adsabs/SciX_UAT_keywords huggingface dataset:
Even More Rapidly Rotating Pre-main-sequence M Dwarfs with Highly Structured Light Curves: An Initial Survey in the Lower Centaurus-Crux and Upper Centaurus-Lupus Associations

Using K2, we recently discovered a new type of periodic photometric variability while analyzing the light curves of members of Upper Sco. The 23 exemplars of this new variability type are all mid-M dwarfs, with short rotation periods. Their phased light curves have one or more broad flux dips or multiple arcuate structures which are not explicable by photospheric spots or eclipses by solid bodies. Now, using Transiting Exoplanet Survey Satellite data, we have searched for this type of variability in the other major sections of Sco-Cen, Upper Centaurus-Lupus (UCL), and Lower Centaurus-Crux (LCC). We identify 28 stars with the same light curve morphologies. We find no obvious difference between the Upper Sco and the UCL/LCC representatives of this class in terms of their light curve morphologies, periods, or variability amplitudes. The physical mechanism behind this variability is unknown, but as a possible clue we show that the rapidly rotating mid-M dwarfs in UCL/LCC have slightly different colors from the slowly rotating M dwarfs—they either have a blue excess (hot spots?) or a red excess (warm dust?). One of the newly identified stars (TIC242407571) has a very striking light curve morphology. At about every 0.05 in phase are features that resemble icicles. The icicles arise because there is a second periodic system whose main feature is a broad flux dip. Using a toy model, we show that the observed light curve morphology results only if the ratio of the two periods and the flux-dip width are carefully arranged.

Keywords: young star clusters; m stars; stellar rotation
An 86 GHz Search for Pulsars in the Galactic Center with the Atacama Large Millimeter / submillimeter Array

We report on the first pulsar and transient survey of the Galactic Center (GC) with the Atacama Large Millimeter/submillimeter Array (ALMA). The observations were conducted during the Global Millimeter VLBI Array campaign in 2017 and 2018. We carry out searches using time series of both total intensity and other polarization components in the form of Stokes parameters. We incorporate acceleration and its derivative in the pulsar search, and also search in segments of the entire observation to compensate for potential orbital motion of the pulsar. While no new pulsar is found, our observations yield the polarization profile of the GC magnetar PSR J1745-2900 at millimeter wavelength for the first time, which turns out to be nearly 100% linearly polarized. Additionally, we estimate the survey sensitivity placed by both system and red noise, and evaluate its capability of finding pulsars in orbital motion with either Sgr A* or a binary companion. We show that the survey is sensitive to only the most luminous pulsars in the known population and future observations with ALMA in Band-1 will deliver significantly deeper survey sensitivity on the GC pulsar population.

Keywords: pulsars; radio pulsars; galactic center
Proton Temperature Anisotropy within the Interplanetary Coronal Mass Ejections Sheath at 1 au

The sheath plasma of interplanetary coronal mass ejections (ICMEs) is highly compressed, heated, turbulent, and magnetically intense relative to the ambient solar wind. In this Letter, we perform a detailed study of proton temperature anisotropy within the 333 ICME sheath regions observed on board the Wind spacecraft spanning the years 1995-2015. Our observations show that marginal stability thresholds of mirror mode and firehose instabilities predominantly constrain the proton temperature anisotropy within these sheath regions regardless of ICME sheath plasma speed. This is true even when the plasma beta values are less than 2, a parameter space that should have favored the prevalence of parallel firehose and proton cyclotron instabilities according to linear stability analysis. This investigation demonstrates the critical role played by distinct plasma instabilities in shaping the evolution of ICME sheath plasma compared to the broader solar wind environment.

Keywords: solar coronal mass ejections; solar wind
Spectral and Jet Properties of the Quasi-thermal-dominated GRB 210121A, GRB 210610B, and GRB 221022B

