adsabs/SciX_UAT_keywords huggingface dataset:
On the Circular Polarization of Repeating Fast Radio Bursts

Fast spinning (e.g., sub-second) neutron star with ultra-strong magnetic fields (or so-called magnetar) is one of the promising origins of repeating fast radio bursts (FRBs). Here we discuss circularly polarized emissions produced by propagation effects in the magnetosphere of fast spinning magnetars. We argue that the polarization-limiting region is well beyond the light cylinder, suggesting that wave mode coupling effects are unlikely to produce strong circular polarization for fast spinning magnetars. Cyclotron absorption could be significant if the secondary plasma density is high. However, high degrees of circular polarization can only be produced with large asymmetries in electrons and positrons. We draw attention to the non-detection of circular polarization in current observations of known repeating FRBs. We suggest that the circular polarization of FRBs could provide key information on their origins and help distinguish different radiation mechanisms.

Keywords: radio transient sources; magnetars; neutron stars; radio bursts; non-thermal radiation sources
ZTF18abhjrcf: The First R Coronae Borealis Star from the Zwicky Transient Facility Public Survey

R Coronae Borealis stars (RCBs) constitute a rare class of stars that are hydrogen depleted and carbon enriched, possibly from white dwarf mergers. RCBs are identified by their stochastic dimming with photometric variations of as much as eight magnitudes. Hence, the discoveries of RCBs were heavily biased to the limited regions monitored by long-term microlensing experiments. However, recent studies of galactic RCBs have shown that they exhibit distinct mid-infrared colors originating from their cool circumstellar shells. The all-sky Wide-field Infrared Survey Explorer (WISE) survey thus facilitates the identification of RCB candidates. Therefore, combining the WISE colors with large-area time-domain surveys will enable us to discover more RCBs. Here we present the results of a first attempt to identify RCBs in the public Zwicky Transient Facility (ZTF) alert stream, using the alert broker ANTARES. We identify a candidate RCB using the infrared color selection by Tisserand et al. and large light curve variations in the optical from ZTF. We then conduct subsequent observations using the FLOYDS spectrograph at the Las Cumbres Observatory and confirm ZTF18abhjrcf as a bona fide RCB. This demonstrates the potential to discover uncharted RCBs as ZTF and other time-domain surveys continue to operate.

Keywords: r coronae borealis variable stars; sky surveys; time series analysis; astrostatistics techniques
TESS Photometric Variability of Young Brown Dwarfs in the Taurus Star-forming Region

We present a comprehensive analysis of TESS high-quality light curves from sectors 43 and 44 of a few samples of young (~2-3 Myr) brown dwarfs in the Taurus molecular cloud. They are well characterized and bona fide members of Taurus. We aim to search for the fast rotations of brown dwarfs and to picturize their dynamic atmosphere and surface features. Out of 11 young BDs, we found that 72% are periodic, in the period range of 1-7 days; among them, three BDs have periods <1.5 day and the period of one object is estimated for the first time. The sinusoidal periodic variations are related to a large spot or group of small spots corotating with the objects. Interestingly, we have detected four flare events in three young BDs, with one object, MHO 4, showing two flares in two different sectors. From the flared light curves, we have estimated the total bolometric flared energy in a range of 1035-1036 erg, which is close to the superflare energy range (≥1034 erg). To produce such kinds of superflare events, we have calculated the required magnetic field strength, which comes out at the order of a few kilogauss. Such superflares have a strong effect on the habitability of planets around M dwarfs.

Keywords: variable stars; stellar flares; stellar activity; m dwarf stars; starspots; stellar rotation; pre-main sequence stars
NEOWISE Observations of the Potentially Hazardous Asteroid (99942) Apophis

Large potentially hazardous asteroids (PHAs) are capable of causing a global catastrophe in the event of a planetary collision. Thus, rapid assessment of such an object's physical characteristics is crucial for determining its potential risk scale. We treated the near-Earth asteroid (99942) Apophis as a newly discovered object during its 2020-2021 close approach as part of a mock planetary defense exercise. The object was detected by the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), and data collected by the two active bands (3.4 and 4.6 μm) were analyzed using thermal and thermophysical modeling. Our results indicate that Apophis is an elongated object with an effective spherical diameter D eff = 340 ± 70 m, a geometric visual albedo p V =0.31 ± 0.09, and a thermal inertia Γ ~ 150-2850 J m-2 ${{\rm{s}}}^{-\tfrac{1}{2}}$ K-1 with a best-fit value of 550 J m-2 ${{\rm{s}}}^{-\tfrac{1}{2}}$ K-1. NEOWISE "discovery" observations reveal that (99942) Apophis is a PHA that would likely cause damage at a regional level and not a global one.

