adsabs/SciX_UAT_keywords huggingface dataset:
Tracking Short-term Variations in the Haze Distribution of Titan's Atmosphere with SINFONI VLT

While it has long been known that Titan's haze and atmosphere are dynamic on seasonal timescales, recent results have revealed that they also exhibit significant subseasonal variations. Here, we report on observations of Titan acquired over an eight-month period between 2014 April and 2015 March with the Spectrograph for Integral Field Observations in the Near Infrared instrument on the Very Large Telescope using adaptive optics. These observations have an average five-day cadence, permitting interrogation of the short-period variability of Titan's atmosphere. Disk-resolved spectra in the H and K bands (1.4-2.4 μm) were analyzed with the PyDISORT radiative transfer model to determine the spatial distribution and variation of stratospheric haze opacity over subseasonal timescales. We observed a uniform decrease in haze opacity at 20°N and an increase in haze opacity at 250-300°E and ~40°N over the span of our observations. Globally, we found variations on the order of 5%-10% on timescales of weeks, as well as a steady, global increase in the amount of haze over timescales of months. The observed variations in haze opacity over the short timescales of our observations were of similar magnitude to long-period variations attributed to seasonal variation, suggesting rapid dynamical processes that may take part in the distribution of hazes in Titan's atmosphere.

Keywords: ground-based astronomy; atmospheric variability; saturnian satellites
Relativistic Outflows from a GRMHD Mean-field Disk Dynamo

In this work, we present simulations of thin accretion disks around black holes, in order to investigate a mean-field disk dynamo, using our resistive GRMHD code, which is able to produce a large-scale magnetic flux. We consider a weak seed field in an initially thin disk, a background (turbulent) magnetic diffusivity, and the dynamo action itself. A standard quenching mechanism is applied to mitigate an otherwise exponential increase in the magnetic field. Comparison simulations of an initial Fishbone-Moncrief torus suggest that reconnection may provide another quenching mechanism. The dynamo-generated magnetic flux expands from the disk interior into the disk corona, becomes advected by disk accretion, and fills the axial region of the domain. The dynamo leads to an initially rapid increase in magnetic energy and flux, while for later evolutionary stages the growth stabilizes. Accretion toward the black hole depends strongly on the type of magnetic-field structure that develops. The radial field component supports extraction of angular momentum, and thus accretion. It also sets the conditions for launching a disk wind, initially from the inner disk area. When a strong field engulfs the disk, strong winds are launched, predominantly driven by the pressure gradient of the toroidal field. For rotating black holes, we identify a Poynting flux-dominated jet, driven by the Blandford-Znajek mechanism. This axial Poynting flux is advected from the disk, and therefore accumulates at the expense of the flux carried by the disk wind, which is itself regenerated by the disk dynamo.

Keywords: magnetohydrodynamics; galaxy accretion disks; general relativity; black holes; galaxy jets
The First Estimation of the Ambipolar Diffusivity Coefficient from Multi-scale Observations of the Class 0/I Protostar, HOPS-370

Protostars are born in magnetized environments. As a consequence, the formation of protostellar disks can be suppressed by the magnetic field, efficiently removing the angular momentum of the infalling material. Nonideal MHD effects are proposed as one way to allow protostellar disks to form. Thus, it is important to understand their contributions to observations of protostellar systems. We derive an analytical equation to estimate the ambipolar diffusivity coefficient at the edge of the protostellar disk in the Class 0/I protostar, HOPS-370, for the first time, under the assumption that the disk radius is set by ambipolar diffusion. Using previous results of the protostellar mass, disk mass, disk radius, density and temperature profiles, and magnetic field strength, we estimate the ambipolar diffusivity coefficient to be . We quantify the contribution of ambipolar diffusion by estimating its dimensionless Elsässer number to be , indicating its dynamical importance in this region. We compare our results to those of the chemical calculations of the ambipolar diffusivity coefficient using the Non-Ideal Magnetohydrodynamics Coefficients and Ionization Library, which are consistent with our results. In addition, we compare our derived ambipolar diffusivity coefficient to the diffusivity coefficients for ohmic dissipation and the Hall effect, and find ambipolar diffusion is dominant in our density regime. These results demonstrate a new methodology to understand nonideal MHD effects in observations of protostellar disks. More detailed modeling of the magnetic field, envelope, and microphysics, along with a larger sample of protostellar systems, is needed to further understand the contributions of nonideal MHD.