Some quasi-thermal (QT)-dominated gamma-ray bursts (GRBs) could be well described by a multicolor blackbody (mBB) function or a combined model of a BB plus a nonthermal (NT) component. In this analysis, two QT radiation-dominated bursts with known emission properties (GRB 210610B, likely from a hybrid jet, and GRB 210121A, with a spectrum consistent with nondissipative photospheric emission from a pure hot fireball) are used to make a comparison between these two models. To diagnose the magnetization properties of the central engine, the "top-down" approach proposed by Gao and Zhang is adopted. It is found that diagnoses based on these two models provide similar conclusions qualitatively; however, the model with mBB (or mBB+NT) may give more reasonable physical explanations. This implies that impacts from the GRB jet structure and geometrical broadening on the observed spectrum should be considered. However, conservatively, these methods may be not sensitive enough to distinguish between a pure hot fireball and a mildly magnetized hybrid jet. Some other information is necessary to provide more evidence when determining the jet properties for similar GRBs. Based on these considerations, we suggest that the photospheric emission of GRB 221022B is from a hot jet, where dissipation is caused by an internal shock mechanism due to the increasing Lorentz factor with time, which makes its prompt emission display typical evolution from thermal to NT.

Keywords: gamma-ray bursts
How Can the Optical Variation Properties of Active Galactic Nuclei Be Unbiasedly Measured?

The variability of active galactic nuclei (AGNs) is ubiquitous, but has not yet been understood. Measuring the optical variation properties of AGNs, such as the variation timescale and amplitude, and then correlating them with their fundamental physical parameters has long served as a critical way of exploring the origin of AGN variability and the associated physics of the accretion process in AGNs. Obtaining accurate variation properties of AGNs is thus essential. It has been found that the damped random walk process can describe the AGN optical variation well, but there is a controversy over how long a minimal monitoring baseline is required to obtain unbiased variation properties. In this work, we settle the controversy by exhaustively scrutinizing the complex combination of assumed priors, adopted best-fit values, ensemble averaging methods, and fitting methods. The new proposal is then an optimized solution where unbiased variation properties of an AGN sample possessing the same variation timescale can be obtained with a minimal baseline of about 10 times their variation timescale. Finally, the new optimized solution is used to demonstrate the positive role of the time-domain surveys to be conducted by the Wide Field Survey Telescope in improving constraints on AGN variation properties.

Keywords: quasars; time series analysis; time domain astronomy
Shock Drift Acceleration of Ions in an Interplanetary Shock Observed by MMS

An interplanetary (IP) shock wave was recorded crossing the Magnetospheric Multiscale constellation on 2018 January 8. Plasma measurements upstream of the shock indicate efficient proton acceleration in the IP shock ramp: 2-7 keV protons are observed upstream for about three minutes (∼8000 km) ahead of the IP shock ramp, outrunning the upstream waves. The differential energy flux of 2-7 keV protons decays slowly with distance from the ramp toward the upstream region (dropping by about half within 8 Earth radii from the ramp) and is lessened by a factor of about four in the downstream compared to the ramp (within a distance comparable to the gyroradius of ∼keV protons). Comparison with test-particle simulations has confirmed that the mechanism accelerating the solar wind protons and injecting them upstream is classical Shock Drift Acceleration (SDA). This example of observed proton acceleration by a low-Mach, quasi-perpendicular shock may be applicable to astrophysical contexts, such as supernova remnants or the acceleration of cosmic rays.

Keywords: interplanetary shocks; shocks; space plasmas
Milky Way Tomography with the SkyMapper Southern Survey. II. Photometric Recalibration of SMSS DR2

We apply the spectroscopy-based stellar-color regression (SCR) method to perform an accurate photometric recalibration of the second data release from the SkyMapper Southern Survey (SMSS DR2). From comparison with a sample of over 200,000 dwarf stars with stellar atmospheric parameters taken from GALAH+ DR3 and with accurate, homogeneous photometry from Gaia DR2, zero-point offsets are detected in the original photometric catalog of SMSS DR2, in particular for the gravity- and metallicity-sensitive uv bands. For the uv bands, the zero-point offsets are close to zero at very low extinction, and then steadily increase with E(B - V), reaching as large as 0.174 and 0.134 mag respectively, at E(B - V) ∼ 0.5 mag. These offsets largely arise from the adopted dust term in the transformations used by SMSS DR2 to construct photometric calibrators from the ATLAS reference catalog. For the gr bands, the zero-point offsets exhibit negligible variations with the E(B - V) of Schlegel et al. due to their tiny coefficients on the dust term in the transformation. Our study also reveals small but significant spatial variations of the zero-point offsets in all uvgr bands. External checks using Strömgren photometry, WD loci, and the SDSS Stripe 82 standard-star catalog independently confirm the zero-points found by our revised SCR method.

