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Physics Astrophysics Solar and Stellar Astrophysics

astro-ph.SR Watch Solar and Stellar Astrophysics

Monday 14 September

Solar and Stellar Astrophysics

Results on young stellar clusters with the WEAVE LIFU

J. Maíz Apellániz, G. Holgado, C. Fariña et al. (+1 more)

We present the first results of two Spanish TAC programs using the WEAVE LIFU that have been obtaining data since October 2023. The first one is obtaining high-resolution multi-epoch spectroscopy of the nine stellar clusters that contain a minimum number of 3 O-type stars (and many more B stars) within the field of view of the WEAVE LIFU and that are accessible from La Palma with the purpose of studying the spectroscopic multiplicity of the OB stars. The second program is obtaining single-shot low-resolution spectroscopy of as many stellar clusters with OB stars as possible with the purpose of characterizing the OB-star population in those regions that are too crowded to be properly studied with the guaranteed time SCIP program. That program has already observed 54 clusters in a wide range of environments, from H II regions to gas-free clusters, and distances, from clusters in the solar neighbourhood to (nearby) extragalactic systems. We also present the tools developed for this program, which include a data cube processing and visualisation utility and UNWIND, a code that fits and subtracts the ISM signature in the observed spectra.
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All authors J. Maíz Apellániz, G. Holgado, C. Fariña, S. R. Berlanas

Solar and Stellar Astrophysics

Riemann Map Operator for Solar Front Diagnostics

Olena Podladchikova

Identifying a shock from a moving solar brightness front requires relating the observed emission to changes in the plasma state. The Riemann framework describes shocks, rarefactions and other waves as parts of a pattern connecting different states. We present the Riemann Map Operator (RMO), an inverse diagnostic framework implemented as a web application to help observers identify magnetohydrodynamic (MHD) shock families from incomplete measurements. RMO tests candidate local connections against conservation laws, entropy requirements and characteristic conditions, while retaining observational uncertainties and model assumptions. We demonstrate the approach with five solar examples and a complementary in-situ comparison. An event combining extreme-ultraviolet (EUV) imaging and spectroscopy admits both fast and slow shock connections within the tested conditions. For an EUV-radio event, we derive an upstream-flow constraint that would exclude ordinary slow shocks within the adopted model, provided both diagnostics sample the same front. A flare-loop benchmark reproduces the published spectral profiles and recovers slow-shock ordering in the isothermal limit. The Solar Orbiter comparison recovers an Alfvenic relation between measured velocity and magnetic-field changes. Together, the examples show how RMO connects observations to admissible MHD interpretations and identifies the additional measurements needed to distinguish them.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA

Tracing Stellar Populations through the Mg-Al Anti-Correlation in Gaia-ESO Globular Clusters

H. Sinclair-Wentworth, C. C. Worley, P. Jofre et al. (+2 more)

Context. Globular clusters (GCs) are fragments of the chemical history and evolution of the Milky Way. Their notable intra-cluster abundance variations have been used to characterise chemical trends and stellar populations. However, the origins of these populations and GCs in general are still largely unknown. Aims. Our aim is to explore the origin of the stellar populations belonging to 14 GCs making use of their abundances derived from the Gaia-ESO Survey (GES) spectra. In particular, we use the Mg-Al anti-correlation to separate the populations. Methods. The GES spectra were revisited to derive additional homogenous chemical abundances for Mg and Al. A Gaussian Mixture Model (GMM) was applied to the [Mg/Al] distribution for each cluster to define a threshold between the two main populations. Results. Eight clusters imply the presence of Mg-Al anti-correlations and dual populations. We find there is a regime transition near [Fe/H] approximately -1.4, where, the Mg-Al anti-correlation disappears at higher metallicity. Several clusters, regardless of their Mg-Al anti- correlation behaviour, exhibit metallicity spreads (e.g., NGC 2808, NGC 6752) and s-process variations (e.g., NGC 7078, NGC 1851), suggesting complex enrichment histories. Conclusions. The presence and spread of the Mg-Al anti-correlation depend on cluster mass and metallicity, with metal poor and/or massive clusters exhibiting the clearest anti-correlation. This work demonstrates additional abundances that can be retrieved from Gaia-ESO Survey spectra and provides further insight into the chemistry of multiple stellar populations within GCs. These findings highlight the chemical complexity of GCs and the need for large, homogeneous datasets to trace their formation histories.
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All authors H. Sinclair-Wentworth, C. C. Worley, P. Jofre, J. Schiappacasse-Ulloa, L. Magrini

Sunday 13 September

Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

Second Generation Planet Formation in Post-AGB Discs - II: Dust Coagulation and Core Accretion

Ali Pourmand, Orsola De Marco, Devika Kamath et al. (+1 more)

Observations have established that the circumbinary discs formed around post-asymptotic giant branch (post-AGB) binaries have striking similarities to protoplanetary discs around young stars. In this study, we examine the feasibility of the different stages of growth in the core accretion framework of planet formation in post-AGB discs, in order to assess whether planet formation is possible in these systems. We find that dust coagulation up to mm sizes is possible within the lifetimes of post-AGB discs, consistent with observations. We then investigate the subsequent growth of planetesimals through pebble and planetesimal accretion. If the streaming instability operates in these discs, the resulting planetesimals formed in higher-mass post-AGB discs ($M_{\rm disc}\sim0.1M_\odot$) can enter rapid pebble accretion and reach their isolation mass within the estimated disc lifetime. By contrast, in lower-mass post-AGB discs ($M_{\rm disc}\sim0.01M_\odot$), planetesimal accretion is too inefficient to produce substantial further growth. We also show that the larger aspect ratios of post-AGB discs compared to protoplanetary discs around young stars imply high pebble isolation masses and therefore potential formation of rocky planets with masses extending up to Jupiter masses. After assessing gas accretion and concurrent migration in these discs, we conclude that second generation planet formation in post-AGB discs is theoretically possible within their estimated lifetimes ($10^4$–$10^5$yr) provided that local dust-to-gas ratio enhancements and sufficiently high disc masses are achieved.
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All authors Ali Pourmand, Orsola De Marco, Devika Kamath, Daniel J. Price

