Sazuna/UAT_keywords huggingface dataset:
Time Delay between Outer Heliosheath Crossing and Observation of Interstellar Neutral Atoms. In situ measurements of the heliospheric particle populations by the Voyager spacecraft can only be put in an appropriate context with remote-sensing observations of energetic and interstellar neutral atoms (ENAs and ISN, respectively) at 1 au when the time delay between the production and the observation times is taken into account. ENA times of flight from the production regions in the heliosheath are relatively easy to estimate because these atoms follow almost constant speed, force-free trajectories. For the ISN populations, dynamical and ballistic selection effects are important, and times of flight are much longer. We estimate these times for ISN He and H atoms observed by IBEX and in the future by IMAP using the WTPM model with synthesis method. We show that for the primary population atoms, the times of flight are on the order of three solar cycle periods, with a spread equivalent to one solar cycle. For the secondary populations, the times of flight are on the order of ten solar cycle periods, and during the past ten years of observations, IBEX has been collecting secondary He atoms produced in the outer heliosheath during almost the entire 19th century. ISN atoms penetrating the heliopause at the time of Voyager crossing will become gradually visible about 2027, during the planned IMAP observations. Hypothetical variations in the ISN flow in the Local Interstellar Medium are currently not detectable. Nevertheless, we expect steady-state heliosphere models used with appropriately averaged solar wind parameters to be suitable for understanding the ISN observations.

Keywords: Heliopause, Astrosphere interstellar medium interactions, Heliosphere, Stellar wind bubbles, Heliosheath
The Magnetic Topology and Eruption Mechanism of a Multiple-ribbon Flare. Multiple-ribbon flares are usually complex in their magnetic topologies and eruption mechanisms. In this paper, we investigate an X2.1 flare (SOL2015-03-11T16:22) that occurred in active region 12297 near the center of the solar disk by both potential and nonlinear force-free field models extrapolated with the data observed by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We calculate the three-dimensional squashing degree distribution. The results reveal that there are two flux ropes in this active region covered by a large-scale hyperbolic flux tube (HFT), which is the intersection of quasi-separatrix layers with a null point embedded in it. When the background magnetic field diminishes due to the separation of the northwest dipole and the flux cancellation, the central flux rope rises up, forming the two brightest central ribbons. It then squeezes the overlying HFT structure to generate further brightenings. This very energetic flare with a complex shape is accompanied by a coronal mass ejection (CME). We adopt the simplified line-tied force-balance equation of the current ring model and assign the observed value of the decay index to the equation to simulate the acceleration profile of the CME in the early stage. It is found that the path with an inclination of 45° from radial best fits the profile of the actual acceleration.

Keywords: Solar magnetic fields, Solar flares, Solar coronal mass ejections
The Information Content in Analytic Spot Models of Broadband Precision Light Curves. II. Spot Distributions and Lifetimes and Global and Differential Rotation. With the advent of space-based precision photometry missions, the quantity and quality of starspot light curves has greatly increased. This paper presents a large number of starspot models and their resulting light curves to (1) better determine light-curve metrics and methods that convey useful physical information and (2) understand how the underlying degeneracies of the translation from physical starspot distributions to the resulting light curves obscure that information. We explore models of relatively active stars at several inclinations while varying the number of (dark) spots, random spot distributions in position and time, timescales of growth and decay, and differential rotation. We examine the behavior of absolute and differential variations of individual intensity dips and overall light curves and demonstrate how complex spot distributions and behaviors result in light curves that typically exhibit only one or two dips per rotation. Unfortunately, simplistic "one- or two-spot" or "active longitude" descriptions or modeling of light curves can often be highly misleading. We also show that short "activity cycles" can easily be simply due to random processes. It turns out to be quite difficult to disentangle the competing effects of spot lifetime and differential rotation, but under most circumstances, spot lifetime is the more influential of the two. Many of the techniques tried to date only work when spots live for many rotations. These include autocorrelation degradation for spot lifetimes and periodograms for both global and differential rotation. Differential rotation may be nearly impossible to accurately infer from light curves alone unless spots live for many rotations. The Sun and solar-type stars its age or older are unfortunately the most difficult type of case. Further work is needed to have increased confidence in light-curve inferences.

