adsabs/SciX_UAT_keywords huggingface dataset:
Cloud Parameterizations and their Effect on Retrievals of Exoplanet Reflection Spectroscopy

Future space-based direct imaging missions will perform low-resolution (R < 100) optical (0.3-1 μm) spectroscopy of planets, thus enabling reflected spectroscopy of cool giants. Reflected light spectroscopy is encoded with rich information about the scattering and absorbing properties of planet atmospheres. Given the diversity of clouds and hazes expected in exoplanets, it is imperative that we solidify the methodology to accurately and precisely retrieve these scattering and absorbing properties that are agnostic to cloud species. In particular, we focus on determining how different cloud parameterizations affect resultant inferences of both cloud and atmospheric composition. We simulate mock observations of the reflected spectra from three top-priority direct imaging cool giant targets with different effective temperatures, ranging from 135 to 533 K. We perform retrievals of cloud structure and molecular abundances on these three planets using four different parameterizations, each with an increasing level of cloud complexity. We find that the retrieved atmospheric and scattering properties depend strongly on the choice of cloud parameterization. For example, parameterizations that are too simplistic tend to overestimate the abundances. Overall, we are unable to retrieve precise/accurate gravity beyond ±50%. Lastly, we find that even reflected light spectroscopy with a low signal-to-noise ratio of 5 and low R = 40 gives cursory zeroth-order insights into the position of the cloud deck relative to the molecular and Rayleigh optical depth level.

Keywords: exoplanet atmospheres; atmospheric clouds; exoplanet atmospheric composition
MAVEN Observations of Whistler-mode Waves Within the Magnetic Dips in the Martian Ionopause/Ionosphere

Whistler-mode waves are natural and common electromagnetic emissions observed both surrounding planets and their moons with and without intrinsic magnetic field. Numerous observations have shown that the magnetic dip is a favorable region for the whistler-mode wave generation in the Earth's magnetosphere. However, up to now, seldom observations of these waves have been reported in such regions at Mars. Based on the measurements from Mars Atmosphere and Volatile EvolutioN mission, quasi-parallel propagating whistler-mode waves are observed within magnetic dip structures in the Martian ionopause and ionosphere. Correspondingly, significant electron beams in the antiparallel direction are observed, and a linear instability analysis shows that f pe/f ce becomes extremely large (~1500) inside the magnetic dip. Thus, the electron minimum resonant energy of whistler-mode waves decreases to several eV, which increases the number density of resonant beam electrons. Such beam electron distribution results in the necessary free energy for the whistler-mode wave growth. Our study indicates magnetic dips may be favorable regions for the whistler-mode wave excitation even in the Martian ionosphere, which has an extremely high f pe/f ce but does not have a global intrinsic magnetic field.

Keywords: space plasmas; planetary magnetospheres; magnetospheric radio emissions
Modeling Grain Rotational Disruption by Radiative Torques and Extinction of Active Galactic Nuclei

Extinction curves observed toward individual Active Galactic Nuclei (AGN) usually show a steep rise toward far-ultraviolet (FUV) wavelengths and can be described by the Small Magellanic Cloud (SMC)-like dust model. This feature suggests the dominance of small dust grains of size a ≤ 0.1 μm in the local environment of AGN, but the origin of such small grains is unclear. In this paper, we aim to explain this observed feature by applying the RAdiative Torque Disruption (RATD) to model the extinction of AGN radiation from FUV to mid-infrared (MIR) wavelengths. We find that in the intense radiation field of AGN, large composite grains of size a ≥ 0.1 μm are significantly disrupted to smaller sizes by RATD up to dRATD > 100 pc in the polar direction and dRATD ~ 10 pc in the torus region. Consequently, optical-MIR extinction decreases, whereas FUV-near-ultraviolet extinction increases, producing a steep far-UV rise extinction curve. The resulting total-to-selective visual extinction ratio thus significantly drops to RV < 3.1 with decreasing distances to AGN center due to the enhancement of small grains. The dependence of RV with the efficiency of RATD will help us to study the dust properties in the AGN environment via photometric observations. In addition, we suggest that the combination of the strength between RATD and other dust destruction mechanisms that are responsible for destroying very small grains of a ≤ 0.05 μm is the key for explaining the dichotomy observed "SMC" and "gray" extinction curve toward many AGN.