Solar and Stellar Astrophysics

Stripped helium stars in UV

L.R. Yungelson, A.G. Kuranov and A.V. Mishakina

The model of Galactic population of stripped helium stars, the remnants of $5 \lesssim M/M_\odot \lesssim 24$ primary components of close binaries after RLOF is presented. These stars emit in the UV range of the spectrum and may be observed by detectors aboard planned "Spektr-UV" (WSO-UV) space telescope. For the current star formation rate in the Galaxy 2$\mathrm {M_\odot yr^{-1}}$, the estimate of the number of 1 to 7 $M_\odot$ objects with $T_{\rm eff} \gtrsim$25,000\,K ranges from 14700 to 28500. The number of 2 to 7 $M_\odot$ objects is 3200 to 5500. The main evolutionary factor influencing the estimate is possible formation of common envelopes and merger of components in them. Detection of candidate stripped helium stars by UV-excess is hampered by the dominance of main-sequence companions to stripped helium stars in the combined spectra of binaries. This effect of observational selection may reduce the number of candidate stripped helium stars potentially detectable by photometry to $\lesssim 1000$.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

NICER neutron stars with dark energy and dark matter: effects on the inferred equation of state

Nathan Rutherford, Chanda Prescod-Weinstein and Anna Watts

[Abridged] In this work, we compare neutron stars where dark energy or dark matter are present in the core, contrasting them against purely baryonic stars with the same central energy density. From this comparison, we find that the dark energy models are more constrained by neutron star observations than the dark matter models, suggesting that neutron stars can place strong constraints on dark energy parameters. We consider four configurations: purely baryonic neutron stars described by a piecewise polytropic (PP) equation of state (EoS), baryonic stars admixed with bosonic or fermionic asymmetric dark matter (ADM) cores, and stars with a dark energy core and baryonic shell, where the dark energy is described by the modified Chaplygin dark fluid (MCDF). Using the masses and radii of PSR J0740$+$6620, PSR J0437$-$4715, and PSR J0030$+$0451, we employ Bayesian inference to investigate how accounting for bosonic/fermionic ADM and MCDF cores affects the inferred neutron star properties. We find that MCDF cores, more than PP and ADM admixed models, substantially broaden neutron star mass-radius and pressure-energy density posterior distributions. Moreover, the MCDF EoS parameters can be tightly constrained. While bosonic and fermionic ADM cores decrease the inferred maximum mass, their posteriors strongly coincide with those of the PP model for ADM mass-fractions $F_\chi \leq 5\%$. These results demonstrate that current mass-radius observations cannot rule out MCDF cores, but they can constrain the MCDF parameter space to narrow regions. ADM admixed neutron stars, however, remain observationally identical to purely baryonic stars for all explored mass-fractions, even with improved mass-radius uncertainties. This suggests that mass-radius measurements alone are insufficient to detect ADM core inside neutron stars and that independent astrophysical or particle physics probes will be required.
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Solar and Stellar Astrophysics · Cross-listed from hep-ph

Probe of Solar Neutrino Magnetic Moments through Spin-Flavor Precession: Resonance Structure and Antineutrino Appearance

Pouya Bakhti, Sudip Jana, Chui-Fan Kong et al. (+2 more)

We investigate solar-neutrino spin–flavor precession (SFP) induced by magnetic moments in the three-active-flavor framework. For Majorana neutrinos, SFP can convert solar neutrinos into antineutrinos of different active flavors. In the Dirac case, SFP instead produces sterile right-handed states and can lead to the disappearance of active neutrinos. Using the full $6\times6$ Hamiltonians and GS98 and AGSS09 solar profiles, we examine propagation-eigenvalue crossings at $B_\perp=0$ and the projected magnetic couplings between the corresponding states. For normal mass ordering and $1\leq E_\nu/\mathrm{MeV}\leq20$, we confirm the absence of finite-density Majorana crossings. A magnetically coupled Dirac crossing emerges above approximately $12~\mathrm{MeV}$ but involves only a subdominant electron-flavor component, limiting resonant disappearance. Nonresonant Majorana conversion nevertheless offers a distinctive lepton-number-violating solar $\bar\nu_e$ signal, motivating our sensitivity study for the Jinping Neutrino Experiment. For a proposed $3~\mathrm{kt}$ detector operating for five to ten years, we project a 90% C.L. sensitivity of $P(\nu_e\to\bar\nu_e)\simeq(0.85-1.3)\times10^{-5}$. In the $\mu_{12}$-only benchmark, optimistic solar-core transverse magnetic fields of $B_\perp=7-10~\mathrm{MG}$ imply a reach of $|\mu_{12}|\simeq(2.3-4.1)\times10^{-13}\,\mu_B$, numerically below existing direct-scattering limits and commonly quoted stellar-cooling bounds. This could enable Jinping to provide one of the most stringent projected terrestrial sensitivities to Majorana transition magnetic moments.
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All authors Pouya Bakhti, Sudip Jana, Chui-Fan Kong, Seodong Shin, Seokhoon Yun

Solar and Stellar Astrophysics

Presence of Solar Neutral Atom Corona and Coronal Heating

Z.Q. Qu, R.Y. Zhou, H. Su et al. (+3 more)

By analyzing slit scanning spectral data obtained during 2024 total solar eclipse, presence of neutral atom corona is revealed along with a hidden inner F-corona detected within heights below half a solar radius. The inner F-corona is found to be essentially different from the Fraunhofer corona formed by dust scattering beyond about 2.3 solar radius heights. The two new kinds of the solar corona are deduced from the intensity difference between the spectral line intensity distribution and their adjacent continuum one acquired during the totality. Through analysis of the recorded spectra ranging from 516.0nm to 540.7nm, the relative depths of strong Fraunhofer lines are found to be changed among the neighboring lines from one place to another and from the photospheric lines acquired before the eclipse. Thus the resonant scattering is regarded to be responsible for the changes. Furthermore, shown in the reconstructed maps at selected spectral lines, the detected inner F-corona is found spreading globally and the neutral atom scattering acts as its dominant source. Both of their distribution patterns generally depend on specific lines and show considerably asymmetrical diffusions. The outward neutral fluxes are observed to be mainly hindered in regions of coronal magnetic loops with roughly estimated concentration above $10^{-5}$. Thus it can play a key role in global coronal heating via Cowling dissipation in these widely spread coronal loops, like heating in Tokamak by neutral beam injection(NBI). The Cowling dissipation can give a consistent interpretation of heating from the chromosphere to the corona. Thus the neutral atom corona is not only a new gradient of the solar corona but also the missed factor critical for concatenating together these coronal heating mechanisms.
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All authors Z.Q. Qu, R.Y. Zhou, H. Su, Y. Liang, L. Chang, X.M. Cheng

