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Physics Astrophysics Solar and Stellar Astrophysics

astro-ph.SR Watch Solar and Stellar Astrophysics

Thursday 3 September

Solar and Stellar Astrophysics

Interferometric Survey of Stellar Parameters: Mass of the metallic A-type binary $\beta$ Aur

J. Jonák, D. Mourard, J. D. Monnier et al. (+23 more)

With the capabilities of the new visible CHARA/SPICA instrument and the multiple spectral band operation of CHARA, our goal is to resolve orbits of short-period binaries and develop a robust framework for combining interferometric, spectroscopic, and photometric observations into a single consistent model. For our target sample, we selected suitable binaries based on brightness, angular separation, and orbital properties based on the expected performance of the CHARA/SPICA instrument. As a case study, we analysed the bright eclipsing binary $\beta$ Aurigae, composed of two slightly evolved A1 stars. We combined new interferometric observations of $\beta$ Aur obtained with CHARA/SPICA, MIRC-X, and MYSTIC with archival MIRC data, radial velocities, and light curves. We first derived astrometric positions from interferometric observables and computed an orbital solution. Afterwards, we implemented a unified model, capable of tying interferometric modelling with the ellc code to simultaneously fit all observables using MCMC sampling. We performed a detailed analysis of the noise statistics of each data set and in the end we adopted a profile likelihood approach to account for underestimated noise and systematics. We derived a consistent orbital and physical solution for $\beta$ Aur through joint modelling. The inclusion of interferometric data tightly constrains the angular semi-major axis and inclination. Using profile likelihood to account for the different intrinsic levels of uncertainty of the fundamentally different observables, we derived the masses of the two stars, $M_1 = 2.359 \pm 0.005$ M$_S$ and $M_2 = 2.293 \pm 0.004$ M$_S$, their radii $R_1 = 2.752 \pm 0.002$ R$_S$ and $R_2 = 2.622 \pm 0.002$ R$_S$, and the distance to the binary, $d = 24.30\pm0.05$ pc.
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All authors J. Jonák, D. Mourard, J. D. Monnier, S. Hoin, M. Brož, A. Oplištilová, K. A. Kubiak, N. Ebrahimkutty, R. V. Ibañez-Bustos, H. Nowacki, M. Vrard, M. Bailleul, P. Bério, J. Dejonghe, R. Ligi, F. Morand, N. Nardetto, D. Salabert, S. Deheuvels, A. Domiciano de Souza, A. Meilland, K. Perraut, M. Wittkowski, S. Kraus, N. Anugu, M. Gutierrez

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

Disk survey in the Serpens star-forming region: Environmental effects in nearby star-forming regions

Simin Tong, Nienke van der Marel, Jonathan P. Williams et al. (+1 more)

The external environment where protoplanetary disks are embedded regulates disk evolution. External irradiation, which heats and evaporates gas, as well as frequent stellar encounters, which truncate disks, can reduce disk sizes and masses. Disk surveys in nearby star-forming regions with various environments can help us understand how external environments impact disk evolution. The Serpens star-forming region, which is thought to be dense but not highly irradiated by massive stars, is an ideal laboratory to study the dynamical effects on disk properties. We aim to study how dynamical interactions modify disk dust masses by comparing Serpens with other nearby star-forming regions through their disk masses and 3D stellar densities. We survey 321 young stellar objects from Class I to III in Serpens using ALMA at 0.25 arcsec. We measure disk dust masses from millimeter observations under the optically thin assumption and recompute the 3D stellar density using Gaia data. We apply the same method to other nearby star-forming regions for direct comparison with Serpens. The cumulative disk dust mass distribution of Serpens is similar to those of similarly aged nearby star-forming regions, such as Lupus and Taurus. Along with re-assessment of the 3D stellar density, it suggests that Serpens is not likely to have experienced strong tidal truncation capable of producing lower disk dust masses. We also find that the low disk dust mass tension in Ophiuchus and Corona Australis can be resolved when only disks that have been explicitly identified as members of young (1-2 Myr) sub-clusters are considered. Disks without Gaia identifications, especially in Ophiuchus, are spatially clustered in the 2D projected sky plane and less massive than nearby disks with Gaia identifications, possibly tracing the effects of tidal truncation in denser environments at earlier evolutionary stages.
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All authors Simin Tong, Nienke van der Marel, Jonathan P. Williams, Alexa R. Anderson

