Sazuna/UAT_keywords huggingface dataset:
Electron Mirror-mode Structure: Magnetospheric Multiscale Observations. The small-scale mirror mode excited by electron dynamics is a fundamental physical process, attracting research interest in space, laboratory, and astrophysical plasma physics over the past half century. However, the investigations of this process were mostly limited to theories and numerical simulations, with no direct observational evidence for their existence. In this study we present clear observations of electron mirror-mode using Magnetospheric Multiscale data at unprecedented high temporal cadence. These structures are train-like, compressible, nonpropagating, and satisfy the theoretical excitation and electron trapping conditions. They were observed near the Earth's foreshock and its downstream turbulence during the corotating interaction region events, which could be involved with the interaction between solar wind and Earth.

Keywords: Space plasmas, Shocks, Solar wind, Planetary magnetospheres
Type I Outbursts in Low-eccentricity Be/X-Ray Binaries. Type I outbursts in Be/X-ray binaries are usually associated with the eccentricity of the binary orbit. The neutron star accretes gas from the outer parts of the decretion disk around the Be star at each periastron passage. However, this mechanism cannot explain type I outbursts that have been observed in nearly circular orbit Be/X-ray binaries. With hydrodynamical simulations and analytic estimates we find that in a circular orbit binary, a nearly coplanar disk around the Be star can become eccentric. The extreme mass ratio of the binary leads to the presence of the 3:1 Lindblad resonance inside the Be star disk and this drives eccentricity growth. Therefore the neutron star can capture material each time it approaches the disk apastron, on a timescale up to a few percent longer than the orbital period. We have found a new application of this mechanism that is able to explain the observed type I outbursts in low-eccentricity Be/X-ray binaries.

Keywords: Accretion, X-ray binary stars, Hydrodynamics, Pulsars, Circumstellar disks, B(e) stars
Detection of Propadiene on Titan. The atmosphere of Titan, the largest moon of Saturn, is rich in organic molecules, and it has been suggested that the moon may serve as an analog for the pre-biotic Earth due to its highly reducing chemistry and existence of global hazes. Photochemical models of Titan have predicted the presence of propadiene (historically referred to as allene), CH2CCH2, an isomer of the well-measured propyne (also called methylacetylene) CH3CCH, but its detection has remained elusive due to insufficient spectroscopic knowledge of the molecule. This has recently been remedied with an updated spectral line list. Here we present the first unambiguous detection of the molecule in any astronomical object, observed with the Texas Echelle Cross Echelle Spectrograph on the NASA Infrared Telescope Facility in 2017 July. We model its emission line near 12 μm and measure a volume mixing ratio of (6.9 ± 0.8) × 10-10 at 175 km, assuming a vertically increasing abundance profile as predicted in photochemical models. Cassini measurements of propyne made during 2017 April indicate that the abundance ratio of propyne to propadiene is 8.2 ± 1.1 at the same altitude. This initial measurement of the molecule in Titan’s stratosphere paves the way toward constraining the amount of atomic hydrogen available on Titan, as well as future mapping of propadiene on Titan from 8 m and larger ground-based observatories, and future detection on other planetary bodies.