Keywords: circumstellar disks; observational astronomy; radio astronomy; star formation; protostars; young stellar objects
Large-scale versus Small-scale Fields in the Shock Front: Effect on the Particle Motion

In situ measurements at heliospheric shocks show that inside the shock front large-scale electric and magnetic fields are accompanied with strong small-scale fields. Until recently, it was widely believed that particle dynamics is determined by the large-scale fields. During the last several years, claims have been made that the small-scale fields are of primary importance. Here we show that the large-scale fields govern the ion motion while both large- and small-scale fields are important for electrons.

Keywords: shocks; planetary bow shocks; interplanetary shocks
Dynamically Tagged Groups of Metal-poor Stars. II. The Radial Velocity Experiment Data Release 6

Orbital characteristics based on Gaia Early Data Release 3 astrometric parameters are analyzed for ~8000 metal-poor stars ([Fe/H] ≤ -0.8) compiled from the Radial Velocity Experiment (RAVE) Data Release 6. Selected as metal-poor candidates based on broadband photometry, RAVE collected moderate-resolution (R ~ 7500) spectra in the region of the Ca triplet for these stars. About 20% of the stars in this sample also have medium-resolution (1200 ≲ R ≲ 2000) validation spectra obtained over a 4 yr campaign from 2014 to 2017 with a variety of telescopes. We match the candidate stars to photometric metallicity determinations from the Huang et al. recalibration of the SkyMapper Southern Survey Data Release 2. We obtain dynamical clusters of these stars from the orbital energy and cylindrical actions using the HDBSCAN unsupervised learning algorithm. We identify 179 dynamically tagged groups (DTGs) with between 5 and 35 members; 67 DTGs have at least 10 member stars. Milky Way (MW) substructures such as Gaia-Sausage-Enceladus, the Metal-weak Thick Disk, the Splashed Disk, Thamnos, the Helmi Stream, and LMS-1 (Wukong) are identified. Associations with MW globular clusters are determined for 10 DTGs; no recognized MW dwarf galaxies were associated with any of our DTGs. Previously identified dynamical groups are also associated with our DTGs, with emphasis placed on their structural determination and possible new identifications. We identify chemically peculiar stars as members of several DTGs; we find 22 DTGs that are associated with r-process-enhanced stars. Carbon-enhanced metal-poor (CEMP) stars are identified among the targets with available spectroscopy, and we assign these to morphological groups following the approach given by Yoon et al.

Keywords: milky way dynamics; galaxy dynamics; galactic archaeology; milky way evolution; milky way stellar halo; milky way formation
Early Stages of Galilean Moon Formation in a Water-depleted Environment

A key feature of the Galilean satellite system is its monotonic decrease in bulk density with growing distance from Jupiter, indicating an ice mass fraction that is zero in the innermost moon Io and about half in the outer moons Ganymede and Callisto. Jupiter-formation models, and perhaps the Juno spacecraft water measurements, are consistent with the possibility that the Jovian system may have formed, at least partly, from ice-poor material. And yet, models of the formation of the Galilean satellites usually assume abundant water ice in the system. Here, we investigate the possibility that the Jovian circumplanetary disk was populated with ice-depleted chondritic minerals, including phyllosilicates. We show that the dehydration of such particles and the outward diffusion of the released water vapor allow condensation of significant amounts of ice in the formation region of Ganymede and Callisto in the Jovian circumplanetary disk. Our model predicts that Europa, Ganymede, and Callisto should have accreted little, if any, volatiles other than water ice, in contrast to the comet-like composition of Saturn's moon Enceladus. This mechanism allows for the presence of ice-rich moons in water-depleted formation environments around exoplanets as well.

