mK between the two viable solvers. More broadly we show that larger ground planes generally perform better than smaller ground planes. Simulated data have a power that is within 4% of real observations, a limitation of net accuracy of the sky and beam models. We observe that residual spectral structures after foreground model fits match qualitatively between simulated data and observations, suggesting that the frequency dependence of the beam is reasonably represented by the models. We find that a soil conductivity of 0.02 S m-1 and relative permittivity of 3.5 yield good agreement between simulated spectra and observations. This is consistent with the soil properties reported by Sutinjo et al. for the Murchison Radio-astronomy Observatory, where EDGES is located.

Keywords: radio astronomy; observational astronomy; astronomical instrumentation; single-dish antennas; radio telescopes; cosmology; bianchi cosmology; reionization; early universe; neutral hydrogen clouds
First Detection of Outflowing Gas in the Outskirts of the Broad-line Region in 1H 0707‑495

We use near-infrared spectroscopy covering simultaneously the zJHK bands to look for outflowing gas from the nuclear environment of 1H 0707‑495 taking advantage that this region is dominated by low-ionization broad-line region lines, most of them isolated. We detect broad components in H I, Fe II, and O I, at rest to the systemic velocity, displaying FWHM values of ∼500 km s‑1, consistent with its classification as a narrow-line Seyfert 1 active galactic nucleus. Moreover, most lines display a conspicuous blue-asymmetric profile, modeled using a blueshifted component, whose velocity shift reaches up to ∼826 km s‑1. This last feature can be interpreted in terms of outflowing gas already observed in X-ray and UV lines in 1H 0707‑495 but not detected before in the low-ionization lines. We discuss the relevance of our findings within the framework of the wind scenario already proposed for this source and suggest that the wind extends well into the narrow-line region owing to the observation of a blueshifted component in the forbidden line of [S III] λ9531. *Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia e Inovações do Brasil (MCTI/LNA), the US National Science Foundation's NOIRLab, the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU).

Keywords: seyfert galaxies; x-ray active galactic nuclei; galaxy spectroscopy
Magnetized Fingering Convection in Stars

Fingering convection (also known as thermohaline convection) is a process that drives the vertical transport of chemical elements in regions of stellar radiative zones where the mean molecular weight increases with radius. Recently, Harrington & Garaud used three-dimensional direct numerical simulations (DNS) to show that a vertical magnetic field can dramatically enhance the rate of chemical mixing by fingering convection. Furthermore, they proposed a so-called "parasitic saturation" theory to model this process. Here, we test their model over a broad range of parameter space, using a suite of DNS of magnetized fingering convection, varying the magnetic Prandtl number, magnetic field strength, and composition gradient. We find that the rate of chemical mixing measured in the simulations is not always predicted accurately by their existing model, in particular when the magnetic diffusivity is large. We then present an extension of the Harrington & Garaud model which resolves this issue. When applied to stellar parameters, it recovers the results of Harrington & Garaud except in the limit where fingering convection becomes marginally stable, where the new model is preferred. We discuss the implications of our findings for stellar structure and evolution.

Keywords: astrophysical fluid dynamics; magnetohydrodynamics; magnetohydrodynamical simulations; stellar abundances; red giant bump; stellar evolutionary models; stellar interiors; stellar magnetic fields; stellar physics; stellar astronomy
Global Energetics of Solar Flares. XI. Flare Magnitude Predictions of the GOES Class

In this study we determine scaling relationships of observed solar flares that can be used to predict upper limits of the Geostationary Orbiting Earth Satellite (GOES)-class magnitude of solar flares. The flare prediction scheme is based on the scaling of the slowly varying potential energy Ep(t), which is extrapolated in time over an interval of Δt ≤ 24 hr. The observed scaling of the dissipated energy Ediss scales with the potential field energy as ${E}_{\mathrm{diss}}\propto {E}_{p}^{1.32}$ . In addition, the observed scaling relationship of the flare volume, $V\propto {E}_{\mathrm{diss}}^{1.17}$ , the multi-thermal energy, Eth ∝ V0.76, the flare emission measure $(\mathrm{EM})\propto {E}_{\mathrm{th}}^{0.79}$ , the EM-weighted temperature Tw, and the GOES flux, ${F}_{8}{(t)\propto {E}_{p}(t)}^{0.92}$ , allows us then to predict an upper limit of the GOES-class flare magnitude in the extrapolated time window. We find a good correlation (cross-correlation coefficient (CCC) ≈ 0.7) between the observed and predicted GOES-class flare magnitudes (in 172 X- and M-class events). This is the first algorithm that employs observed scaling laws of physical flare parameters to predict GOES flux upper limits, an important capability that complements previous flare prediction methods based on machine-learning algorithms used in space-weather forecasting.