Keywords: Planetary science, Planetary atmospheres, Saturnian satellites, Pre-biotic astrochemistry
The MOSDEF Survey: Sulfur Emission-line Ratios Provide New Insights into Evolving Interstellar Medium Conditions at High Redshift. We present results on the emission-line properties of 1.3 ≤ z ≤ 2.7 galaxies drawn from the complete the MOSFIRE Deep Evolution Field (MOSDEF) survey. Specifically, we use observations of the emission-line diagnostic diagram of [O III]λ 5007/Hβ versus [S II]λλ6717,6731/Hα, i.e., the “[S II] BPT diagram,” to gain insight into the physical properties of high-redshift star-forming regions. High-redshift MOSDEF galaxies are offset toward lower [S II]λλ6717,6731/Hα at fixed [O III]λ5007/Hβ, relative to local galaxies from the Sloan Digital Sky Survey (SDSS). Furthermore, at fixed [O III]λ5007/Hβ, local SDSS galaxies follow a trend of decreasing [S II]λλ6717,6731/Hα as the surface density of star formation (ΣSFR) increases. We explain this trend in terms of the decreasing fractional contribution from diffuse ionized gas (f DIG) as ΣSFR increases in galaxies, which causes galaxy-integrated line ratios to shift toward the locus of pure H II-region emission. The z ∼ 0 relationship between f DIG and ΣSFR implies that high-redshift galaxies have lower f DIG values than typical local systems, given their significantly higher typical ΣSFR. When an appropriate low-redshift benchmark with zero or minimal f DIG is used, high-redshift MOSDEF galaxies appear offset toward higher [S II]λλ6717,6731/Hα and/or [O III]λ 5007/Hβ. The joint shifts of high-redshift galaxies in the [S II] and [N II] BPT diagrams are best explained in terms of the harder spectra ionizing their star-forming regions at fixed nebular oxygen abundance (expected for chemically young galaxies), as opposed to large variations in N/O ratios or higher ionization parameters. The evolving mixture of H II regions and diffuse ionized gas is an essential ingredient of our description of the interstellar medium over cosmic time. Based on data obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration, and was made possible by the generous financial support of the W.M. Keck Foundation.

Keywords: High-redshift galaxies, Galaxy evolution, Interstellar medium
On the Fast Solar Wind Heating and Acceleration Processes: A Statistical Study Based on the UVCS Survey Data. The UltraViolet Coronagraph Spectrometer (UVCS) on board the SOlar and Heliospheric Observatory has almost continuously observed, throughout the whole solar cycle 23, the UV solar corona. This work addresses the first-ever statistical analysis of the daily UVCS observations, performed in the O VI channel, of the northern polar coronal hole, between 1.5 and 3 R ⊙, during the period of low solar activity from 1996 April to 1997 December. The study is based on the investigation, at different heights, of the correlation between the variance of the O VI 1031.92 Å spectral line and the O VI 1031.92, 1037.61 Å doublet intensity ratio, which are proxies of the kinetic temperature of the O5+ ions and of the speed of the oxygen component of the fast solar wind, respectively. This analysis allows the clear identification of the sonic point in polar coronal holes at the distance of 1.9 R ⊙. The results show that heat addition below the sonic point does not lead to an increase of the outflow speed. As a matter of fact, the coronal plasma is heated more efficiently in the subsonic region, while its acceleration occurs more effectively in the region of supersonic flow. So, within the panorama of the Parker Solar Probe and Solar Orbiter missions, the statistical analysis of the historical UVCS data appears to be very promising in providing unique clues to some still unsolved problems, as the coronal heating, in the solar corona.

Keywords: Solar wind, The Sun, Solar coronal heating, Solar corona
Viewing Angle Constraints on S190425z and S190426c and the Joint Gravitational-wave/Gamma-Ray Detection Fractions for Binary Neutron Star Mergers. The Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo scientific collaboration (LVC) detected two binary neutron star (BNS) merger candidates, S190425z and S190426c. The Fermi-Gamma-ray Burst Monitor (GBM) observed 55.6% (for S190425z) and 100% (for S190426c) of the probability regions of both events at the respective merger times, but no gamma-ray burst (GRB) was detected in either case. The derived luminosity upper limits suggest that a short GRB similar to GRB 170817A would not be detectable for both cases due to their distances, which are larger than that of GW170817. Assuming that the jet profile obtained from GW170817/GRB 170817A is quasi-universal for all BNS-GRB associations, we derive that the viewing angles of S190425z and S190426c should be &gt;(0.11-0.41) and &gt;(0.09-0.39), respectively. Through Monte Carlo simulations, we show that with the GRB 170817A-like jet structure, all sky gamma-ray detectors, such as GBM and the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor, are expected to detect ∼4.6%, 3.9%, 1.7%, and 6.6%, 5.7%, 2.8% of BNS mergers triggered by advanced LIGO, A+, and the Einstein Telescope, respectively. The joint detection fraction would be largely reduced for Swift-BAT, SVOM-ECLAIRS, and the Einstein Probe, whose sensitivities are better but whose FOVs are smaller.

