adsabs/SciX_UAT_keywords huggingface dataset:
Magnetohydrodynamic Perspective on the Disappearance of Mercury's Bow Shock by Helios Data Exploration

In the magnetohydrodynamic (MHD) perspective, the planet's bow shock would disappear when the fast-mode Mach number (M F) of the solar wind is less than one. Compared to Earth, Mercury is subject to a lower M F solar wind due to its proximity to the Sun, resulting in a higher possibility of the disappearance of its bow shock. To examine the variability of Mercury's bow shock in response to the solar wind properties, analyses of the observations by the Helios spacecraft at 0.30-0.50 au during 1975-1983, covering solar cycle 21, together with the theoretical solutions and MHD simulations are conducted in this study. Our observational analyses show that more solar wind data with extremely low fast-mode Mach numbers (say, M F ≤ 1.5) are observed during the rising and maximum phases and are characterized by a significantly low proton number density. It is also found that approximately 35% of the extremely low fast-mode Mach number solar wind events (M F ≤ 1.5) occur within the main body of interplanetary coronal mass ejections (ICMEs), while about 58% of them are unrelated to ICMEs. Three of these events are selected to demonstrate that the occurrences of the solar wind with M F ≤ 1.5 may not be necessarily affected by ICMEs. Our theoretical and numerical results indicate that when Mercury encounters the solar wind with M F ≤ 1.5, its bow shock would move farther away, become flattened, and even disappear. Furthermore, our calculations suggest that Mercury's bow shock would become a slow-mode shock with a concave-upward structure under such extreme solar wind conditions.

Keywords: planetary bow shocks; mercury (planet); solar wind; magnetohydrodynamics; solar cycle
Will the Mars Helicopter Induce Local Martian Atmospheric Breakdown?

Any rotorcraft on Mars will fly in a low-pressure and dusty environment. It is well known that helicopters on Earth become highly charged due, in part, to triboelectric effects when flying in sandy conditions. We consider the possibility that the Mars Helicopter Scout (MHS), called Ingenuity, flying at Mars as part of the Mars 2020 Perseverance mission, will also become charged due to grain-rotor triboelectric interactions. Given the low Martian atmospheric pressure of ∼5 Torr, the tribocharge on the blade could become intense enough to stimulate gas breakdown near the surface of the rotorcraft. We modeled the grain-blade interaction as a line of current that forms along the blade edge in the region where grain-blade contacts are the greatest. This current then spreads throughout the connected quasi-conductive regions of the rotorcraft. Charge builds up on the craft, and the dissipative pathway to remove charge is back into the atmosphere. We find that for blade tribocharging currents that form in an ambient atmospheric dust load, system current balance and charge dissipation can be accomplished via the nominal atmospheric conductive currents. However, at takeoff and landing, the rotorcraft could be in a rotor-created particulate cloud, leading to local atmospheric electrical breakdown near the rotorcraft. We especially note that the atmospheric currents in the breakdown are not large enough to create any hazard to Ingenuity itself, but Ingenuity operations can be considered a unique experiment that provides a test of the electrical properties of the Martian near-surface atmosphere.

Keywords: mars; planetary atmospheres; atmospheric effects; atmospheric science
Probability of Forming Gaps in the GD-1 Stream by Close Encounters of Globular Clusters

One of the most intriguing properties of the GD-1 stellar stream is the existence of three gaps. If these gaps were formed by close encounters with dark matter subhalos, the GD-1 stream opens an exciting window through which we can see the size, mass, and velocity distributions of the dark matter subhalos in the Milky Way. However, in order to use the GD-1 stream as a probe of the dark matter substructure, we need to disprove that these gaps are not due to the perturbations from baryonic components of the Milky Way. Here we ran a large number of test-particle simulations to investigate the probability that each of the known globular clusters (GCs) can form a GD-1-like gap, by using the kinematical data of the GD-1 stream and GCs from Gaia early data release 3 and by fully taking the observational uncertainty into account. We found that the probability that all of the three gaps were formed by GCs is as low as 1.7 × 10-5, and the expected number of gaps formed by GCs is only 0.057 in our fiducial model. Our result highly disfavors a scenario in which GCs form the gaps. Given that other baryonic perturbers (e.g., giant molecular clouds) are even less likely to form a gap in the retrograde-moving GD-1 stream, we conclude that at least one of the gaps in the GD-1 stream was formed by dark matter subhalos if the gaps were formed by flyby perturbations.

