adsabs/SciX_UAT_keywords huggingface dataset:
Kinetic Simulations of Electron Pre-energization by Magnetized Collisionless Shocks in Expanding Laboratory Plasmas

Collisionless shocks are common features in space and astrophysical systems where supersonic plasma flows interact, such as in the solar wind, the heliopause, and supernova remnants. Recent experimental capabilities and diagnostics allow detailed laboratory investigations of high-Mach-number shocks, which therefore can become a valuable way to understand shock dynamics in various astrophysical environments. Using 2D particle-in-cell simulations with a Coulomb binary collision operator, we demonstrate the mechanism for generation of energetic electrons and experimental requirements for detecting this process in the laboratory high-Mach-number collisionless shocks. We show through a parameter study that electron acceleration by magnetized collisionless shocks is feasible in laboratory experiments with laser-driven expanding plasmas.

Keywords: shocks; planetary bow shocks; laboratory astrophysics; plasma physics
StraKLIP: A Novel Pipeline for Detection and Characterization of Close-in Faint Companions through the Karhunen-Loéve Image Processing Algorithm

We present a new pipeline developed to detect and characterize faint astronomical companions at small angular separation from the host star using sets of wide-field imaging observations not specifically designed for high-contrast imaging analysis. The core of the pipeline relies on Karhunen-Loéve truncated transformation of the reference point-spread function (PSF) library to perform PSF subtraction and identify candidates. Tests of reliability of detections and characterization of companions are made through simulation of binaries and generation of receiver operating characteristic curves for false-positive/true-positive analysis. The algorithm has been successfully tested on large HST/ACS and WFC3 data sets acquired for two HST Treasury Programs on the Orion Nebula Cluster. Based on these extensive numerical experiments we find that, despite being based on methods designed for observations of a single star at a time, our pipeline performs very well on mosaic space-based data. In fact, we are able to detect brown-dwarf-mass companions almost down to the planetary-mass limit. The pipeline is able to reliably detect signals at separations as close as ≳0.″1 with a completeness of ≳10%, or ~0.″2 with a completeness of ~30%. This approach can potentially be applied to a wide variety of space-based imaging surveys, from data in the existing HST archive to near-future JWST mosaics and future wide-field Roman images.

Keywords: astronomy software; direct imaging; exoplanet detection methods
The Direct Mid-infrared Detectability of Habitable-zone Exoplanets around Nearby Stars

Giant planets within the habitable zones of the closest several stars can currently be imaged with ground-based telescopes. Within the next decade, the Extremely Large Telescopes (ELTs) will begin to image the habitable zones of a greater number of nearby stars with much higher sensitivity, potentially imaging exo-Earths around the closest stars. To determine the most promising candidates for observations over the next decade, we establish a theoretical framework for the direct detectability of Earth- to super-Jovian-mass exoplanets in the mid-infrared based on available atmospheric and evolutionary models. Of the 83 closest BAFGK-type stars, we select 37 FGK-type stars within 10 pc and 34 BA-type stars within 30 pc with reliable age constraints. We prioritize targets based on a parametric model of a planet's effective temperature based on a star's luminosity, distance, and age, and on the planet's orbital semimajor axis, radius, and albedo. We then predict the most likely planets to be detectable with current 8 m telescopes and with a 39 m ELT with up to 100 hr of observation per star. Putting this together, we recommend observation times needed for the detection of habitable-zone exoplanets spanning the range from very nearby temperate Earth-sized planets to more distant young giant planets. We then recommend ideal initial targets for current telescopes and the upcoming ELTs.

Keywords: exoplanet astronomy; direct imaging; extrasolar rocky planets
Spectro-temporal Studies of Rapid Transition of the Quasi-periodic Oscillations in the Black Hole Source H1743-322

An appearance or disappearance of quasi-periodic oscillations (QPOs) associated with the variation of X-ray flux can be used to decipher the accretion-ejection mechanism of black hole X-ray sources. We searched for and studied such rapid transitions in H1743-322 using archival data from the Rossi X-ray Timing Explorer and found eight such events, where a QPO vanishes suddenly along with the variation of X-ray flux. The appearance/disappearance of QPOs was associated with four events exhibiting type-B QPOs at ∼4.5 Hz: one was a type-A QPO at ν ∼ 3.5 Hz, and the other three were connected to type-C QPOs at ∼9.5 Hz. Spectral studies of the data revealed that the inner disk radius remained at the same location around 2-9 rg, depending on the model used, but power-law indices were varying, indicating that either a corona or a jet is responsible for the events. The probable ejection radii of coronas were estimated to be around 4.2-15.4 rg based on the plasma ejection model. Our X-ray and quasi-simultaneous radio correlation studies suggest that the type-B QPOs are probably related to the precession of a weak jet, though a small and weak corona is present at its base, and the type-C QPOs are associated with the base of a relatively strong jet, which is acting like a corona.