Keywords: Gravitational wave astronomy, Gamma-ray bursts
Prospects for Gravitational Wave Measurement of ZTF J1539+5027. The short-period eclipsing binary ZTF J1539+5027 discovered by Burdge et al. will be a strong gravitational-wave source for the Laser Interferometer Space Antenna (LISA). We study how well LISA will constrain the parameters of this system by analyzing simulated gravitational-wave data, and find that LISA observations will significantly improve measurements of the distance and inclination of the source, and allow for novel constraints to be placed on the speed of gravity.

Keywords: White dwarf stars, Gravitational waves, Relativistic binary stars
The Kennicutt-Schmidt Law and Gas Scale Height in Luminous and Ultraluminous Infrared Galaxies. A new analysis of high-resolution data from the Atacama Large Millimeter/submillimeter Array for five luminous or ultraluminous infrared galaxies gives a slope for the Kennicutt-Schmidt (KS) relation equal to {1.74}-0.07+0.09 for gas surface densities Σmol &gt; 103 M ☉ pc-2 and an assumed constant CO-to-H2 conversion factor. The velocity dispersion of the CO line, σ  v , scales approximately as the inverse square root of Σmol, making the empirical gas scale height determined from H∼ 0.5{σ }2/(π G{{{Σ }}}mol}) nearly constant, 150-190 pc, over 1.5 orders of magnitude in Σmol. This constancy of H implies that the average midplane density, which is presumably dominated by CO-emitting gas for these extreme star-forming galaxies, scales linearly with the gas surface density, which in turn implies that the gas dynamical rate (the inverse of the freefall time) varies with {{{Σ }}}mol}1/2, thereby explaining most of the super-linear slope in the KS relation. Consistent with these relations, we also find that the mean efficiency of star formation per freefall time is roughly constant, 5%-7%, and the gas depletion time decreases at high Σmol, reaching only ∼16 Myr at Σmol ∼ 104 M ☉ pc-2. The variation of σ  v  with Σmol and the constancy of H are in tension with some feedback-driven models, which predict σ  v  to be more constant and H to be more variable. However, these results are consistent with simulations in which large-scale gravity drives turbulence through a feedback process that maintains an approximately constant Toomre Q instability parameter.

Keywords: Starburst galaxies, Star formation, Infrared galaxies, Interstellar medium
X-Ray and Gamma-Ray Emission from Core-collapse Supernovae: Comparison of Three-dimensional Neutrino-driven Explosions with SN 1987A. During the first few hundred days after the explosion, core-collapse supernovae (SNe) emit down-scattered X-rays and gamma-rays originating from radioactive line emissions, primarily from the 56Ni → 56Co → 56Fe chain. We use supernova (SN) models based on three-dimensional neutrino-driven explosion simulations of single stars and mergers to compute this emission and compare the predictions with observations of SN 1987A. A number of models are clearly excluded, showing that high-energy emission is a powerful way of discriminating between models. The best models are almost consistent with the observations, but differences that cannot be matched by a suitable choice of viewing angle are evident. Therefore, our self-consistent models suggest that neutrino-driven explosions are able to produce, in principle, sufficient mixing, although remaining discrepancies may require small changes to the progenitor structures. The soft X-ray cutoff is primarily determined by the metallicity of the progenitor envelope. The main effect of asymmetries is to vary the flux level by a factor of ∼3. For the more asymmetric models, the shapes of the light curves also change. In addition to the models of SN 1987A, we investigate two models of SNe II-P and one model of a stripped-envelope SN IIb. The Type II-P models have observables similar to those of the models of SN 1987A, but the stripped-envelope SN model is significantly more luminous and evolves faster. Finally, we make simple predictions for future observations of nearby SNe.

