adsabs/SciX_UAT_keywords huggingface dataset:
Spectroscopy of the 2015 Outburst of AG Pegasi

Between 2015 September and 2016 January, we obtained 18 spectra of the 2015 classical outburst of AG Pegasi—a symbiotic star consisting of a white dwarf (WD), red giant, and surrounding nebula. Modelling the flux contributions of these components reveals that nebular emission, from the reprocessing of high energy WD photons, dominates the 3200-6300 Å range. Nebular emission rises and falls in line with changes seen in the WD, whose properties have been derived using Hβ and He II (4686 A) line flux, and emission measure calculations. WD parameters follow changes seen in visual band light curves. During the second peak of the outburst, WD temperatures reach 166,000 ± 6000 K, with a luminosity and radius of 14,000 ± 2000 L⊙ and 0.149 ± 0.014 R⊙ respectively. These features are consistent with an expansion of the WD pseudo-photosphere due to an accretion rate exceeding that required for stable hydrogen burning.

Keywords: symbiotic binary stars; symbiotic novae; transient sources; time domain astronomy
Finite System-size Effects in Self-organized Criticality Systems

We explore upper limits for the largest avalanches or catastrophes in nonlinear energy dissipation systems governed by self-organized criticality. We generalize the idealized "straight" power-law size distribution and Pareto distribution functions in order to accommodate incomplete sampling, limited instrumental sensitivity, finite system-size effects, and "Black Swan" and "Dragon King" extreme events. Our findings are as follows. (i) Solar flares show no finite system-size limits up to L ≲ 200 Mm, but solar flare durations reveal an upper flare duration limit of ≲6 hr. (ii) Stellar flares observed with Kepler exhibit inertial ranges of E ≈ 1034-1037 erg, finite system-size ranges of E ≈ 1037-1038 erg, and extreme events at E ≈ (1-5) × 1038 erg. (iii) The maximum flare energies of different spectral type stars (M, K, G, F, A, giants) reveal a positive correlation with the stellar radius, which indicates a finite system-size limit imposed by the stellar surface area. Fitting our finite system-size models to terrestrial data sets (earthquakes, wildfires, city sizes, blackouts, terrorism, words, surnames, web links) yields evidence (in half of the cases) for finite system-size limits and extreme events, which can be modeled with dual power-law size distributions.

Keywords: solar flares; stellar flares; stellar phenomena; astrostatistics distributions
NICER, NuSTAR, and Insight-HXMT Views to the Newly Discovered Black Hole X-Ray Binary Swift J1727.8-1613

Swift J1727.8-1613 is a black hole X-ray binary newly discovered in 2023. We perform spectral analysis with simultaneous Insight-HXMT, NICER, and NuSTAR observations when the source was approaching the hard intermediate state. Such a joint view reveals an additional hard component apart from the normally observed hard component with reflection in the spectrum, to be distinguished from the usual black hole X-ray binary systems. By including this extra component in the spectrum, we have measured a high spin of ${0.98}_{-0.07}^{+0.02}$ and an inclination of around ${40}_{-0.8}^{+1.2}$ °, which is consistent with NICER results reported before. However, we find that the additional spectral component cannot be exclusively determined due to the model degeneracy. Accordingly, a possible jet/corona configuration is adjusted to account for the spectral fitting with different model trials. The extra component may originate either from a relativistic jet or a jet base/corona underneath a slow jet.

Keywords: x-ray binary stars
Typhon: A Polar Stream from the Outer Halo Raining through the Solar Neighborhood

We report on the discovery in the Gaia DR3 astrometric and spectroscopic catalog of a new polar stream that is found as an overdensity in action space. This structure is unique as it has an extremely large apocenter distance, reaching beyond 100 kpc, and yet is detected as a coherent moving structure in the solar neighborhood with a width of ~4 kpc. A subsample of these stars that was fortuitously observed by LAMOST has a mean spectroscopic metallicity of $\langle [\mathrm{Fe}/{\rm{H}}]\rangle =-{1.60}_{-0.16}^{+0.15}$ dex and possesses a resolved metallicity dispersion of $\sigma ([\mathrm{Fe}/{\rm{H}}])={0.32}_{-0.06}^{+0.17}$ dex. The physical width of the stream, the metallicity dispersion, and the vertical action spread indicate that the progenitor was a dwarf galaxy. The existence of such a coherent and highly radial structure at their pericenters in the vicinity of the Sun suggests that many other dwarf galaxy fragments may be lurking in the outer halo.

