adsabs/SciX_UAT_keywords huggingface dataset:
Constraining Saturn's Interior with Ring Seismology: Effects of Differential Rotation and Stable Stratification

Normal mode oscillations in Saturn excite density and bending waves in the C ring, providing a valuable window into the planet's interior. Saturn's fundamental modes (f-modes) excite the majority of the observed waves, while gravito-inertial modes (rotationally modified g-modes) associated with stable stratification in the deep interior provide a compelling explanation for additional density waves with low azimuthal wavenumbers m. However, multiplets of density waves with nearly degenerate frequencies, including an m = 3 triplet, still lack a definitive explanation. We investigate the effects of rapid and differential rotation on Saturn's oscillations, calculating normal modes for independently constrained interior models. We use a non-perturbative treatment of rotation that captures the full effects of the Coriolis and centrifugal forces, and consequently the mixing of sectoral f-modes with g-modes characterized by very different spherical harmonic degrees. Realistic profiles for differential rotation associated with Saturn's zonal winds can enhance these mode interactions, producing detectable oscillations with frequencies separated by less than 1%. Our calculations demonstrate that a three-mode interaction involving an f-mode and two g-modes can feasibly explain the finely split m = 3 triplet, although the fine-tuning required to produce such an interaction generally worsens agreement with seismological constraints provided by m = 2 density waves. Our calculations additionally demonstrate that sectoral f-mode frequencies are measurably sensitive to differential rotation in Saturn's convective envelope. Finally, we find that including realistic equatorial antisymmetry in Saturn's differential rotation profile couples modes with even and odd equatorial parity, producing oscillations that could in principle excite both density and bending waves simultaneously.

Keywords: planetary science; planetary structure; solar system; saturn; hydrodynamics; pulsation modes
Hinode/EIS Coronal Magnetic Field Measurements at the Onset of a C2 Flare

We study Hinode/EIS observations of an active region taken before, during, and after a small C2.0 flare in order to monitor the evolution of the magnetic field and its relation to the flare event. We find that while the flare left the active region itself unaltered, the event included a large magnetic field enhancement (MFE), which consisted of a large magnetic field strength increase to values just short of 500 G in a rather small region where no magnetic field was measured before. This MFE is observed during the impulsive phase of the flare at the footpoints of flare loops, its magnetic energy is sufficient to power the radiative losses of the entire flare, and has completely dissipated after the flare. We argue that the MFE might occur at the location of the reconnection event triggering the flare, and note that it formed within 22 minutes of the flare start (as given by the EIS raster return time). These results open the door to a new line of studies aimed at determining whether MFEs can be flare precursor events or used for Space Weather forecasts, what advance warning time they could provide and if this time is long enough to allow for mitigation procedures to be implemented; as well as to explore which physical processes lead to MFE formation and dissipation, whether such processes are the same in both long-duration and impulsive flares, and whether they can be predicted by theoretical models.

Keywords: solar magnetic fields; solar flares
Spin Dynamics of Planets in Resonant Chains

About a dozen exoplanetary systems have been discovered with three or more planets participating in a sequence of mean-motion resonances. The unique and complex architectures of these so-called "resonant chains" motivate efforts to characterize their planets holistically. In this work, we perform a comprehensive exploration of the spin-axis dynamics of planets in resonant chains. Planetary spin states are closely linked with atmospheric dynamics and habitability and are thus especially relevant to resonant chains like TRAPPIST-1, which hosts several temperate planets. Considering a set of observed resonant chains, we calculate the equilibrium states of the planetary axial tilts ("obliquities"). We show that high-obliquity states exist for ∼60% of planets in our sample, and many of these states can be stable in the presence of tidal dissipation. Using case studies of two observed systems (Kepler-223 and TOI-1136), we demonstrate how these high-obliquity states could have been attained during the initial epoch of disk-driven orbital migration that established the resonant orbital architectures. We show that the TRAPPIST-1 planets most likely have zero obliquities, with the possible exception of planet d. Overall, our results highlight that both the orbital and spin states of resonant chains are valuable relics of the early stages of planet formation and evolution.

