adsabs/SciX_UAT_keywords huggingface dataset:
The Evolution of a Newborn Millisecond Magnetar with a Propeller-recycling Disk

A rapidly rotating and highly magnetized neutron star (NS) could be formed from explosive phenomena such as superluminous supernovae and gamma-ray bursts. This newborn NS can substantially influence the emission of these explosive transients through its spin-down. The spin-down evolution of the NS can sometimes be affected by fallback accretion, although it is usually regulated by the magnetic dipole radiation and gravitational wave radiation of the NS. Under appropriate conditions, the accreting material can be first ejected and subsequently recycled back, so that the accretion disk can remain in a quasi-steady state for a long time. Here we describe the interaction of the NS with such a propeller-recycling disk and their coevolution. Our result shows that the spin-down of the NS can be initially dominated by the propeller, which prevents the disk material from falling onto the NS until hundreds or thousands of seconds later. It is suggested that the abrupt fall of the disk material onto the NS could significantly suppress the magnetic dipole radiation and then convert the NS from a normal magnetar to a low-field magnetar. This evolution behavior of the newborn NS can help us understand the very different influence of the NS on the early GRB afterglows and the late supernova/kilonova emission.

Keywords: neutron stars; magnetars; stellar accretion disks; gamma-ray bursts
Eccentricity without Measuring Eccentricity: Discriminating among Stellar Mass Black Hole Binary Formation Channels

We show how the observable number of binaries in LISA is affected by eccentricity through its influence on the peak gravitational-wave frequency, enhanced binary number density required to produce the LIGO observed rate, and the reduced signal-to-noise ratio (S/N) for an eccentric event. We also demonstrate how these effects should make it possible to learn about the eccentricity distribution and formation channels by counting the number of binaries as a function of frequency, even with no explicit detection of eccentricity. We also provide a simplified calculation for S/N of eccentric binaries.

Keywords: gravitational wave astronomy
Mining S-PLUS for Metal-poor Stars in the Milky Way

This work presents the medium-resolution (R ~ 1500) spectroscopic follow-up of 522 low-metallicity star candidates from the Southern Photometric Local Universe Survey (S-PLUS). The objects were selected from narrowband photometry, taking advantage of the metallicity-sensitive S-PLUS colors. The follow-up observations were conducted with the Blanco and Gemini South telescopes, using the COSMOS and GMOS spectrographs, respectively. The stellar atmospheric parameters (T eff, $\mathrm{log}\,g$ , and [Fe/H]), as well as carbon and α-element abundances, were calculated for the program stars in order to assess the efficacy of the color selection. Results show that ${92}_{-3}^{+2} \% $ of the observed stars have [Fe/H] ≤ -1.0, ${83}_{-3}^{+3} \% $ have [Fe/H] ≤ -2.0, and ${15}_{-3}^{+3} \% $ have [Fe/H] ≤ -3.0, including two ultra metal-poor stars ([Fe/H] ≤ -4.0). The 80th percentile for the metallicity cumulative distribution function of the observed sample is [Fe/H] = -2.04. The sample also includes 68 carbon-enhanced metal-poor stars. Based on the calculated metallicities, further S-PLUS color cuts are proposed, which can increase the fractions of stars with [Fe/H] ≤ -1.0 and ≤ -2.0 to 98% and 88%, respectively. Such high success rates enable targeted high-resolution spectroscopic follow-up efforts, as well as provide selection criteria for fiber-fed multiplex spectroscopic surveys.

Keywords: narrow band photometry; metallicity; stellar atmospheres; chemical abundances
Dynamic Kink Instability and Transverse Motions of Solar Spicules

Hydrodynamic jets are unstable to the kink instability (m = 1 mode in cylindrical geometry) owing to the centripetal force, which increases the transverse displacement of the jet. When the jet moves along a magnetic field, the Lorentz force tries to decrease the displacement and stabilize the instability of sub-Alfvénic flows. The threshold of the instability depends on the Alfvén Mach number (the ratio of Alfvén and jet speeds). We suggest that the dynamic kink instability may be important to explain observed transverse motions of type II spicules in the solar atmosphere. We show that the instability may begin for spicules that rise up at the peripheries of vertically expanding magnetic flux tubes because of the decrease of the Alfvén speed in both the vertical and the radial directions. Therefore, inclined spicules may be more unstable and have higher transverse speeds. Periods and growth times of unstable modes in the conditions of type II spicules have values of 30 s and 25-100 s, respectively, which are comparable to the lifetime of the structures. This may indicate an interconnection between high-speed flow and the rapid disappearance of type II spicules in chromospheric spectral lines.