Solar and Stellar Astrophysics

New scaling relations from MESA models for mass and radius of subgiant stars: application to Kepler targets

T. Çak{\i}r Alsaç, M. Y{\i}ld{\i}z and S. Örtel

The precise determination of the fundamental parameters of stars is crucial for understanding stellar structure and evolution. In this regard, the asteroseismic parameters of solar-like oscillating stars obtained from space telescopes are very useful for determining the mass ($M$) and radius ($R$) of single stars using classical scaling relations. These relations still need to be improved. In this study, we develop alternative scaling relations based on reference frequencies ($\nu_{\rm min}$) due to helium ionization zone glitches, specifically for subgiant (SG) stars. We compile main-sequence and SG {\small{MESA}} models from the literature and derive new scaling relations for stellar $R$ and surface gravity ($g$) of evolved stars. The expressions for $R$ and $g$ are obtained simultaneously as functions of the large frequency separation, $\nu_{\rm min}$, and metallicity. These new relations allow $M$ and $R$ to be reliably determined only from observational parameters, without the need for detailed stellar modelling. The resulting $\nu_{\rm min}$-based relations are applied to a sample of 31 SG stars observed by the Kepler mission, with masses in the range 0.85-1.74 M$_\odot$ and radii between 1.03 and 2.82 R$_\odot$. Using simultaneous solutions, we estimate new mass and radius ranges of 0.89-1.62 M$_\odot$ and 1.04-2.95 R$_\odot$, respectively. The results demonstrate significant improvements in the determination of $M$ and $R$ for evolved Kepler target stars, highlighting the potential of the new scaling relations in asteroseismic analyses.
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Solar and Stellar Astrophysics

The Role of Preceding CMEs and SIRs in Enhancing Shock Acceleration of Electrons

Xiaomin Chen, Seve Nyberg, Rami Vainio et al. (+3 more)

The role of large-scale pre-existing interplanetary structures, including preceding coronal mass ejections (CMEs) and stream interaction regions (SIRs), in shaping the shock acceleration environment for energetic electrons remains not fully understood. In this study, we investigate nine interplanetary shocks observed by the Solar Terrestrial Relations Observatory (STEREO) that are associated with significant MeV electron enhancements, as such enhancements are rarely observed at interplanetary shocks. We combine remote-sensing observations, drag-based modeling, and in-situ measurements to analyze the shock propagation through pre-existing interplanetary structures. Eight of the nine events are associated with a preceding slow or intermediate-speed CME, while six shocks are in-situ observed propagating within preceding ICMEs, indicating that large-scale upstream trapping structures are a common feature of these events. Further analysis identifies three distinct scenarios associated with enhanced electron acceleration: shocks propagating through preceding ICMEs, shock-SIR interactions, and direct injection of flare-accelerated electrons into SIRs. As a representative shock-in-ICME event, the 2012 January 29 low-$\beta$, quasi-perpendicular shock ($\theta_{Bn}\sim87^\circ$) is further investigated using observations together with one-dimensional Monte Carlo test-particle simulations of a shock propagating into a large-scale upstream magnetic loop. The simulation suggests that the upstream loop prolongs electron residence near the shock and substantially enhances acceleration efficiency. These results demonstrate that large-scale interplanetary structures can precondition the upstream magnetic environment, providing favorable conditions for prolonged electron residence and efficient shock acceleration.
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All authors Xiaomin Chen, Seve Nyberg, Rami Vainio, Immanuel Christopher Jebaraj, Chuan Li, Nina Dresing

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

Trans-Neptunian Object dynamics even better explained by a stellar flyby after 4.5 Gyr of evolution

