adsabs/SciX_UAT_keywords huggingface dataset:
The Radiation Environments of Middle-aged F-type Stars

Far-ultraviolet (FUV) emission lines from dwarf stars are important driving sources of photochemistry in planetary atmospheres. Properly interpreting spectral features of planetary atmospheres critically depends on the emission of its host star. While the spectral energy distributions (SEDs) of K- and M-type stars have been extensively characterized by previous observational programs, the full X-ray to infrared SED of F-type stars has not been assembled to support atmospheric modeling. On the second flight of the Suborbital Imaging Spectrograph for Transition-region Irradiance from Nearby Exoplanet host stars (SISTINE-2) rocket-borne spectrograph, we successfully captured the FUV spectrum of Procyon A (F5 IV-V) and made the first simultaneous observation of several emission features across the FUV bandpass (1010-1270 and 1300-1565 Å) of any cool star. We combine flight data with stellar models and archival observations to develop the first SED of a mid-F star. We model the response of a modern Earth-like exoplanet's upper atmosphere to the heightened X-ray and extreme UV radiation within the habitable zone of Procyon A. These models indicate that this planet would not experience significant atmospheric escape. We simulate observations of the Lyα transit signal of this exoplanet with the Hubble Space Telescope (HST) and the Habitable Worlds Observatory (HWO). While marginally detectable with HST, we find that H I Lyα transits of potentially habitable exoplanets orbiting high radial velocity F-type stars could be observed with HWO for targets up to 150 pc away.

Keywords: hubble space telescope; late-type dwarf stars; astronomical instrumentation; exoplanet atmospheres; spectral energy distribution; ultraviolet spectroscopy; space observatories; radiative transfer
Observational Characteristics of Radiation-mediated Shocks in Photospheric Gamma-Ray Burst Emission

Emission from the photosphere in gamma-ray burst jets can be substantially affected by subphotospheric energy dissipation, which is typically caused by radiation-mediated shocks. We study the observational characteristics of such emission, in particular the spectral signatures. Relevant shock initial conditions are estimated using a simple internal collision framework, which then serve as inputs for a radiation-mediated shock model that generates synthetic photospheric spectra. Within this framework, we find that if the free fireball acceleration starts at r 0 ~ 1010 cm, in agreement with hydrodynamical simulations, then the typical spectrum consists of a broad, soft power-law segment with a cutoff at high energies and a hardening in X-rays. The synthetic spectra are generally well fitted with a standard cutoff power-law (CPL) function, as the hardening in X-rays is commonly outside the observable energy range of current detectors. The CPL-fits yield values for the low-energy index, α, and the peak energy, E peak, that are centered around ~ -0.8 and ~220 keV, respectively, similar to typical observed values. We also identify a nonnegligible parameter region for what we call optically shallow shocks: shocks that do not accumulate enough scatterings to reach a steady-state spectrum before decoupling and thereby produce more complex spectra. These occur for optical depths $\tau \lesssim 55\,{u}_{u}^{-2}$ , where u u = γ u β u is the dimensionless specific momentum of the upstream as measured in the shock rest frame.

Keywords: gamma-ray bursts; shocks
Formation of Chromospheric Spicules in Magnetic Bright Points: An Analytical Approach Using Cartesian Slab Geometry

We aim to provide insight into chromospheric spicules by suggesting a new formation mechanism. A magnetic field boundary condition is imposed, generating an Alfvén wave that shears a magnetic slab and propagates up the slab. The resulting Lorentz force accelerates material vertically, potentially nonlinearly driving a jet-like feature. This formation mechanism is applied to take place in a magnetic bright point embedded in the photosphere, providing motivation to use the simplifying assumption of a zero-β plasma. After deriving an analytical expression describing the vertical mass flux that constitutes the spicular jet, further understanding is gained by examining a model example of a magnetic field boundary condition in terms of standard functions. By visualizing the vertical mass flux through 3D plots, we demonstrate that the jet properties capture the observed properties of chromospheric spicules during their formation. This vindicates the model and simplifying assumptions used. Although we do not provide insight into the full evolution of a spicule, we show that the role of Alfvén waves triggered by shear in fact could be a viable formation mechanism for at least some chromospheric spicules. Consequently, we provide a starting point for further studies of this formation mechanism, which will lead to a greater understanding of the vast variety of chromospheric jets.