Keywords: Starspots, Light curve classification , Solar differential rotation, Stellar magnetic fields, Stellar activity, Stellar rotation
Pulsations of the Rapidly Oscillating Ap Star KIC 10685175 Revisited by TESS. KIC 10685175 (TIC 264509538) was discovered to be a rapidly oscillating Ap star from Kepler long-cadence data using super-Nyquist frequency analysis. It was reobserved by the Transiting Exoplanet Survey Satellite (TESS) with 2 minute cadence data in Sectors 14 and 15. We analyzed the TESS light curves, finding that the previously determined frequency is a Nyquist alias. The revised pulsation frequency is 191.5151 ± 0.0005 day-1 (P = 7.52 minutes) and the rotation frequency is 0.32229 ± 0.00005 day-1 (Prot = 3.1028 days). The star is an oblique pulsator with a pulsation amplitude modulated by the rotation, reaching a maximum pulsation amplitude at the time of the rotational light minimum. The oblique pulsation generates a frequency quintuplet split by exactly the rotation frequency. The phases of sidelobes, the pulsation phase modulation, and a spherical harmonic decomposition all show this star to be pulsating in a distorted quadrupole mode. Following the oblique pulsator model, we calculated the rotation inclination i and magnetic oblique β of this star, which provide detailed information of pulsation geometry. The i and β derived by the best fit of pulsation amplitude and phase modulation through a theoretical model differ from those calculated for a pure quadrupole, indicating the existence of strong magnetic distortion. The model also predicts the polar magnetic field strength is as high as about 6 kG which is predicted to be observed in a high-resolution spectrum of this star.

Keywords: Oblique rotators, Asteroseismology, Rapidly oscillating Ap star
Modeling the Survival of Population III Stars to the Present Day. Recent numerical simulations have suggested the probability of a fraction of the primordial stars being ejected from the cluster of their origin. We explore the possibility that some of these can remain on the main sequence until the present epoch. We develop a semianalytical model guided by results of cosmological simulations to study the mass accretion by these protostars as a function of the original stellar mass and other parameters such as angular momentum and gravitational drag due to ambient gas. We also explore whether some of the protostars remain sufficiently low mass and long-lived to survive to the present day. This requires that the protostars are ejected from the star-forming region while their mass is less than $0.8\,{M}_{\odot }$ . Assuming that the protostars gain mass via the spherical Bondi-Hoyle accretion from the ambient medium, we show that Population III protostars that initially form within a certain range of mass and are ejected with velocity larger than the escape velocity may survive to the present day on the main sequence. Thus, they may even be found in our Milky Way or its satellites. Our calculations also reveal that protostars that do not get ejected from the parent gas clump accrete a large amount of gas. We predict that these can become massive enough to be progenitors of black holes.

Keywords: Theoretical models, Bondi accretion, Population III stars, Dynamical evolution
Kelvin-Helmholtz Instability at Proton Scales with an Exact Kinetic Equilibrium. The Kelvin-Helmholtz instability (KHI) is a ubiquitous physical process in ordinary fluids and plasmas, frequently observed also in space environments. In this paper, kinetic effects at proton scales in the nonlinear and turbulent stage of the KHI have been studied in magnetized collisionless plasmas by means of hybrid Vlasov-Maxwell simulations. The main goal of this work is to point out the back-reaction on particles triggered by the evolution of such instability, as energy reaches kinetic scales along the turbulent cascade. Interestingly, turbulence is inhibited when KHI develops over an initial state that is not an exact equilibrium state. On the other hand, when an initial equilibrium condition is considered, energy can be efficiently transferred toward short scales, reaches the typical proton wavelengths, and drives the dynamics of particles. As a consequence of the interaction of particles with the turbulent fluctuating fields, the proton velocity distribution deviates significantly from the local thermodynamic equilibrium, the degree of deviation increasing with the level of turbulence in the system and being located near regions of strong magnetic stresses. These numerical results support recent space observations from the Magnetospheric MultiScale mission of ion kinetic effects driven by the turbulent dynamics at Earth's magnetosheath and by the KHI in Earth's magnetosphere.