Keywords: stellar dynamics; milky way dark matter halo; milky way dynamics; globular star clusters; stellar streams
The Supernova Remnant Population of NGC 6946 as Observed in [Fe II] 1.644 μm with HST

NGC 6946 is a high-star-formation-rate, face-on, spiral galaxy that has hosted 10 supernovae since 1917. Not surprisingly, a large number of supernova remnants and candidates have been identified either as optical nebulae with high [S II]:Hα line ratios (147) or as compact non-thermal radio sources (35). However, there are only seven overlaps between these two samples. Here, we apply [Fe II] 1.644 μm emission as a new diagnostic to search for supernova remnants in an attempt to resolve this discrepancy. [Fe II] is expected to be relatively strong in the radiative shocks of supernova remnants and almost absent in H II regions. It is less susceptible to the effects of absorption along the line of sight than the optical lines normally used to identify remnants. Using data from the WFC3 camera on Hubble Space Telescope (HST), we identify 132 [Fe II] emission nebulae in NGC 6946 as likely supernova remnants. Of these, 54 align with previously known optical supernova remnants. The remaining 78 objects are new; of these 44 are visible in new HST imagery in Hα and [S II]. This brings the total number of supernova remnant candidates (from optical and/or IR data) in NGC 6946 to 225. A total of 14 coincidences with radio supernova remnant candidates (out of 30 in our search area) are found in this expanded list. The identification of so many new remnant candidates validates the use of [Fe II] imagery for finding remnants, and suggests that previous remnant searches in other galaxies may be far from complete. * Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. Support for program numbers 14638 and 15216 was provided through a grant from the STScI under NASA contract NAS5-26555.

Keywords: supernova remnants; interstellar medium
Coherent Cherenkov Radiation by Bunches in Fast Radio Bursts

Fast radio bursts (FRBs) are extragalactic radio transients with extremely high brightness temperature, which strongly suggests the presence of coherent emission mechanisms. In this study, we introduce a novel radiation mechanism for FRBs involving coherent Cherenkov radiation (ChR) emitted by bunched particles that may originate within the magnetosphere of a magnetar. We assume that some relativistic particles are emitted from the polar cap of a magnetar and move along magnetic field lines through a charge-separated magnetic plasma, emitting coherent ChR along their trajectory. The crucial condition for ChR to occur is that the refractive index of the plasma medium, denoted as n r , must satisfy the condition ${n}_{r}^{2}\gt 1$ . We conduct comprehensive calculations to determine various characteristics of ChR, including its characteristic frequency, emission power, required parallel electric field, and coherence factor. Notably, our proposed bunched coherent ChR mechanism has the remarkable advantage of generating a narrower-band spectrum. Furthermore, a frequency downward-drifting pattern and ~100% linearly polarized emission can be predicted within the framework of this emission mechanism.