Keywords: galaxies; stellar streams; milky way dynamics; local group; milky way stellar halo
Light Curves and Period Analysis of Beta Lyrae Stars

We present the light-curve period analysis of 36 Beta Lyrae stars from the OMC Archive. We check the reference period of the sample by applying the phase dispersion minimization periodogram. From our research, we determine the orbital period of five stars in the sample. One of them is CP Psc, showing a prominent hump in the light curve before the deeper eclipse, and revealing a period of 0.6847936 day. Additionally, we improved the previously published orbital period for 27 stars. Within this sample, we discovered three stars with the O'Connell effect, and we identified a system with a second period known as II Per, with an orbital period of 0.4798512 day and a long period 9.2724776 days. The light curve of V 747 Cen can be reproduced with a dark spot of temperature about 70% its surroundings, which covers one of the poles of the more massive star.

Keywords: beta lyrae stars; eclipsing binary stars
Early Time Spectropolarimetry of the Aspherical Type II Supernova SN 2023ixf

We present six epochs of optical spectropolarimetry of the Type II supernova (SN) 2023ixf ranging from ~2 to 15 days after the explosion. Polarimetry was obtained with the Kast double spectrograph on the Shane 3 m telescope at Lick Observatory, representing the earliest such observations ever captured for an SN. We observe a high continuum polarization p cont ≈ 1% on days +1.4 and +2.5 before dropping to 0.5% on day +3.5, persisting at that level up to day +14.5. Remarkably, this change coincides temporally with the disappearance of highly ionized "flash" features. The decrease of the continuum polarization is accompanied by a ~70° rotation of the polarization position angle (PA) as seen across the continuum. The early evolution of the polarization may indicate different geometric configurations of the electron-scattering atmosphere as seen before and after the disappearance of the emission lines associated with highly ionized species (e.g., He II, C IV, and N III), which are likely produced by elevated mass loss shortly prior to the SN explosion. We interpret the rapid change of polarization and PA from days +2.5 to +4.5 as the time when the SN ejecta emerge from the dense asymmetric circumstellar material (CSM). The temporal evolution of the continuum polarization and the PA is consistent with an aspherical SN explosion that exhibits a distinct geometry compared to the CSM. The rapid follow-up spectropolarimetry of SN 2023ixf during the shock ionization phase reveals an exceptionally asymmetric mass-loss process leading up to the explosion.

Keywords: spectropolarimetry; core-collapse supernovae; type ii supernovae; flash spectra; circumstellar matter
Phase Mixing of Propagating Alfvén Waves in a Single-fluid Partially Ionized Solar Plasma

The phase mixing of Alfvén waves is one of the most promising mechanisms for the heating of the solar atmosphere. The damping of waves in this case requires small transversal scales, relative to the magnetic field direction; this requirement is achieved by considering a transversal inhomogeneity in the equilibrium plasma density profile. Using a single-fluid approximation of a partially ionized chromospheric plasma, we study the effectiveness of the damping of phase-mixed shear Alfvén waves and investigate the effect of varying the ionization degree on the dissipation of waves. Our results show that the dissipation length of shear Alfvén waves strongly depends on the ionization degree of the plasma, but more importantly, in a partially ionized plasma, the damping length of shear Alfvén waves is several orders of magnitude shorter than in the case of a fully ionized plasma, providing evidence that phase mixing could be a large contributor to heating the solar chromosphere. The effectiveness of phase mixing is investigated for various ionization degrees, ranging from very weakly to very strongly ionized plasmas. Our results show that phase-mixed propagating Alfvén waves in a partially ionized plasma with ionization degrees in the range μ = 0.518-0.657, corresponding to heights of 1916-2150 km above the solar surface, can provide sufficient heating to balance chromospheric radiative losses in the quiet Sun.