Keywords: galilean satellites; natural satellites (solar system); natural satellite formation; planetary system formation
Radioactively Powered Gamma-Ray Transient Associated with a Kilonova from Neutron Star Merger

The association of GW170817/GRB 170817A/AT2017gfo provides the first direct evidence for neutron star mergers as significant sources of r-process nucleosynthesis. A gamma-ray transient (GRT) would be powered by the radioactive decay of the freshly synthesized r-process elements. By analyzing the composition and gamma-ray opacity of the kilonova ejecta in detail, we calculate the lightcurve and spectrum of the GRT for a range of spherically symmetric merger ejecta models with mass M ej = 0.001 to ~0.05M ⊙ and expansion velocity v ej = 0.1c to ~0.4c. It is found that the peak of the GRT lightcurve depends on M ej and v ej as ${t}_{\mathrm{pk}}\approx 0.5\,\mathrm{days}\,{({M}_{\mathrm{ej}}/0.01{M}_{\odot })}^{1/2}{({v}_{\mathrm{ej}}/0.1c)}^{-1}$ and ${L}_{\mathrm{pk}}\approx 2.0\times {10}^{41}\,\mathrm{erg}\,{{\rm{s}}}^{-1}{({M}_{\mathrm{ej}}/0.01{M}_{\odot })}^{1/2}({v}_{\mathrm{ej}}/0.1c)$ . Most radiating photons are in the 100-3000 keV band and the spectrum peaks at about 800 keV for different nuclear physics inputs. The line features are blurred out by the Doppler broadening effect. Adopting the ejecta parameters reported in the literature, we examine the detection probability of the possible GRT associated with AT2017gfo. We show that the GRT cannot be convincingly detected with either current or proposed missions in the MeV band, such as ETCC and AMEGO. The low gamma-ray flux, together with the extremely low event rate at local universe, makes a discovery of GRTs a great challenge.

Keywords: gamma-ray transient sources; stellar mergers; r-process; gamma-ray bursts; explosive nucleosynthesis
Evidence for Multiple Shocks from the γ-Ray Emission of RS Ophiuchi

In 2021 August, the Fermi Large Area Telescope, H.E.S.S., and MAGIC detected GeV and TeV γ-ray emission from an outburst of recurrent nova RS Ophiuchi. This detection represents the first very high-energy γ-rays observed from a nova, and it opens a new window to study particle acceleration. Both H.E.S.S. and MAGIC described the observed γ-rays as arising from a single, external shock. In this paper, we perform detailed, multi-zone modeling of RS Ophiuchi's 2021 outburst, including a self-consistent prescription for particle acceleration and magnetic field amplification. We demonstrate that, contrary to previous work, a single shock cannot simultaneously explain RS Ophiuchi's GeV and TeV emission, in particular the spectral shape and distinct light-curve peaks. Instead, we put forward a model involving multiple shocks that reproduces the observed γ-ray spectrum and temporal evolution. The simultaneous appearance of multiple distinct velocity components in the nova optical spectrum over the first several days of the outburst supports the presence of distinct shocks, which may arise either from the strong latitudinal dependence of the density of the external circumbinary medium (e.g., in the binary equatorial plane versus the poles) or due to internal collisions within the white dwarf ejecta (which power the γ-ray emission in classical novae).

Keywords: symbiotic novae; gamma-rays; cosmic rays; galactic cosmic rays; novae; recurrent novae; shocks; plasma astrophysics; stellar winds; classical novae; gamma-ray transient sources; gamma-ray sources
Hyper Illumination of Exoplanets: Analytical and Numerical Approaches

This work describes the illumination of exoplanets whose orbits are close enough to their host star that the finite angular size of their host star causes hyper illumination, in which more than 50% of the planet receives light. Such exoplanets include the hot Jupiters KELT-9 b (64.5% illuminated) and Kepler-91 b (69.6% illuminated). We describe the geometry of three primary illumination zones: the fully illuminated zone, penumbral zone, and unilluminated zone. The integrals required to determine the incident radiation as a function of position from the substellar point on the exoplanet are explained and derived, and the analytical solution is presented within the fully illuminated zone. We find that the illumination predicted by our model is greater at the substellar point than the typical plane-parallel ray model used would suggest. In addition, it is greater within the region of the penumbral zone extending into the antistellar side of the exoplanet. Finally, we compare our model to that used in starry, an open-source software package used to create albedo maps. It appears that starry may be overestimating the illumination of closely orbiting exoplanets because the foreshortening of the area element of the host star is not included in its calculation.