Keywords: solar spicules; solar chromosphere; alfven waves; solar photosphere; magnetohydrodynamics; solar physics; the sun; solar magnetic bright points
Momentum Enhancement during Kinetic Impacts in the Low-intermediate-strength Regime: Benchmarking and Validation of Impact Shock Physics Codes

In 2022 September, the DART spacecraft (NASA's contribution to the Asteroid Impact & Deflection Assessment (AIDA) collaboration) will impact the asteroid Dimorphos, the secondary in the Didymos system. The crater formation and material ejection will affect the orbital period. In 2027, Hera (ESA's contribution to AIDA) will investigate the system, observe the crater caused by DART, and characterize Dimorphos. Before Hera's arrival, the target properties will not be well-constrained. The relationships between observed orbital change and specific target properties are not unique, but Hera's observations will add additional constraints for the analysis of the impact event, which will narrow the range of feasible target properties. In this study, we use three different shock physics codes to simulate momentum transfer from impactor to target and investigate the agreement between the results from the codes for well-defined target materials. In contrast to previous studies, care is taken to use consistent crushing behavior (e.g., distension as a function of pressure) for a given porosity for all codes. First, we validate the codes against impact experiments into a regolith simulant. Second, we benchmark the codes at the DART impact scale for a range of target material parameters (10%-50% porosity, 1.4-100 kPa cohesion). Aligning the crushing behavior improves the consistency of the derived momentum enhancement between the three codes to within +/-5% for most materials used. Based on the derived mass-velocity distributions from all three codes, we derive scaling parameters that can be used for studies of the ejecta curtain.

Keywords: impact phenomena; a subgiant stars; near-earth objects; asteroid satellites; planetary science
Inferring the Solar Wind Velocity in the Outer Corona Based on Multiview Observations of Small-scale Transients by STEREO/COR2

Based on the Heliospheric Imager-1 images of the STEREO twin spacecraft, we established the CORrelation-Aided Reconstruction (CORAR) technique to locate and reconstruct the 3D structures of solar wind transients in interplanetary space. Here, we extend the CORAR method to images of COR2 on board STEREO to study the evolution of small-scale transients in the outer corona from 2010 January to May. We confirm that the transients can be located and reconstructed well by comparing the results with those of a self-similar expanding model. The speed distribution of the reconstructed transients generally shows the typical characteristics of the slow solar wind. We further study the sources of the transients on the Sun, and find that most reconstructed transients are located near the top of streamer belts or the heliospheric current sheet and can be tracked back to the boundaries of the closed-field and open-field regions along the field lines extrapolated by corona models. The formation mechanisms of these transients in the slow solar wind are also discussed.

Keywords: solar coronal transients; coronagraphic imaging; solar wind
A Model for the Possible Connection Between a Tidal Disruption Event and Quasi-periodic Eruption in GSN 069

Quasi-periodic eruptions (QPEs) are found in the center of five galaxies, where a tidal disruption event (TDE)-like event has been reported in GSN 069, which occurred a couple of years before the QPEs. We explain the connection of these phenomena based on a model of a highly eccentric white dwarf (WD) 104-6 M ⊙ massive black hole (MBH) binary formed by the Hill mechanism. In this system, the tidally induced internal oscillation of a WD can heat the WD envelope thereby inducing tidal nova and inflating the WD envelope, which can be captured by the MBH and form a TDE. The tidal stripping of the surviving WD in the eccentric orbit can produce QPEs. We also apply this model to the other four QPE sources. Based on the estimated fallback rate, we find that the remaining time after the QPE-observed time for these QPEs is only around 1-2 yr based on our simple model estimation, after which the WD will be fully disrupted. We also show that the accretion rate can be much higher than the Eddington accretion rate in the final stage of these QPE sources. The peak frequency of the spectral energy distribution of the disk stays in the soft X-ray band (~0.1-1 keV), which is consistent with observational results.