Keywords: magnetohydrodynamics; solar magnetic fields; space plasmas; alfven waves
Gas and Dust Shadows in the TW Hydrae Disk

We present new observations of CO J = 2 - 1 emission from the protoplanetary disk around TW Hya. Emission is detected out to 240 au (4″) and found to exhibit azimuthal variations up to 20% beyond 180 au (3″), with the west side of the disk brighter than the east. This asymmetry is interpreted as tracing the shadow previously seen in scattered light. A re-analysis of the multi-epoch observations of the dust shadow in scattered light from Debes et al. suggests that an oscillatory motion would provide a better model of the temporal evolution of the dust shadow rather than orbital motion. Both models predict an angular offset between the dust shadow and the gas shadow of up to ~100°. We attribute this offset to the finite rate at which dust grains and gas molecules can exchange heat, dominated by the collisional rate between gas molecules and dust grains, t coll. The angular offsets derived are equivalent to collisional timescales that range from the near-instantaneous up to t coll ~ 10 yr, depending on whether a straight or curved dust shadow, as suggested by Hubble Space Telescope observations reported by Debes et al., is adopted. The inferred range of t coll are consistent with those predictions based on representative gas densities, temperatures, gas-to-dust ratios and grain sizes. These results represent the first time empirical constraints can be placed on t coll.

Keywords: protoplanetary disks; interferometry; planet formation
Slow Magnetoacoustic Oscillations in Stellar Coronal Loops

Slow magnetoacoustic oscillations in stellar coronal loops with gravitational stratification are analyzed with a numerical solution of the boundary value problem for eigenvalues and eigenfunctions. In this study, we only focus on the resonant periods. The effects of the gravitational stratification, star mass, loop temperature, and loop length on the properties of slow magnetoacoustic oscillations are investigated. It is shown that the discrepancy between stratified and nonstratified loops is higher in density perturbations than in velocity perturbations. When the star has a larger mass, higher coronal temperature, and longer loop, the density perturbations in the stratified loop are significantly different from the harmonic functions. The periods in the stratified loop are slightly longer than in the nonstratified loop. The periods calculated in our model (14-644 minutes) are consistent with the periods of stellar quasi-periodic pulsations observed in both soft X-rays (2-70 minutes) and white lights (8-390 minutes).

Keywords: stellar coronal loops; stellar flares; stellar oscillations; magnetohydrodynamics; solar coronal loops; solar oscillations
Investigation of the Diamagnetic Drift Condition for the Suppression of Magnetic Reconnection in 3D Interlinked Reconnection Events with Magnetic Flux Pileup

We have performed a statistical survey to investigate possible diamagnetic drift suppression of magnetic reconnection using Magnetospheric Multiscale observations of interlinked magnetic field line events at Earth's magnetopause. Our goal is to investigate a possible cause for the observed magnetic field pileup on the two sides of the thin reconnecting current sheets, at the interface of the converging field lines. We compare whether the diamagnetic drift condition for the suppression of reconnection is satisfied before and after magnetic field pileup. We find that for a majority of events in our data set, the pre-pileup plasma β gradient and magnetic shear values were in the reconnection-suppressed regime, whereas the post-pileup values were in the reconnection-allowed regime. A possible interpretation is that reconnection was suppressed under the pre-pileup conditions, and this led to magnetic field pileup. The magnetic field pileup subsequently generated boundary conditions at the interlinked current sheet that overcame the diamagnetic drift suppression condition, allowing reconnection to happen in the interlinked field line structure. However, in one third of the events, the pre-pileup conditions were already in the reconnection-allowed regime, indicating that in such events, the diamagnetic suppression condition was probably not responsible for field pileup.