Susanne Pfalzner, Frank W. Wagner and Marco Bischoff

The Trans-Neptunian objects (TNOs) formed together with the planets from a flat disc of gas and dust but now orbit the Sun mostly on inclined, eccentric orbits. One explanation for the TNOs' orbits is a close flyby of another star. One with a mass of $M_p =$0.8 Msun at $q_p =$ 110 au and $i_p =$70{\deg} fits the observations. However, such close encounters were more frequent when the Sun was young and still part of its birth cluster. Assuming that the flyby happened then, we use numerical $N$-body simulations to model how the TNOs' orbits changed due to interactions with the planets over the 4.56 Gyr since the Sun formed. We find that the cold Kuiper belt region lost about 80% and the hot Kuiper belt population about 40% of its initial population. About 7% –8% of the TNOs were injected into the planet region, but almost all (99%) were ejected afterwards. By contrast, the orbital parameters of distant Sedna-like objects ($q >$60 au) remained basically unchanged. Surprisingly, in sum, these changes improve the fit to the observed TNO population even more, strengthening the argument for a close flyby to the Solar System. The remaining differences concern the location of the cold population and the inclination distribution of the Sedna population. We discuss possible reasons and steps to resolve these issues.
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Solar and Stellar Astrophysics

NGC 1901: A new benchmark cluster for gyrochronology in the TESS Southern continuous viewing zone

Mara Bernizzoni, Timothy R. Bedding, Prasad Mani et al. (+5 more)

We use Gaia DR3 astrometry and TESS time-series photometry to study NGC 1901, a sparse open cluster in the TESS Southern continuous viewing zone, that has been neglected due to its proximity on the sky to the Large Magellanic Cloud. We refine its membership list and measure rotation periods for 32 high-confidence rotating stars. By applying the gyro-interp empirical model, we calculate a gyrochronal age of 830 $\pm$ 50 Myr. Because NGC 1901 possesses a sub-solar metallicity of [Fe/H]$\approx -0.11$, its excellent agreement with empirical models suggests that temperature-period relations mitigate metallicity effects for Sun-like stars. Our results place NGC 1901 in the gap between the benchmark open clusters Praesepe (670 Myr) and NGC 6811 (1 Gyr). We find that the rotation sequence exhibits significant spin-down stalling in late K dwarfs, supporting models of core–envelope recoupling. Furthermore, our findings provide constraints for the cluster's structural evolution, including its recently identified tidal tail and corona. This study establishes NGC 1901 as a new benchmark for calibrating the next generation of gyrochronal models.
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All authors Mara Bernizzoni, Timothy R. Bedding, Prasad Mani, Luke G. Bouma, Courtney L. Crawford, Jason Lee Curtis, Benjamin T. Montet, Samir Nepal

Solar and Stellar Astrophysics

Bifurcations in the Heliosphere: Global-scale Coronal Pseudostreamer Evolution and the Creation of Secondary Heliospheric Current Sheets

Lizet S. Casillas, Benjamin J. Lynch, Victor Réville et al. (+2 more)

The heliospheric current sheet (HCS) is the boundary between open magnetic fields of opposite polarity in the solar wind that connect back to their respective open flux regions in the corona. The HCS is often described as a disc-like sheet warped by the combined effects of the inclination of the magnetic equator on the Sun, solar rotation, and solar wind expansion. The HCS is an extension of the helmet streamer belt-the closed-flux region resulting from the global dipole component of the observed photospheric magnetic field. In the absence of a strong dipole, a dominant quadrupole can result in two separate HCSs. Here we report on a set of idealized, axisymmetric solar wind simulations designed to study the transition of the single-HCS corona and inner heliosphere into a multiple-HCS configuration, run with the WindPredict-AW PLUTO MHD code. We systematically vary the relative strengths of the dipole/quadrupole contributions to the global field and show this results in the formation of a large-scale, closed-flux pseudostreamer that interacts with the ambient solar wind. We find that when the dipole and quadrupole magnitudes are approximately equal, our model corona can oscillate between a one- and two-HCS configuration on the timescale of days-to-weeks. We characterize the plasma dynamics of the transition of a global-scale pseudostreamer into a localized, secondary helmet-streamer belt with its own HCS surrounding a newly-opened, opposite-polarity coronal hole. We discuss the implications of our results for solar cycle evolution and the dynamic coupling of the highly structured corona and inner heliosphere.
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All authors Lizet S. Casillas, Benjamin J. Lynch, Victor Réville, Olga A. Panasenco, Marco Velli

Wednesday 2 September

Solar and Stellar Astrophysics

Infrared-excess sources in VVV: candidate debris discs, SED characterization, and variability analysis