Keywords: accretion; gravitational wave sources; white dwarf stars; tidal disruption; black hole physics
Inferences Regarding a Stellar Metallicity Distribution Following a Single Primordial Chemical Enrichment Event

An idealized scenario is presented according to which a power-law distribution in metallicity might have characterized the first metal-poor stars. The scenario assumes an initial synthesis of heavy elements within a first source of metals, followed by a subsequent ejection of these metals and their dilution within a metal-free gas environment.

Keywords: stellar abundances; stellar populations; stellar nucleosynthesis
Late Type Stars in the Michigan Spectral Catalogue: Conflation of Luminosity Classes IV and V

While computing a comprehensive set of mean values and uncertainties for key stellar parameters, particularly absolute V magnitudes and (B - V)0 colors, for main sequence stars drawn from the Michigan Spectral Catalogue (MSC), we discovered a significant co-mingling between luminosity classes IV and V in spectral types G8 and later. Indeed, among the ~200 nearby K0 dwarfs examined, as many 20% may be misclassified subgiants. We present data to substantiate this view and offer some commentary on the observed conflation of these two luminosity classes. We conclude that while the MSC is a unique and largely reliable database for spectral classifications of predominantly southern hemisphere HD stars across the H-R diagram, it does contain some ambiguities in the assignment of luminosity classes IV and V among the late G and early K stars of which users should be aware.

Keywords: hertzsprung russell diagram; k dwarf stars; k subgiant stars
Mass–Radius Relationships and Contraction of Condensed Planets by Cooling or Despinning

Condensed planets contract or expand as their temperature changes. With the exception of the effect of phase changes, this phenomenon is generally interpreted as being solely related to the thermal expansivity of the planet's components. However, changes in density affect pressure and gravity and, consequently, the planet's compressibility. A planet's radius is also linked to its rate of rotation. Here again, changes in pressure, gravity, and compressibility are coupled. In this article we clarify how the radius of a condensed planet changes with temperature and rotation, using a simple and rigorous thermodynamic model. We consider condensed materials to obey a simple equation of state which generalizes a polytopic EoS as temperature varies. Using this equation, we build simple models of condensed planet's interiors including exoplanets, derive their mass–radius relationships, and study the dependence of their radius on temperature and rotation rate. We show that it depends crucially on the value of ρ s gR/K s (ρ s being surface density, g gravity, R radius, K s surface incompressibility). This nondimensional number is also the ratio of the dissipation number which appears in compressible convection and the Gruneïsen mineralogic parameter. While the radius of small planets depends on temperature, this is not the case for large planets with large dissipation numbers; Earth and a super-Earth like CoRoT-7b are in something of an intermediate state, with a moderately temperature-dependent radius. Similarly, while the radius of these two planets is a function of their rotation rates, this is not the case for smaller or larger planets.

Keywords: solar system terrestrial planets; planetary thermal histories; extrasolar rocky planets
Galaxy Clusters from the DESI Legacy Imaging Surveys. II. Environmental Effects on the Size–Mass Relation

To investigate the environmental effects on the growth of galaxies, we study the size–mass relation across a broad range of environments with a vast sample of approximately 32 million galaxies at z < 0.5 from the DESI Legacy Imaging Surveys. This sample is divided into three subsamples representing galaxies within three different environments: brightest cluster galaxies (BCGs), other cluster galaxies, and field galaxies. The BCGs in our large sample are dominated by quiescent galaxies (QGs), while only a minority (∼13%) of BCGs are star-forming galaxies (SFGs). To demonstrate the influence of the environment on size growth, we attempt to observe the difference in size–mass relation for these three subsamples. In general, the slope differences between QGs and SFGs within various environments are significant and tend to be greater at higher redshifts. For the mass-complete subsamples at z < 0.5, BCGs are found to have the highest slope of size–mass relation, and no difference in size–mass relation is found between cluster members and field galaxies. To assess whether the observed slope differences stem from the variations in environment or mass distribution, we construct the mass-matched subsamples for QGs and SFGs. As a result, both QGs and SFGs show negligible differences in the slope of the size–mass relation among the galaxies within three distinct environments, indicating that stellar mass is the most fundamental factor driving the size evolution at z < 0.5, though the mass growth mode for QGs and SFGs may have been affected by galaxy environment.