Keywords: interstellar dust extinction; ccd photometry; flux calibration; sky surveys; spectroscopy; astronomy data analysis
Expansion and Spectral Softening of the Dust-scattering Rings of GRB 221009A

An expanding X-ray halo or rings appear when short pulses of X-ray radiation from a background source are scattered by clouds of dust in the Milky Way. We study the X-ray rings of the brightest gamma-ray burst (GRB) 221009A, detected by the Swift X-Ray Telescope. The rings center on the GRB position, and their angular radii increase with time. We identify five major expanding rings, and our modeling of their expansion history suggests that they are scattered off from five dusty clouds at distances of 0.4–13 kpc from the observer. Given an assumed prompt X-ray fluence of this GRB, the fluxes of those rings suggest that these clouds have dust grain column densities of 107∼8 cm‑2. More interestingly, our time-dependent spectral analysis of these rings shows that they all experience spectral softening, i.e., getting softer as they expand, with spectral indices ranging from 2.2 to 5, consistent with what the dust-scattering model predicts.

Keywords: gamma-ray bursts
Population of X-Ray Sources in the Intermediate-age Cluster NGC 3532: a Test Bed for Machine-learning Classification

Open clusters are thought to be the birthplace of most stars in the galaxy. Thus, they are excellent laboratories for investigating stellar evolution, and X-ray properties of various types of stars (including binary stars, evolved stars, and compact objects). In this work, we investigate the population of X-ray sources in the nearby 300 Myr old open cluster NGC 3532 using Chandra X-ray Observatory and multiwavelength data from several surveys. We apply a random-forest machine-learning pipeline (MUWCLASS) to classify all confidently detected X-ray sources (signal-to-noise ratio, hereafter S/N, > 5) in the field of NGC 3532. We also perform a more detailed investigation of brighter sources, including their X-ray spectra and lightcurves. Most X-ray sources are confirmed as coronally active low-mass stars, many of which are confidently identified by MUWCLASS. Several late B- or early A-type stars are relatively bright in X-rays, most of which are likely binaries. We do not find any compact objects among X-ray sources reliably associated with NGC 3532, down to the limiting X-ray flux of ~2 × 10-15 erg s-1cm-2, corresponding to L X ~ 6 × 1028 erg s-1 at the cluster's distance. We also identify several Galactic sources beyond NGC 3532 that differ from typical coronally active stars, and were classified by MUWCLASS as potential compact objects. Detailed investigation reveals that these sources may indeed belong to rarer classes, and deserve follow-up observations.

Keywords: open star clusters; x-ray sources; classification; random forests; x-ray stars; astronomy data visualization; astrostatistics tools; x-ray surveys; compact objects; low mass stars; catalogs
Metal Enrichment in the Circumgalactic Medium and Lyα Halos around Quasars at z ∼ 3

Deep observations have detected extended Lyα emission nebulae surrounding tens of quasars at redshift 2-6. However, the metallicity of such extended halos is still poorly understood. We perform a detailed analysis on a large sample of 80 quasars at z ∼ 3 based on MUSE-VLT data. We find clear evidence of extended emission of the UV nebular lines such as C IV λ1549 or He II λ1640 for about 20% of the sample, while C III] λ1909 is only marginally detected in a few objects. By stacking the cubes, we detect emission of C IV, He II, and C III] out to a radius of about 45 kpc. C IV and He II show a radial decline much steeper than Lyα, while C III] shows a shallower profile similar to Lyα in the inner 45 kpc. We infer that the average metallicity of the circumgalactic gas within the central 30-50 kpc is ∼0.5 solar, or even higher. However, we also find evidence of a component of the Lyα halos, which has much weaker metal emission lines relative to Lyα. We suggest that the high metallicity of the circumgalactic medium within the central 30-50 kpc is associated with chemical pre-enrichment by past quasar-driven outflows and that there is a more extended component of the circumgalactic medium that has much lower metallicity and is likely associated with near-pristine gas accreted from the intergalactic medium. We show that our observational results are in good agreement with the expectations of the FABLE zoom-in cosmological simulations.