Keywords: solar spicules; solar chromosphere; solar mottles
On the Rigidity Spectrum of Cosmic-Ray Variations within Propagating Interplanetary Disturbances: Neutron Monitor and SOHO/EPHIN Observations at ∼1-10 GV

The rigidity dependence of all Forbush decreases (FDs) recorded from 1995 to 2015 has been determined using neutron monitor (NM) and Solar and Heliospheric Observatory (SOHO) (EPHIN) spacecraft data, covering the energy (rigidity) range from ∼433 MeV (1 GV) to 9.10 GeV (10 GV). We analyzed a total of 421 events and determined the spectrum in rigidity with an inverse power-law fit. As a result, the mean spectral index was identified to be ⟨γF⟩ = 0.46 ± 0.02. The majority (∼66%) of the FDs have γF within the range 0.3-0.7. The remaining one-third of the events (∼33%) have either (very) soft or hard FD spectra, with the latter being more common than the former. Significant variations of γF occur within almost every FD event. During the initial FD decay phase the spectrum becomes gradually harder, in contrast to the recovery phase, when it becomes softer. Additionally, low energies (rigidities) seem to be better suited for studying the fine structure of interplanetary disturbances (primarily interplanetary coronal mass ejections) that lead to FDs. In particular, FDs recorded by the EPHIN instrument on SOHO better capture a two-step structure than FDs observed by NMs. Finally, the ejecta of an ICME, especially when identified as a magnetic cloud, often leads to abrupt changes in the slope of γF.

Keywords: solar coronal mass ejections; forbush effect; corotating streams
Scalaron Gravity near Sagittarius A*: Investigation of Spin of the Black Hole and Observing Requirements

In this paper the author applies the scalaron gravity field and corresponding Yukawa coupling (derived by Kalita from the consideration of quantum vacuum fluctuations with UV and IR scales) to examine the scales of stellar orbits near the Galactic Center black hole, which can be probed by upcoming astrometric facilities for constraining modified gravity. Through the assumption that the pericenter shift of stellar orbits becomes of the order of spin and quadrupole moment effects of the black hole, it is found that for semimajor axes bounded below by time scales of gravitational wave emission and stellar age and above by S-2 like orbits (a = 990 au) the black hole spin with 0.1 ≤ χ ≤ 0.980 is eligible to probe scalaron masses within (10-22-10-18) eV and also the scalaron coupling, α = 2.73 × 10-4 derived earlier from quantum vacuum fluctuations. The orbital eccentricities are considered as e = 0.1, 0.5, and 0.9. Astrometric categories with σ = 10, 50, and 100 μas are used to probe the time scales and number of observing campaigns required for simultaneously constraining scalaron mass and black hole spin. It is found that extraction of black hole spin is possible within a = (74-433) au through 10 μas facilities. The present analysis is realized to be an independent opportunity to simultaneously constrain scalaron coupling, black hole spin, and tidal charge and hence to reveal the true nature of the spacetime structure of our nearest supermassive black hole.

Keywords: black hole physics; galactic center
An Analysis of Soft X-Ray Structures at Kiloparsec Distances from the Active Nucleus of Centaurus A Galaxy

Here we reanalyze the archival Chandra data for the central parts of the Centaurus A radio galaxy, aiming for a systematic investigation of the X-ray emission associated with the inner radio lobes in the system, and their immediate surroundings. In particular, we focus on four distinct features characterized by the soft excess with respect to the adjacent fields. Those include the two regions located at kiloparsec distances from the nucleus to the west and east, the extended bow-shock structure to the south, and a fragment of a thin arc north of the center. The selected north, west, and south features coincide with the edges of the radio lobes, while the east structure is seemingly displaced from the radio-emitting plasma. Our X-ray spectral analysis reveals (i) a power-law emission component with photon index Γ ∼ 2 in the north, east, and south regions, and (ii) a dense (number density ∼0.3 cm-3) and relatively cold (temperature ∼ 0.2 keV) gas in the east and west regions. The power-law emission is consistent with the synchrotron continuum generated at the edges of the radio structure, and implies that the efficiency of the electron acceleration at the terminal bow shock does not vary dramatically over the inner lobes' extension. The presence of gaseous condensations, on the other hand, could possibly be understood in terms of a massive outflow from the central regions of the galaxy.