Keywords: radio bursts; radio transient sources; magnetars; radiative processes
On the Hadronic Origin of High-energy Emission of γ-Ray-loud Narrow-line Seyfert 1 PKS 1502+036

The radiation mechanism of Radio-Loud Narrow-Line Seyfert 1 (RL-NLS1) galaxies from X-ray to γ-ray bands remains an open question. While the leptonic model has been employed to explain the spectral energy distribution (SED), the hadronic process may potentially account for the high-energy radiation of some γ-ray-loud Narrow-Line Seyfert 1 (NLS1) galaxies as well. We study one of such RL-NLS1s, PKS 1502+036, comparing the theoretical SEDs predicted by the leptonic model and the lepto-hadronic model to the observed one. For the hadronic processes, we take into account the proton synchrotron radiation and proton-photon interactions (including the Bethe-Heitler process and the photopion process) including the emission of pairs generated in the electromagnetic cascade initiated by these processes. Our results show that the leptonic model can reproduce the SED of this source, in which the X-ray to γ-ray radiation can be interpreted as the inverse Compton scattering. On the other hand, the proton synchrotron radiation can also explain the high-energy component of SED although extreme parameters are needed. We also demonstrate that the p γ interactions as well as the cascade process cannot explain SED. Our results imply that a leptonic origin is favored for the multiwavelength emission of PKS 1502+036.

Keywords: high energy astrophysics; active galactic nuclei
Possible Discrimination of Black Hole Origins from the Lensing Rate of DECIGO and B-DECIGO Sources

In this paper, we forecast the expected detection rates and redshift distributions of gravitationally lensed gravitational waves (GWs) from three different mass distributions of primordial black holes (PBHs) and two stellar formation models of astrophysical black holes (ABHs) in the context of the DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) and its smaller-scale version B-DECIGO. It suggests that DECIGO will be able to detect 104-105 GW signals from such binary black holes each year and the event rate distributions for PBHs will differ from those for ABHs due to their different merger rate with respect to redshift. The large number of event rates makes 5-70 detections of lensed GW signals possible. After considering the gravitational lensing effect, the difference between the detection rates and distributions for PBHs and ABHs will be more significant. Therefore, this can be served as a complementary method to distinguish PBHs from ABHs.

Keywords: primordial black holes; strong gravitational lensing; gravitational waves
Dust in Comet 67P/Churyumov-Gerasimenko: Interrelation between in situ Findings by Rosetta and Ground-based Polarimetry

We present a comprehensive model of the ground-based polarization observations from comet 67P/Churyumov-Gerasimenko using two different apertures on its apparitions of 1982 and 2016. Our modeling suggests that the dust composition viewed within the two apertures is the same, but that they have different size distributions, with smaller particles more concentrated in the innermost coma. The best fits to the measurements using the smallest aperture are obtained using a dust size distribution similar to the Rosetta findings. Furthermore, the model parameters providing the best fit to polarimetric observations using the large aperture appear remarkably consistent with what was previously inferred in other comets. This represents an attempt to reconcile the Rosetta results with ground-based observations.

Keywords: comets; coma dust; short period comets
Stellar Obliquities in Exoplanetary Systems

The rotation of a star and the revolutions of its planets are not necessarily aligned. This article reviews the measurement techniques, key findings, and theoretical interpretations related to the obliquities (spin-orbit angles) of planet-hosting stars. The best measurements are for stars with short-period giant planets, which have been found on prograde, polar, and retrograde orbits. It seems likely that dynamical processes such as planet-planet scattering and secular perturbations are responsible for tilting the orbits of close-in giant planets, just as those processes are implicated in exciting orbital eccentricities. The observed dependence of the obliquity on orbital separation, planet mass, and stellar structure suggests that in some cases, tidal dissipation damps a star's obliquity within its main-sequence lifetime. The situation is not as clear for stars with smaller or wider-orbiting planets. Although the earliest measurements of such systems tended to find low obliquities, some glaring exceptions are now known in which the star's rotation is misaligned with respect to the coplanar orbits of multiple planets. In addition, statistical analyses based on projected rotation velocities and photometric variability have found a broad range of obliquities for F-type stars hosting compact multiple-planet systems. The results suggest it is unsafe to assume that stars and their protoplanetary disks are aligned. Primordial misalignments might be produced by neighboring stars or more complex events that occur during the epoch of planet formation.