Keywords: geomagnetic fields; solar magnetic reconnection
A Search for H-Dropout Lyman Break Galaxies at z 12-16

We present two bright galaxy candidates at z ~ 12-13 identified in our H-dropout Lyman break selection with 2.3 deg2 near-infrared deep imaging data. These galaxy candidates, selected after careful screening of foreground interlopers, have spectral energy distributions showing a sharp discontinuity around 1.7 μm, a flat continuum at 2-5 μm, and nondetections at <1.2 μm in the available photometric data sets, all of which are consistent with a z > 12 galaxy. An ALMA program targeting one of the candidates shows a tentative 4σ [O III] 88 μm line at z = 13.27, in agreement with its photometric redshift estimate. The number density of the z ~ 12-13 candidates is comparable to that of bright z ~ 10 galaxies and is consistent with a recently proposed double-power-law luminosity function rather than the Schechter function, indicating little evolution in the abundance of bright galaxies from z ~ 4 to 13. Comparisons with theoretical models show that the models cannot reproduce the bright end of rest-frame ultraviolet luminosity functions at z ~ 10-13. Combined with recent studies reporting similarly bright galaxies at z ~ 9-11 and mature stellar populations at z ~ 6-9, our results indicate the existence of a number of star-forming galaxies at z > 10, which will be detected with upcoming space missions such as the James Webb Space Telescope, Nancy Grace Roman Space Telescope, and GREX-PLUS.

Keywords: galaxy evolution; galaxy formation; high-redshift galaxies
HAWC and Fermi-LAT Detection of Extended Emission from the Unidentified Source 2HWC J2006+341

The discovery of the TeV point source 2HWC J2006+341 was reported in the second HAWC gamma-ray catalog. We present a follow-up study of this source here. The TeV emission is best described by an extended source with a soft spectrum. At GeV energies, an extended source is significantly detected in Fermi-LAT data. The matching locations, sizes, and spectra suggest that both gamma-ray detections correspond to the same source. Different scenarios for the origin of the emission are considered and we rule out an association to the pulsar PSR J2004+3429 due to extreme energetics required, if located at a distance of 10.8 kpc.

Keywords: gamma-rays; interstellar medium
Observational Evidence for Rotational Desorption of Complex Molecules by Radiative Torques from Orion BN/KL

Complex organic molecules (COMs) are believed to form in the ice mantle of dust grains and are released to the gas by thermal sublimation when grain mantles are heated to temperatures of ${T}_{{\rm{d}}}\gtrsim 100\,{\rm{K}}$ . However, some COMs are detected in regions with temperatures below 100 K. Recently, a new mechanism of rotational desorption due to centrifugal stress induced by radiative torques (RATs) was proposed by Hoang & Tram (2020) that can desorb COMs at low temperatures. In this paper, we report observational evidence for rotational desorption of COMs toward the nearest massive star-forming region, Orion BN/KL. We compare the abundance of three representative COMs that have very high binding energy computed by the rotational desorption mechanism with observations by ALMA, and demonstrate that the rotational desorption mechanism can explain the existence of such COMs. We also analyze the polarization data from SOFIA/HAWC+ and JCMT/SCUBA-2 and find that the polarization degree at far-infrared/submillimeter decreases with increasing the grain temperature for ${T}_{{\rm{d}}}\gtrsim 71\,{\rm{K}}$ . This is consistent with the theoretical prediction using the RAT alignment theory and Radiative Torque Disruption mechanism. Such an anticorrelation between dust polarization and dust temperature supports the rotational disruption as well as rotational desorption mechanism of COMs induced by RATs.