Keywords: exoplanets; orbital resonances; spin-orbit resonances; exoplanet dynamics; exoplanet tides; orbits; atmospheric structure; exoplanet migration; exoplanet formation
Measurements of Coronal Magnetic Field Strengths in Solar Active Region Loops

The characteristic electron densities, temperatures, and thermal distributions of 1 MK active region loops are now fairly well established, but their coronal magnetic field strengths remain undetermined. Here we present measurements from a sample of coronal loops observed by the Extreme-ultraviolet Imaging Spectrometer on Hinode. We use a recently developed diagnostic technique that involves atomic radiation modeling of the contribution of a magnetically induced transition to the Fe X 257.262 Å spectral line intensity. We find coronal magnetic field strengths in the range of 60-150 G. We discuss some aspects of these new results in the context of previous measurements using different spectropolarimetric techniques, and their influence on the derived Alfvén speeds and plasma β in coronal loops.

Keywords: solar physics; solar active region magnetic fields
Constraints on Neutron Star Structure from the Clocked X-Ray Burster 1RXS J180408.9-342058

Type I X-ray bursts are rapid-brightening transient phenomena on the surfaces of accreting neutron stars (NSs). Some X-ray bursts, called clocked bursters, exhibit regular behavior with similar light-curve profiles in their burst sequences. The periodic nature of clocked bursters has the advantage of constraining X-ray binary parameters and physics inside the NS. In the present study, we compute numerical models, based on different equations of state and NS masses, which are compared with the observations of a recently identified clocked burster, 1RXS J180408.9-342058. We find that the relation between the accretion rate and the recurrence time is highly sensitive to the NS mass and radius. We determine, in particular, that 1RXS J180408.9-342058 appears to possess a mass less than 1.7M ⊙ and favors a stiffer nuclear equation of state (with an NS radius ≳12.7 km). Consequently, the observations of this new clocked burster may provide additional constraints for probing the structure of NSs.

Keywords: x-ray bursts; neutron stars; low-mass x-ray binary stars
Investigating Coronal Holes and CMEs as Sources of Brightness Depletion Detected in PSP/WISPR Images

The Parker Solar Probe (PSP) mission provides a unique opportunity to observe the solar corona from distances below 20 R ☉. In this work, we utilize white light images from the Wide-field Imager for Solar PRobe aboard the PSP from solar encounters 10 through 13 to examine the causes of brightness depletions of the corona during the rapid transit of PSP through the perihelia of its orbit. We analyze the effect of (1) coronal holes (CHs) and (2) energetic coronal mass ejection (CME) events on the observed brightness of the images. We speculate on the causes of the brightness depletions, ascribing them to the evacuation of (1) free electrons (reduced K-corona) and (2) interplanetary dust (reduced F-corona). In particular, we show that (1) the presence of CHs in all of the orbits is directly correlated with the depletion of the global white light emission recorded, and (2) a huge CME event in encounter 13 caused a very deep depletion in its wake that removed the electron content as well as some of the interplanetary dust.

Keywords: solar f corona; solar k corona; solar coronal holes; solar wind; solar coronal mass ejections; interplanetary dust
The Launching of Cold Clouds by Galaxy Outflows. III. The Influence of Magnetic Fields

Motivated by observations of outflowing galaxies, we investigate the combined impact of magnetic fields and radiative cooling on the evolution of cold clouds embedded in a hot wind. We perform a collection of three-dimensional adaptive mesh refinement, magnetohydrodynamical simulations that span two resolutions, and include fields that are aligned and transverse to the oncoming, super-Alfvénic material. Aligned fields have little impact on the overall lifetime of the clouds over the non-magnetized case, although they do increase the mixing between the wind and cloud material by a factor of ≈3. Transverse fields lead to magnetic draping, which isolates the clouds, but they also squeeze material in the direction perpendicular to the field lines, which leads to rapid mass loss. A resolution study suggests that the magnetized simulations have somewhat better convergence properties than non-magnetized simulations, and that a resolution of 64 zones per cloud radius is sufficient to accurately describe these interactions. We conclude that the combined effects of radiative cooling and magnetic fields are dependent on field orientation, but are unlikely to enhance cloud lifetimes beyond the effect of radiative cooling alone.