Keywords: high-redshift galaxies; circumgalactic medium; quasars
The Invariance of the Diffusion Coefficient with Iterative Operations of the Charged Particle Transport Equation

The spatial parallel diffusion coefficient (SPDC) is one of the important quantities describing energetic charged particle transport. There are three different definitions for the SPDC: the displacement variance definition ${\kappa }_{{zz}}^{\mathrm{DV}}={\mathrm{lim}}_{t\to {t}_{\infty }}d{\sigma }^{2}/(2{dt})$ , the Fick's law definition ${\kappa }_{{zz}}^{\mathrm{FL}}=J/X$ with $X=\partial F/\partial z$ , and the Taylor-Green-Kubo (TGK) formula definition ${\kappa }_{{zz}}^{\mathrm{TGK}}={\int }_{0}^{\infty }{dt}\langle {v}_{z}(t){v}_{z}(0)\rangle $ . For a constant mean magnetic field, the three different definitions of the SPDC give the same result. However, for a focusing field, it is demonstrated that the results of the different definitions are not the same. In this paper, from the Fokker-Planck equation, we find that different methods, e.g., the general Fourier expansion and iteration method, can give different equations of the isotropic distribution function (EIDFs). But it is shown that one EIDF can be transformed into another by some derivative iterative operations (DIOs). If one definition of the SPDC is invariant for the DIOs, it is clear that the definition is also invariant for different EIDFs; therefore, it is an invariant quantity for the different derivation methods of the EIDF. For the focusing field, we suggest that the TGK definition ${\kappa }_{{zz}}^{\mathrm{TGK}}$ is only an approximate formula, and the Fick's law definition ${\kappa }_{{zz}}^{\mathrm{FL}}$ is not invariant to some DIOs. However, at least for the special condition, in this paper we show that the definition ${\kappa }_{{zz}}^{\mathrm{DV}}$ is an invariant quantity to the DIOs. Therefore, for a spatially varying field, the displacement variance definition ${\kappa }_{{zz}}^{\mathrm{DV}}$ , rather than the Fick's law definition ${\kappa }_{{zz}}^{\mathrm{FL}}$ and TGK formula definition ${\kappa }_{{zz}}^{\mathrm{TGK}}$ , is the most appropriate definition of the SPDCs.

Keywords: interplanetary turbulence; interplanetary magnetic fields; particle astrophysics; cosmic rays
Radical Recombination during the Phase Transition of Interstellar CO Ice

Complex organic molecules (COMs) can be produced efficiently in ice mixtures that simulate the ice mantle on cosmic dust grains, according to prior experimental studies. However, the mechanism that brings the reactive species together in the ice has been debated. Thermal diffusion, which is widely regarded as the main mechanism to bring reactants together, is inefficient at a typical dense cloud temperature of 10 K. A recent experimental study found that the transition of a thin CO ice film from the amorphous to crystalline phase happens at about 10 K. When a small fraction of CO2 was mixed with CO, the CO2 molecules can separate and form clusters during CO phase transition. It was further proposed that the separation of minor species in the CO ice during phase transition may be an important mechanism to form interstellar COMs without the need for thermal diffusion. In this study, we try to verify this new mechanism through laboratory experiments. An ice mixture of CH3OH and CO, which is an analog of the outer layer of the ice mantle on cosmic dust grains, was exposed to UV irradiation to produce radicals such as HCO and CH2OH, whose concentration was monitored during the subsequent warm-up of the ice. We find clear evidence that during the CO phase transition, most of the radicals recombine to form other molecular species, therefore supporting the recently proposed mechanism of COM formation via CO phase transition.

Keywords: interstellar dust; interstellar molecules; molecule formation; dense interstellar clouds; laboratory astrophysics
Virial Velocities for Stellar Flybys with Planetary Disks in Star Formation Regions

In this work, we show the effect of different velocities of stellar encounters with planetary disks to support the use of virial or constant velocities in simulations of typical star-forming clusters; the variety of observed member velocity values in such objects can cause concern when selecting velocities for simulations. In this work, we employ different velocities from 1.5 to 10 km s‑1 with constant flyby masses, approach distances, and impact angles for encounters with a disk. We found that these velocities produce similar results, which suggests that such simulations can use virial or constant velocities without loss of accuracy.