Keywords: planet hosting stars; stellar rotation; tidal interaction; exoplanet dynamics; exoplanet formation; exoplanet migration
The MOSDEF Survey: Neon as a Probe of ISM Physical Conditions at High Redshift

We present results on the properties of neon emission in z ∼ 2 star-forming galaxies drawn from the MOSFIRE Deep Evolution Field (MOSDEF) survey. Doubly ionized neon ([Ne III]λ3869) is detected at ≥3σ in 61 galaxies, representing ∼25% of the MOSDEF sample with Hα, Hβ, and [O III]λ5007 detections at similar redshifts. We consider the neon emission-line properties of both individual galaxies with [Ne III]λ3869 detections and composite z ∼ 2 spectra binned by stellar mass. With no requirement of [Ne III]λ3869 detection, the latter provide a more representative picture of neon emission-line properties in the MOSDEF sample. The [Ne III]λ3869/[O II]λ3727 ratio (Ne3O2) is anticorrelated with stellar mass in z ∼ 2 galaxies, as expected based on the mass-metallicity relation. It is also positively correlated with the [O III]λ5007/[O II]λ3727 ratio (O32), but z ∼ 2 line ratios are offset toward higher Ne3O2 at fixed O32, compared with both local star-forming galaxies and individual H II regions. Despite the offset toward higher Ne3O2 at fixed O32 at z ∼ 2, biases in inferred Ne3O2-based metallicity are small. Accordingly, Ne3O2 may serve as an important metallicity indicator deep into the reionization epoch. Analyzing additional rest-optical line ratios including [Ne III]λ3869/[O III]λ5007 (Ne3O3) and [O III]λ5007/Hβ (O3Hβ), we conclude that the nebular emission-line ratios of z ∼ 2 star-forming galaxies suggest a harder ionizing spectrum (lower stellar metallicity, i.e., Fe/H) at fixed gas-phase oxygen abundance, compared to systems at z ∼ 0. These new results based on neon lend support to the physical picture painted by oxygen, nitrogen, hydrogen, and sulfur emission of an ionized interstellar medium in high-redshift star-forming galaxies irradiated by chemically young, α-enhanced massive stars. * 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: galaxy evolution; high-redshift galaxies; interstellar medium
On the Impulsive Heating of Quiet Solar Corona

The solar corona consists of a million degree Kelvin plasma. A complete understanding of this phenomenon demands the study of quiet Sun (QS) regions. In this work, we study QS regions in the 171 Å, 193 Å, and 211 Å passbands of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory, by combining the empirical impulsive heating forward model of Pauluhn & Solanki with a machine-learning inversion model that allows uncertainty quantification. We find that there are &approx;2-3 impulsive events per minute, with a lifetime of about 10-20 minutes. Moreover, for all the three passbands, the distribution of power-law slope α peaks above 2. Our exploration of correlations among the frequency of impulsive events and their timescales and peak energy suggests that conduction losses dominate over radiative cooling losses. All these findings suggest that impulsive heating is a viable heating mechanism in QS corona.

Keywords: quiet sun; quiet solar corona; convolutional neural networks
An Estimate of the Binary Star Fraction among Young Stars at the Galactic Center: Possible Evidence of a Radial Dependence

