adsabs/SciX_UAT_keywords huggingface dataset:
Earth as a Proxy Exoplanet: Simulating DSCOVR/EPIC Observations Using the Earth Spectrum Simulator

Analyzing time-resolved disk-integrated spectral images of the Earth can provide a baseline for future exoplanet characterization. The Earth Polychromatic Imaging Camera (EPIC) onboard the Deep Space Climate Observatory (DSCOVR) provides ~5000 full-disk sunlit Earth images each year in ten wavelengths from the ultraviolet to the near-infrared. A whole-disk radiative transfer model can improve our understanding of the temporal variation of Earth's disk-integrated reflected radiance ("light curves") at different wavelengths and create a pool of possible observations of Earth-like exoplanets. We use the two-stream-exact-single-scattering line-by-line radiative transfer model to build the Earth Spectrum Simulator (ESS) and reconstruct DSCOVR/EPIC spectral observations. Atmospheric effects, such as scattering by air molecules, clouds, aerosols, and gaseous absorption, are included. Surface contributions are treated using appropriate bidirectional reflectance distribution functions. We simulate ~300 images in each channel for observations collected in 2016, with a spatial resolution of ~2000 pixels over the visible disk. ESS provides a simultaneous fit to the observed light curves, with time-averaged reflectance differences typically less than 7% and root-mean-square errors less than 1%. The only exceptions are in the oxygen absorption channels, where reflectance biases can be as large as 19.55%; this is a consequence of simplified assumptions about clouds; especially their vertical placement. We also recover principal components of the spectrophotometric light curves and correlate them with atmospheric and surface features.

Keywords: spectrophotometry; radiative transfer; exoplanets; exoplanet surface variability; exoplanet atmospheres; exoplanet astronomy; habitable planets; earth (planet); exoplanet atmospheric composition; exoplanet atmospheric variability; exoplanet surface characteristics; exoplanet surface composition
Measuring the Soft Excess Region Size Relative to the Corona in Active Galactic Nuclei with NICER

The soft excess is a significant emission component in the soft (<1 keV) X-ray spectra of many active galactic nuclei (AGN). It has been explained by disk reflection, a warm corona, and other models. Understanding its origin is crucial for the energy budget of AGN emission, and for using it to study the inner accretion disk. Here, we track the weeks-to-months variability of several AGN that show different levels of soft excess strength with NICER. We use the variability timescales to compare the relative size of the soft excess emission region to the corona producing the hard X-ray emission above 1 keV. We find that the size of the soft excess emission region relative to the corona is not the same for the three sources studied. For TON S180, the soft excess region is comparable in size to the hard corona. While for MRK 335 and 1H0707-495, the soft excess region is larger than the corona by a factor of 2-4. This is the first time the relative sizes are quantified independently of the assumptions of the spectral models.

Keywords: x-ray active galactic nuclei; active galactic nuclei; seyfert galaxies; supermassive black holes; black hole physics
Exploring Mutual Information between IRIS Spectral Lines. II. Calculating the Most Probable Response in all Spectral Windows

A three-dimensional picture of the solar atmosphere's thermodynamics can be obtained by jointly analyzing multiple spectral lines that span many formation heights. In Paper I, we found strong correlations between spectral shapes from a variety of different ions during solar flares in comparison to the quiet Sun. We extend these techniques to address the following questions: which regions of the solar atmosphere are most connected during a solar flare, and what are the most likely responses across several spectral windows based on the observation of a single Mg II spectrum? Our models are derived from several million IRIS spectra collected from 21 M- and X-class flares. We applied this framework to archetypal Mg II flare spectra and analyzed the results from a multiline perspective. We find that (1) the line correlations from the photosphere to the transition region are highest in flare ribbons. (2) Blueshifted reversals appear simultaneously in Mg II, C II, and Si IV during the impulsive phase, with Si IV displaying possible optical depth effects. Fe II shows signs of strong emission, indicating deep early heating. (3) The Mg II line appears to typically evolve a blueshifted reversal that later returns to line center and becomes single peaked within 1-3 minutes. The widths of these single-peaked profiles slowly erode with time. During the later flare stages, strong red-wing enhancements indicating coronal rain are evident in Mg II, C II, and Si IV. Our framework is easily adaptable to any multiline data set and enables comprehensive statistical analyses of the atmospheric behavior in different spectral windows.