Keywords: Core-collapse supernovae, Gamma-ray lines, X-ray transient sources, Gamma-ray transient sources, Supernova dynamics
Temperature Fluctuation at the Sun and Large-scale Electric Field in Solar Wind: A Challenge for the Parker Solar Probe Mission. Velocity distributions of particles are key elements in the study of solar wind. The physical mechanisms that regulate their many features are a matter of debate. The present work addresses the subject with a fully analytical method in order to establish the shape of particle velocity distributions in solar wind. The method consists of solving the steady-state kinetic equation for particles and the related fluid equations, with spatial profiles for density and temperature that match general observational data. The model is one-dimensional in configuration-space and two-dimensional in velocity-space, and accounts for large-scale processes, namely, advection, gravity, magnetic mirroring, and the large-scale ambipolar electric field. The findings reported add to the general understanding of regulation of particle distributions in solar wind and to the predictions of their shape in regions restricted for in situ measurements. In particular, the results suggest that fluctuations of temperature at the Sun might play a key role in shaping solar wind velocity distributions via large-scale ambipolar electric field.

Keywords: Solar coronal transients, Parallel field, Solar wind, Kappa mechanism, Interplanetary particle acceleration
The Lanthanide Fraction Distribution in Metal-poor Stars: A Test of Neutron Star Mergers as the Dominant r-process Site. Multimessenger observations of the neutron star merger GW170817 and its kilonova proved that neutron star mergers can synthesize large quantities of r-process elements. If neutron star mergers in fact dominate all r-process element production, then the distribution of kilonova ejecta compositions should match the distribution of r-process abundance patterns observed in stars. The lanthanide fraction (X La) is a measurable quantity in both kilonovae and metal-poor stars, but it has not previously been explicitly calculated for stars. Here we compute the lanthanide fraction distribution of metal-poor stars ([Fe/H] &lt; - 2.5) to enable comparison to current and future kilonovae. The full distribution peaks at log X La ∼ -1.8, but r-process-enhanced stars ([Eu/Fe] &gt; 0.7) have distinctly higher lanthanide fractions: {log}{X}La}≳ -1.5. We review observations of GW170817 and find general consensus that the total {log}{X}La}=-2.2+/- 0.5, somewhat lower than the typical metal-poor star and inconsistent with the most highly r-enhanced stars. For neutron star mergers to remain viable as the dominant r-process site, future kilonova observations should be preferentially lanthanide-rich (including a population of ∼10% with {log}{X}La}&gt; -1.5). These high-X La kilonovae may be fainter and more rapidly evolving than GW170817, posing a challenge for discovery and follow-up observations. Both optical and (mid-)infrared observations will be required to robustly constrain kilonova lanthanide fractions. If such high-X La kilonovae are not found in the next few years, that likely implies that the stars with the highest r-process enhancements have a different origin for their r-process elements.

Keywords: Neutron stars, Stellar abundances, R-process, Transient sources, Nuclear astrophysics
Differentiation of Enceladus and Retention of a Porous Core. The Cassini mission revealed gas plumes associated with surface features called “tiger stripes” at the south pole of Saturn’s moon Enceladus. The composition of plume particles and local cryovolcanism suggested as a possible cause for the activity are typically considered in the context of hydrothermal circulation in the rocky core within a differentiated core-ocean-ice crust structure. We model the internal evolution and differentiation of Enceladus heated by radioactive nuclides and tidal dissipation. Calculating the core formation, we investigate its compaction by modeling the evolution of porosity, thereby varying the rock rheology based on different assumptions on the composition, such as grain size, creep activation energy, degree of hydration, and oxygen fugacity. We present final structures with a core radius of 185-205 km, a porous core layer of 4-70 km, an ocean of ≈10-27 km, and an ice crust layer of ≈30-40 km, that are largely consistent with the current estimates for Enceladus. By fitting the model results to these observations, we determine an accretion time of 1.3-2.3 Ma after calcium-aluminum-rich inclusions for Enceladus. Our models produce a porous outer core for wet and dry olivine rock rheologies supporting the hypothesis of hydrothermal circulation of oceanic water in the core. No porosity is retained for an antigorite rheology, implying that the core of Enceladus is not dominated by this mineral.