Keywords: orbits; debris disks; protoplanetary disks; stellar dynamics
Hall Cascade with Fractional Magnetic Helicity in Neutron Star Crusts

The ohmic decay of magnetic fields in the crusts of neutron stars is generally believed to be governed by Hall drift, which leads to what is known as a Hall cascade. Here we show that helical and fractionally helical magnetic fields undergo strong inverse cascading like in magnetohydrodynamics (MHD), but the magnetic energy decays more slowly with time t: $\propto \,{t}^{-2/5}$ instead of ∝t-2/3 in MHD. Even for a nonhelical magnetic field there is a certain degree of inverse cascading for sufficiently strong magnetic fields. The inertial range scaling with wavenumber k is compatible with earlier findings for the forced Hall cascade, i.e., proportional to k-7/3, but in the decaying cases, the subinertial range spectrum steepens to a novel k5 slope instead of the k4 slope in MHD. The energy of the large-scale magnetic field can increase quadratically in time through inverse cascading. For helical fields, the energy dissipation is found to be inversely proportional to the large-scale magnetic field and proportional to the fifth power of the rms magnetic field. For neutron star conditions with an rms magnetic field of a few times ${10}^{14}\,{\rm{G}}$ , the large-scale magnetic field might only be ${10}^{11}\,{\rm{G}}$ , while still producing magnetic dissipation of ${10}^{33}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ for thousands of years, which could manifest itself through X-ray emission. Finally, it is shown that the conclusions from local unstratified models agree rather well with those from stratified models with boundaries.

Keywords: magnetohydrodynamical simulations; magnetars
The Imprint of Clump Formation at High Redshift. II. The Chemistry of the Bulge

In Paper I, we showed that clumps in high-redshift galaxies, having a high star formation rate density (ΣSFR), produce disks with two tracks in the [Fe/H]-[α/Fe] chemical space, similar to that of the Milky Way's (MW's) thin+thick disks. Here we investigate the effect of clumps on the bulge's chemistry. The chemistry of the MW's bulge is comprised of a single track with two density peaks separated by a trough. We show that the bulge chemistry of an N-body + smoothed particle hydrodynamics clumpy simulation also has a single track. Star formation within the bulge is itself in the high-ΣSFR clumpy mode, which ensures that the bulge's chemical track follows that of the thick disk at low [Fe/H] and then extends to high [Fe/H], where it peaks. The peak at low metallicity instead is comprised of a mixture of in situ stars and stars accreted via clumps. As a result, the trough between the peaks occurs at the end of the thick disk track. We find that the high-metallicity peak dominates near the mid-plane and declines in relative importance with height, as in the MW. The bulge is already rapidly rotating by the end of the clump epoch, with higher rotation at low [α/Fe]. Thus clumpy star formation is able to simultaneously explain the chemodynamic trends of the MW's bulge, thin+thick disks, and the splash.

Keywords: galactic bulge; milky way formation; milky way evolution; milky way dynamics; galaxy bulges
The SAGA Survey. II. Building a Statistical Sample of Satellite Systems around Milky Way-like Galaxies

We present the Stage II results from the ongoing Satellites Around Galactic Analogs (SAGA) Survey. Upon completion, the SAGA Survey will spectroscopically identify satellite galaxies brighter than Mr,o = -12.3 around 100 Milky Way (MW) analogs at z ∼ 0.01. In Stage II, we have more than quadrupled the sample size of Stage I, delivering results from 127 satellites around 36 MW analogs with an improved target selection strategy and deep photometric imaging catalogs from the Dark Energy Survey and the Legacy Surveys. We have obtained 25,372 galaxy redshifts, peaking around z = 0.2. These data significantly increase spectroscopic coverage for very low redshift objects in 17 < ro < 20.75 around SAGA hosts, creating a unique data set that places the Local Group in a wider context. The number of confirmed satellites per system ranges from zero to nine and correlates with host galaxy and brightest satellite luminosities. We find that the number and luminosities of MW satellites are consistent with being drawn from the same underlying distribution as SAGA systems. The majority of confirmed SAGA satellites are star-forming, and the quenched fraction increases as satellite stellar mass and projected radius from the host galaxy decrease. Overall, the satellite quenched fraction among SAGA systems is lower than that in the Local Group. We compare the luminosity functions and radial distributions of SAGA satellites with theoretical predictions based on cold dark matter simulations and an empirical galaxy-halo connection model and find that the results are broadly in agreement.

Keywords: redshift surveys; dwarf galaxies; luminosity function; galaxy physics; galaxy dark matter halos