Keywords: circumgalactic medium; galactic winds; radiative magnetohydrodynamics; magnetohydrodynamics
Relativistic Envelopes and Gamma-Rays from Neutron Star Mergers

We suggest that neutron star mergers eject an ultrarelativistic envelope of mass m ∼ 10-7 M⊙, which helps explain the gamma-ray burst from GW170817. One ejection mechanism is the ablation of the neutron star surface by the burst of neutrinos in the first 30 μs of the merger. Another, more efficient mechanism for inflating the ultrarelativistic envelope is an internal shock in the massive ejecta from the merger. A strong shock is expected if the merger product is a magnetar, which emits a centrifugally accelerated wind. The shock propagates outward through the ejecta and accelerates in its outer layers at radii r ∼ 109-1010 cm, launching an ultrarelativistic opaque envelope filled with ∼104 photons per nucleon. The Lorentz factor profile of the envelope rises outward and determines its homologous expansion, which adiabatically cools the trapped photons. Once the magnetar loses its differential rotation and collapses into a black hole, a powerful jet forms. It drives a blast wave into the envelope, chasing its outer layers, and eventually catching up with the envelope photosphere at r ∼ 1012 cm. The ultrarelativistic photospheric breakout of the delayed blast wave emits a gamma-ray burst in a broad solid angle around the merger axis. This model explains the gamma-ray pulse from merger GW170817 with luminosity Lγ ∼ 1047 erg s-1, duration ${\rm{\Delta }}{t}_{\mathrm{obs}}\sim 0.5\,{\rm{s}}$ , and characteristic photon energy ∼100 keV. The blast-wave Lorentz factor at the envelope photosphere is consistent with Γ ≳ 5, which we derive from the observed light curve of the burst. We suggest future tests of the model.

Keywords: relativistic fluid dynamics; gamma-ray bursts; non-thermal radiation sources; neutron stars; gravitational wave sources; shocks
Evidence of a Decreased Binary Fraction for Massive Stars within 20 milliparsecs of the Supermassive Black Hole at the Galactic Center

We present the results of the first systematic search for spectroscopic binaries within the central 2 × 3 arcsec2 around the supermassive black hole at the center of the Milky Way galaxy. This survey is based primarily on over a decade of adaptive optics-fed integral-field spectroscopy (R ~ 4000), obtained as part of the Galactic Center Orbits Initiative at Keck Observatory, and it has a limiting K'-band magnitude of 15.8, which is at least 4 mag deeper than previous spectroscopic searches for binaries at larger radii within the central nuclear star cluster. From this primary data set, over 600 new radial velocities are extracted and reported, increasing by a factor of 3 the number of such measurements. We find no significant periodic signals in our sample of 28 stars, of which 16 are massive, young (main-sequence B) stars and 12 are low-mass, old (M and K giant) stars. Using Monte Carlo simulations, we derive upper limits on the intrinsic binary star fraction for the young star population at 47% (at 95% confidence) located ~20 mpc from the black hole. The young star binary fraction is significantly lower than that observed in the field (70%). This result is consistent with a scenario in which the central supermassive black hole drives nearby stellar binaries to merge or be disrupted, and it may have important implications for the production of gravitational waves and hypervelocity stars.

Keywords: galactic center; infrared spectroscopy; adaptive optics; binary stars
Early Spectroscopy and Dense Circumstellar Medium Interaction in SN 2023ixf

We present the optical spectroscopic evolution of SN 2023ixf seen in subnight cadence spectra from 1.18 to 15 days after explosion. We identify high-ionization emission features, signatures of interaction with material surrounding the progenitor star, that fade over the first 7 days, with rapid evolution between spectra observed within the same night. We compare the emission lines present and their relative strength to those of other supernovae with early interaction, finding a close match to SN 2020pni and SN 2017ahn in the first spectrum and SN 2014G at later epochs. To physically interpret our observations, we compare them to CMFGEN models with confined, dense circumstellar material around a red supergiant (RSG) progenitor from the literature. We find that very few models reproduce the blended N III (λλ4634.0,4640.6)/C III (λλ4647.5,4650.0) emission lines observed in the first few spectra and their rapid disappearance thereafter, making this a unique diagnostic. From the best models, we find a mass-loss rate of 10-3-10-2 M ⊙ yr-1, which far exceeds the mass-loss rate for any steady wind, especially for an RSG in the initial mass range of the detected progenitor. These mass-loss rates are, however, similar to rates inferred for other supernovae with early circumstellar interaction. Using the phase when the narrow emission features disappear, we calculate an outer dense radius of circumstellar material R CSM,out ≈ 5 × 1014 cm, and a mean circumstellar material density of ρ = 5.6 × 10-14 g cm-3. This is consistent with the lower limit on the outer radius of the circumstellar material we calculate from the peak Hα emission flux, R CSM,out ≳ 9 × 1013 cm.