Keywords: active galactic nuclei; astrophysical dust processes; interstellar dust extinction
Analyzing the Habitable Zones of Circumbinary Planets Using Machine Learning

Exoplanet detection in the past decade by efforts including NASA's Kepler and TESS missions has revealed many worlds that differ substantially from planets in our own solar system, including more than 150 exoplanets orbiting binary or multi-star systems. This not only broadens our understanding of the diversity of exoplanets, but also promotes our study of exoplanets in the complex binary systems and provides motivation to explore their habitability. In this study, we investigate the habitable zones of circumbinary planets (P-type) based on planetary trajectory and dynamically informed habitable zones. Our results indicate that the mass ratio and orbital eccentricity of binary stars are important factors affecting the orbital stability and habitability of planetary systems. Moreover, planetary trajectory and dynamically informed habitable zones divide planetary habitability into three categories: habitable, periodic habitable, and non-habitable. Therefore, we successfully train a machine-learning model to quickly and efficiently classify these planetary systems, which provides more useful constraints.

Keywords: exoplanet evolution; habitable zone; binary stars
Dynamical Response of Solar Wind Charge Exchange Soft X-Ray Emission in Earth's Magnetosphere to the Solar Wind Proton Flux

This work studies the dynamic response of solar wind charge exchange (SWCX) soft X-ray emission in the Earth's magnetosphere to the solar wind proton flux. Unlike previous studies that attempted to use complex magnetohydrodynamic models to match the details of observed SWCX of a necessarily limited number of cases, this work focuses on determining the changes over individual observations in a much larger sample. To provide the cleanest test, we selected XMM-Newton observations when the solar wind proton flux changed suddenly by a factor greater than 1.5 and calculated the correlation coefficient between the SWCX emission in the 0.5-0.7 keV band and the proton flux. We find that the dynamical response is weak when the solar wind proton flux is low (<10,000 n*km/cc/s) because its variation is smaller than the uncertainty due to other emission components, but this response increases with the proton flux and its change value. The response is improved when the valence state of solar wind ions is high, as a higher abundance of ions generating SWCX can produce a greater correlation even though the proton flux is relatively low. It is conducive to the study of interplanetary coronal mass ejections (ICMEs) because ions in ICMEs are usually highly ionized. For XMM-Newton, the 0.5-0.7 keV band shows the strongest correlation, as the instrumental response decreases at lower energies and the SWCX emission decreases at higher energies. Moreover, the closer the satellite line of sight is to the subsolar magnetopause with the strongest SWCX emissivity, the better the correlation.

Keywords: x-ray astronomy; diffuse x-ray background; solar coronal mass ejections; solar wind; solar-terrestrial interactions
Statistical Analyses of Solar Prominences and Active Region Features in 304 Å Filtergrams Detected via Deep Learning

Solar active regions (ARs) are areas on the Sun with very strong magnetic fields where various activities take place. Prominences are one of the typical solar features in the solar atmosphere, whose eruptions often lead to solar flares and coronal mass ejections. Therefore, studying their morphological features and their relationship with solar activity is useful in predicting eruptive events and in understanding the long-term evolution of solar activities. A huge amount of data have been collected from various ground-based telescopes and satellites. The massive amounts of data make human inspection difficult. For this purpose, we developed an automated detection method for prominences and ARs above the solar limb based on deep-learning techniques. We applied it to process the 304 Å data obtained by SDO/AIA from 2010 May 13 to 2020 December 31. Besides the butterfly diagrams and latitudinal migrations of the prominences and ARs during solar cycle 24, the variations of their morphological features (such as the locations, areas, heights, and widths) with the calendar years and the latitude bands were analyzed. Most of these statistical results based on our new method are in agreement with previous studies, which also guarantees the validity of our method. The N–S asymmetry indices of the prominences and ARs show that the northern hemisphere is the dominant hemisphere in solar cycle 24, except for 2012–2015, and 2020 for ARs. The high-latitude prominences show much stronger N–S asymmetry, where the northern hemisphere is dominant in ∼2011 and ∼2015, and the southern hemisphere is dominant from 2016–2019.