Solar and Stellar Astrophysics

Thermodynamic exchange and mode stability in strongly nonadiabatic radial pulsations

Jan Zalewski

We apply the quadratic balance relation for linear nonadiabatic pulsations to radial modes of a sequence of post-AGB envelope models covering $3.52\leq\log(T_{\rm eff})\leq4.6$. The relation separates the thermodynamic exchange power $\mathcal{P}_{\rm ex}$, response power $\mathcal{P}_{\rm rsp}$, and surface contribution $\Delta B$, and introduces an effective norm through $\mathcal{P}_{\rm ex}+\Delta B=2\gamma\chi_{\rm eff}\mathcal{E}_{\rm kin}$, where $\gamma$ is the mode growth rate. For nearly adiabatic modes $\chi_{\rm eff}$ is close to unity, whereas in strongly nonadiabatic pulsations the compression and horizontal area deformation contributions can make it change magnitude or even reverse its sign. In the cooler models, a broad family of overtone modes has $\chi_{\rm eff}<0$ and is damped despite positive thermodynamic exchange power. For these modes the response power supplies the opposing damping. Conversely, two low frequency modes in the hotter models are excited over a range of effective temperatures, even though their thermodynamic exchange power is negative. Their instability is produced by the positive response power. Strange modes generally have $\chi_{\rm eff}>0$, apart from a narrow boundary-sensitive transition during which a cold strange mode transforms into an ordinary p-mode. The excited low frequency solution with $\chi_{\rm eff}<0$ persists with a finer numerical mesh and such modes are found under the alternative outer boundary formulations examined, although their frequencies are boundary condition dependent. Thus, for strongly nonadiabatic pulsations, the sign of the thermodynamic exchange power alone does not determine mode stability.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA

Decoding RR Lyrae light curves with deep learning for accurate absolute magnitude estimation

Shunxuan He, Huiwen Wu, Yang Huang et al. (+6 more)

RR Lyrae stars are essential standard candles for distance measurements in the Milky Way and nearby galaxies. Traditional estimates rely on the Period–Absolute Magnitude–Metallicity relation but are limited by uncertainties in metallicity determinations. We present a deep learning approach that directly predicts absolute magnitudes from RRab and RRc light curves, eliminating the need for metallicity estimates. Our model achieves validation precisions of 0.053 mag and 0.036 mag (approximately 2.5% and 1.7% in distance) for individual RRab and RRc stars, respectively. Tests on globular clusters yield typical distance precisions of 1.0% for RRab and 1.7% for RRc stars. For the benchmark systems, combining the RRab- and RRc-based models yields distance moduli of 18.498 $\pm$ 0.001$_{\text{stat}}$ $\pm$ 0.018$_{\text{sys}}$ mag for the Large Magellanic Cloud and 19.564 $\pm$ 0.003$_{\text{stat}}$ $\pm$ 0.019$_{\text{sys}}$ mag for the Sculptor dwarf spheroidal galaxy. These measurements are in excellent agreement with previous results, achieve a distance precision of approximately 1%, and represent a 1.8-fold improvement over traditional RR Lyrae calibration relations. Our approach showcases the ability of AI to directly extract key physical parameters from complex, information-rich light curves, resolve degeneracies, and scale to broader applications.
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All authors Shunxuan He, Huiwen Wu, Yang Huang, Deyi Zhang, Guirong Xue, Jifeng Liu, Xiaodian Chen, Xinyu Qi, Xiaoyu Tang

Solar and Stellar Astrophysics

Supersonic flows observed by THEMIS related to a coronal bright point and filament

Garima Karki, Brigitte Schmieder, Ramesh Chandra et al. (+3 more)

In this paper, we report on the dynamics of the fine structure of a solar quiescent filament observed on September 28, 2023, with the Télescope Héliographique pour l'Etude du Magnétisme et des Instabilités Solaires (THEMIS). The main aim is to understand the relationship between the supersonic downflows measured in H$\alpha$ at the filament end and an associated coronal bright point. We use a cloud-model method to derive the supersonic velocity of the falling, elongated cool blob. Besides, we use H$\alpha$ Global Oscillation Network Group (GONG) data to track the plasma along the filament. Repetitive plasma motions are observed along the northern end of the filament. During one event of plasma motion, the THEMIS field of view was centred on the filament end, where supersonic downflows of 89.8 km s$^{-1}$ were measured with a standard deviation of $\pm$0.25 km s$^{-1}$. We suggest that the plasma moving along the filament could be falling towards the chromosphere. A ballistic trajectory could confirm this first scenario. However, we could not rule out the second scenario, in which coronal rain forms due to the thermal instability of coronal plasma. In the hot AIA channels, we identify a bright point at the same location, with a temperature reaching about 6 MK. In addition, we confirm a counter-streaming flow pattern along the fine filament strands, with widths of less than an arc second, as measured by the high-spatial- and spectral-resolution spectra of THEMIS.
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All authors Garima Karki, Brigitte Schmieder, Ramesh Chandra, Pooja Devi, Pascal Demoulin, Stefaan Poedts

Saturday 12 September

Solar and Stellar Astrophysics

Reconstruction of pretelescopic and early telescopic solar activity cycles from auroral records