Keywords: galaxy evolution; galaxy environments; brightest cluster galaxies
Bumpy Declining Light Curves Are Common in Hydrogen-poor Superluminous Supernovae

Recent work has revealed that the light curves of hydrogen-poor (Type I) superluminous supernovae (SLSNe), thought to be powered by magnetar central engines, do not always follow the smooth decline predicted by a simple magnetar spin-down model. Here we present the first systematic study of the prevalence and properties of "bumps" in the post-peak light curves of 34 SLSNe. We find that the majority (44%-76%) of events cannot be explained by a smooth magnetar model alone. We do not find any difference in supernova properties between events with and without bumps. By fitting a simple Gaussian model to the light-curve residuals, we characterize each bump with an amplitude, temperature, phase, and duration. We find that most bumps correspond with an increase in the photospheric temperature of the ejecta, although we do not see drastic changes in spectroscopic features during the bump. We also find a moderate correlation (ρ ≈ 0.5; p ≈ 0.01) between the phase of the bumps and the rise time, implying that such bumps tend to happen at a certain "evolutionary phase," (3.7 ± 1.4)t rise. Most bumps are consistent with having diffused from a central source of variable luminosity, although sources further out in the ejecta are not excluded. With this evidence, we explore whether the cause of these bumps is intrinsic to the supernova (e.g., a variable central engine) or extrinsic (e.g., circumstellar interaction). Both cases are plausible, requiring low-level variability in the magnetar input luminosity, small decreases in the ejecta opacity, or a thin circumstellar shell or disk.

Keywords: circumstellar matter; circumstellar shells; magnetars; supernovae
Investigation of Surface Effects of Simple Flux Tubes Using Numerical Simulations

We use the SPARC code for MHD simulations with monolithic flux tubes of varying subsurface topology. Our studies involve the interactions of waves caused by a single source with subsurface magnetic fields. Mode conversion causing acoustic power to trickle downwards along the flux tube has been described before and can be visualized in our simulations. We show that this downward propagation causes the flux tube to act as an isolated source, creating a characteristic surface wave field. Measuring this wave field at the surface reveals subsurface properties of the magnetic field topology. Using time-distance helioseismology, we demonstrate how to detect such a flux tube signal based on a group travel time delay of Δt = 282.6 s due to the wave packet spending time subsurface as a slow mode wave. Although the amplitude is small and generally superimposed by the full wave field, it can be detected if assumptions about Δt are made. We demonstrate this for a simulation with solar-like sources. This kind of study has the potential to reveal subsurface information of sunspots based on the analysis of a surface signal.

Keywords: magnetohydrodynamics; magnetohydrodynamical simulations; sunspots; solar physics; solar magnetic fields; helioseismology
Neutrinos from the Galactic Center Hosting a Hypernova Remnant