Keywords: solar prominences; solar active regions; convolutional neural networks; solar cycle
Classifying Be Star Variability With TESS. I. The Southern Ecliptic

TESS photometry is analyzed for 430 classical Be stars observed in the first year of the mission. The often complex and diverse variability of each object in this sample is classified to obtain an understanding of the behavior of this class as a population. Ninety-seven percent of the systems are variable above the noise level, with timescales spanning nearly the entire range of what is accessible with TESS, from tens of minutes to tens of days. The variability seen with TESS is summarized as follows. Nearly every system contains multiple periodic signals in the frequency regime between about 0.5 and 4 day-1. One or more groups of closely spaced frequencies is the most common feature, present in 87% of the sample. Among the Be stars with brightening events that are characteristic of mass ejection episodes (18% of the full sample, or 31% of early-type stars), all have at least one frequency group, and the majority of these (83%) show a concurrent temporary amplitude enhancement in one or more frequency groups. About one-third (34%) of the sample is dominated by longer-term trends (timescales >2 day). Low-frequency stochastic signals are prominent in about 25% of the sample, with varying degrees of intensity. Higher-frequency signals (6 < f < 15 day-1) are sometimes seen (in 15% of the sample) and in most cases likely reflect p-mode pulsation. In rare cases (~3%), even higher frequencies beyond the traditional p-mode regime (f > 15 day-1) are observed.

Keywords: be stars; pulsating variable stars; stellar mass loss; photometry; light curves; early-type emission stars; circumstellar disks; multi-periodic variable stars
Salt, Hot Water, and Silicon Compounds Tracing Massive Twin Disks

We report results of $0\buildrel{\prime\prime}\over{.} 05$-resolution observations toward the O-type proto-binary system IRAS 16547-4247 with the Atacama Large Millimeter/submillimeter Array. We present dynamical and chemical structures of the circumbinary disk, circumstellar disks, outflows, and jets, illustrated by multi-wavelength continuum and various molecular lines. In particular, we detect sodium chloride, silicon compounds, and vibrationally excited water lines as probes of the individual protostellar disks at a scale of 100 au. These are complementary to typical hot-core molecules tracing the circumbinary structures on a 1000 au scale. The H2O line tracing inner disks has an upper-state energy of ${E}_{u}/k\gt 3000\,{\rm{K}}$, indicating a high temperature of the disks. On the other hand, despite the detected transitions of NaCl, SiO, and SiS not necessarily having high upper-state energies, they are enhanced only in the vicinity of the protostars. We posit that these molecules are the products of dust destruction, which only happens in the inner disks. This is the second detection of alkali metal halide in protostellar systems after the case of the disk of Orion Source I, and also one of few massive protostellar disks associated with high-energy transition water and silicon compounds. These new results suggest that these "hot-disk" lines may be common in innermost disks around massive protostars, and have great potential for future research of massive star formation. We also tentatively find that the twin disks are counter-rotating, which might give a hint of the origin of the massive proto-binary system IRAS 16547-4247.