Kristof Petrovay, Laura G. Magyar and Hisashi Hayakawa

The historical record of low-latitude aurorae is essentially a poorly sampled record of the largest space weather events (SWEs). Its use for the identification of individual solar cycles is hindered by the low event rate and by the fact that the solar cycle profile of the occurrence of SWEs does not closely follow the variation of sunspot numbers. Based on recent studies of the solar cycle dependence of the occurrence rates of large SWEs, here we construct Monte-Carlo simulations of a large number of activity cycles to identify the optimal procedure to infer the characteristics of underlying solar cycles from the sparse record. We find that a reliable reconstruction of the cycle phase ($>90$% of reconstructed minima corresponding to actual minima within $\pm 2$ years) is possible whenever the long-term mean event rate (annual mean number of space weather events resulting in low-latitude auroral sightings) reaches or exceeds a value around 3. This condition is found to be satisfied during most of the the Early Modern Active Period (EMAP), a century-long period of normal solar activity between the Spörer and Maunder Minima. For the numbering of solar cycles in the EMAP we introduce the "telescopic era", where T$n$ denotes the $n$th cycle from the first telescopically observed cycle, T$0$, ongoing in 1610. Using our optimal procedure we reconstruct a series of 8 solar activity cycles from T$-5$ to T$2$ (1560-1640). Earlier cycles starting from 1540 can be reconstructed with a somewhat lower degree of reliability. Comparing our results with radionuclide-based reconstructions and sunspot observations we find a good overall correspondence, with the exception of the last cycle before the Maunder Minimum.
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Solar and Stellar Astrophysics

Investigating Variability in Long-lived Protoplanetary Disks (>10 Myr) using NEOWISE

Gregory Mathews Ben, Jessy Jose and Jesús Hernández

We present the mid-infrared variability study of 120 long-lived disk candidates (ages 10-100 Myr) and 13 confirmed "Peter Pan" disks using NEOWISE to constrain the longevity of dynamic disk processes beyond the standard 10 Myr dispersal threshold. We identified 32 sources exhibiting significant variability by combining stochastic flux metrics and the Stetson $J$ index. The majority of these variables belong to the 10-25 Myr age range, suggesting a sharp decline in disk brightness variability after 25 Myr. Morphologically, the variability is characterized by irregular fluctuations with mean amplitudes of $\Delta W2 \approx 0.5$ mag, consistent with variable extinction or inner disk warping induced by embedded planetesimals. We also report a strong correspondence between mid-infrared and optical variability for the Peter Pan disks, with all 9 mid-IR variables confirmed as optical variables in the literature. Our findings reveal a population of 23 new long-lived disk candidates that retain active, gas-rich environments, supporting the existence of a rarefied class of systems that survive well into the debris disk era.
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Solar and Stellar Astrophysics

Investigating r-Process Diversity in Mildly Metal-Poor Stars Using the Th II 5989 Angstrom Line

Kurumi Furutsuka and Satoshi Honda

The origin of the heaviest elements remains a fundamental mystery, with the "actinide-boost" (Thorium overabundance) serving as a critical diagnostic of r-process nucleosynthesis conditions. While this diversity is well documented in extremely metal-poor (EMP) stars, its frequency in the mildly metal-poor (MMP) regime ($-2.0 < $ [Fe/H] $< -0.5$) is poorly constrained due to blending issues in the commonly used 4019 angstrom line. We present a systematic abundance analysis of mildly metal-poor stars, exploiting the relatively unblended Th II 5989 angstrom line. High-dispersion spectra were obtained using the 1.5 m telescope at Gunma Astronomical Observatory with GAOES, complemented by archival data from the Subaru Telescope/HDS retrieved via SMOKA. We successfully detected Th in 32 out of 42 sample stars. Our analysis demonstrates that the 5989 angstrom line is a useful diagnostic for Th abundance measurements in cool red giants. The mean [Th/Eu] ratio remains nearly constant over the MMP metallicity range, suggesting that progressive homogenization of the interstellar medium during Galactic chemical evolution would reduce abundance variations inherited from individual r-process events, or the dominant r-process events may share similar initial Th/Eu production ratios. While the [Th/Eu] ratio is nearly constant overall in the MMP regime, its dispersion appears to increase toward the metal-rich end of this regime and reaches approximately 1 dex near solar metallicity. This behavior may reflect the combined effects of variations in the initial Th/Eu production ratios of the progenitor objects, the mixing of r-process products in the interstellar medium, and the radioactive decay of Th over stellar lifetimes.
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Solar and Stellar Astrophysics

Multi-Instrument Analysis of NOAA AR 12781: Coupling Surface Evolution of the AR with Its In-Situ Solar Wind Signatures

Dharanesh Palaniappan and Khagendra Katuwal

Understanding the relationship between photospheric magnetic flux evolution and coronal emission remains an important problem in heliophysics. We investigate the decay of NOAA Active Region 12781 from 7 to 11 November 2020 using observations from the Solar Dynamics Observatory (SDO). Coronal emission was measured from the Atmospheric Imaging Assembly (AIA) 193\~Å images, while the photospheric signed and unsigned magnetic flux was derived from Helioseismic and Magnetic Imager (HMI) observations of the FeĨ 6173\~Å line. The unsigned magnetic flux and mean 193\~Å intensity showed a strong inverse relationship, with Pearson and Spearman correlation coefficients of $r\_p=-0.771$ and $r\_s=-0.758$, respectively. During the five-day decay phase, the unsigned magnetic flux decreased by 36.54\% from its maximum value, while the coronal emission increased by 21.96\%. We also compared the remote-sensing observations with near-Earth solar-wind and interplanetary magnetic-field measurements from the OMNI database using an estimated propagation delay. The results show that the coronal emission did not vary directly with the photospheric magnetic flux during the decay of AR\~12781. The possible association with the solar wind is discussed cautiously because a direct source connection cannot be established from the time-delay estimate alone.
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Friday 11 September

Solar and Stellar Astrophysics

SDSS-IV MaStar: Determination of Stellar Parameters Using Bayesian Averaging

Yan-Ping Chen, Renbin Yan, Szabolcs Mészáros et al. (+12 more)