Keywords: high energy astrophysics; x-ray active galactic nuclei; non-thermal radiation sources; active galaxies; x-ray astronomy; x-ray sources
Metal-poor Stars Observed with the Southern African Large Telescope II. An Extended Sample

We present results from high-resolution (R ~ 40,000) spectroscopic observations of over 200 metal-poor stars, mostly selected from the RAVE survey, using the Southern African Large Telescope. We were able to derive stellar parameters for a total of 108 stars; an additional sample of 50 stars from this same effort was previously reported on by Rasmussen et al. Among our newly reported observations, we identify 84 very metal-poor (VMP; [Fe/H] < -2.0, 53 newly identified) stars and three extremely metal-poor (EMP; [Fe/H] < -3.0, one newly identified) stars. The elemental abundances were measured for carbon, as well as several other α-elements (Mg, Ca, Sc, and Ti), iron-peak elements (Mn, Co, Ni, and Zn), and neutron-capture elements (Sr, Ba, and Eu). Based on these measurements, the stars are classified by their carbon and neutron-capture abundances into carbon-enhanced metal-poor (CEMP; [C/Fe] > +0.70), CEMP subclasses, and by the level of their r-process abundances. A total of 17 are classified as CEMP stars. There are 11 CEMP-r stars (eight newly identified), one CEMP-s star (newly identified), two possible CEMP-i stars (one newly identified), and three CEMP-no stars (all newly identified) in this work. We found 11 stars (eight newly identified) that are strongly enhanced in r-process elements (r-II; [Eu/Fe] > +0.70), 38 stars (31 newly identified) that are moderately enhanced in r-process elements (r-I; +0.30 < [Eu/Fe] ≤ + 0.70), and one newly identified limited-r star.

Keywords: milky way stellar halo; stellar abundances; stellar atmospheres; cemp stars; r-process; population ii stars
Is There an Enormous Cold Front at the Virial Radius of the Perseus Cluster?

We present new XMM-Newton observations extending the mosaic of the Perseus cluster out to the virial radius to the west. Previous studies with ROSAT have reported a large excess in surface brightness to the west, possibly the result of large-scale gas sloshing. In our new XMM-Newton observations we have found two X-ray surface brightness edges at 1.2 and 1.7 Mpc to the west. The temperature measurements obtained with Suzaku data indicate that the temperature increases sharply at each edge, consistent with what would be expected from cold fronts. However the the XMM-Newton data are affected by stray light, which at present is a poorly understood source of systematic error that can also lead to curved features in X-ray images. To test our results, we compared our X-ray surface brightness profile with that obtained from ROSAT PSPC data. While the edge at 1.2 Mpc is confirmed by ROSAT PSPC, the ROSAT data quality is insufficient to confirm the outer edge at 1.7 Mpc. Further observations with future X-ray telescopes will be needed to confirm the existence of the outer edge at 1.7 Mpc. By comparing with numerical simulations, we find that these large cold fronts require a large impact parameter, and low-mass ratio mergers that can produce fast gas motions without destroying the cluster core.

Keywords: galaxy clusters; intracluster medium; perseus cluster; x-ray astronomy; high energy astrophysics
Two X-Ray Plateaus of Gamma-Ray Bursts: Energy Injection from Nascent Magnetars with an Evolving Magnetic Inclination Angle

As the central engine of gamma-ray bursts (GRBs), a nascent magnetar can release its rotational energy by magnetic dipole radiation. This energy is injected into the outflow of the GRB jet. With the rapid evolution of the magnetic inclination angle, the perpendicular component of the magnetar's surface magnetic field decreases rapidly. The energy loss rate of the magnetar is characterized by two different timescales. This peculiar two-stage energy injection model will affect the shape of the X-ray afterglow light curve. It is shown that a new shallow decay phase will emerge following the initial shallow decay phase. As a result, there could be two plateaus in the X-ray light curve. The effects of various physical parameters on the light curves are studied in detail. It is found that the observed afterglows of GRB 161217A and GRB 190114A, both having two plateaus in the light curve, can be well fitted in our framework.