Keywords: low-mass x-ray binary stars
Relative Abundances of CO2, CO, and CH4 in Atmospheres of Earth-like Lifeless Planets

Carbon is an essential element for life on Earth, and the relative abundances of major carbon species (CO2, CO, and CH4) in the atmosphere exert fundamental controls on planetary climate and biogeochemistry. Here we employed a theoretical model of atmospheric chemistry to investigate diversity in the atmospheric abundances of CO2, CO, and CH4 on Earth-like lifeless planets orbiting Sun-like (F-, G-, and K-type) stars. We focused on the conditions for the formation of a CO-rich atmosphere, which would be favorable for the origin of life. Results demonstrated that elevated atmospheric CO2 levels trigger photochemical instability of the CO budget in the atmosphere (i.e., CO runaway) owing to enhanced CO2 photolysis relative to H2O photolysis. Higher volcanic outgassing fluxes of reduced C (CO and CH4) also tend to initiate CO runaway. Our systematic examinations revealed that anoxic atmospheres of Earth-like lifeless planets could be classified in the phase space of CH4/CO2 versus CO/CO2, where a distinct gap in atmospheric carbon chemistry is expected to be observed. Our findings indicate that the gap structure is a general feature of Earth-like lifeless planets with reducing atmospheres orbiting Sun-like (F-, G-, and K-type) stars.

Keywords: earth atmosphere; exoplanet atmospheres; planetary atmospheres
A Multiwavelength Light-curve Model of the Classical Nova V339 Del: A Mechanism for the Coexistence of Dust Dip and Supersoft X-Rays

The classical nova V339 Del 2013 is characterized by a 1.5 mag dip of the V light curve owing to a dust shell formation, with which soft X-ray emissions coexist. We present a Strömgren y-band light curve, which represents continuum emission, not influenced by strong [O III] emission lines. The y light curve monotonically decreases in marked contrast to the V light curve that shows a 1.5 mag dip. We propose a multiwavelength light-curve model that reproduces the y and V light curves as well as the gamma-ray and X-ray light curves. In our model, a strong shock arises far outside the photosphere after optical maximum, because later ejected matter collides with earlier ejected gas. Our shocked shell model explains optical emission lines, Hα, hard X-ray, and gamma-ray fluxes. A dust shell forms behind the shock that suppresses [O III]. This low flux of [O III] shapes a 1.5 mag drop in the V light curve. Then, the V flux recovers with an increasing contribution from [O III] lines, while the y flux does not. However, the optical depth of the dust shell is too small to absorb the photospheric (X-ray) emission of the white dwarf. This is the reason that a dust shell and soft X-ray radiation coexist. We determined the white dwarf mass to be M WD = 1.25 ± 0.05 M ☉ and the distance modulus in the V band to be (m ‑ M) V = 12.2 ± 0.2; the distance is d = 2.1 ± 0.2 kpc for the reddening of E(B ‑ V) = 0.18.

Keywords: classical novae; fast novae; x-ray astronomy; high energy astrophysics; shocks; gamma-ray astronomy; dust formation
ALMA Observations of Giant Molecular Clouds in M33. II. Triggered High-mass Star Formation by Multiple Gas Colliding Events at the NGC 604 Complex