Keywords: near-earth objects; close encounters; infrared astronomical satellite; photometry; computational astronomy; markov chain monte carlo; astronomy data modeling
A Global Nonhydrostatic Atmospheric Model with a Mass- and Energy-conserving Vertically Implicit Correction (VIC) Scheme

Global nonhydrostatic atmospheric models are becoming increasingly important for studying the climates of planets and exoplanets. However, such models suffer from computational difficulties due to the large aspect ratio between the horizontal and vertical directions. To overcome this problem, we developed a global model using a vertically implicit correction (VIC) scheme in which the integration time step is no longer limited by the vertical propagation of acoustic waves. We proved that our model, based on the Athena++ framework and its extension for planetary atmospheres—SNAP (Simulating Nonhydrostatic Atmospheres on Planets), rigorously conserves mass and energy in finite-volume simulations. We found that traditional numerical stabilizers such as hyperviscosity and divergence damping are not needed when using the VIC scheme, which greatly simplifies the numerical implementation and improves stability. We present simulation results ranging from 1D linear waves to 3D global circulations with and without the VIC scheme. These tests demonstrate that our formulation correctly tracks local turbulent motions, produces Kelvin-Helmholtz instability, and generates a super-rotating jet on hot Jupiters. Employing this VIC scheme improves the computational efficiency of global simulations by more than two orders of magnitude compared to an explicit model and facilitates the capability of simulating a wide range of planetary atmospheres both regionally and globally.

Keywords: atmospheric science; exoplanet atmospheres; planetary atmospheres; computational methods
COMAP Early Science. II. Pathfinder Instrument

Line intensity mapping (LIM) is a new technique for tracing the global properties of galaxies over cosmic time. Detection of the very faint signals from redshifted carbon monoxide (CO), a tracer of star formation, pushes the limits of what is feasible with a total-power instrument. The CO Mapping Project Pathfinder is a first-generation instrument aiming to prove the concept and develop the technology for future experiments, as well as delivering early science products. With 19 receiver channels in a hexagonal focal plane arrangement on a 10.4 m antenna and an instantaneous 26-34 GHz frequency range with 2 MHz resolution, it is ideally suited to measuring CO (J = 1-0) from z ~ 3. In this paper we discuss strategies for designing and building the Pathfinder and the challenges that were encountered. The design of the instrument prioritized LIM requirements over those of ancillary science. After a couple of years of operation, the instrument is well understood, and the first year of data is already yielding useful science results. Experience with this Pathfinder will guide the design of the next generations of experiments.

Keywords: astronomical instrumentation; cassegrain telescopes; galaxy clusters; galaxy formation; high-redshift galaxy clusters; molecular gas; radio astronomy; radio telescopes
Carbon-Oxygen Phase Separation in Modules for Experiments in Stellar Astrophysics (MESA) White Dwarf Models

We enhance the treatment of crystallization for models of white dwarfs (WDs) in the stellar evolution software Modules for Experiments in Stellar Astrophysics (MESA) by implementing carbon-oxygen (C/O) phase separation. The phase separation process during crystallization leads to transport of oxygen toward the centers of WDs, resulting in a more compact structure that liberates gravitational energy as additional heating that modestly slows WD cooling timescales. We quantify this cooling delay in MESA C/O WD models over the mass range 0.5-1.0 M ⊙, finding delays of 0.5-0.8 Gyr for typical C/O interior profiles. MESA WD cooling timescales including this effect are generally comparable to other WD evolution models that make similar assumptions about input physics. When considering phase separation alongside 22Ne sedimentation, however, we find that both MESA and BaSTI WD cooling models predict a more modest sedimentation delay than the latest LPCODE models, and this may therefore require a reevaluation of previously proposed solutions to some WD cooling anomalies that were based on LPCODE models of 22Ne sedimentation. Our implementation of C/O phase separation in the open-source stellar evolution software MESA provides an important tool for building realistic grids of WD cooling models, as well as a framework for expanding on our implementation to explore additional physical processes related to phase transitions and associated fluid motions in WD interiors.