We present the first estimate of the intrinsic binary fraction of young stars across the central ≈0.4 pc surrounding the supermassive black hole (SMBH) at the Milky Way Galactic center (GC). This experiment searched for photometric variability in 102 spectroscopically confirmed young stars, using 119 nights of 10″ wide adaptive optics imaging observations taken at W. M. Keck Observatory over 16 yr in the $K^{\prime} $ -[2.1 μm] and H-[1.6 μm] bands. We photometrically detected three binary stars, all of which are situated more than 1″ (0.04 pc) from the SMBH and one of which, S2-36, is newly reported here with spectroscopic confirmation. All are contact binaries or have photometric variability originating from stellar irradiation. To convert the observed binary fraction into an estimate of the underlying binary fraction, we determined the experimental sensitivity through detailed light-curve simulations, incorporating photometric effects of eclipses, irradiation, and tidal distortion in binaries. The simulations assumed a population of young binaries, with stellar ages (4 Myr) and masses matched to the most probable values measured for the GC young star population, and underlying binary system parameters (periods, mass ratios, and eccentricities) similar to those of local massive stars. As might be expected, our experimental sensitivity decreases for eclipses narrower in phase. The detections and simulations imply that the young, massive stars in the GC have a stellar binary fraction ≥71% (68% confidence), or ≥42% (95% confidence). This inferred GC young star binary fraction is consistent with that typically seen in young stellar populations in the solar neighborhood. Furthermore, our measured binary fraction is significantly higher than that recently reported by Chu et al. based on radial velocity measurements for stars ≲1″ of the SMBH. Constrained with these two studies, the probability that the same underlying young star binary fraction extends across the entire region is <1.4%. This tension provides support for a radial dependence of the binary star fraction, and therefore, for the dynamical predictions of binary merger and evaporation events close to the SMBH.

Keywords: galactic center; binary stars; eclipsing binary stars; star formation; dynamical evolution
Origin of High-velocity Ejecta, Excess Emission, and Redward Color Evolution in the Infant Type Ia Supernova 2021aefx

SN 2021aefx is a normal Type Ia supernova (SN) showing excess emission and redward color evolution over the first ~ 2 days. We present analyses of this SN using our high-cadence KMTNet multiband photometry, spectroscopy, and publicly available data, including first measurements of its explosion epoch (MJD 59529.32 ± 0.16) and onset of power-law rise (t PL = MJD 59529.85 ± 0.55; often called first light) associated with the main ejecta 56Ni distribution. The first KMTNet detection of SN 2021aefx precedes t PL by ~ 0.5 hr, indicating presence of additional power sources. Our peak-spectrum confirms its intermediate Type Ia subclassification between core-normal and broad-Line, and we estimate an ejecta mass of ~ 1.34 M ⊙. The spectral evolution identifies material reaching >40,000 km s-1 (fastest ever observed in Type Ia SNe) and at least two split-velocity ejecta components expanding homologously: (1) a normal-velocity (~ 12,400 km s-1) component consistent with typical photospheric evolution of near-Chandrasekhar-mass ejecta; and (2) a high-velocity (~ 23,500 km s-1) secondary component visible during the first ~ 3.6 days post-explosion, which locates the component within the outer <16% of the ejecta mass. Asymmetric subsonic explosion processes producing a nonspherical secondary photosphere provide an explanation for the simultaneous appearance of the two components, and may also explain the excess emission via a slight 56Ni enrichment in the outer ~ 0.5% of the ejecta mass. Our 300 days post-peak nebular-phase spectrum advances constraints against nondegenerate companions and further supports a near-Chandrasekhar-mass explosion origin. Off-center ignited delayed-detonations are likely responsible for the observed features of SN 2021aefx in some normal Type Ia SNe.

Keywords: binary stars; supernovae; type ia supernovae; white dwarf stars; transient sources; time domain astronomy
GRB 200415A: A Short Gamma-Ray Burst from a Magnetar Giant Flare?

The giant flares of soft gamma-ray repeaters (SGRs) have long been proposed to contribute to at least a subsample of the observed short gamma-ray bursts (GRBs). In this paper, we perform a comprehensive analysis of the high-energy data of the recent bright short GRB 200415A, which was located close to the Sculptor galaxy. Our results suggest that a magnetar giant flare provides the most natural explanation for most observational properties of GRB 200415A, including its location, temporal and spectral features, energy, statistical correlations, and high-energy emissions. On the other hand, the compact star merger GRB model is found to have difficulty reproducing such an event in a nearby distance. Future detections and follow-up observations of similar events are essential to firmly establish the connection between SGR giant flares and a subsample of nearby short GRBs.

Keywords: gamma-ray bursts; magnetars; gamma-ray transient sources