adsabs/SciX_UAT_keywords huggingface dataset:
Constraints on Weak Supernova Kicks from Observed Pulsar Velocities

Observations of binary pulsars and pulsars in globular clusters suggest that at least some pulsars must receive weak natal kicks at birth. If all pulsars received strong natal kicks above 50 km s-1, those born in globular clusters would predominantly escape, while wide binaries would be disrupted. On the other hand, observations of transverse velocities of isolated radio pulsars indicate that only 5 ± 2% have velocities below 50 km s-1. We explore this apparent tension with rapid binary population synthesis modeling. We propose a model in which supernovae with characteristically low natal kicks (e.g., electron-capture supernovae) only occur if the progenitor star has been stripped via binary interaction with a companion. We show that this model naturally reproduces the observed pulsar speed distribution and without reducing the predicted merging double neutron star yield. We estimate that the zero-age main-sequence mass range for noninteracting progenitors of electron-capture supernovae should be no wider than ≈0.2 M⊙.

Keywords: neutron stars; pulsars; binary pulsars; supernovae
The Chemical Composition of Extreme-velocity Stars

Little is known about the origin of the fastest stars in the Galaxy. Our understanding of the chemical evolution history of the Milky Way and surrounding dwarf galaxies allows us to use the chemical composition of a star to investigate its origin and to say whether it was formed in situ or was accreted. However, the fastest stars, the hypervelocity stars, are young and massive and their chemical composition has not yet been analyzed. Though it is difficult to analyze the chemical composition of a massive young star, we are well versed in the analysis of late-type stars. We have used high-resolution ARCES/3.5 m Apache Point Observatory, MIKE/Magellan spectra to study the chemical details of 15 late-type hypervelocity star candidates. With Gaia EDR3 astrometry and spectroscopically determined radial velocities we found total velocities with a range of 274-520 km s-1 and mean value of 381 km s-1. Therefore, our sample stars are not fast enough to be classified as hypervelocity stars, and are what is known as extreme-velocity stars. Our sample has a wide iron abundance range of -2.5 ≤ [Fe/H] ≤ -0.9. Their chemistry indicates that at least 50% of them are accreted extragalactic stars, with iron-peak elements consistent with prior enrichment by sub-Chandrasekhar mass Type Ia supernovae. Without indication of binary companions, their chemical abundances and orbital parameters indicate that they are the accelerated tidal debris of disrupted dwarf galaxies. * This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. † This paper includes data gathered with the 3.5 m Telescope at Apache Point Observatory.

Keywords: hypervelocity stars; population ii stars; stellar abundances; stellar populations
Electron Heating in Perpendicular Low-beta Shocks

Collisionless shocks heat electrons in the solar wind, interstellar blast waves, and hot gas permeating galaxy clusters. How much shock heating goes to electrons instead of ions, and what plasma physics controls electron heating? We simulate 2D perpendicular shocks with a fully kinetic particle-in-cell code. For magnetosonic Mach number ${{ \mathcal M }}_{\mathrm{ms}}\sim 1\mbox{--}10$ and plasma beta ${\beta }_{{\rm{p}}}\lesssim 4$ , the post-shock electron/ion temperature ratio ${T}_{{\rm{e}}}/{T}_{{\rm{i}}}$ decreases from 1 to 0.1 with increasing ${{ \mathcal M }}_{\mathrm{ms}}$ . In a representative ${{ \mathcal M }}_{\mathrm{ms}}=3.1$ , ${\beta }_{{\rm{p}}}=0.25$ shock, electrons heat above adiabatic compression in two steps: ion-scale ${E}_{\parallel }={\boldsymbol{E}}\cdot \hat{{\boldsymbol{b}}}$ accelerates electrons into streams along ${\boldsymbol{B}}$ , which then relax via two-stream-like instability. The ${\boldsymbol{B}}$ -parallel heating is mostly induced by waves; ${\boldsymbol{B}}$ -perpendicular heating is mostly adiabatic compression by quasi-static fields.