Keywords: gamma-ray bursts; magnetars; light curves
Star Formation Efficiency and Dispersal of Giant Molecular Clouds with UV Radiation Feedback: Dependence on Gravitational Boundedness and Magnetic Fields

Molecular clouds are supported by turbulence and magnetic fields, but quantifying their influence on cloud life cycle and star formation efficiency (SFE) remains an open question. We perform radiation magnetohydrodynamic simulations of star-forming giant molecular clouds (GMCs) with UV radiation feedback, in which the propagation of UV radiation via ray tracing is coupled to hydrogen photochemistry. We consider 10 GMC models that vary in either initial virial parameter (1 ≤ αvir,0 ≤ 5) or dimensionless mass-to-magnetic flux ratio (0.5 ≤ μΦ,0 ≤ 8 and ∞ ); the initial mass 105 M⊙ and radius 20 pc are fixed. Each model is run with five different initial turbulence realizations. In most models, the duration of star formation and the timescale for molecular gas removal (primarily by photoevaporation) are 4-8 Myr. Both the final SFE (ɛ*) and time-averaged SFE per freefall time (ɛff) are reduced by strong turbulence and magnetic fields. The median ɛ* ranges between 2.1% and 9.5%. The median ɛff ranges between 1.0% and 8.0%, and anticorrelates with αvir,0, in qualitative agreement with previous analytic theory and simulations. However, the time-dependent αvir(t) and ɛff,obs(t) based on instantaneous gas properties and cluster luminosity are positively correlated due to rapid evolution, making observational validation of star formation theory difficult. Our median ɛff,obs(t) ≈ 2% is similar to observed values. We show that the traditional virial parameter estimates the true gravitational boundedness within a factor of 2 on average, but neglect of magnetic support and velocity anisotropy can sometimes produce large departures from traditional virial parameter estimates. Magnetically subcritical GMCs are unlikely to represent sites of massive star formation given their unrealistic columnar outflows, prolonged lifetime, and low escape fraction of radiation.

Keywords: giant molecular clouds; h ii regions; radiative magnetohydrodynamics; star formation; magnetic fields; stellar feedback; interstellar medium
A Three-dimensional Model for the Evolution of Magnetohydrodynamic Turbulence in the Outer Heliosphere

We present a time-dependent, three-dimensional single-fluid model for the transport of magnetohydrodynamic (MHD) turbulence that is self-consistently evolving with a dynamic large-scale solar wind in the outer heliosphere. The emphasis is on the region beyond the termination shock, where the solar wind expands subsonically, as well as sub-Alfvénically and nonradially. In extension of earlier work, we refine the treatment of turbulence by considering, in addition to the Elsässer energies, a nonconstant energy difference (or residual energy) and by allowing each of these quantities its own characteristic correlation length scale. While the nonlinear effects in the equations for the Elsässer energies and their length scales are implemented using familiar von Kármán-Howarth style modeling of homogeneous MHD turbulence, the energy difference, which is not conserved in the absence of dissipation, and its length scale are modeled using distinct approaches. We also clarify the impact of the choice of measurement direction for correlation functions associated with two-dimensional fluctuations in transport models. Finally, we illustrate and study the solutions of the resulting six-equation model in detail.

Keywords: heliosheath; interplanetary turbulence; solar wind termination
Deconfusing Detections in Directly Imaged Multiplanet Systems

High-contrast images from future space-based telescopes may contain several planets from multiplanet systems and potentially a few planet-like speckles. When taken several months apart, the short-period planets and speckles will appear to move significantly, to the point that it might not be clear which point source (detection) in the image belongs to which object. In this work, we develop a tool, the deconfuser, to test quickly all the plausible partitions of detections by planets based on orbital mechanics. We then apply the deconfuser to a large set of simulated observations to estimate "confusion" rates, i.e., how often there are multiple distinct orbit combinations that describe the data well. We find that in the absence of missed and false detections, four observations are sufficient to avoid confusion, except for systems with high inclinations (above 75°). In future work, the deconfuser will be integrated into mission simulation tools, such as EXOSIMS, to assess the risk of confusion in missions such as the IR/O/UV large telescope recommended by the Astro2020 decadal survey. * Submitted on April, 1st, 2022

Keywords: astrometric exoplanet detection; orbit determination; coronagraphic imaging; regression
Identifying RR Lyrae Variable Stars in Six Years of the Dark Energy Survey