We present the stellar parameters for 59,266 high-quality spectra of 24,130 unique stars from the MaNGA Stellar Library (MaStar) in the Sloan Digital Sky Survey (SDSS) Data Release 17 (DR17). The median signal-to noise ratio per pixel of the spectra is 96. We derive four stellar parameters, effective temperature (Teff), surface gravity (log g), metallicity ([M/H]), and {$\alpha$}-enhancement ratio ($\rm [\alpha/M]$), by comparing the data with BOSZ (ATLAS-9 based) and MARCS theoretical atmospheric models. We adopt a Bayesian method and use color and absolute magnitude derived from Gaia to select a subset of theoretical models for each star. We then perform full-spectrum fitting to estimate the likelihood of each model in the subset and then compute their likelihood-weighted mean parameters as the final parameters. We set stellar-parameter quality flags to facilitate the use of the derived stellar parameters. The MaStar stellar parameters derived herein span an effective temperature range of $\rm 2,600 \leq T_{eff} \leq 29,861 K$, a surface gravity range of $0 \leq \log g \leq 5.5$, a metallicity range of $-4.9 \leq \rm[M/H] \leq 1.0$, and an {$\alpha$}-abundance range of $\rm -0.98 \leq [\alpha/M] \leq 1.0$. We compare these parameters with those from APOGEE and Gaia for stars in common, finding general consistency within the uncertainties. However, some artifacts and systematic differences are present, and we discuss their potential causes. These new stellar parameters are available through the MaStar SDSS-IV DR17 value-added catalog (https://www.sdss4.org/dr17/mastar/mastar-stellar-parameters/).
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All authors Yan-Ping Chen, Renbin Yan, Szabolcs Mészáros, Claudia Maraston, Daniel Thomas, Daniel Lazarz, Guy S. Stringfellow, Lewis Hill, Julie Imig, Joseph D. Gelfand, Jon A. Holtzman, Matthew Bershady, Dmitry Bizyaev, Niv Drory, Keivan G. Stassun

Solar and Stellar Astrophysics

Sinking Silicates I: Characterizing the benchmark system containing the T0 brown dwarf CWISE J210640.16+250729.0 using JWST

Austin Rothermich, Jacqueline K. Faherty, Ben Burningham et al. (+16 more)

In this study, we present the full (97.3% complete) 0.8–12.5 $\mu$m spectral energy distribution (SED) of an L/T transition object, CWISE J210640.16+250729.0 (CW2106), using the James Webb Space Telescope (JWST). We provide a full characterization of the host star's elemental abundances and age. We empirically derive the bolometric luminosity ($L_{\rm bol}\approx-4.825$ $\textup{L}_\odot$) of CW2106, and obtain estimates of its mass (M$\approx50-62$ M$_{\rm Jup}$), radius (R$\approx0.83-0.87$ R$_{\rm Jup}$), effective temperature ($T_{\rm eff}$$\approx1213$ K), and surface gravity ($\log~g$$\approx5.28$ dex). We find the near-infrared (near-IR) spectrum ($0.8-2.5 ~\mu$m) is best reproduced with cloudy atmospheric models while the mid-infrared (mid-IR) spectrum ($5-12.5 ~\mu$m) is best reproduced with cloudless models. This suggests a cloud layer restricted to only the deepest observable parts of the atmosphere and is qualified by the lack of a 9 $\mu$m silicate feature. Making use of the Mg/Si ratio of the primary, alongside thermochemical models, we predict the clouds in CW2106 to be composed primarily of enstatite (MgSiO$_3$), removing $\sim23\%$ of the bulk oxygen out of the atmosphere. Future retrieval studies will be able to help investigate the existence and full impact of these cloud species.
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All authors Austin Rothermich, Jacqueline K. Faherty, Ben Burningham, Catherine Manea, Emily Calamari, Genaro Suárez, Daniella C. Bardalez Gagliuffi, John M. Brewer, Kelle L. Cruz, Josefine Gaarn, Jonathan Gagné, Eileen C. Gonzales, Marina E. Gemma, Viktória Kecskeméthy, Mark S. Marley, Caprice L. Phillips, Channon Visscher, Johanna M. Vos, Niall Whiteford

Solar and Stellar Astrophysics

Classical Cases of Non-solar Mixing Length in Low Mass Stars

Rebecca Woody, Charlie Conroy, Phillip A. Cargile et al. (+1 more)

The treatment of convection is one of the largest remaining uncertainties in models of low mass stars. The Mixing Length Theory (MLT) for super-adiabatic convection has come under particular scrutiny. Evidence is mounting that a single value of the mixing length parameter ($\alpha_\mathrm{MLT}$) should not be applied universally across stellar model grids, although simulations and asteroseismic calibrations disagree on how it should vary with stellar parameters. A variable $\alpha_\mathrm{MLT}$ causes critical systematic changes to the ages of low mass stars, the key age tracers for galactic and exoplanetary studies. In this work we use classical (non-seismic) geometric radii and specialized MIST isochrones with a variable $\alpha_\mathrm{MLT}$ to measure mixing length in detached eclipsing binaries (DEBs) and interferometrically resolved stars, finding plenty of evidence for non-solar $\alpha_\mathrm{MLT}$. In particular we find that standard evolutionary models overpredict the radii of metal-poor interferometric stars and that a sub-solar $\alpha_\mathrm{MLT}$/$\alpha_{\mathrm{MLT},\odot}\sim0.50-0.75$ can neatly explain this discrepancy, consistent with other empirical calibrations and in stark contrast to simulations. We infer low convective efficiency in cool, rapidly rotating lower main-sequence stars, consistent with the known phenomenon of magnetically-induced radius inflation. We also identify several slowly rotating DEBs whose measurements cannot be reproduced by standard isochrones, and we propose these systems as useful case studies of non-solar mixing length in stars outside of the Sun. However, we find no definitive population level correlations between $\alpha_\mathrm{MLT}$ and stellar parameters among the DEBs, calling into question the existence of a ubiquitous mixing length calibration in low mass stars.
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All authors Rebecca Woody, Charlie Conroy, Phillip A. Cargile, Aaron Dotter

Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA

Chemo-dynamical Analysis of a CNO-Enhanced Ultra Metal-poor Star ($\rm[Fe/H] < -4$): Insights into Early Enrichment by Faint Population III Supernova

Guilherme Limberg, Vinicius M. Placco, Alexander P. Ji et al. (+10 more)