Keywords: shocks; planetary bow shocks; space plasmas; plasma astrophysics
Observation of the Hall Magnetic Reconnection As Close As 56 Solar Radii from the Sun

A few thin current layers were detected in the rear boundary of an interplanetary coronal mass ejection (ICME) observed at 56 solar radii from the Sun as the Parker Solar Probe spacecraft approached the perihelion for the first time, and were caused by the interaction between the background solar wind and the rear boundary of the ICME. Among two of the current layers, the ion diffusion region of the Hall magnetic reconnection was directly detected, based on opposite ion jets, low-speed inflows, and the Hall effect. Both reconnection events were fast and occurred in the current layer with a small magnetic field shear angle and with significantly asymmetric magnetic field intensity as well as plasma between their two sides, i.e., an asymmetric magnetic reconnection with a strong guide field. A magnetic flux rope was detected inside one of the diffusion regions, indicating bursty reconnection. Additionally, multiple reconnection jets were detected inside the ICME and its rear boundary. Thus, we speculate that more ongoing reconnection events were occurring inside the ICME and its boundary. The observations suggested that fast Hall magnetic reconnection can occur as close as 56 solar radii from the Sun and plays a crucial role in ICME evolution.

Keywords: solar wind; heliosphere
Very-high-energy Emission and Cascade Radiation of Gamma-Ray Burst Afterglows: Homogeneous versus Wind External Media

Recent detection of sub-TeV emission from gamma-ray bursts (GRBs) represents a breakthrough in the GRB study. The multiwavelength data of the afterglows of GRB 190114C support the synchrotron self-Compton (SSC) origin for its sub-TeV emission. We present a comparative analysis on the SSC emission of GRB afterglows in the homogeneous and wind environments in the framework of the forward shock model. The γγ absorption of very-high-energy photons due to pair production within the source and the Klein-Nishina effect on the inverse Compton scattering are considered. Generally a higher SSC flux is expected for a larger circum-burst density due to a larger Compton parameter, but meanwhile the internal γγ absorption is more severe for sub-TeV emission. The flux ratio between the SSC component and the synchrotron component decreases more quickly with time in the wind medium case than in the homogeneous density medium case. The light curves of the SSC emission are also different for the two types of media. We also calculate the cascade emission resulting from the absorbed high-energy photons. In the ISM environment with n ≳ 1 cm-3, the cascade synchrotron emission could be comparable to the synchrotron emission of the primary electrons in the optical band, which may flatten the optical afterglow light curve at an early time (t < 1 hr). In the wind medium with A* ≳ 0.1, the cascade emission in the eV-GeV band is comparable or even larger than the emission of the primary electrons at the early time.

Keywords: gamma-ray bursts; gamma-rays
Pluto's Haze Abundance and Size Distribution from Limb Scatter Observations by MVIC

The New Horizons spacecraft observed Pluto and Charon at solar-phase angles between 16° and 169°. In this work, we use the Multispectral Visible Imaging Camera (MVIC) observations to construct multiwavelength phase curves of Pluto's atmosphere, using the limb scatter technique. Observational artifacts and biases were removed using Charon as a representative airless body. The size and distribution of the haze particles were constrained using a Titan fractal aggregate phase function. We find that monodispersed and log-normal populations cannot simultaneously describe the observed steep forward scattering, indicative of wavelength-scale particles, and the non-negligible backscattering indicative of particles much smaller than the wavelength. Instead, we find it necessary to use bimodal or power-law distributions, especially below ∼200 km, to properly describe the MVIC observations. Above 200 km, where the atmosphere is isotropically scattering, a monodisperse, log-normal, or a bimodal/power law approximating a monodispersed population is able to fit the phase curves well. As compared to the results of previously published articles, we find that Pluto's atmosphere must contain haze particle number densities an order of magnitude greater for small (∼10 nm) and large (∼1 μm) radii, and relatively fewer intermediate sizes (∼100 nm). These conclusions support a lower aggregate aerosol growth rate than that found by Gao et al., indicating a higher charge-to-radius ratio, upwards of 60e- μm-1. In order to generate large particles with a lower growth rate, the atmosphere must also have a lower sedimentation velocity (<∼0.01 m s-1 at 200 km), which is possible with a fractal dimension of less than 2.