We present a search for RR Lyrae stars using the full six-year data set from the Dark Energy Survey covering ∼5000 deg2 of the southern sky. Using a multistage multivariate classification and light-curve template-fitting scheme, we identify RR Lyrae candidates with a median of 35 observations per candidate. We detect 6971 RR Lyrae candidates out to ∼335 kpc, and we estimate that our sample is >70% complete at ∼150 kpc. We find excellent agreement with other wide-area RR Lyrae catalogs and RR Lyrae studies targeting the Magellanic Clouds and other Milky Way satellite galaxies. We fit the smooth stellar halo density profile using a broken-power-law model with fixed halo flattening (q = 0.7), and we find strong evidence for a break at ${R}_{0}={32.1}_{-0.9}^{+1.1}\,\mathrm{kpc}$ with an inner slope of ${n}_{1}=-{2.54}_{-0.09}^{+0.09}$ and an outer slope of ${n}_{2}=-{5.42}_{-0.14}^{+0.13}$ . We use our catalog to perform a search for Milky Way satellite galaxies with large sizes and low luminosities. Using a set of simulated satellite galaxies, we find that our RR Lyrae-based search is more sensitive than those using resolved stellar populations in the regime of large (rh ≳ 500 pc), low-surface-brightness dwarf galaxies. A blind search for large, diffuse satellites yields three candidate substructures. The first can be confidently associated with the dwarf galaxy Eridanus II. The second has a distance and proper motion similar to the ultrafaint dwarf galaxy Tucana II but is separated by ∼5 deg. The third is close in projection to the globular cluster NGC 1851 but is ∼10 kpc more distant and appears to differ in proper motion.

Keywords: rr lyrae variable stars; rrab variable stars; milky way stellar halo
Disk Galaxies Are Self-similar: The Universality of the H I-to-Halo Mass Ratio for Isolated Disks

Observed scaling relations in galaxies between baryons and dark matter global properties are key to shed light on the process of galaxy formation and on the nature of dark matter. Here, we study the scaling relation between the neutral hydrogen (H I) and dark matter mass in isolated rotationally supported disk galaxies at low redshift. We first show that state-of-the-art galaxy formation simulations predict that the H I-to-dark-halo mass ratio decreases with stellar mass for the most massive disk galaxies. We then infer dark matter halo masses from high-quality rotation curve data for isolated disk galaxies in the local Universe and report on the actual universality of the H I-to-dark halo mass ratio for these observed galaxies. This scaling relation holds for disks spanning a range of 4 orders of magnitude in stellar mass and 3 orders of magnitude in surface brightness. Accounting for the diversity of rotation curve shapes in our observational fits decreases the scatter of the H I-to-dark halo mass ratio while keeping it constant. This finding extends the previously reported discrepancy for the stellar-to-halo mass relation of massive disk galaxies within galaxy formation simulations to the realm of neutral atomic gas. Our result reveals that isolated galaxies with regularly rotating extended H I disks are surprisingly self-similar up to high masses, which hints at mass-independent self-regulation mechanisms that have yet to be fully understood.

Keywords: spiral galaxies; dark matter distribution; galaxy kinematics; galaxy dynamics; irregular galaxies
Testing Horndeski Gravity from EHT Observational Results for Rotating Black Holes

The Event Horizon Telescope (EHT) collaboration recently unveiled the first image of the supermassive black hole M87*, which exhibited a ring of angular diameter θ d = 42 ± 3 μas, a circularity deviation of ΔC ≤ 0.1, and also inferred a black hole mass of M = (6.5 ± 0.7) × 109 M ⊙. This provides a new window onto tests of theories of gravity in the strong-field regime, including probes of violations of the no-hair theorem. It is widely believed that the Kerr metric describes astrophysical black holes, as encapsulated in the critical but untested no-hair theorem. Modeling Horndeski gravity black holes-with an additional hair parameter h besides the mass M and spin a of the Kerr black hole-as the supermassive black hole M87*, we observe that to be a viable astrophysical black hole candidate, the EHT result constrains the (a, h) parameter space. However, a systematic bias analysis indicates that rotating Horndeski black hole shadows may or may not capture Kerr black hole shadows, depending on the parameter values; the latter is the case over a substantial part of the constrained parameter space, allowing Horndeski gravity and general relativity to be distinguishable in the said space, and opening up the possibility of potential modifications to the Kerr metric.

Keywords: astrophysical black holes; black hole physics; galactic center; gravitation; gravitational lensing