Keywords: solar flare spectra; solar flares; neural networks; bayesian information criterion
Electrons in the Young Solar Wind: First Results from the Parker Solar Probe

The Solar Wind Electrons Alphas and Protons experiment on the Parker Solar Probe (PSP) mission measures the three-dimensional electron velocity distribution function. We derive the parameters of the core, halo, and strahl populations utilizing a combination of fitting to model distributions and numerical integration for ∼100,000 electron distributions measured near the Sun on the first two PSP orbits, which reached heliocentric distances as small as ∼0.17 au. As expected, the electron core density and temperature increase with decreasing heliocentric distance, while the ratio of electron thermal pressure to magnetic pressure (βe) decreases. These quantities have radial scaling consistent with previous observations farther from the Sun, with superposed variations associated with different solar wind streams. The density in the strahl also increases; however, the density of the halo plateaus and even decreases at perihelion, leading to a large strahl/halo ratio near the Sun. As at greater heliocentric distances, the core has a sunward drift relative to the proton frame, which balances the current carried by the strahl, satisfying the zero-current condition necessary to maintain quasi-neutrality. Many characteristics of the electron distributions near perihelion have trends with solar wind flow speed, βe, and/or collisional age. Near the Sun, some trends not clearly seen at 1 au become apparent, including anticorrelations between wind speed and both electron temperature and heat flux. These trends help us understand the mechanisms that shape the solar wind electron distributions at an early stage of their evolution.

Keywords: solar physics; solar wind; slow solar wind
Spectral Background-subtracted Activity Maps

High-resolution solar spectroscopy provides a wealth of information from photospheric and chromospheric spectral lines. However, the volume of data easily exceeds hundreds of millions of spectra on a single observation day. Therefore, methods are needed to identify spectral signatures of interest in multidimensional data sets. Background-subtracted activity maps (BaSAMs) have previously been used to locate features of solar activity in time series of images and filtergrams. This research note shows how this method can be extended and adapted to spectral data.

Keywords: solar physics; solar chromosphere; astronomical methods; spectroscopy; astronomy image processing
Evolutionary Models for the Remnant of the Merger of Two Carbon-Oxygen Core White Dwarfs

We construct evolutionary models of the remnant of the merger of two carbon-oxygen (CO) core white dwarfs (WDs). With total masses in the range 1-2 M⊙, these remnants may either leave behind a single massive WD or undergo a merger-induced collapse to a neutron star (NS). On the way to their final fate, these objects generally experience a ∼10 kyr luminous giant phase, which may be extended if sufficient helium remains to set up a stable shell-burning configuration. The uncertain, but likely significant, mass-loss rate during this phase influences the final remnant mass and fate (WD or NS). We find that the initial CO core composition of the WD is converted to oxygen-neon (ONe) in remnants with final masses ≳1.05 M⊙. This implies that the CO core/ONe core transition in single WDs formed via mergers occurs at a similar mass as in WDs descended from single stars and thus that WD-WD mergers do not naturally provide a route to producing ultramassive CO-core WDs. As the remnant contracts toward a compact configuration, it experiences a "bottleneck" that sets the characteristic total angular momentum that can be retained. This limit predicts that single WDs formed from WD-WD mergers have rotational periods of ≈10-20 minutes on the WD cooling track. Similarly, it predicts remnants that collapse can form NSs with rotational periods ∼10 ms.