Keywords: pluto; planetary atmospheres; atmospheric composition; planetary science; phase angle; radiative transfer equation
Planetary Spin and Obliquity from Mergers

In planetary systems with sufficiently small inter-planet spacing, close encounters can lead to planetary collisions/mergers or ejections. We study the spin property of the merger products of two giant planets in a statistical manner using numerical simulations and analytical modeling. Planetary collisions lead to rapidly rotating objects and a broad range of obliquities. We find that, under typical conditions for two-planet scatterings, the distributions of spin magnitude S and obliquity ${\theta }_{\mathrm{SL}}$ of the merger products have simple analytical forms: fS ∝ S and ${f}_{\cos {\theta }_{\mathrm{SL}}}\propto {(1-{\cos }^{2}{\theta }_{\mathrm{SL}})}^{-1/2}$ . Through parameter studies, we determine the regime of validity for the analytical distributions of spin and obliquity. Since planetary mergers are a major outcome of planet-planet scatterings, observational search for the spin/obliquity signatures of exoplanets would provide important constraints on the dynamical history of planetary systems.

Keywords: exoplanet astronomy; exoplanet dynamics; n-body simulations
ν Gem: A Hierarchical Triple System with an Outer Be Star

Time series of spectroscopic, speckle-interferometric, and optical long-baseline-interferometric observations confirm that ν Gem is a hierarchical triple system. It consists of an inner binary composed of two B-type stars and an outer classical Be star. Several photospheric spectral lines of the inner components were disentangled, revealing two stars with very different rotational broadening (~260 and ~140 km s-1, respectively), while the photospheric lines of the Be star remain undetected. From the combined spectroscopic and astrometric orbital solution it is not possible to unambiguously cross identify the inner astrometric components with the spectroscopic components. In the preferred solution based on modeling of the disentangled line profiles, the inner binary is composed of two stars with nearly identical masses of 3.3 M⊙ and the more rapidly rotating star is the fainter one. These two stars are in a marginally elliptical orbit (e = 0.06) about each other with a period of 53.8 days. The third star also has a mass of 3.3 M⊙ and follows a more eccentric (e = 0.24) orbit with a period of 19.1 yr. The two orbits are codirectional, and at inclinations of 79° and 76° of the inner and the outer orbit, respectively, about coplanar. No astrometric or spectroscopic evidence could be found that the Be star itself is double. The system appears dynamically stable and not subject to eccentric Lidov-Kozai oscillations. After disentangling, the spectra of the components of the inner binary do not exhibit peculiarities that would be indicative of past interactions. Motivations for a wide range of follow-up studies are suggested.

Keywords: be stars; circumstellar disks; optical interferometry; orbit determination; spectroscopy; trinary stars
Multiwavelength Radio Observations of Two Repeating Fast Radio Burst Sources: FRB 121102 and FRB 180916.J0158+65

The spectra of fast radio bursts (FRBs) encode valuable information about the source's local environment, underlying emission mechanism(s), and the intervening media along the line of sight. We present results from a long-term multiwavelength radio monitoring campaign of two repeating FRB sources, FRB 121102 and FRB 180916.J0158+65, with the NASA Deep Space Network (DSN) 70 m radio telescopes (DSS-63 and DSS-14). The observations of FRB 121102 were performed simultaneously at 2.3 and 8.4 GHz, and spanned a total of 27.3 hr between 2019 September 19 and 2020 February 11. We detected two radio bursts in the 2.3 GHz frequency band from FRB 121102, but no evidence of radio emission was found at 8.4 GHz during any of our observations. We observed FRB 180916.J0158+65 simultaneously at 2.3 and 8.4 GHz, and also separately in the 1.5 GHz frequency band, for a total of 101.8 hr between 2019 September 19 and 2020 May 14. Our observations of FRB 180916.J0158+65 spanned multiple activity cycles during which the source was known to be active and covered a wide range of activity phases. Several of our observations occurred during times when bursts were detected from the source between 400 and 800 MHz with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope. However, no radio bursts were detected from FRB 180916.J0158+65 at any of the frequencies used during our observations with the DSN radio telescopes. We find that FRB 180916.J0158+65's apparent activity is strongly frequency-dependent due to the narrowband nature of its radio bursts, which have less spectral occupancy at high radio frequencies (≳ 2 GHz). We also find that fewer or fainter bursts are emitted from the source at high radio frequencies. We discuss the implications of these results for possible progenitor models of repeating FRBs.