Keywords: Interplanetary turbulence, Space plasmas
Hall Cascade with Fractional Magnetic Helicity in Neutron Star Crusts. The ohmic decay of magnetic fields in the crusts of neutron stars is generally believed to be governed by Hall drift, which leads to what is known as a Hall cascade. Here we show that helical and fractionally helical magnetic fields undergo strong inverse cascading like in magnetohydrodynamics (MHD), but the magnetic energy decays more slowly with time t: $\propto \,{t}^{-2/5}$ instead of ∝t-2/3 in MHD. Even for a nonhelical magnetic field there is a certain degree of inverse cascading for sufficiently strong magnetic fields. The inertial range scaling with wavenumber k is compatible with earlier findings for the forced Hall cascade, i.e., proportional to k-7/3, but in the decaying cases, the subinertial range spectrum steepens to a novel k5 slope instead of the k4 slope in MHD. The energy of the large-scale magnetic field can increase quadratically in time through inverse cascading. For helical fields, the energy dissipation is found to be inversely proportional to the large-scale magnetic field and proportional to the fifth power of the rms magnetic field. For neutron star conditions with an rms magnetic field of a few times ${10}^{14}\,{\rm{G}}$ , the large-scale magnetic field might only be ${10}^{11}\,{\rm{G}}$ , while still producing magnetic dissipation of ${10}^{33}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ for thousands of years, which could manifest itself through X-ray emission. Finally, it is shown that the conclusions from local unstratified models agree rather well with those from stratified models with boundaries.

Keywords: Magnetars, Magnetohydrodynamical simulations
PHAT XX. AGB Stars and Other Cool Giants in M31 Star Clusters. The presence of asymptotic giant branch (AGB) stars in clusters provides key constraints for stellar models, as has been demonstrated with historical data from the Magellanic Clouds. In this work, we look for candidate AGB stars in M31 star clusters from the Panchromatic Hubble Andromeda Treasury survey. Our photometric criteria selects stars brighter than the tip of the red giant branch, which includes the bulk of the thermally pulsing AGB stars as well as early-AGB stars and other luminous cool giants expected in young stellar populations (e.g., massive red supergiants, and intermediate-mass red helium-burning stars). The AGB stars can be differentiated, a posteriori, using the ages already estimated for our cluster sample. 937 candidates are found within the cluster aperture radii, half (∼450) of which are very likely cluster members. Cross-matching with additional databases reveals two carbon stars and 10 secure variables among them. The field-corrected age distribution reveals the presence of young supergiants peaking at ages smaller than 108 yr, followed by a long tail of AGB stars extending up to the oldest possible ages. This long tail reveals the general decrease in the numbers of AGB stars from initial values of $\sim 50\times {10}^{-6}{M}_{\odot }^{-1}$ at 108 yr down to $\sim 5\times {10}^{-6}{M}_{\odot }^{-1}$ at 1010 yr. Theoretical models of near-solar metallicity reproduce this general trend, although with localized discrepancies over some age intervals, whose origin is not yet identified. The entire catalog is released together with finding charts to facilitate follow-up studies.

Keywords: Asymptotic giant branch stars, Andromeda Galaxy, Star clusters
In Situ Evidence of a Magnetic Hole Downstream of the Magnetic Reconnection in the Magnetopause Reconnection. Recently, an electron-scale coherent structure known as a magnetic hole (MH) was observed in the Earth's magnetosheath. It has been found that the MH is characterized by decreasing magnetic field strength and increasing parameters such as electron density, temperature, and temperature anisotropy. Observations suggest that electron temperature anisotropy within an MH structure provides free energy to generate whistler waves. In this study, a small-scale MH formed in the downstream region of the magnetic reconnection, indicating that the region far away from the X-line may be another important electron heating channel. A strong energy-dissipating current layer is observed near the boundary within MH, implying that boundary may be the energy source for MH heating electrons. These characteristics indicate that the observed MH is still highly dynamic and has multiscale coupling processes.