Keywords: white dwarf stars; stellar mergers; supernovae; neutron stars
Thermal Instability in Radiation Hydrodynamics: Instability Mechanisms, Position-dependent S-curves, and Attenuation Curves

Local thermal instability can plausibly explain the formation of multiphase gas in many different astrophysical environments, but the theory of local TI is only well-understood in the optically thin limit of the equations of radiation hydrodynamics (RHD). Here, we lay groundwork for transitioning from this limit to a full RHD treatment assuming a gray opacity formalism. We consider a situation where the gas becomes thermally unstable due to the hardening of the radiation field when the main radiative processes are free-free cooling and Compton heating. We identify two ways in which this can happen: (i) when the Compton temperature increases with time, through a rise in either the intensity or energy of a hard X-ray component; and (ii) when attenuation reduces the flux of the thermal component such that the Compton temperature increases with depth through the slab. Both ways likely occur in the broad-line region of active galactic nuclei where columns of gas can be ionization-bounded. In such instances where attenuation is significant, thermal equilibrium solution curves become position-dependent and it no longer suffices to assess the stability of an irradiated column of gas at all depths using a single equilibrium curve. We demonstrate how to analyze a new equilibrium curve-the attenuation curve-for this purpose, and we show that, by Field's instability criterion, a negative slope along this curve indicates that constant-density slabs are thermally unstable whenever the gas temperature increases with depth.

Keywords: active galactic nuclei
A Broad-line Quasar with Unexplained Extreme Velocity Offsets: Post-shock Outflow?

The quasar SDSS 0956 + 5128 exhibits three distinct velocity components with large offsets in emission: the systemic velocity of [O II], [O III], and [Ne III] narrow lines have redshift z = 0.7142; the broad Mg II line is shifted by -1200 km s-1 with respect to the narrow lines; the broad Hα and Hβ lines are at -4100 km s-1. We present new Hubble Space Telescope spectra of Lyα and C IV emission lines and high-resolution images of the quasar. The offsets of these lines are consistent with the velocity component of the Balmer emission, and the photometry in optical and near-infrared wavelengths does not show any signs of recent mergers in the host galaxy or irregularities in the location of the quasar. The data do not confirm predictions of the previous most likely hypotheses involving a special orientation and morphology of the quasar disk, such as in the recoiling black hole scenario, neither it is consistent with accretion disk winds. Instead, based on the cumulative evidence, we propose a new scenario, in which the broad-line region is in the state of outflow caused by a strong shock wave, with a supernova as a possible event for producing the shock ejecta.

Keywords: quasars; shocks; supernovae
(When) Can Wave Heating Balance Optically Thin Radiative Losses in the Corona?

Why the atmosphere of the Sun is orders of magnitudes hotter than its surface is a long standing question in solar physics. Over the years, many studies have looked at the potential role of magnetohydrodynamic (MHD) waves in sustaining these high temperatures. In this study, we use 3D MHD simulations to investigate (driven) transverse waves in a coronal loop. As the boundary-driven transverse waves propagate along the flux tube, the radial density profile leads to resonant absorption (or mode coupling) and phase mixing in the boundaries of the flux tube and the large velocity shears are subject to the Kelvin-Helmholtz instability (KHI). The combination of these effects leads to enhanced energy dissipation and wave heating. Considering both resonant and nonresonant boundary driving as well as different densities for the flux tube, we show that only wave heating associated with a resonant driver in a lower-density loop (with a loop core density ~5 × 10-13 kg m-3) is able to balance radiative losses in the loop shell. Changing the model parameters to consider a denser loop or a driver with a nonresonant frequency, or both, leads to cooling of the coronal loop as the energy losses are greater than the energy injection and dissipation rates.

Keywords: solar coronal loops; solar coronal heating; solar coronal waves
The Periodic Signals of Nova V1674 Herculis (2021)

We present time-series photometry during the early decline phase of the extremely fast nova V1674 Herculis. The 2021 light curve showed periodic signals at 0.152921(3) days and 501.486(5) s, which we interpret as respectively the orbital and white dwarf spin periods in the underlying binary. We also detected a sideband signal at the difference frequency between these two clocks. During the first 15 days of outburst, the spin period appears to have increased by 0.014(1)%. This increase probably arose from the sudden loss of high-angular-momentum gas ("the nova explosion") from the rotating, magnetic white dwarf. Both periodic signals appeared remarkably early in the outburst, which we attribute to the extreme speed with which the nova evolved (and became transparent to radiation from the inner binary). After that very fast initial period increase of 71 ms, the period subsequently decreased-at 182(18) ms yr-1 in 2021, and 88(18) ms yr-1 in 2022. These rates are ~100× faster than typically seen in intermediate polars. This could be due to high accretion torques from very high mass-transfer rates, which might be common when low-mass donor stars are strongly irradiated by a nova outburst.