Keywords: interstellar clouds; interstellar dust; interstellar dust extinction; star formation; astrochemistry; starlight polarization
Variability of the Sun's Luminosity Places Constraints on the Thermal Equilibrium of the Convection Zone

Luminosity, which is the total amount of radiant energy emitted by an object, is one of the most critical quantities in astrophysics for characterizing stars. Equally important is the temporal evolution of a star's luminosity because of its intimate connection with the stellar energy budget, large-scale convective motion, and heat storage in the stellar interior. The Sun's luminosity and its variation have not been measured to date because current observations of the solar radiative output have been restricted to vantage points near the Earth. Here, we model the solar luminosity by extending a semiempirical total solar irradiance (TSI) model that uses solar-surface magnetism to reconstruct solar irradiance over the entire 4π solid angle around the Sun. This model was constrained by comparing its output to the irradiance in the Earth's direction with the measured TSI. Comparing the solar luminosity to the TSI on timescales from days to solar cycles for cycles 23 and 24, we find poor agreement on short timescales (<solar rotation). This is not unexpected due to the Earth-centric viewing geometry and short-term irradiance dependence on surface features on the Earth-facing solar disk. On longer timescales, however, we find good agreement between the luminosity model and the TSI, which suggests that the extrapolation of luminosities to multicycle timescales based on TSI reconstructions may be possible. We show that the solar luminosity is not constant but varies in phase with the solar cycle. This variation has an amplitude of 0.14% from minimum to maximum for Solar Cycle 23. Considering the energetics in the solar convection zone, it is therefore obvious that a steady-state input from the radiative zone at the solar minimum level would lead to a gradual reduction in the energy content in the convection zone over multicentury timescales. We show that the luminosity at the base of the convection zone should be approximately 0.032% higher than that at the solar surface during solar minimum to maintain net energy equilibrium through the solar cycle. These different energy-input scenarios place constraints on the long-term evolution of the TSI and its impact on the solar forcing of climate variability. These results highlight the convection zone's role as an energy reservoir on solar-cycle timescales and set constraints for dynamo models intending to understand the long-term evolution of the Sun and solar analogs.

Keywords: solar spectral irradiance; solar magnetic flux emergence; stellar luminosities; solar cycle; solar dynamo; stellar convective zones; solar convective zone
ÆSOPUS 2.0: Low-temperature Opacities with Solid Grains

In this study we compute the equation of state and Rosseland mean opacity from temperatures of T ≃ 30,000 K down to T ≃ 400 K, pushing the capabilities of the ÆSOPUS code into the regime where solid grains can form. The GGchem code is used to solve the chemistry for temperatures less than ≃3000 K. Atoms, molecules, and dust grains in thermodynamic equilibrium are all included in the equation of state. To incorporate monochromatic atomic and molecular cross sections, an optimized opacity sampling technique is used. The Mie theory is employed to calculate the opacity of 43 grain species. Tables of Rosseland mean opacities for scaled-solar compositions are provided. Based on our computing resources, opacities for other chemical patterns, as well as various grain sizes, porosities, and shapes, can be easily computed upon user request to the corresponding author.

Keywords: stellar atmospheric opacity; astrochemistry; silicate grains; carbonaceous grains
Radiative Transfer Modeling of EC 53: An Episodically Accreting Class I Young Stellar Object

In the episodic accretion scenario, a large fraction of the protostellar mass accretes during repeated and large bursts of accretion. Since outbursts on protostars are typically identified at specific wavelengths, interpreting these outbursts requires converting this change in flux to a change in total luminosity. The Class I young stellar object EC 53 in the Serpens Main cloud has undergone repeated increases in brightness at 850 μm that are likely caused by bursts of accretion. In this study, we perform two- and three-dimensional continuum radiative transfer modeling to quantify the internal luminosity rise in EC 53 that corresponds to the factor of ∼1.5 enhancement in flux at 850 μm. We model the spectral energy distribution and radial intensity profile in both the quiescent and outburst phases. The internal luminosity in the outburst phase is ∼3.3 times brighter than the luminosity in the quiescent phase. The radial intensity profile analysis demonstrates that the detected submillimeter flux variation of EC 53 comes from the heated envelope by the accretion burst. We also find that the role of external heating of the EC 53 envelope by the interstellar radiation field is insignificant.

Keywords: astronomy data modeling; infrared astronomy; radiative transfer; star formation