Keywords: Space plasmas, Magnetic fields, Plasma jets
A New Method to Model Magnetic Cloud-driven Forbush Decreases: The 2016 August 2 Event. Interplanetary coronal mass ejections (ICMEs), generally containing magnetic clouds (MCs), are associated with galactic-cosmic ray (GCR) intensity depressions known as Forbush decreases (FDs). An ICME was observed at L1 between 2016 August 2 at 14:00 UT and August 3 at 03:00 UT. The MC region was identified and its magnetic configuration was retrieved by using the Grad-Shafranov (GS) reconstruction. A weak FD in the GCR count-rate was observed on 2016 August 2 by a particle detector on board the European Space Agency LISA Pathfinder mission. The spacecraft orbited around L1 and the particle detector allowed us to monitor the GCR intensity at energies above 70 MeV n-1. A 9% decrease in the cosmic-ray intensity was observed during the ICME passage. The first structure of the ICME caused a 6.4% sharp decrease, while the MC produced a 2.6% decrease. A suited full-orbit test-particle simulation was performed on the MC configuration obtained through the GS reconstruction. The FD amplitude and time profile obtained through the simulation show an excellent agreement with observations. The test-particle simulation allows us to derive the energy dependence of the MC-driven FD providing an estimate of the amplitude at different rigidities, here compared with several neutron monitor observations. This work points out the importance of the large-scale MC configuration in the interaction between GCRs and ICMEs and suggests that particle drifts have a primary role in modulating the GCR intensity within the MC under study and possibly in at least all slowly expanding ICMEs lacking a shock/sheath region.

Keywords: Ejecta, Cosmic ray detectors, Forbush effect, Galactic cosmic rays, Solar coronal mass ejections, Interplanetary physics
A Search for Light Hydrides in the Envelopes of Evolved Stars. We report a search for the diatomic hydrides SiH, PH, and FeH along the line of sight toward the chemically rich circumstellar envelopes of IRC+10216 and VY Canis Majoris. These molecules are thought to form in high-temperature regions near the photospheres of those stars, and may then further react via gas-phase and dust-grain interactions leading to more complex species, but have yet to be constrained by observation. We used the German Receiver for Astronomy at Terahertz Frequencies spectrometer on the Stratospheric Observatory for Infrared Astronomy to search for rotational emission lines of these molecules in four spectral windows ranging from 600 to 1500 GHz. Although none of the targeted species was detected in our search, we report their upper limit abundances in each source and discuss how they influence the current understanding of hydride chemistry in dense circumstellar media. We attribute the nondetections of these hydrides to their compact source sizes, high barriers of formation, and proclivity to react with other molecules in the winds.

Keywords: Interstellar molecules, Circumstellar envelopes, Red supergiant stars, Chemical abundances, Astrochemistry, Asymptotic giant branch stars
A Chemo-dynamical Link between the Gjöll Stream and NGC 3201. Recent analysis of the second data release of Gaia has revealed a number of new stellar streams surrounding the Milky Way. It has been suggested that one of these streams, Gjöll, is associated with the globular cluster NGC 3201, but thus far the association has only been based on kinematics of the stream stars. In this work we identify five likely stream members in the Gaia data that are bright enough for high-resolution spectroscopic follow-up with the Harlan J. Smith telescope at McDonald Observatory. One star is ruled out as a member based on its radial velocity. Abundance analysis of the remaining four kinematic members reveals a good chemical match to NGC 3201 for two of the stars, driven by their similar Al and α-element abundances. Of the remaining two, one shows no chemical similarity to NGC 3201 and is likely an unassociated Milky Way halo field star, while the other exhibits a similar Al abundance but has somewhat lower α-element abundances. The chemical tagging of stars in the Gjöll stream to NGC 3201 confirms this association and provides direct proof of the hierarchical formation of the Milky Way.