Keywords: cataclysmic variable stars; classical novae; close binary stars; interacting binary stars; novae; recurrent novae; stellar accretion; stellar accretion disks
The IGRINS YSO Survey. I. Stellar Parameters of Pre-main-sequence Stars in Taurus-Auriga

We present fundamental parameters for 110 canonical K- and M-type (1.3-0.13 M⊙) Taurus-Auriga young stellar objects (YSOs). The analysis produces a simultaneous determination of effective temperature (Teff), surface gravity (log g), magnetic-field strength (B), and projected rotational velocity ( $v\sin i$ ). Our method employed synthetic spectra and high-resolution (R ~ 45,000) near-infrared spectra taken with the Immersion GRating INfrared Spectrometer (IGRINS) to fit specific K-band spectral regions most sensitive to those parameters. The use of these high-resolution spectra reduces the influence of distance uncertainties, reddening, and non-photospheric continuum emission on the parameter determinations. The median total (fit + systematic) uncertainties were 170 K, 0.28 dex, 0.60 kG, 2.5 km s-1 for Teff, log g, B, and $v\sin i$ , respectively. We determined B for 41 Taurus YSOs (upper limits for the remainder) and find systematic offsets (lower Teff, higher log g and $v\sin i$ ) in parameters when B is measurable but not considered in the fit. The average log g for the Class II and Class III objects differs by 0.23 ± 0.05 dex, which is consistent with Class III objects being the more evolved members of the star-forming region. However, the dispersion in log g is greater than the uncertainties, which highlights how the YSO classification correlates with age (log g), yet there are exceptionally young (lower log g) Class III YSOs and relatively old (higher log g) Class II YSOs with unexplained evolutionary histories. The spectra from this work are provided in an online repository along with TW Hydrae Association comparison objects and the model grid used in our analysis.

Keywords: fundamental parameters of stars; infrared sources; pre-main sequence stars; high resolution spectroscopy
LARS XIII: High Angular Resolution 21 cm H I Observations of Lyα Emitting Galaxies

The Lyα emission line is one of the main observables of galaxies at high redshift, but its output depends strongly on the neutral gas distribution and kinematics around the star-forming regions where UV photons are produced. We present observations of Lyα and 21 cm H I emission at comparable scales with the goal to qualitatively investigate how the neutral interstellar medium (ISM) properties impact Lyα transfer in galaxies. We have observed 21 cm H I at the highest possible angular resolution (≈3″ beam) with the Very Large Array in two local galaxies from the Lyman Alpha Reference Sample. We compare these data with Hubble Space Telescope Lyα imaging and spectroscopy, and Multi Unit Spectroscopic Explorer and Potsdam MultiAperture Spectrophotometer ionized gas observations. In LARS08, high-intensity Lyα emission is cospatial with high column density H I where the dust content is the lowest. The Lyα line is strongly redshifted, consistent with a velocity redistribution that allows Lyα escape from a high column density neutral medium with a low dust content. In eLARS01, high-intensity Lyα emission is located in regions of low column density H I, below the H I data sensitivity limit ( < 2 × 1020 cm-2). The perturbed ISM distribution with low column density gas in front of the Lyα emission region plays an important role in the escape. In both galaxies, the faint Lyα emission (~1×10-16 erg s-1cm-2 arcsec-2) traces intermediate Hα emission regions where H I is found, regardless of the dust content. Dust seems to modulate, but not prevent, the formation of a faint Lyα halo. This study suggests the existence of scaling relations between dust, Hα, H I, and Lyα emission in galaxies.