Keywords: solar physics; solar flares
Imaging of HH80-81 Jet in the Near-infrared Shock Tracers H2 and [Fe II]

The HH80-81 system is one of the most powerful jets driven by a massive protostar. We present new near-infrared (NIR) line imaging observations of the HH80-81 jet in the H2 (2.122 μm) and [Fe II] (1.644 μm) lines. These lines trace not only the jet close to the exciting source but also the knots located farther away. We have detected nine groups of knot-like structures in the jet including HH80 and HH81 spaced 0.2-0.9 pc apart. The knots in the northern arm of the jet show only [Fe II] emission closer to the exciting source, a combination of [Fe II] and H2 at intermediate distances, and solely H2 emission farther outwards. Toward the southern arm, all the knots exhibit both H2 and [Fe II] emission. The nature of the shocks is inferred by combining the NIR observations with radio and X-ray observations from the literature. In the northern arm, we infer the presence of strong dissociative shocks, in the knots located close to the exciting source. The knots in the southern arm that include HH80 and HH81 are explicable as a combination of strong and weak shocks. The mass-loss rates of the knots determined from [Fe II] luminosities are in the range ~3.0 × 10-7-5.2 × 10-5 M ⊙ yr-1, consistent with those from massive protostars. Toward the central region, close to the driving source of the jet, we have observed various arcs in H2 emission that resemble bow shocks, and strings of H2 knots that reveal traces of multiple outflows.

Keywords: shocks; herbig-haro objects; jets; protostars
Interaction of Inner Heliosheath Ions with Electromagnetic Ion Cyclotron Waves

Inner heliosheath (IHS) ions are expected to be exposed to various waves, shocks, and turbulence, which can affect ion distributions and thus their charge-exchange rates with interstellar neutral atoms. This work addresses the potential significance of electromagnetic ion cyclotron (EMIC) waves under expected IHS conditions. From a kinetic dispersion relation, we find the possibility of frequent triggering of EMIC instability in the IHS. The threshold anisotropy of proton temperatures required for the instability is small, (T ⊥-T ∣∣)/T ∣∣ ≈ 0.1 or less, mainly due to high plasma β (ratio of the plasma to magnetic pressures). Numerical calculations on the scattering of ions (protons, He+, and He2+ with energy of 0.01-50 keV) based on two models for EMIC waves with a moderate intensity indicate significant scattering in the pitch angle (mostly a few tens of degrees) and energy (mostly a few tens of percent) although details depend on the energy and pitch angle of each species and adopted EMIC wave models. This occurs on a short timescale (<100 times the gyro-period of each ion species). Resonant scattering in a few to a few tens of keV (corresponding to the expected pickup ion energy in the IHS) is easily expected unless the wavenumber is too large. The scattering effect is distinguished among different species such that for lower gyrofrequency ions (He+ versus He2+and He2+ versus protons), the main scattering effect moves toward a lower energy domain. All these results imply continuous disturbance of ion distributions by possibly prevailing EMIC waves in the IHS.

Keywords: heliosheath; termination shock; solar wind
Discovery and Timing Analysis of New Pulsars in Globular Cluster NGC 5024: New Observations from FAST

The most distant globular cluster (GC) with known pulsars is NGC 5024 (M53). In this paper, we report the discovery of a new binary millisecond pulsar, PSR J1312+1810E (M53E), and present the new timing solutions for M53B-E based on 22 observations from the Five-hundred-meter Aperture Spherical radio Telescope (FAST). These discoveries and timing work benefit from FAST's high sensitivity. We find that M53C is the only isolated millisecond pulsar known in this distant GC, with a spin period of 12.53 ms and spin period derivative of 5.26 × 10-20 s s-1. Our results reveal orbital periods of 47.7, 5.8, and 2.4 days for M53B, D, and E, respectively. The companions, with masses of 0.25, 0.27, and 0.18 M ⊙, respectively, are likely to be white dwarf stars; if they are extended objects, they do not eclipse the pulsars. We find no X-ray counterparts for these millisecond pulsars in archival Chandra images in the band of 0.3-8 keV. The characteristics of this pulsar population are similar to the population of millisecond pulsars in the Galactic disk, as expected from the low stellar density of M53.

Keywords: globular star clusters; millisecond pulsars; binary pulsars; radio telescopes
On the Nature of the Energy-dependent Morphology of the Composite Multi-TeV Gamma-Ray Source J1702-420

HESS J1702-420 is a multi-TeV gamma-ray source with an unusual energy-dependent morphology. The recent H.E.S.S. observations suggest that the emission is well described by a combination of the point-like HESS J1702-420A (dominating at highest energies, ≳30 TeV) and diffuse (~0.°3) HESS J1702-420B (dominating below ≲5 TeV) sources with very hard (Γ ~ 1.5) and soft (Γ ~ 2.6) power-law spectra, respectively. Here, we propose a model that postulates that the proton accelerator is located at the position of HESS J1702-420A and is embedded into a dense molecular cloud that coincides with HESS J1702-420B. In the proposed model, the very-high-energy radiation of HESS J1702-420 is explained by pion-decay emission from the continuously injected relativistic protons propagating through a dense cloud. The energy-dependent morphology is defined by the diffusive nature of the low-energy proton propagation, transiting sharply to (quasi) ballistic propagation at higher energies. Adopting a strong energy dependence of the diffusion coefficient, D ∝ E β with β ≥ 1, we argue that HESS J1702-420 as a system of two gamma-ray sources is the result of the propagation effect. Protons injected by a single accelerator at a rate ${Q}_{0}\simeq {10}^{38}\,{({n}_{0}/100\,\,{\mathrm{cm}}^{-3})}^{-1}\,{(d/0.25\,\mathrm{kpc})}^{-1}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ can reasonably reproduce the morphology and fluxes of the two gamma-ray components.

Keywords: gamma-ray sources; high energy astrophysics