Keywords: exoplanets; analytical mathematics
Evidence of High-latitude Emission in the Prompt Phase of GRBs: How Far from the Central Engine are the GRBs Produced?

One of the difficulties in nailing down the physical mechanism of gamma-ray bursts (GRBs) comes from the fact that there has been no clear observational evidence on how far from the central engine the prompt gamma rays of GRBs are emitted. Here we present a simple study addressing this question by making use of the "high-latitude emission" (HLE). We show that our detailed numerical modeling exhibits a clear signature of HLE in the decaying phase of "broad pulses" of GRBs. We show that the HLE can emerge as a prominent spectral break in F ν spectra and dominate the peak of ν F ν spectra even while the "line-of-sight emission" (LoSE) is still ongoing. This finding provides a new view of HLE emergence since it has been believed so far that the HLE can show up and dominate the spectra only after the LoSE is turned off. We remark, however, that this "HLE break" can be hidden in some broad pulses, depending on the proximity between the peak energies of the LoSE and the HLE. Therefore, this new picture of HLE emergence explains both the detection and nondetection of HLE signature in observations of broad pulses. Also, we present three examples of Fermi Gamma-ray Burst Monitor GRBs with broad pulses that exhibit the HLE signature. We show that their gamma-ray-emitting region should be located at ∼1016 cm from the central engine, which places a constraint on the GRB models.

Keywords: gamma-ray bursts; non-thermal radiation sources; relativistic jets
Chemical Equilibrium between Cores, Mantles, and Atmospheres of Super-Earths and Sub-Neptunes and Implications for Their Compositions, Interiors, and Evolution

We investigate the equilibrium chemistry between molten metal and silicate and a hydrogen-rich envelope using 18 independent reactions among 25 phase components for sub-Neptune-like exoplanets. Both reactive and unreactive metal sequestered in an isolated core are modeled. The overarching effects of equilibration are oxidation of the envelope and reduction of the mantle and core. Hydrogen and oxygen typically comprise significant fractions of metal cores at chemical equilibrium, leading to density deficits that offer a possible alternative explanation for the low densities of the Trappist-1 planets. Reactions with the magma ocean produce significant amounts of SiO and H2O in the envelopes directly above the magma ocean. Molar concentrations in the envelopes of planets with reactive metal are H2 > SiO > CO ~ Na ~ Mg > H2O ≫ CO2 ~ CH4 ≫ O2, while for the unreactive metal case, H2O becomes the second most abundant species, after H2, providing an arbiter for the two scenarios amenable to observation. The water abundances in the atmospheres exceed those in the mantles by at least an order of magnitude in both scenarios. The water concentrations in the silicate mantles are ~0.01 and ~0.1 wt% in the reactive and unreactive metal core cases, respectively, limiting the H2O that might be outgassed in a future super-Earth. Less dissolved water in the reactive core case is due to sequestration of H and O in the Fe-rich metal. The total hydrogen budget of most sub-Neptunes can, to first order, be estimated from their atmospheres alone, as the atmospheres typically contain more than 90% of all H.