We present the results of ALMA observations in 12CO(J = 2 - 1), 13CO(J = 2 - 1), and C18O(J = 2 - 1) lines and 1.3 mm continuum emission toward a massive (∼106 M⊙) giant molecular cloud associated with the giant H II region NGC 604 in one of the nearest spiral galaxies, M33, at an angular resolution of 0"44 × 0"27 (1.8 pc × 1.1 pc). The 12CO and 13CO images show highly complicated molecular structures composed of a lot of filaments and shells whose lengths are 5-20 pc. We found three 1.3 mm continuum sources to be dense clumps at the edges of two shells and also at the intersection of several filaments. We examined the velocity structures of the 12CO(J = 2 - 1) emission in the shells and filaments containing dense clumps, and concluded that expansion of the H II regions cannot explain the formation of such dense cores. Alternatively, we suggest that cloud-cloud collisions induced by an external H I gas flow and the galaxy's rotation compressed the molecular material into dense filaments/shells which are ongoing high-mass star formation sites. We propose that multiple gas converging/colliding events with a velocity of a few tens of kilometers per second are necessary to build up NGC 604, the most significant cluster-forming complex in the Local Group of galaxies.

Keywords: interstellar medium; star formation; giant molecular clouds; triangulum galaxy; local group; h ii regions
Neutron Star Kicks and Implications for Their Rotation at Birth

Neutron stars are born out of core-collapse supernovae, and they are imparted natal kicks at birth as a consequence of asymmetric ejection of matter and possibly neutrinos. Unless the force resulting from the kicks is exerted exactly at their center, it will also cause the neutron star to rotate. In this paper, we discuss the possibility that neutron stars may receive off-center natal kicks at birth, which imprint a natal rotation. In this scenario, the observed pulsar spin and transverse velocity in the Galaxy are expected to correlate. We develop a model of the natal rotation imparted to neutron stars and constrain it by the observed population of pulsars in our Galaxy. When considering a single-kick position parameter, we find that the location of the off-center kick is ${R}_{\mathrm{kick}}={2.03}_{-1.69}^{+3.74}$ km at 90% confidence, and is robust when considering pulsars with different observed periods, transverse velocities, and ages. Nonetheless, the model encounters challenges in effectively fitting the data, particularly at small transverse velocities, prompting the exploration of alternative models that include more complex physics. Our framework could be used as a guide for core-collapse simulations of massive stars.

Keywords: neutron stars; neutron star cores; core-collapse supernovae; supernovae; rotation powered pulsars; pulsars; radio pulsars
Does the Disk in the Hard State of XTE J1752-223 Extend to the Innermost Stable Circular Orbit?

The accreting black hole binary XTE J1752-223 was observed in a stable hard state for 25 days by the Rossi X-ray Timing Explorer (RXTE), yielding a 3-140 keV spectrum of unprecedented statistical quality. Its published model required a single-Comptonization spectrum reflecting from a disk close to the innermost stable circular orbit. We studied that model as well as a number of other single-Comptonization models (yielding similarly low inner radii), but found they violate a number of basic physical constraints, e.g., their compactness is much above the maximum allowed by pair equilibrium. We also studied the contemporaneous 0.55-6 keV spectrum from the Swift/X-ray Telescope and found it well fitted by an absorbed power law and a disk blackbody with the innermost temperature of 0.1 keV. The normalization of the disk blackbody corresponds to an inner radius of ≳20 gravitational radii and its temperature, to irradiation of the truncated disk by a hot inner flow. We have also developed a Comptonization/reflection model including the disk irradiation and intrinsic dissipation, but found that it does not yield any satisfactory fits. On the other hand, we found that the ≤10 keV band from RXTE is much better fitted by a reflection from a disk with the inner radius ≳100 gravitational radii, which model then underpredicts the spectrum at >10 keV by <10%. We argue that the most plausible explanation of the above results is inhomogeneity of the source, with the local spectra hardening with the decreasing radius. Our results support the presence of a complex Comptonization region and a large disk truncation radius in this source.

Keywords: accretion; non-thermal radiation sources; x-ray binary stars
K2-138 g: Spitzer Spots a Sixth Planet for the Citizen Science System