Keywords: Globular star clusters, Milky Way dynamics, Chemical abundances
Possible Time Correlation between Jet Ejection and Mass Accretion for RW Aur A. For the active T-Taur star RW Aur A we have performed long-term (∼10 yr) monitoring observations of (1) jet imaging in the [Fe II] 1.644 μm emission line using Gemini-NIFS and VLT-SINFONI; (2) optical high-resolution spectroscopy using CFHT-ESPaDOnS; and (3) V-band photometry using the CrAO 1.25-m telescope and AAVSO. The latter two observations confirm the correlation of time variabilities between (A) the Ca II 8542 Šand O I 7772 Šline profiles associated with magnetospheric accretion, and (B) optical continuum fluxes. The jet images and their proper motions show that four knot ejections occurred at the star over the past ∼15 yr with an irregular interval of 2-6 yr. The timescale and irregularity of these intervals are similar to those of the dimming events seen in the optical photometry data. Our observations show a possible link between remarkable (∆V &lt; -1) photometric rises and jet knot ejections. Observations over another few years may confirm or reject this trend. If confirmed, this would imply that the location of the jet launching region is very close to the star (r ≲ 0.1 au) as predicted by some jet launching models. Such a conclusion would be crucial for understanding disk evolution within a few astronomical units of the star, and therefore possible ongoing planet formation at these radii. * Based on observations collected at the European Southern Observatory under ESO program 2100.C-5015.

Keywords: Stellar accretion disks, T Tauri stars, Stellar jets
Circumbinary Disks: Accretion and Torque as a Function of Mass Ratio and Disk Viscosity. Using numerical hydrodynamics calculations and a novel method for densely sampling parameter space, we measure the accretion and torque on a binary system from a circumbinary disk. In agreement with some earlier studies, we find that the net torque on the binary is positive for mass ratios close to unity, and that accretion always drives the binary toward equal mass. Accretion variability depends sensitively on the numerical sink prescription, but the torque and relative accretion onto each component do not depend on the sink timescale. Positive torque and highly variable accretion occurs only for mass ratios greater than around 0.05. This means that for mass ratios below 0.05, the binary would migrate inward until the secondary accreted sufficient mass, after which it would execute a U-turn and migrate outward. We explore a range of viscosities, from α = 0.03 to α = 0.15, and find that this outward torque is proportional to the viscous torque, so that torque per unit accreted mass is independent of α. Dependence of accretion and torque on mass ratio is explored in detail, densely sampling mass ratios between 0.01 and unity. For mass ratio q &gt; 0.2, accretion variability is found to exhibit a distinct sawtooth pattern, typically with a five-orbit cycle that provides a smoking gun prediction for variable quasars observed over long periods, as a potential means to confirm the presence of a binary.

Keywords: Supermassive black holes, Binary stars, Black holes, Quasars, Close binary stars, Galaxy accretion disks, Active galactic nuclei, Galaxy mergers, Circumstellar disks, Wide binary stars, Shocks, Astrophysical fluid dynamics
Polytropic Behavior of Solar Wind Protons Observed by Parker Solar Probe. A polytropic process describes the transition of a fluid from one state to another through a specific relationship between the fluid density and temperature. The value of the polytropic index that governs this relationship determines the heat transfer and the effective degrees of freedom during a specific process. In this study, we analyze solar wind proton plasma measurements, obtained by the Faraday cup instrument on board the Parker Solar Probe. We examine the large-scale variations of the proton plasma density and temperature within the inner heliosphere explored by the spacecraft. We then address the polytropic behavior in the density and temperature fluctuations in short time intervals, which we analyze in order to derive the effective polytropic index of small-scale processes. The large-scale variations of the solar wind proton density and temperature, which are associated with the plasma expansion into the heliosphere, follow a polytropic model with a polytropic index ∼5/3. On the other hand, the short-scale fluctuations, which are potentially associated with turbulence, follow a model with a larger polytropic index. We investigate possible correlations between the polytropic index of short-scale fluctuations and the plasma speed, plasma β, and the magnetic field direction. We discuss candidate mechanisms leading to this behavior including energy transfer and possible mechanisms restricting the effective particle degrees of freedom at smaller scales.

Keywords: Space plasmas, Solar physics, Solar wind