Keywords: exoplanet structure; exoplanet evolution; exoplanet atmospheric composition; exoplanet formation; exoplanet atmospheres; exoplanets; extrasolar rocky planets; solar system terrestrial planets
Fast Magnetic Reconnection Structures in Poynting Flux-dominated Jets

The ubiquitous relativistic jet phenomena associated with black holes play a major role in high and very-high-energy (VHE) astrophysics. In particular, observations have demonstrated that blazars show VHE emission with time variability from days to minutes (in the gigaelectronvolt and teraelectronvolt bands), implying very compact emission regions. The real mechanism of the particle acceleration process responsible for this emission is still being debated, but magnetic reconnection has lately been discussed as a strong potential candidate. In this work, we present the results of three-dimensional special relativistic magnetohydrodynamic simulations of the development of reconnection events driven by turbulence induced by current-driven kink instability along a relativistic jet. We have performed a systematic identification of all reconnection regions in the system, characterizing their local magnetic field topology and quantifying the reconnection rates. We obtained average rates of 0.051 ± 0.026 (in units of Alfvén speed), which are comparable to the predictions of the theory of turbulence-induced fast reconnection. A detailed statistical analysis also demonstrated that the fast reconnection events follow a log-normal distribution, which is a signature of its turbulent origin. To probe the robustness of our method, we have applied our results to the blazar Mrk 421. Building a synthetic light curve from the integrated magnetic reconnection power, we evaluated the time variability from a power spectral density analysis, obtaining good agreement with observations in the gigaelectronvolt band. This suggests that turbulent fast magnetic reconnection can be a possible process behind the high-energy emission variability phenomena observed in blazars.

Keywords: high energy astrophysics; plasma astrophysics; special relativity; magnetohydrodynamical simulations; magnetohydrodynamics; relativistic jets
The Effects of Biconical Outflows on Lyα Escape from Green Peas

We analyze the spectra of 10 Green Pea galaxies, previously studied by Henry et al., using a semi-analytical line transfer model to interpret emission and absorption features observed in UV galactic spectra. We focus our analysis on various ionization states of silicon, associated with the cool (∼104 K) and warm (∼105 K) gas. By analyzing low-ionization lines, we study the relationships between the distribution and kinematics of the outflowing H I gas and the observed Lyα escape fraction, ${f}_{\mathrm{esc}}^{\mathrm{Ly}\alpha }$ , as well as the Lyα emission peak separation, Δpeak. We find that outflow geometries that leave a portion of the source uncovered along the line of sight create the best conditions for Lyα escape and have narrow peak separations, while geometries that block the observer's view of the source create the worst conditions for Lyα escape and have large peak separations. To isolate the effects of outflow kinematics, we restrict our testing set to galaxies with spherical outflows and find that ${f}_{\mathrm{esc}}^{\mathrm{Ly}\alpha }$ and the Lyα luminosity both increase with the extent of the galactic winds. A simple estimate suggests that the collisional excitation of neutral hydrogen by free electrons in the cool gas of the winds can account for the Lyα luminosity observed in these objects. Finally, we speculate on the relationship between outflows and the escape of ionizing radiation from the circumgalactic medium.

Keywords: circumgalactic medium; ultraviolet astronomy; spectroscopy; galactic winds; reionization; compact galaxies; interstellar medium; cooling flows; galaxy dynamics; galaxy evolution; radiative transfer
The Rarity of Repeating Fast Radio Bursts from Binary Neutron Star Mergers

Fast radio bursts (FRBs) are extragalactic, bright pulses of emission at radio frequencies with millisecond durations. Observationally, FRBs can be divided into two classes, repeating FRBs and non-repeating FRBs. At present, 20 repeating FRBs have been discovered with unknown physical origins. Localization of the first repeating FRB 121102 and discovery of an associated persistent radio source support that FRBs are powered by young millisecond magnetars, which could be formed by the core-collapses of massive stars or binary neutron star (BNS) mergers. These two formation channels can be distinguished by the gravitational waves generated by BNSs mergers. We first calculate the lower limit of the local formation rate of repeating FRBs observed by the Canadian Hydrogen Intensity Mapping Experiment (CHIME). Then we show that only a small fraction (6%) of repeating FRBs are produced by young magnetars from BNS mergers, based on the gravitational-wave detections by the third observing run (O3) of the Advanced LIGO/Virgo gravitational-wave detectors. Therefore, we believe that repeating FRBs are more likely produced by newborn magnetars newborn from the core-collapses of massive stars rather than magnetars from BNS mergers.

Keywords: radio transient sources; gravitational waves; gamma-ray bursts; magnetars