K2 greatly extended Kepler's ability to find new planets, but it was typically limited to identifying transiting planets with orbital periods below 40 days. While analyzing K2 data through the Exoplanet Explorers project, citizen scientists helped discover one super-Earth and four sub-Neptune sized planets in the relatively bright (V = 12.21, K = 10.3) K2-138 system, all which orbit near 3:2 mean-motion resonances. The K2 light curve showed two additional transit events consistent with a sixth planet. Using Spitzer photometry, we validate the sixth planet's orbital period of 41.966 ± 0.006 days and measure a radius of ${3.44}_{-0.31}^{+0.32}\,{R}_{\oplus }$, solidifying K2-138 as the K2 system with the most currently known planets. There is a sizeable gap between the outer two planets, since the fifth planet in the system, K2-138 f, orbits at 12.76 days. We explore the possibility of additional nontransiting planets in the gap between f and g. Due to the relative brightness of the K2-138 host star, and the near resonance of the inner planets, K2-138 could be a key benchmark system for both radial velocity and transit-timing variation mass measurements, and indeed radial velocity masses for the inner four planets have already been obtained. With its five sub-Neptunes and one super-Earth, the K2-138 system provides a unique test bed for comparative atmospheric studies of warm to temperate planets of similar size, dynamical studies of near-resonant planets, and models of planet formation and migration.

Keywords: exoplanet systems; exoplanets; mini neptunes; super earths
NuSTAR Spectral Analysis beyond 79 keV with Stray Light

Due to the structure of the NuSTAR telescope, photons at a large off axis (>1°) can reach the detectors directly (stray light), without passing through the instrument optics. At these off-axis angles NuSTAR essentially turns into a collimated instrument and the spectrum can extend to energies above the Pt K edge (79 keV) of the multilayers, which limits the effective area bandpass of the optics. We present the first scientific spectral analysis beyond 79 keV using a Cygnus X-1 observation in StrayCats: the catalog of stray light observations. This serendipitous stray light observation occurred simultaneously with an INTEGRAL observation. When the spectra are modeled together in the 30-120 keV energy band, we find that the NuSTAR stray light flux is well calibrated and constrained to be consistent with the INTEGRAL flux at the 90% confidence level. Furthermore, we explain how to treat the background of the stray light spectral analysis, which is especially important at high energies.

Keywords: high energy astrophysics; calibration; black hole physics
Origin of Sources of Repeating Fast Radio Bursts with Periodicity in Close Binary Systems

It is proposed that sources of repeating fast radio bursts with detected periodicity are magnetars with extremely short initial spin periods at the protoneutron star stage, formed in binaries after tidal synchronization of their progenitor cores during late stages of thermonuclear burning in very close systems with orbital periods about a few days. This formation channel is in several respects different from evolution of progenitors of normal magnetars, and thus, it explains some differences between repeating and non-repeating sources of fast radio bursts.

Keywords: neutron stars; magnetars; compact objects
Atomic Hydrogen Scaling Relations at z ≈ 0.35

The atomic hydrogen (H I) properties of star-forming galaxies in the local universe are known to correlate with other galaxy properties via the "H I scaling relations." The redshift evolution of these relations serves as an important constraint on models of galaxy evolution. However, until recently, there were no estimates of the H I scaling relations at cosmological distances. Using data from a deep Giant Metrewave Radio Telescope H I 21 cm survey of the Extended Groth Strip, and the technique of spectral-line stacking, we determine the scaling relation between the H I mass and the stellar mass for star-forming galaxies at z ≈ 0.35. We use this measurement, along with the main-sequence relation in galaxies, to infer the dependence of the H I depletion timescale of these galaxies on their stellar mass. We find that massive star-forming galaxies at z ≈ 0.35, with stellar mass M * ≳ 109.5 M ⊙, are H I-poor compared to local star-forming galaxies of a similar stellar mass. However, their characteristic H I depletion time is lower by a factor of ≈5 than that of their local analogs, indicating a higher star formation efficiency at intermediate redshifts (similar to that at z ≈ 1). While our results are based on a relatively small cosmic volume and could thus be affected by cosmic variance, the short characteristic H I depletion timescales (≲3 Gyr) of massive star-forming galaxies at z ≈ 0.35 indicate that they must have acquired a significant amount of neutral gas through accretion from the circumgalactic medium over the past 4 Gyr, to avoid quenching of their star formation activity.