Keywords: core-collapse supernovae; type ii supernovae; circumstellar matter; stellar mass loss; red supergiant stars
Near-infrared and Optical Observations of Type Ic SN 2021krf: Luminous Late-time Emission and Dust Formation

We present near-infrared (NIR) and optical observations of the Type Ic supernova (SN Ic) SN 2021krf obtained between days 13 and 259 at several ground-based telescopes. The NIR spectrum at day 68 exhibits a rising K-band continuum flux density longward of ~2.0 μm, and a late-time optical spectrum at day 259 shows strong [O I] 6300 and 6364 Å emission-line asymmetry, both indicating the presence of dust, likely formed in the SN ejecta. We estimate a carbon-grain dust mass of ~2 × 10-5 M ⊙ and a dust temperature of ~900-1200 K associated with this rising continuum and suggest the dust has formed in SN ejecta. Utilizing the one-dimensional multigroup radiation-hydrodynamics code STELLA, we present two degenerate progenitor solutions for SN 2021krf, characterized by C-O star masses of 3.93 and 5.74 M ⊙, but with the same best-fit 56Ni mass of 0.11 M ⊙ for early times (0-70 days). At late times (70-300 days), optical light curves of SN 2021krf decline substantially more slowly than those expected from 56Co radioactive decay. Lack of H and He lines in the late-time SN spectrum suggests the absence of significant interaction of the ejecta with the circumstellar medium. We reproduce the entire bolometric light curve with a combination of radioactive decay and an additional powering source in the form of a central engine of a millisecond pulsar with a magnetic field smaller than that of a typical magnetar.

Keywords: supernovae; core-collapse supernovae; explosive nucleosynthesis; type ic supernovae
Molecular Gas Excitation of the Massive Dusty Starburst CRLE and the Main-sequence Galaxy HZ10 at z = 5.7 in the COSMOS Field

We report CO(5 → 4) and CO(6 → 5) line observations in the dusty starbursting galaxy CRLE (z = 5.667) and the main-sequence (MS) galaxy HZ10 (z = 5.654) with the Northern Extended Millimeter Array. CRLE is the most luminous z > 5 starburst in the COSMOS field and HZ10 is the most gas-rich "normal" galaxy currently known at z > 5. We find line luminosities for CO(5 → 4) and CO(6 → 5) of (4.9 ± 0.5) and (3.8 ± 0.4) × 1010 K km s-1 pc2 for CRLE and upper limits of < 0.76 and < 0.60 × 1010 K km s-1 pc2 for HZ10, respectively. The CO excitation of CRLE appears comparable to other z > 5 dusty star-forming galaxies. For HZ10, these line luminosity limits provide the first significant constraints of this kind for an MS galaxy at z > 5. We find the upper limit of ${L}_{5\to 4}^{{\prime} }/{L}_{2\to 1}^{{\prime} }$ in HZ10 could be similar to the average value for MS galaxies around z ≈ 1.5, suggesting that MS galaxies with comparable gas excitation may already have existed one billion years after the Big Bang. For CRLE we determine the most likely values for the H2 density, kinetic temperature, and dust temperature based on excitation modeling of the CO line ladder. We also derive a total gas mass of (7.1 ± 1.3) × 1010 M ⊙. Our findings provide some of the currently most detailed constraints on the gas excitation that sets the conditions for star formation in a galaxy protocluster environment at z > 5.