We report an independent identification of the ultra metal-poor (UMP) star ($\rm[Fe/H] =-4.06$) $Gaia$ DR3 $\texttt{source_id}$ 4795913112968206720 (GDR3_479591) in the $Gaia$ mission's Blue and Red Photometer 'XP' spectro-photometric catalog. We combine multi-band photometry, astrometry, and high-resolution spectroscopy to confirm GDR3_479591 as a red giant-branch (RGB) star located at ${\sim} 6\,{\rm kpc}$ from the Sun. Abundance analysis under local thermodynamic equilibrium reveals significant enhancements, relative to the solar level, in carbon ($\rm[C/Fe] = +1.54$, after evolutionary correction for depletion in the RGB), nitrogen ($\rm[N/Fe] = +2.64$), and oxygen ($\rm[O/Fe] = +2.62$). The CNO-enhanced GDR3_479591 is thus one of only 5 UMP stars with a detected oxygen abundance. The CNO excess is accompanied by enhancements in several other light elements, such as Na, Mg, Al, and Si. We demonstrate that the chemical pattern of GDR3_479591 can be reproduced by the yields of a single Population~III 'faint' supernova with progenitor mass of $\sim$21-to-28$\,M_\odot$ and a low explosion energy of ($0.3 \leq E_{\rm SN}/(10^{51}\,{\rm erg}) \leq 0.9$). Additionally, we use literature metal-poor stars to show that, contrary to recent propositions for high-redshift galaxies, a mild enhancement in [C/O] does not automatically translate to the high [C/Fe] typically observed in UMP stars in the Milky Way and its satellites. GDR3_479591 could not be dynamically associated with any of the most relevant accreted substructures in the Galactic halo, and we speculate that it was formed in an ultra-faint dwarf galaxy environment that later merged with the Milky Way.
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All authors Guilherme Limberg, Vinicius M. Placco, Alexander P. Ji, Yupeng Yao, Friedrich Anders, Wendy Q. Sun, Anna de Graaff, Rohan P. Naidu, Harley Katz, Pierre N. Thibodeaux, Anirudh Chiti, Mohammad K. Mardini, Anna Frebel

Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA

Magnetized interstellar molecular clouds - III. Filament Collisions and Core Formation: Insights into Substructures and Evolution

Pak Shing Li, Christopher F. McKee and Richard I. Klein

We present the results of a driven, ideal magnetohydrodynamics simulation of a molecular cloud to study the formation of filamentary structures in molecular clouds and their fragmentation into dense cloud cores. Shock compression results in the formation of sub-parsec long filamentary structures. Large-scale supersonic flows push these small filaments together and form massive filamentary clouds. The main filament that forms stretches across the full 4.4 pc length of the simulation and has a median FWHM of 0.075 pc, similar to that observed in many molecular clouds. Subfilaments inside the main filament have a median FWHM of 0.03-0.04 pc, similar to the “fibers" observed in molecular lines. The collision and merger of subcritical subfilaments is an important mechanism for the formation of prestellar cores. Colliding subfilaments and the resulting core at their intersection is a hub-filament system similar to that associated with massive star formation, but on a smaller scale. Core mergers can be significant in the growth of pre-stellar cores and may play a role in the formation of binary stars.
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Solar and Stellar Astrophysics

Late-Time Evolution of the Massive Stellar Merger M101 OT2015-1

Marco A. Gómez-Muñoz, Nadejda Blagorodnova, Tomasz Kamiński et al. (+6 more)

Luminous red novae (LRNe) are astronomical transients arising from unstable mass transfer and common envelope phases in close binary systems, culminating in stellar mergers. Their cold, expanding ejecta provide the ideal conditions for molecule and dust formation. We investigate the late-time dust and molecular evolution of the LRN M101 OT2015-1, a high-mass LRN progenitor ($18\pm 1$M$_\odot$), using multi-band near-infrared (NIR) and mid-infrared (MIR) observations spanning up to ~5 years post-outburst. Archival and unpublished photometry from NEOWISE, Spitzer, Keck NIRC2 and MOSFIRE, are interpolated via 2D Gaussian process regression. Spectral energy distributions (SEDs) are modelled with 1D spherical radiative transfer models to track the evolution of dust mass, dust temperature, and central source properties. At +150d post-peak, the SED is well-fit by a pure stellar blackbody ($T_{\rm eff}=3330$K) with negligible dust (log$M_{\rm dust}/$M$_\odot<-6.36$). Rapid dust condensation begins around day +200 ($T_{\rm dust}=1591$K), with dust mass increasing to log$M_{\rm dust}/$M$_\odot =-3.65$ and optical depth reaching $\tau_V =64$ by +1300d. A prominent NIR re-brightening at ~600d ($M_K=-12.04$), accompanied by a dust reheating bump ($T_{\rm dust}=1567$K), highlights a second phase of dust nucleation likely driven by shock interactions. Low-resolution NIR MOSFIRE spectra are analysed for line identification and also using an LTE isothermal slab model to characterise CO molecular features. The MOSFIRE spectra reveal an O-rich environment with photospheric expansion velocities of ~$430-530$km s$^{-1}$, as measured in the Pa$\beta$ emission line, and high CO column densities (log$N_{\rm CO}$/cm$^{-2}=20.9$ and $20.7$ at +142d and +197d, respectively). These results confirm that LRNe from massive progenitors are prolific producers of cosmic dust and are efficient molecular factories.
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All authors Marco A. Gómez-Muñoz, Nadejda Blagorodnova, Tomasz Kamiński, Gerard Garcia-Moreno, Maxime Wavasseur, Hugo Tranin, Grace Katusiime, Jacob E. Jencson, Mansi M. Kasliwal

Solar and Stellar Astrophysics · Cross-listed from astro-ph.IM

XMST: An Extended Minimum Spanning Tree Framework with Objective Fracture-Scale Selection

Mark Gallaway

We present XMST, an extended minimum spanning tree framework for identifying spatially coherent stellar structures using an objective, data-driven fracture-scale criterion. XMST combines percolation analysis with Jenks Natural Breaks optimisation, restricting the MST edge-length distribution to the subcritical regime before determining the final fracture scale. The method was validated using 1000 controlled Monte Carlo realisations containing eight empirical stellar-association templates, including a deliberately overlapping pair. The Percolation-Jenks fracture scale was highly reproducible, with a median of 17.028 pc and a between-realisation standard deviation of 0.066 pc. The injected structures were recovered with a mean completeness of 0.9996 and mean purity of 0.7413, while alternative CDF and Mean Edge criteria selected substantially larger fracture scales and produced markedly lower purities. Propagation of the published distance uncertainties through 10,000 additional XMST reconstructions produced a median absolute fracture-scale change of only 0.282 pc. Under these perturbations, mean completeness was 0.8605 and mean purity 0.7126, while 99.30 per cent of injected-template cases continued to satisfy the adopted detection criterion. Individual membership was less stable, with peripheral and locally sparse members showing lower persistence than structural cores. The deliberately overlapping pair was resolved spatially in only 15 of 1000 realisations; a reddening-based post-processing diagnostic increased this to 446, although with a corresponding completeness-purity trade-off. XMST therefore provides a robust method for identifying candidate stellar structures and their stable cores, while detailed outer membership and strongly overlapping populations require additional astrophysical information.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