Keywords: galaxy evolution; radio spectroscopy; h i line emission; high-redshift galaxies
Spectral Analysis of Clay-bearing Outcrops in Northern Xanthe Terra, Mars: Comparison with Oxia Planum, the Landing Site for the ExoMars Rover Mission

Clay minerals detected on Mars are valuable targets to seek traces of life on the planet, where biosignatures might be preserved. Here, we report an in-depth spectral analysis of clay-rich outcrops identified in northern Xanthe Terra (300°-320° E, 10°-20° N). We focused particularly on the absorptions centered in the 1.0-2.6 μm spectral range to (1) constrain the mineralogy of the clay outcrops, (2) map their strength and distribution throughout the region, and thus (3) develop a better understanding of the geologic environment at circum-Chryse Planitia. We then compared the infrared signatures in Xanthe Terra and Oxia Planum. Like in Oxia Planum, Xanthe's clays are consistent with either Fe-bearing saponites or vermiculites. However, the spectral signatures in Xanthe are weaker relative to Oxia Planum, perhaps due to significant dust cover in the region. Besides the spectral signatures, northern Xanthe Terra displays several morphological features similar to Oxia Planum, indicating long-lasting aqueous activity (fluvial channels and fan deltas). Clays found at the fan deltas could be detrital (fluvial transport) or authigenic (lacustrine or deltaic sedimentation), while the origin of clays seen elsewhere on the surrounding plateaus remains undetermined. Oxia Planum has been selected as the landing site for ESA's ExoMars "Rosalind Franklin" rover, where its instruments will search for signs of life and constrain the nature and origin of the clays. This exploration will indubitably provide new clues on the clays found in the circum-Chryse basin.

Keywords: mars
Thermodynamic Predictions of Hydrogen Generation during the Serpentinization of Harzburgite with Seawater-derived Brines

Salty aqueous solutions (brines) occur on Earth and may be prevalent elsewhere. Serpentinization represents a family of geochemical reactions where the hydration of olivine-rich rocks can release aqueous hydrogen, H2(aq), as a byproduct, and hydrogen is a known basal electron donor for terrestrial biology. While the effects of lithological differences on serpentinization products have been thoroughly investigated, effects focusing on compositional differences of the reacting fluid have received less attention. In this contribution, we investigate how the chemistry of seawater-derived brines affects the generation of biologically available hydrogen resulting from the serpentinization of harzburgite. We numerically investigate the serpentinization of ultramafic rocks at equilibrium with an array of brines at different water activities (a proxy for salt concentration in aqueous fluids and a determinant for habitability) derived from seawater evaporation. Because the existing supersaturation of aqueous calcium carbonate, a contributor to dissolved inorganic carbon (DIC) in natural seawater, cannot be captured in equilibrium calculations, we bookend our calculations by enabling and suppressing carbonate minerals when simulating serpentinization. We find that the extent of DIC supersaturation can provide an important control of hydrogen availability. Increased DIC becomes a major sink for hydrogen by producing formate and associated complexes when the reacting fluids are acidic enough to allow for CO2. Indeed, H2(aq) reduces CO2(aq) to formate, leading to a hydrogen deficit. These conclusions provide additional insights into the habitability of brine systems, given their potential for serpentinization across diverse planetary bodies such as on Mars and ocean worlds.

Keywords: astrobiology; planetary geology
The Behavior of HgMn Stars in the Far UV-Paper 25: HD 77350

The analysis of five spectra of the Hg Mn spectroscopic binary star HD 77350 recorded by the SWP camera on board the International Ultraviolet Explorer reveals one instance of variation of the far-ultraviolet flux of this star shortwards of 1800 Å. The analysis of the coadded SWP spectrum built from the high resolution SWP spectra obtained through the small aperture shows the importance of low excitation and resonance Si II lines shortwards of 1540 Å, of the Ga II and Ga III resonance lines at 1414 and 1495 Å and of the Al II resonance line at 1670 Å. A very rough estimate of a period from the currently available Transiting Exoplanet Survey Satellite lightcurves yields a value of 9.63 ± 0.18 days which could be the rotation period but this needs confirmation.

Keywords: chemically peculiar stars; mercury-manganese stars
Are the Magnetic Fields Radial in the Solar Polar Region?

We investigate the orientation of the photospheric magnetic fields in the solar polar region using observations from the Helioseismic and Magnetic Imager (HMI). Inside small patches of significant polarization, the inferred magnetic field vectors at 1″ scale appear to systematically deviate from the radial direction. Most tilt toward the pole; all are more inclined toward the plane of sky compared to the radial vector. These results, however, depend on the "filling factor" f that characterizes the unresolved magnetic structures. The default, uninformative f ≡ 1 for HMI will incur larger inclination and less radial fields than f < 1. The observed trend may be a systematic bias inherent to the limited resolution.

Keywords: solar magnetic fields; solar photosphere