Keywords: stellar physics; white dwarf stars
Synthetic Polarization Maps of an Outflow Zone from Magnetohydrodynamic Simulations

The canonical theory of star formation in a magnetized environment predicts the formation of hourglass-shaped magnetic fields during the prestellar collapse phase. In protostellar cores, recent observations reveal complex and strongly distorted magnetic fields in the inner regions that are sculpted by rotation and outflows. We conduct resistive, nonideal magnetohydrodynamic simulations of a protostellar core and employ the radiative transfer code POLARIS to produce synthetic polarization segment maps. A comparison of our mock-polarization maps based on the toroidal-dominated magnetic field in the outflow zone with the observed polarization vectors of SiO lines in Orion Source I shows a reasonable agreement when the magnetic axis is tilted at an angle θ = 15° with respect to the plane of the sky and if the SiO lines have a net polarization parallel to the local magnetic field. Although the observed polarization is from SiO lines and our synthetic maps are due to polarized dust emission, a comparison is useful and allows us to resolve the ambiguity of whether the line polarization is parallel or perpendicular to the local magnetic field direction.

Keywords: interstellar clouds; interstellar dust processes; interstellar magnetic fields; star forming regions; radiative transfer; magnetohydrodynamics
Fan-beamed X-Ray Emission from 1 to above 130 keV from the Ultraluminous X-Ray Pulsar RX J0209.6-7427 in the Small Magellanic Cloud

We present detailed timing and spectral analyses of the transient X-ray pulsar RX J0209.6-7427 in the Small Magellanic Cloud during its 2019 giant outburst. With a better known distance than most galactic X-ray pulsars, its peak luminosity is determined to be (1.11 ± 0.06) × 1039 erg s-1; it is thus a bona fide pulsating ultraluminous X-ray source (PULX). Owing to the broad energy band of Insight-HXMT, its pulsed X-ray emission was detected from 1 keV up to the 130-180 keV band, which is the highest energy emission detected from any PULXs outside the Milky Way. This allows us to conclude that its main pulsed X-ray emission is from the fan beam of the accretion column, and its luminosity is thus intrinsic. We also estimate its magnetic field of (4.8-8.6) × 1012 or (1.7-2.2) × 1013 G, from its spin evolution or transition in the accretion column structure during the outburst; we suggest that the two values of the magnetic field strength correspond to the dipole and multipole magnetic fields of the neutron star, similar to the recent discovery in the Galactic PULX Swift J0243.6+6124. Therefore, the nature of the neutron star and its ULX emission can be understood within the current theoretical frame of accreting neutron stars. This may have implications for understanding the nature of those farther away extragalactic PULXs.

Keywords: neutron stars; high mass x-ray binary stars; binary pulsars
True Masses of the Long-period Companions to HD 92987 and HD 221420 from Hipparcos-Gaia Astrometry

The extensive time span of modern radial velocity surveys has made the discovery of long-period substellar companions more common in recent years; however, measuring the true masses of these objects remains challenging. Astrometry from the Gaia mission is expected to provide mass measurements for many of these long-period companions, but these data are not yet available. However, combining proper-motion data from Gaia DR2 and the earlier Hipparcos mission makes it possible to measure true masses of substellar companions in favorable cases. In this work, we combine radial velocities with Hipparcos-Gaia astrometry to measure the true masses of two recently discovered long-period substellar companion candidates, HD 92987 B and HD 221420 b. In both cases, we find that the true masses are significantly higher than implied by radial velocities alone. A 2087 ± 19 m s-1 astrometric signal reveals that HD 92987 B is not close to its 17 MJ minimum mass but is instead a 0.2562 ± 0.0045 M⊙ star viewed at a near-polar orbital inclination, whereas the 22.9 ± 2.2 MJ HD 221420 b can be plausibly interpreted as a high-mass "superplanet" or a low-mass brown dwarf. With semimajor axes of ∼10 au, both companions are interesting targets for direct imaging, and HD 221420 b in particular would be a benchmark metal-rich substellar object if it proves possible to directly detect. Our results demonstrate the power of Hipparcos-Gaia astrometry for studying long-period planet and brown dwarf candidates discovered from radial velocity surveys.