The influence of free-free absorption on the radio spectrum of Particle-Accelerating Colliding-Wind Binaries

Michaël De Becker, Agustina Belen Blanco, Mathilde Tasseroul et al. (+4 more)

The study of massive stars in binary and higher-multiplicity systems that participate in particle acceleration is a key topic at the crossroads of massive star physics, shock physics, and galactic cosmic ray astrophysics. From an observational perspective, radio measurements are our pri- mary tool for identifying these systems through their synchrotron radio emission. Out of the 54 such systems known to date, all but two have been discovered via their non-thermal radio emission. However, identifying these systems is not straightforward. The main challenge lies in free-free absorption, which can obscure the synchrotron signature and hinder detection. This paper summarizes recent developments in our understanding of these systems, with a particular focus on the strong observational bias introduced by free-free absorption. This bias significantly limits our ability to determine the true fraction of particle accelerators among colliding-wind binaries.
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All authors Michaël De Becker, Agustina Belen Blanco, Mathilde Tasseroul, Anandmayee Tej, Anindya Saha, Paula Benaglia, Santiago del Palacio

Solar and Stellar Astrophysics · Cross-listed from physics.atom-ph

Experimental Ni II Oscillator Strengths for Transitions from $3d^84d$ to $3d^84p$ Levels Measured Using High-resolution Fourier Transform Spectroscopy

Ruijie Chen, Christian P. Clear, Gillian Nave et al. (+1 more)

We present the first experimentally measured oscillator strengths for Ni II 3d$^8$4d - 3d$^8$4p transitions. High-resolution Fourier transform spectra were recorded of Ni-He hollow cathode lamps, and intensity calibrated using deuterium standard lamps to determine relative intensities of Ni II emission lines between 200 and 395 nm. Branching fractions were determined for observed lines from 35 3d$^8$4d upper levels in total, and account for at least 95% of the predicted transition probability for 31 of these levels, with sufficient completeness to give reliable branching fractions. Combining our measured branching fractions with four experimental and 27 theoretically calculated energy level lifetimes provided absolute oscillator strengths for 174 lines, for which no previous experimental values exist. We compare the results with three sets of previously published theoretical calculations, illustrating the improvement of laboratory measurements for these atomic data. Please note: The transition probability datasets are withheld from this pre-print to ensure that only the peer-reviewed, definitive versions are released. This approach prevents the propagation of duplicate or inconsistent data across widely used atomic databases and modelling frameworks.
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All authors Ruijie Chen, Christian P. Clear, Gillian Nave, Juliet C. Pickering

Solar and Stellar Astrophysics

Plasmoid-Trapped Condensation Associated with a Transient Thermal-Instability-Like Process in Chromospheric Magnetic Reconnection

Abdullah Zafar, Lei Ni, Samrat Sen et al. (+4 more)

We perform high-resolution 2.5D single-fluid magnetohydrodynamic (MHD) simulations to explore the development of cool-dense structures within plasmoids during chromospheric magnetic reconnection. The model incorporates temperature-dependent ionization degrees of hydrogen and helium, resulting in improved diffusivities and viscosity, as well as adequate radiative cooling. The numerical results show that plasma is significantly hot with temperatures reaching several tens of thousands of Kelvin in newly developed plasmoids in the low initial plasma-$\beta$ cases, $\beta_0$ = 0.05. Later, the localized accumulation of plasma and decreasing temperature result in an explosive much stronger radiative cooling process. Radiative cooling peaks over the temperature range 8000 K$-$20,000 K. The co-spatial density enhancement, substantial radiative cooling, shorter radiative cooling timescales, and a significant temperature decrease support the existence of thermal-instability-like condensation within the plasmoids. Similar cool-dense structures are identified in all low-$\beta_0$ cases at different chromospheric altitudes, suggesting that this occurrence is not limited to a single chromospheric layer. In contrast, in the high initial plasma-$\beta$, $\beta_0$ = 0.5 case, the maximum temperature inside the dense plasmoids is only about 8000 K. Therefore, the radiative cooling has never entered into the explosive increasing stage and the thermal-instability-like condensation does not happen. These findings reveal that plasmoid-trapped condensation may develop in chromospheric reconnection once radiative losses become self-amplifying under low-$\beta_0$ conditions.
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All authors Abdullah Zafar, Lei Ni, Samrat Sen, Yuhao Chen, Mohamed Sedik, Mehdi Yousefzadeh, Jun Lin

Solar and Stellar Astrophysics

Large-Scale Latitude-time Relationships Between the Green-Line Corona and Sunspot Activity During Solar Cycles 18-24

Jouni J. Takalo

We investigate the latitude-time relationship between the solar green-line corona and sunspot activity during Solar Cycles 18-24 using homogeneous coronal observations and a Gaussian representation of sunspot activity fields. The activity-field model is constructed from individual sunspot areas and latitudes in order to describe the large-scale spatial organization of solar magnetic activity more realistically than traditional sunspot number or sunspot area indices. The analysis reveals a stable double-peaked latitude-dependent correlation structure centered on the active-region belts, where the strongest corona-sunspot correlations are observed. The latitude profiles remain remarkably similar from cycle to cycle, indicating that the large-scale relationship between coronal emission and photospheric activity is largely independent of cycle amplitude and parity. The lag-correlation profiles exhibit broad positive lag plateaus in most cycles. Although formal lag-correlation maxima occur at positive lags, surrogate-data tests indicate that these maxima are generally not statistically distinguishable from neighboring lag values. The results therefore suggest temporal persistence and memory of large-scale coronal magnetic structures rather than a precisely defined physical delay. The Gaussian activity-field representation produces substantially stronger and more coherent correlations with the green-line corona than conventional sunspot measures, supporting the interpretation that the large-scale distribution of magnetic activity, rather than individual sunspots alone, governs the evolution of the large-scale corona.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