Keywords: radio jets; circumstellar disks; massive stars; star formation; astrochemistry; binary stars; protostars; young stellar objects; stellar jets
Data-driven Modeling of a Coronal Magnetic Flux Rope: From Birth to Death

Magnetic flux ropes are a bundle of twisted magnetic field lines produced by internal electric currents, which are responsible for solar eruptions and are the major drivers of geomagnetic storms. As such, it is crucial to develop a numerical model that can capture the entire evolution of a flux rope, from its birth to death, in order to predict whether adverse space weather events might occur or not. In this paper, we develop a data-driven modeling that combines a time-dependent magnetofrictional approach with a thermodynamic magnetohydrodynamic model. Our numerical modeling successfully reproduces the formation and confined eruption of an observed flux rope, and unveils the physical details behind the observations. Regarding the long-term evolution of the active region, our simulation results indicate that the flux cancellation due to collisional shearing plays a critical role in the formation of the flux rope, corresponding to a substantial increase in magnetic free energy and helicity. Regarding the eruption stage, the deformation of the flux rope during its eruption can cause an increase in the downward tension force, which suppresses it from further rising. This finding may shed light on why some torus-unstable flux ropes lead to failed eruptions after large-angle rotations. Moreover, we find that twisted fluxes can accumulate during confined eruptions, which would breed the subsequent eruptive flares.

Keywords: magnetohydrodynamical simulations; solar coronal mass ejections; solar magnetic fields; solar flares
TESS Observations of TX Col: Rapidly Varying Accretion Flow

Using the first long-term photometry from the Transiting Exoplanet Survey Satellite, we have carried out a detailed time-resolved timing analysis of an intermediate polar TX Col. The power spectra of almost 52 days of continuous time-series data reveal the orbital period of 5.691 ± 0.006 hr, spin period of 1909.5 ± 0.2 s, and beat period of 2105.76 ± 0.25 s, which is consistent with the earlier results. We have also found the presence of quasi-periodic oscillations (QPOs) for a few days with a period of 5850-5950 s, which appears to be due to the beating of the Keplerian period with the spin period of the white dwarf. The continuous data allowed us to look thoroughly at the day-wise evolution of the system's accretion geometry. We report here that the TX Col changes its accretion mechanism even on a timescale of one day, confirming its variable disk-overflow accretion nature. For the majority of the time, it was found to be disk-overflow system with stream-fed dominance; however, pure disk-fed and pure stream-fed accretions cannot be ruled out during the observations.

Keywords: cataclysmic variable stars; semi-detached binary stars; dq herculis stars
SN 2023ixf-Temporal Development of Hα Emission

I present a series of spectra of the Type II supernova SN 2023ixf in M101 covering a temporal period of 6 weeks. The latest spectra show a prominent Hα P-Cygni profile with an expansion velocity of 7100 km s-1. In addition to an expanding gas shell, evidence of a possible additional absorption was found. This could be interpreted as an asymmetrical feature in the expanding material of the supernova.

Keywords: supernovae
Direct Measurement of the [C I] Luminosity to Molecular Gas Mass Conversion Factor in High-redshift Star-forming Galaxies

The amount of cold, molecular gas in high-redshift galaxies is typically inferred from proxies of molecular hydrogen (H2), such as carbon monoxide (CO) or neutral atomic carbon ([C I]) and molecular gas mass conversion factors. The use of these proxies, however, relies on modeling and observations that have not been directly measured outside the local universe. Here, we use recent samples of high-redshift gamma-ray burst (GRB) and quasar molecular gas absorbers to determine this conversion factor ${\alpha }_{[{\rm{C}}{\rm{I}}]}={M}_{\mathrm{mol}}/{L}_{[{\rm{C}}\,{\rm{I}}](1-0)}^{{\prime} }$ from the column density of H2, which gives us the mass per unit column, and the [C I](J = 1) column density, which provides the luminosity per unit column. This technique allows us to make direct measurements of the relative abundances in high-redshift absorption-selected galaxies. Our sample spans redshifts of z = 1.9-3.4 and covers two orders of magnitude in gas-phase metallicity. We find that the [C I]-to-Mmol conversion factor is metallicity dependent, with α[C I] scaling linearly with the metallicity: $\mathrm{log}{\alpha }_{[{\rm{C}}{\rm{I}}]}=-1.13\times \mathrm{log}(Z/{Z}_{\odot })+1.33$ , with a scatter of σ α[CI] = 0.2 dex. Using a sample of emission-selected galaxies at z ∼ 0-5, with both [C I] and CO line detections, we apply the α[C I] conversion to derive independent estimates of the molecular gas mass and the CO-to-Mmol, αCO, conversion factor. We find a remarkable agreement between the molecular gas masses inferred from the absorption-derived α[C I] compared to typical αCO-based estimates, which we confirm here to be metallicity-dependent as well, with an inferred slope that is consistent with αCI and previous estimates from the literature. These results thus support the use of the absorption-derived α[C I] conversion factor for emission-selected star-forming galaxies and demonstrate that both methods probe the same universal properties of molecular gas in the local and high-redshift universe.