Keywords: exoplanets; astrometry; binary stars; brown dwarfs; low mass stars
Spiral Arms and a Massive Dust Disk with Non-Keplerian Kinematics: Possible Evidence for Gravitational Instability in the Disk of Elias 2-27

To determine the origin of the spiral structure observed in the dust continuum emission of Elias 2-27 we analyze multiwavelength continuum ALMA data with a resolution of ~0"2 (~23 au) at 0.89, 1.3, and 3.3 mm. We also study the kinematics of the disk with 13CO and C18O ALMA observations in the J = 3-2 transition. The spiral arm morphology is recovered at all wavelengths in the dust continuum observations, where we measure contrast and spectral index variations along the spiral arms and detect subtle dust-trapping signatures. We determine that the emission from the midplane is cold and interpret the optical depth results as signatures of a disk mass higher than previous constraints. From the gas data, we search for deviations from Keplerian motion and trace the morphology of the emitting surfaces and the velocity profiles. We find an azimuthally varying emission layer height in the system, large-scale emission surrounding the disk, and strong perturbations in the channel maps, colocated with the spirals. Additionally, we develop multigrain dust and gas hydrodynamical simulations of a gravitationally unstable disk and compare them to the observations. Given the large-scale emission and highly perturbed gas structure, together with the comparison of continuum observations to theoretical predictions, we propose infall-triggered gravitational instabilities as the origin for the observed spiral structure.

Keywords: planet formation; planetary system formation; protoplanetary disks; hydrodynamical simulations; gravitational instability
Higher Compact Multiple Occurrence around Metal-poor M-dwarfs and Late-K-dwarfs

The planet-metallicity correlation serves as a potential link between exoplanet systems as we observe them today and the effects of bulk composition on the planet formation process. Many observers have noted a tendency for Jovian planets to form around stars with higher metallicities; however, there is no consensus on a trend for smaller planets. Here, we investigate the planet-metallicity correlation for rocky planets in single- and multi-planet systems around Kepler M-dwarf and late-K-dwarf stars. Due to molecular blanketing and the dim nature of these low-mass stars, it is difficult to make direct elemental abundance measurements via spectroscopy. We instead use a combination of accurate and uniformly measured parallaxes and photometry to obtain relative metallicities and validate this method with a subsample of spectroscopically determined metallicities. We use the Kolmogorov-Smirnov (K-S) test, Mann-Whitney U-test, and Anderson-Darling (AD) test to compare the compact multiple planetary systems with single-transiting planet systems and systems with no detected transiting planets. We find that the compact multiple planetary systems are derived from a statistically more metal-poor population, with a p-value of 0.015 in the K-S test, a p-value of 0.005 in the Mann-Whitney U-test, and a value of 2.574 in the AD test statistic, which exceeds the derived threshold for significance by a factor of 25. We conclude that metallicity plays a significant role in determining the architecture of rocky planet systems. Compact multiples either form more readily, or are more likely to survive on gigayear timescales, around metal-poor stars.

Keywords: exoplanet astronomy; planet hosting stars; exoplanets; main sequence; m dwarf stars; sloan photometry
On Its Way to the Neutron Star–White Dwarf Binary Graveyard, IGR J16194‑2810, A First Ascent M Giant X-Ray Binary

A single-lined spectroscopic orbit for the M giant in the X-ray binary IGR J16194‑2810 is determined from a time series of optical spectra. The spectroscopic orbital period of 192.5 days is twice that of the photometric period, confirming that the M giant in the system is an ellipsoidal variable. The giant is identified as a first ascent giant approaching the red giant tip. The primary is a neutron star (NS) with its M giant companion filling its Roche lobe, verifying the system classification as a low-mass X-ray binary (LMXB). Stellar C, N, O, and Fe abundances are derived for the M giant with the C, N, and O values typical for a field giant with [Fe/H] = ‑0.14. The system does not have a large kick velocity. Models for the evolution of the system into a binary NS–white dwarf are presented. The X-ray properties are discussed in the context of this model. This binary is a rare example of a luminous, long orbital period LMXB early in the transient ellipsoidal phase.