Keywords: interstellar atomic gas; lyman-alpha galaxies; starburst galaxies; interacting galaxies
Event Rate of Strongly Lensed Gravitational Waves of Stellar Binary Black Hole Mergers Produced by Dynamical Interactions

Gravitational waves emitted from stellar binary black hole (sBBH) mergers can be gravitationally lensed by intervening galaxies and detected by future ground-based detectors. A great amount of effort has been put into the estimation of the detection rate of lensed sBBH originating from the evolution of massive binary stars (EMBS channel). However, sBBHs produced by the dynamical interaction in dense clusters (dynamical channel) may also be dominant in our universe and their intrinsic distribution of physical properties can be significantly different from those produced by massive stars, especially mass and redshift distribution. In this paper, we investigate the event rate of lensed sBBHs produced via the dynamical channel by Monte Carlo simulations and the number is ${16}_{-12}^{+4.7}$ yr-1 for the Einstein telescope and ${24}_{-17}^{+6.8}$ yr-1 for Cosmic Explorer, of which the median is about ~2 times the rate of sBBHs originating from the EMBS channel (calibrated by the local merger rate density estimated for the dynamical and the EMBS channel, i.e., $\sim {14}_{-10}^{+4.0}$ and ${19}_{-3.0}^{+42}\,\,{\mathrm{Gpc}}^{-3}\,{\mathrm{yr}}^{-1}$ , respectively). Therefore, one may constrain the fraction of both the EMBS and dynamical channels through the comparison of the predicted and observed number of lensed sBBH events statistically.

Keywords: gravitational wave astronomy
An Experimental and Theoretical Investigation into the Formation of Ketene (H2CCO) and Ethynol (HCCOH) in Interstellar Analog Ices

The formation of isomers of C2H2O—ketene (H2CCO), ethynol (HCCOH), and oxirene (c-CHCHO)—was investigated in interstellar ice analogs composed of carbon monoxide and water. Using tunable photoionization time-of-flight mass spectrometry to selectively ionize the isomer of interest, ketene and ethynol were detected as reaction products, but oxirene remains elusive. These findings demonstrate that organic compounds that are precursors to complex organic molecules can form without an organic source of carbon. Furthermore, we report the first plausible detection of ethynol in astrophysically relevant ices. These investigations were supported by theoretical calculations describing reaction energies, pathways, ionization energies, and harmonic frequencies.

Keywords: laboratory astrophysics; interdisciplinary astronomy; chemical abundances; interstellar molecules; astrochemistry
Magnetic Reconnection Invoked by Sweeping of the CME-driven Fast-mode Shock

Coronal waves exist ubiquitously in the solar atmosphere. They are important not only because of their rich physics but also because they are essential candidates for triggering remote magnetic eruptions. However, the latter mechanism has never been directly confirmed. By revisiting the successive eruptions on 2012 March 7, fast-mode shocks are identified to account for the X5.4 flare-related Extreme UltraViolet wave with a velocity of 550 km s-1, and appeared faster than 2060 ± 270 km s-1 at the front of the corresponding coronal mass ejection in the slow-rising phase. They not only propagated much faster than the local Alfvén speed of about 260 km s-1, but also were simultaneously accompanied by a type II radio burst, i.e., a typical feature of a shock wave. The observations show that the shock wave disturbs the coronal loops C1 connecting active regions (ARs) 11429 and 11430, which neighbor a null point region. Following a 40 minute oscillation, an external magnetic reconnection (EMR) occurred in the null point region. About 10 minutes later, a large-scale magnetic flux rope overlaid by C1 became unstable and erupted quickly. It is thought that the fast-mode shock triggered EMR in the null point region and caused the subsequent eruptions. This scenario is directly observed for the first time, and provides new hints for understanding the physics of solar activities and eruptions.

Keywords: solar magnetic reconnection; solar magnetic fields; solar coronal waves; solar activity; solar flares
Multicolor Photometry of the Semiregular Variable V698 Tau

We present photometric B, V, and R observations of the semiregular variable V698 Tau taken in 2020-2021. The star varies by ∼1 mag in each filter with a mean period of ∼185 days. At minimum light the star is noticeably redder in V - R than at maximum while slightly bluer in B - V. The magnitudes and colors are consistent with those of a reddened M8 giant.

Keywords: semi-regular variable stars