Keywords: high-redshift galaxies; galaxies; interstellar medium; interstellar molecules; interstellar absorption; interstellar abundances; quasar absorption line spectroscopy; gamma-ray bursts; star formation
Coronal Heating as Determined by the Solar Flare Frequency Distribution Obtained by Aggregating Case Studies

Flare frequency distributions represent a key approach to addressing one of the largest problems in solar and stellar physics: determining the mechanism that counterintuitively heats coronae to temperatures that are orders of magnitude hotter than the corresponding photospheres. It is widely accepted that the magnetic field is responsible for the heating, but there are two competing mechanisms that could explain it: nanoflares or Alfvén waves. To date, neither can be directly observed. Nanoflares are, by definition, extremely small, but their aggregate energy release could represent a substantial heating mechanism, presuming they are sufficiently abundant. One way to test this presumption is via the flare frequency distribution, which describes how often flares of various energies occur. If the slope of the power law fitting the flare frequency distribution is above a critical threshold, α = 2 as established in prior literature, then there should be a sufficient abundance of nanoflares to explain coronal heating. We performed >600 case studies of solar flares, made possible by an unprecedented number of data analysts via three semesters of an undergraduate physics laboratory course. This allowed us to include two crucial, but nontrivial, analysis methods: preflare baseline subtraction and computation of the flare energy, which requires determining flare start and stop times. We aggregated the results of these analyses into a statistical study to determine that α = 1.63 ± 0.03. This is below the critical threshold, suggesting that Alfvén waves are an important driver of coronal heating.

Keywords: solar physics; solar flares; astrostatistics distributions
The Advanced Spectral Library: The Evolution of Chromospheric Wind Characteristics from Noncoronal to Hybrid Giant Stars

The Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph treasury program Advanced Spectral Library enables investigations of the characteristics and dynamics of the wind and chromosphere of cool stars using high-quality spectral data. This paper shows how the wind features change with spectral class when transferring across the Linsky-Haisch dividing line by comparing the noncoronal M3.5 giant γ Cru with the K5 hybrid γ Dra. We compare the presence of high-ionization diagnostics in the two objects in addition to the prominent wind spectrum in predominantly singly ionized species. We study the intrinsic strength and variation of the numerous Fe II profiles observed in the near-ultraviolet HST spectrum that are sensitive to the wind opacity, turbulence, and flow velocity. The Fe II relative strengths and wavelength shifts between the absorption and emission components reflect the acceleration of the wind from the base of the chromosphere. Furthermore, we have modeled the wind spectra using the SEI code to derive terminal velocities, wind acceleration, and mass-loss rates. Our results indicate that the chromosphere is denser when moving toward the hybrid objects. The derived average mass-loss rates in the two objects are similar but with a significantly greater terminal wind velocity and acceleration in the hybrid star. We present the analysis of the outflowing wind characteristics when transitioning from the cool noncoronal objects toward the warmer objects with chromospheric emission from a hotter environment. * This material is based upon work supported by NASA under award No. 80GSFC21M0002.

Keywords: stellar atmospheres; stellar mass loss; late-type supergiant stars