Keywords: stellar abundances; neutron stars; ellipsoidal variable stars; low-mass x-ray binary stars; multiple star evolution; symbiotic binary stars
Multithermal Jet Formation Triggered by Flux Emergence

Flux emergence is responsible for various solar eruptions. Combining observation and simulations, we investigate the influence of flux emergence at one footpoint of an arcade on coronal rain as well as induced eruptions. The emergence changes the pressure in the loops, and the internal coronal rain all moves to the other side. The emerging flux reconnects with the overlying magnetic field, forming a current sheet and magnetic islands. The plasma is ejected outwards and heated, forming a cool jet ~6000 K and a hot X-ray jet ~4 million Kelvin (MK) simultaneously. The jet dynamical properties agree very well between observation and simulation. In the simulation, the jet also displays transverse oscillations with a period of 8 minutes, in a so-called whiplike motion. The movement of the jet and dense plasmoids changes the configuration of the local magnetic field, facilitating the occurrence of the Kelvin-Helmholtz instability, and vortex-like structures form at the boundary of the jet. Our simulation clearly demonstrates the effect of emergence on coronal rain, the dynamical details of reconnecting plasmoid chains, the formation of multithermal jets, and the cycling of cool mass between the chromosphere and the corona.

Keywords: solar atmosphere; magnetohydrodynamical simulations; solar activity; solar prominences; solar magnetic flux emergence
Do All Interplanetary Coronal Mass Ejections Have a Magnetic Flux Rope Structure Near 1 au?

Interplanetary coronal mass ejections (ICMEs) often consist of a shock wave, sheath region, and ejecta region. The ejecta regions are divided into two broad classes: magnetic clouds (MCs) that exhibit the characteristics of magnetic flux ropes, and non-magnetic clouds (NMCs) that do not. As CMEs result from eruption of magnetic flux ropes, it is important to answer why NMCs do not have the flux rope features. One claims that NMCs lose their original flux rope features due to the interactions between ICMEs or ICMEs and other large-scale structures during their transit in the heliosphere. The other attributes this phenomenon to the geometric selection effect; i.e., when an ICME has its nose (flank, including leg and non-leg flanks) pass through the observing spacecraft, the MC (NMC) features will be detected along the spacecraft trajectory within the ejecta. In this Letter, we examine which explanation is more reasonable through the geometric properties of ICMEs. If the selection effect leads to different ejecta types, MCs should have narrower sheath region compared to NMCs from the statistical point of view, which is confirmed by our statistics. In addition, we find that NMCs have similar sizes in solar cycles 23 and 24, and NMCs are smaller than MCs in cycle 23 but larger than MCs in cycle 24. This suggests that most NMCs have their leg flank pass through the spacecraft. Our geometric analyses support that all ICMEs should have a magnetic flux rope structure near 1 au.

Keywords: solar coronal mass ejections; solar coronal mass ejection shocks; solar storm
The Late-epoch, Intermediate Excitation Spectrum of the Nova V612 Scuti

Optical and optical/infrared spectra of V612 Scuti from three and four years after outburst show a nova that has sustained nuclear burning and slowly increased in excitation but has not reached the excitation levels exhibited by many novae. The emission lines range in excitation from the nebular lines (e.g., [S III], [O III]), up to the coronal lines [Ca VIII] and [Si VII]. Distinctive spectral features are particularly strong lines of [Fe VII] and several features of [Fe VI] that include rarely seen infrared lines. A line at 2.04 μm frequently identified with [Al IX] in other novae, is shown to be [Fe V] 2.0401 in V496 Scuti. An analysis with the photoionization code CLOUDY suggests that a largely matter-bounded emission line region is excited by a photosphere of temperature 2.6 × 105 K and luminosity 1 × 104 L ⊙.

Keywords: classical novae