Similar to star-forming galaxies or starburst galaxies, star-forming regions in our Galaxy can host cosmic-ray (CR) accelerators and rich gas as targets of hadronuclear interaction. By our estimations, the IceCube neutrino observatory might detect muon neutrinos from a CR accelerator associated with a molecular cloud complex in our Galaxy. The associated high-energy gamma-ray emission might be observed by the Cherenkov Telescope Array (CTA), High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC), and Large High Altitude Air Shower Observatory (LHAASO). Furthermore, taking the Galactic Center (GC) region as an example, we assume that a hypernova exploded in the past in the GC. We simulate the acceleration of CRs in the hypernova remnant (HNR) as well as their confinement and escape. The high-energy protons escape from the HNR, diffuse around the GC, interact with molecular clouds, and then produce gamma-rays and neutrinos. In the optimal cases, the GC would be a promising 100 TeV gamma-ray source for LHAASO's one-month observation. We propose that neutrino-induced searching for starting track-like and high-energy starting events (HESEs) observed by IceCube, from the GC region with a radius of 1°8, would help us discover the particle accelerator in the GC or constrain our models. Under the constraint from high-energy gamma-ray observations by the H.E.S.S. telescope, we estimate the exposure time needed to make a significant discovery for the optimal cases. The analysis combining observations of IceCube and ANTARES, starting track-like events and HESEs, future observations by neutrino detectors IceCube-Gen2 and KM3net, and gamma-ray telescopes CTA, HAWC, and LHAASO would help to constrain our models.

Keywords: neutrino astronomy; galactic center; hypernovae; high energy astrophysics; supernova remnants; molecular clouds; neutrino telescopes; gamma-ray astronomy; gamma-ray telescopes; cosmic rays; hydrodynamical simulations; extended radiation sources
What Is the Role of Stellar Radiative Feedback in Setting the Stellar Mass Spectrum?

In spite of decades of theoretical efforts, the physical origin of the stellar initial mass function (IMF) is still debated. Particularly crucial is the question of what sets the peak of the distribution. To investigate this issue, we perform high-resolution numerical simulations with radiative feedback exploring, in particular, the role of the stellar and accretion luminosities. We also perform simulations with a simple effective equation of state (EOS), and we investigate 1000 solar-mass clumps having, respectively, 0.1 and 0.4 pc of initial radii. We found that most runs, both with radiative transfer or an EOS, present similar mass spectra with a peak broadly located around 0.3-0.5 M⊙ and a power-law-like mass distribution at higher masses. However, when accretion luminosity is accounted for, the resulting mass spectrum of the most compact clump tends to be moderately top-heavy. The effect remains limited for the less compact one, which overall remains colder. Our results support the idea that rather than the radiative stellar feedback, this is the transition from the isothermal to the adiabatic regime, which occurs at a gas density of about 1010 cm-3, that is responsible for setting the peak of the IMF. This stems from (i) the fact that extremely compact clumps for which the accretion luminosity has a significant influence are very rare and (ii) the luminosity problem, which indicates that the effective accretion luminosity is likely weaker than expected.

Keywords: initial mass function; star formation; collapsing clouds; stellar feedback; stellar accretion; radiative transfer; hydrodynamical simulations
Habitable Zones around Almost Extremely Spinning Black Holes (Black Sun Revisited)

We analyzed the thermodynamics of hypothetical exoplanets at very low Keplerian circular orbits in close vicinity of rapidly spinning supermassive black holes. Such black hole exoplanets are heated by strongly blueshifted and focused flux of the incoming cosmic microwave background (CMB) and cooled by the cold part of the local sky containing the black hole shadow. This gives rise to a temperature difference, which can drive processes far from thermodynamic equilibrium in a hypothetical life form inhabiting black hole exoplanets, similar to the case of a planet heated by the radiation of the parent star and cooled by the night sky. We found that for a narrow range of radii of very low Keplerian circular orbits and for very high spin of a supermassive black hole, the temperature regime of the black hole exoplanets corresponds to the habitable zone around standard stars. The thermodynamics of black hole exoplanets therefore, in principle, does not exclude the existence of life based on known biology. The peak of the multiblackbody spectral profile of the CMB heating the exoplanet is located in the ultraviolet band, but a significant fraction of the flux comes also in the visible and infrared bands. The minimum mass of a black hole ensuring the resistance to tidal disruption of an Earth-like exoplanet orbiting in the habitable zone is estimated to 1.63 × 108 m⊙.

Keywords: kerr black holes; supermassive black holes; exoplanets; gravitational lensing; cosmic microwave background radiation