Keywords: radio transient sources; radio bursts
Ultra-impulsive Solar Flare Seismology

We examine a strong, coherent, highly impulsive acoustic transient radiated into the solar interior by the flare SOL20110730T02:04-M9.3. The acoustic spectrum of this transient extends out to 10 mHz. The fine diffraction limit of this high-frequency component of the flare acoustic transient allows us to discriminate different source components in operation during the flare. Acoustic-source power density maps of the 10 mHz component show sources that are compact to within the now 760 km diffraction limit of local helioseismic diagnostics for this frequency. One of the acoustic sources found is bifurcated across a sharp penumbral magnetic boundary, the component in the stronger magnetic field temporally lagging its partner. The facility to discriminate this level of acoustic-source detail could open the door to a long sought after understanding of the mechanics of transient emission from solar flares, still a mystery two decades after its discovery. It also suggests that helioseismic observations of higher cadence and spatial resolution could reveal coherent acoustic emission at even higher frequencies, with proportionately further potential benefits to solar seismology and its growing domain of applications.

Keywords: solar flares; helioseismology; solar interior
GJ 1252 b: A 1.2 R⊕ Planet Transiting an M3 Dwarf at 20.4 pc

We report the discovery of GJ 1252 b, a planet with a radius of 1.193 ± 0.074 ${R}_{\oplus }$ and an orbital period of 0.52 days around an M3-type star (0.381 ± 0.019 ${M}_{\odot }$ , 0.391 ± 0.020 ${R}_{\odot }$ ) located 20.385 ± 0.019 pc away. We use Transiting Exoplanet Survey Satellite (TESS) data, ground-based photometry and spectroscopy, Gaia astrometry, and high angular resolution imaging to show that the transit signal seen in the TESS data must originate from a transiting planet. We do so by ruling out all false-positive scenarios that attempt to explain the transit signal as originating from an eclipsing stellar binary. Precise Doppler monitoring also leads to a tentative mass measurement of 2.09 ± 0.56 M⊕. The host star proximity, brightness (V = 12.19 mag, K = 7.92 mag), low stellar activity, and the system's short orbital period make this planet an attractive target for detailed characterization, including precise mass measurement, looking for other objects in the system, and planet atmosphere characterization.

Keywords: exoplanet astronomy; m dwarf stars; exoplanets
LHS 1815b: The First Thick-disk Planet Detected by TESS

We report the first discovery of a thick-disk planet, LHS 1815b (TOI-704b, TIC 260004324), detected in the Transiting Exoplanet Survey Satellite (TESS) survey. LHS 1815b transits a bright (V = 12.19 mag, K = 7.99 mag) and quiet M dwarf located 29.87 ± 0.02 pc away with a mass of 0.502 ± 0.015 M⊙ and a radius of 0.501 ± 0.030 R⊙. We validate the planet by combining space- and ground-based photometry, spectroscopy, and imaging. The planet has a radius of 1.088 ± 0.064 R⊕ with a 3σ mass upper limit of 8.7 M⊕. We analyze the galactic kinematics and orbit of the host star LHS 1815 and find that it has a large probability (Pthick/Pthin = 6482) to be in the thick disk with a much higher expected maximal height (Zmax = 1.8 kpc) above the Galactic plane compared with other TESS planet host stars. Future studies of the interior structure and atmospheric properties of planets in such systems using, for example, the upcoming James Webb Space Telescope, can investigate the differences in formation efficiency and evolution for planetary systems between different Galactic components (thick disks, thin disks, and halo).

Keywords: astrometric exoplanet detection; astrometry; stellar kinematics; transit photometry; radial velocity
A Pilot Survey of an M Dwarf Flare Star with Swift's UV Grism

The near-ultraviolet (NUV) spectral region is a useful diagnostic for stellar flare physics and assessing the energy environment of young exoplanets, especially as relates to prebiotic chemistry. We conducted a pilot NUV spectroscopic flare survey of the young M dwarf AU Mic with the Neil Gehrels Swift Observatory's UltraViolet and Optical Telescope. We detected four flares and three other epochs of significantly elevated count rates during the 9.6 hr of total exposure time, consistent with a NUV flare rate of ~0.5 hr-1. The largest flare we observed released a minimum energy of 6 × 1033 erg between 1730 and 5000 Å. All flares had durations longer than the ~14-17 minutes duration of each Swift visit, making measuring total flare energy and duration infeasible.