Keywords: Water vapor, Planetesimals, Planetary cores, Saturnian satellites, Habitable planets
Understanding Uniturbulence: Self-cascade of MHD Waves in the Presence of Inhomogeneities. It is widely accepted in the MHD turbulence community that the nonlinear cascade of wave energy requires counterpropagating Alfvénic wave packets, along some mean magnetic field. This fact is an obvious outcome of the MHD equations under the assumptions of incompressibility and homogeneity. Despite attempts to relax these assumptions in the context of MHD turbulence, the central idea of turbulence generation persists. However, once the assumptions of incompressiblity and homogeneity break down, the generally accepted picture of turbulent cascade generation is not universal. In this paper, we show that perpendicular inhomogeneities (across the mean magnetic field) lead to propagating wave solutions that are necessarily described by co-propagating Elsässer fields, already in the incompressible case. One simple example of these wave solutions is the surface Alfvén wave on a planar discontinuity across the magnetic field. We show through numerical simulations how the nonlinear self-deformation of these unidirectionally propagating waves leads to a cascade of wave energy across the magnetic field. The existence of this type of unidirectional cascade might have an additional strong effect on the turbulent dissipation rate of dominantly outward-propagating Alfvénic waves in structured plasma, as in the solar corona and solar wind.

Keywords: Magnetohydrodynamics, Plasma physics, Alfven waves, Magnetohydrodynamical simulations
In Situ Measurements of the Variable Slow Solar Wind near Sector Boundaries. The release of density structures at the tip of the coronal helmet streamers, likely as a consequence of magnetic reconnection, contributes to the mass flux of the slow solar wind (SSW). In situ measurements in the vicinity of the heliospheric plasma sheet of the magnetic field, protons, and suprathermal electrons reveal details of the processes at play during the formation of density structures near the Sun. In a previous article, we exploited remote-sensing observations to derive a 3D picture of the dynamic evolution of a streamer. We found evidence of the recurrent and continual release of dense blobs from the tip of the streamers. In the present paper, we interpret in situ measurements of the SSW during solar maximum. Through both case and statistical analysis, we show that in situ signatures (magnetic field magnitude, smoothness and rotation, proton density, and suprathermal electrons, in the first place) are consistent with the helmet streamers producing, in alternation, high-density regions (mostly disconnected) separated by magnetic flux ropes (mostly connected to the Sun). This sequence of emission of dense blobs and flux ropes also seems repeated at smaller scales inside each of the high-density regions. These properties are further confirmed with in situ measurements much closer to the Sun using Helios observations. We conclude on a model for the formation of dense blobs and flux ropes that explains both the in situ measurements and the remote-sensing observations presented in our previous studies.

Keywords: Slow solar wind, Solar corona, Solar magnetic reconnection
Galactic Cosmic Rays Modulation in the Vicinity of Corotating Interaction Regions: Observations During the Last Two Solar Minima. Corotating interaction regions (CIRs) are responsible for short-term recurrent cosmic-ray modulation, prominent near solar minima. Using the OMNI data sets for two periods of low solar activity near the beginning and end of solar cycle 24, superposed epoch analysis was performed on the solar wind plasma features for 53 and 43 events during periods 2007-2008 and 2017-2018, respectively. Turbulent properties of the solar wind were studied using the variance method for each CIR. Power spectra have been constructed for overlapped subintervals in the vicinity of stream interfaces (SIs). Using measured correlation lengths and turbulent energies, parallel and perpendicular diffusion mean free paths for cosmic-ray ions have been inferred based on two distinct theoretical formulations. For the two periods with opposite solar polarities, our results show that unlike solar wind speed, magnetic field strength, flow pressure, and proton density are relatively higher during the latest period. Increased turbulent energy and reduced parallel transport coefficients of energetic particles at the SIs are observed. The diffusion coefficients follow the same trends during both periods. The perpendicular diffusion starts increasing nearly a day before SIs and is higher in the fast wind. Superposed epoch analysis is performed on the &gt;120 MeV proton count rate obtained from the CRIS instrument on board the ACE spacecraft for the same events. The recorded proton rates have peaks half a day before a SI and reach their minimum more than a day after a SI and have a high anticorrelation with the perpendicular diffusion coefficient.

Keywords: Galactic cosmic rays, Solar wind, Corotating streams, Interplanetary turbulence