Terminal instability of the Solar System triggered by stochastic solar mass loss

Konstantin Batygin, Jim Fuller and Fred C. Adams

From its birth, celestial mechanics has been deeply intertwined with the question of the Solar System's dynamical stability. For the inner planets, this question is now statistically settled: Mercury's orbit carries a $\sim$1% chance of destabilization before the Sun leaves the main sequence. The outer Solar System has seemed more secure, with its intrinsic dynamical lifetime estimated at $\sim10^{18}$ years. Even accounting for the Sun's mass loss and stellar flybys, the orbital architecture of the giant planets had been expected to persist for $\sim$100 Gyr. Here we show that these estimates rest on the assumption that solar mass loss is smooth. The recently measured white dwarf recoil demands asymmetric mass loss that is most readily attributed to discrete, independently directed ejections that impulsively perturb stellar motion. As the Sun sheds its envelope in such parcels, the planets' orbits random-walk with amplitude set by the mass-loss granularity. At a coarseness corresponding to observationally permitted kicks, this stochastic forcing restructures the outer Solar System concurrently with the Sun's death. In particular, our numerical experiments reveal that orbit crossings can commence on the red giant branch, with $\sim$40% of realizations undergoing disruption or violent scattering before the white dwarf forms and $\sim$90% self-destructing within 3 Gyr. The outer Solar System's dynamical lifetime thus collapses from $10^{18}$ years to approximately a gigayear after white dwarf formation.
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Solar and Stellar Astrophysics

Magnetic Fields and Asymmetric Accretion in the Class 0 Protostellar System L1527 IRS

Hanju Nam, Woojin Kwon, Youngwoo Choi et al. (+2 more)

Magnetic fields play a crucial role in regulating the collapse of dense molecular clouds, shaping the early stages of protostellar evolution. We present polarization observations in submillimeter wavelengths with the SCUBA-2/POL-2 instrument on James Clerk Maxwell Telescope toward the Class 0 protostellar system L1527 IRS (IRAS 04368+2557). The magnetic field morphology varies across the core of L1527 IRS. Magnetic fields in the eastern region are perpendicular to the outflow axis, while those in the western region show a pinched morphology, overall aligned with the outflow cavity. These distinct bipolar outflow regions also have a clear color difference in near infrared observed by James Webb Space Telescope. In addition, the spectral index derived from 450 {\mu}m and 850 {\mu}m observations reveals that the northwestern region of the protostellar system is colder and denser compared to the southeastern region. Furthermore, in the large-scale structures and magnetic fields revealed by Herschel and Planck, we find a ~0.1 pc scale filamentary structure parallel to the large-scale magnetic fields in the eastern region and a relatively isotropic mass distribution in the western region. Based on these results, we propose an accretion scenario of the protostellar system in which an asymmetric mass distribution causes the distinct features observed in near-infrared and submillimeter wavelengths.
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All authors Hanju Nam, Woojin Kwon, Youngwoo Choi, Ian W. Stephens, Leslie W. Looney

Solar and Stellar Astrophysics

Superoutbursts and Superhumps of Cataclysmic Variables observed with TESS

Qi-Bin Sun, Sheng-Bang Qian, Li-Ying Zhu et al. (+10 more)

Superoutbursts and superhumps are characteristic signatures of SU UMa-type cataclysmic variables, yet the full diversity of superhump evolutionary behaviour remains poorly constrained owing to the limitations of ground-based photometric monitoring. Here we report a systematic analysis of 30 SU UMa-type dwarf novae observed with the Transiting Exoplanet Survey Satellite, covering 37 superoutburst events. We detect coherent superhump signals in 29 systems-including five first-time detections-and determine or refine orbital periods for 13 objects. The high-cadence light curves clearly resolve the canonical three-stage (A-B-C) superhump period evolution. Using Stage A periods, we derive dynamical mass ratios for 18 systems, consistent with established cataclysmic variable evolutionary tracks. The measured Stage B period derivatives match the range of values from ground-based campaigns, with prominent positive drifts concentrated in short-period, low-mass-ratio systems. Repeated superoutbursts yield consistent evolutionary patterns, demonstrating that superhump morphology is an intrinsic, repeatable property. Two systems break the standard template: RZ LMi shows an inverted stage sequence inconsistent with classical precession theory, while ASASSN-14kj exhibits no stage evolution but a coherent long-period modulation. Persistent superhumps during normal outbursts in three systems provide direct evidence that eccentric disk structures can survive beyond their parent superoutbursts, revealing a decoupling between thermal accretion and tidal eccentricity cycles. These results demonstrate the power of space-borne photometry for probing accretion disk dynamics and testing precession models across the cataclysmic variable parameter space.
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All authors Qi-Bin Sun, Sheng-Bang Qian, Li-Ying Zhu, Qin-Mei Li, Wen-Ping Liao, Fu-Xing Li, Wen-Xu Lin, Min-Yu Li, Ping Li, Fang-Zhou Yan, Zi-Mo Li, Tao Wang, Mao-Han Zhang

Thursday 10 September

Solar and Stellar Astrophysics

Plasma Heating and Energization in Hot-Onset Flare Precursor Events

H. Da, J. F. Drake and M. Swisdak

Recent observations of solar flares have revealed a distinct 'hot onset' phase preceding the impulsive phase, characterized by elevated plasma temperature (>10 MK), a steady increase in emission measure, and the absence of detectable nonthermal emission. The standard flare model attributes the formation of hot flare plasma to chromospheric evaporation driven by energy deposition from nonthermal electrons, and therefore does not naturally explain why thermal heating precedes nonthermal emission. Motivated by recent observations of guide-field evolution in two-ribbon flares and by large-scale MHD simulations, we investigate hot-onset energization using simulations of magnetic reconnection with the kinetic macroscale model kglobal initialized with a spatially non-uniform guide field. Because the efficiency of electron non-thermal energy production from Fermi acceleration decreases with guide-field strength, the evolving guide field produces two distinct energization regimes. During the initial high-guide-field phase, the bulk electron temperature rises to approximately twice its initial value, while non-thermal particle production remains weak. Upon entering the low-guide-field regime, energetic particle production is significantly enhanced, with the population of high-energy electrons growing rapidly. These findings provide a self-consistent physical explanation for the hot onset, demonstrating that the macroscopic evolution of the guide field naturally bridges the hot-onset and impulsive phases of solar flares without requiring a separate trigger mechanism.
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