Keywords: stellar flares; m dwarf stars; pre-main sequence stars; pre-biotic astrochemistry; near ultraviolet astronomy
TeV Cosmic-Ray Nucleus Acceleration in Shell-type Supernova Remnants with Hard γ-Ray Spectra

The emission mechanism for hard γ-ray spectra from supernova remnants (SNRs) is still a matter of debate. Recent multiwavelength observations of the TeV source HESS J1912+101 show that it is associated with an SNR with an age of ∼100 kyr, making it unlikely produce the TeV γ-ray emission via leptonic processes. We analyzed Fermi observations of it and found an extended source with a hard spectrum. HESS J1912+101 may represent a peculiar stage of SNR evolution that dominates the acceleration of TeV cosmic rays. By fitting the multiwavelength spectra of 13 SNRs with hard GeV γ-ray spectra with simple emission models with a density ratio of GeV electrons to protons of ∼10-2, we obtain reasonable mean densities and magnetic fields with a total energy of ∼1050 erg for relativistic ions in each SNR. Among these sources, only two of them, namely SN 1006 and RCW 86, favor a leptonic origin for the γ-ray emission. The magnetic field energy is found to be comparable to that of accelerated relativistic ions and their ratio has a tendency to increase with the age of SNRs. These results suggest that TeV cosmic rays mainly originate from SNRs with hard γ-ray spectra.

Keywords: galactic cosmic rays; gamma-ray sources; non-thermal radiation sources; gamma-ray astronomy; supernova remnants
PPDONet: Deep Operator Networks for Fast Prediction of Steady-state Solutions in Disk-Planet Systems

We develop a tool, which we name Protoplanetary Disk Operator Network (PPDONet), that can predict the solution of disk-planet interactions in protoplanetary disks in real time. We base our tool on Deep Operator Networks, a class of neural networks capable of learning nonlinear operators to represent deterministic and stochastic differential equations. With PPDONet we map three scalar parameters in a disk-planet system-the Shakura-Sunyaev viscosity α, the disk aspect ratio h 0, and the planet-star mass ratio q-to steady-state solutions of the disk surface density, radial velocity, and azimuthal velocity. We demonstrate the accuracy of the PPDONet solutions using a comprehensive set of tests. Our tool is able to predict the outcome of disk-planet interaction for one system in less than a second on a laptop. A public implementation of PPDONet is available at https://github.com/smao-astro/PPDONet.

Keywords: protoplanetary disks; planetary-disk interactions; hydrodynamical simulations; neural networks; open source software
An Extension of Gmunu: General-relativistic Resistive Magnetohydrodynamics Based on Staggered-meshed Constrained Transport with Elliptic Cleaning

We present the implementation of general-relativistic resistive magnetohydrodynamics solvers and three divergence-free handling approaches adopted in the General-relativistic multigrid numerical (Gmunu) code. In particular, implicit-explicit Runge-Kutta schemes are used to deal with the stiff terms in the evolution equations for small resistivity. The three divergence-free handling methods are (i) hyperbolic divergence cleaning (also known as the generalized Lagrange multiplier), (ii) staggered-meshed constrained transport schemes, and (iii) elliptic cleaning through a multigrid solver, which is applicable in both cell-centered and face-centered (stagger grid) magnetic fields. The implementation has been tested with a number of numerical benchmarks from special-relativistic to general-relativistic cases. We demonstrate that our code can robustly recover from the ideal magnetohydrodynamics limit to a highly resistive limit. We also illustrate the applications in modeling magnetized neutron stars, and compare how different divergence-free handling methods affect the evolution of the stars. Furthermore, we show that the preservation of the divergence-free condition of the magnetic field when using staggered-meshed constrained transport schemes can be significantly improved by applying elliptic cleaning.

Keywords: magnetohydrodynamics; general relativity; plasma astrophysics; computational methods; magnetars