Keywords: molecular gas; galaxy masses; galaxy luminosities
Extracting the True Cosmic-ray Relative Intensity Sky Map from Normalized Relative Intensity Data Measured by Air Shower Experiments

We use data from the Tibet AS γ experiment for 4 teraelectronvolt (TeV) cosmic rays as an example to perform a nonlinear interstellar distribution model regression according to the way the observed anisotropy is typically presented, from which we extract normalization factors that allow us to obtain a true relative intensity sky map from the measurements. By using various test statistics, we show that the nonlinear fit significantly outperforms the direct linear fit in its ability to model cosmic-ray anisotropy. The procedure also allows us to produce normalization constants that can trace minute latitudinal variations of experimental response to cosmic-ray intensity. Applying the correction of the latitudinal response function to the Tibet ASγ data, we generate a sky map of true relative intensity. As a result, we observe that the measured and corrected sky maps show significant differences in intensity and angular spectral power. Our full anisotropy sky map of true relative intensity contradicts the assumption that the latitudinal variation in longitudinally averaged flux is negligible. The result further confirms that TeV cosmic-ray anisotropy is dominated by a dipole (ℓ = 1) aligned with the interstellar magnetic field's direction. Our results also confirm the existence of much weaker middle-scale interstellar anisotropy between ℓ = 2 and ℓ = 13.

Keywords: heliosphere; galactic cosmic rays; computational methods
The Blue Compact Dwarf Galaxy VCC 848 Formed by Dwarf-Dwarf Merging: H I Gas, Star Formation, and Numerical Simulations

A clear link between a dwarf-dwarf merger event and enhanced star formation (SF) in the recent past was recently identified in the gas-dominated merger remnant VCC 848, offering by far the clearest view of a gas-rich late-stage dwarf-dwarf merger. We present a joint analysis of JVLA H I emission line mapping, optical imaging, and numerical simulations of VCC 848 in order to examine the effect of the merger on the stellar and gaseous distributions. VCC 848 has less than 30% of its H I gas concentrated within the central high-surface-brightness star-forming region, while the remaining H I is entrained in outlying tidal features. Particularly, a well-defined tidal arm reaches N(H I) comparable to the galaxy center but lacks SF. The molecular gas mass inferred from the current SF rate (SFR) dominates over the atomic gas mass in the central ∼1.5 kpc. VCC 848 is consistent with being a main-sequence star-forming galaxy for its current stellar mass and SFR. The H II region luminosity distribution largely agrees with that of normal dwarf irregulars with similar luminosities, except that the brightest H II region is extraordinarily luminous. Our N-body/hydrodynamical simulations imply that VCC 848 is a merger between a gas-dominated primary progenitor and a gas-bearing star-dominated secondary. The progenitors had their first passage on a near-radial noncoplanar orbit more than 1 Gyr ago. The merger did not build up a core as compact as typical compact dwarfs with a centralized starburst, which may be partly ascribed to the star-dominated nature of the secondary and, in a general sense, a negative stellar feedback following intense starbursts triggered at early stages of the merger.

Keywords: blue compact dwarf galaxies; disk galaxies; dwarf galaxies; dwarf irregular galaxies; irregular galaxies; galaxy interactions; galaxy mergers; interstellar medium; hydrodynamical simulations; h i line emission
Conditions for Justifying Single-fluid Approximation for Charged and Neutral Dust Fluids and a Smoothed Particle Magnetohydrodynamics Method for Dust-Gas Mixture

We describe a numerical scheme for magnetohydrodynamics simulations of dust-gas mixture by extending smoothed particle magnetohydrodynamics. We employ the single-species particle approach to describe dust-gas mixture with several modifications from the previous studies. We assume that the charged and neutral dust can be treated as single-fluid, that the electromagnetic force acts on the gas, and that that acting on the charged dust is negligible. The validity of these assumptions in the context of protostar formation is not obvious and is extensively evaluated. By investigating the electromagnetic force and electric current with terminal velocity approximation, it is found that as the dust size increases, the contribution of dust to them becomes smaller and negligible. We conclude that our assumption that the electromagnetic force on the dusts is negligible is valid for the dust size with ad ≳ 10 μm. On the other hand, they do not produce the numerical artifact for the dust ad ≲ 10 μm in the envelope and disk, where the perfect coupling between gas and dust is realized. However, we also found that our assumptions may break down in outflow (or under an environment with very strong magnetic field and low density) for the dust ad ≲ 10 μm. We conclude that our assumptions are valid in almost all cases where macroscopic dust dynamics is important in the context of protostar formation. We conduct numerical tests of dusty waves, dusty magnetohydrodynamics shocks, and gravitational collapse of magnetized cloud cores with our simulation code. The results show that our numerical scheme well reproduces the dust dynamics in the magnetized medium.

Keywords: star formation; magnetohydrodynamical simulations; astrophysical dust processes