P. Esteves, R. K. Saito, C. Chavero et al. (+9 more)

Debris disks around main-sequence stars provide important insights into planetary formation and evolution; however, identifying these systems in the Galactic plane, where high interstellar extinction and source crowding are significant, remains challenging. Deep, wide-area surveys such as the VISTA Variables in the Via Lactea (VVV) ESO Public Survey provide multi-epoch infrared observations, enabling systematic searches in these environments. We present a method to construct a refined sample of main-sequence candidates by cross-matching data from Gaia DR3, GLIMPSE, DECaPS, and VIRAC2, to identify infrared excesses through spectral energy distribution (SED) analysis, and to analyse time-series data to detect variability potentially associated with circumstellar material or other astrophysical phenomena. Although infrared excess is commonly associated with debris disks, contributions from unresolved binaries or line-of-sight contamination cannot be excluded. Applying this methodology to the VVV tile d077 using the VIRAC2 catalogue, we analyse an initial sample of 879,556 sources and identify 446 sources flagged by the VO SED Analyser (VOSA) excess-detection algorithm as having infrared excess, of which 171 show candidate periodic variability in the VIRAC2 Ks-band light curves. We further perform SED fitting using theoretical models to derive physical parameters and approximate spectral types for these sources. This work provides a catalogue of infrared-excess candidates in a Galactic field affected by high extinction, along with a reproducible method suitable for future large-scale applications.
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All authors P. Esteves, R. K. Saito, C. Chavero, V. Fermiano, D. Minniti, B. W. Borges, C. Cáceres, Z. Guo, V. D. Ivanov, R. G. Kurtev, D. Quispe, R. M. Torres

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

Direct Four-Spacecraft Measurement of Reconnection Exhaust Thickness in the Solar Wind

Mani K Chettri, Rupak Mukherjee and Hemam D. Singh

The thickness of a reconnecting solar wind current sheet is normally inferred from a single spacecraft as the product of the boundary-normal speed and the crossing duration. Multipoint measurements have tested this method for generic solar wind current sheets and constrained the geometry of reconnection exhausts, but a direct comparison with single-spacecraft thickness estimates for the same reconnection-associated crossings is still lacking. We analyze four current-sheet crossings observed by Magnetospheric Multiscale in the pristine solar wind on 2017 November 10 the two boundaries of a confirmed reconnection exhaust and two nearby current sheets. The tetrahedron separation was $\approx 16$ km. Cross-correlation of the magnetic-field ramps gives millisecond-level lag uncertainties, boundary normals to $4^\circ-8^\circ$ and speeds to $5\%-8\%$, while single-threshold timing at these separations is noise dominated. The exhaust edges are 29-31 ion inertial lengths ($d_i$) thick. The two edge normals differ by $22^\circ$ (68% interval [$19^\circ,26^\circ$]), resolving a nonparallel exhaust geometry. At the exhaust edges, the measured boundary speeds agree with three standard single-spacecraft estimates to 4%-15% and remain within about one Alfvén speed of the plasma motion. Estimating the normal from one spacecraft is the larger source of error, though the thickness is still recovered to within tens of percent when a degenerate minimum-variance solution is rejected. At the actively reconnecting sheet, the boundary speed exceeds the local plasma speed along the normal by $29\% \pm 10\%$.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

Single-pulse reanalysis of the 2024 Vela glitch and new observations of PSR~J0437$-$4715 and PSR~J1644$-$4559

Carlos O. Lousto, Ruby Shrestha, Ezequiel Zubieta et al. (+6 more)

The Pulsar Monitoring in Argentina (PuMA) collaboration systematically monitors southern glitching pulsars, maintaining high-cadence single-pulse records of the Vela pulsar. We present a pulse-per-pulse reanalysis of the 2024 major glitch of Vela (PSR~J0835$-$4510) with the 400~MHz-bandwidth ROACH backend of the Argentine Institute of Radioastronomy, and extend our machine-learning single-pulse pipeline—Isolation Forest outlier rejection, $\beta$-Variational-AutoEncoder denoising, and Self-Organizing-Map clustering—to new observations of PSR~J1644$-$4559 and the millisecond pulsar PSR~J0437$-$4715. For Vela, the 4- and 6-cluster decompositions of the eight days bracketing the glitch reproduce, with seven times the previous bandwidth, the behavior found with the narrow-band ETTUS receivers: higher-amplitude clusters peak earlier in phase, are narrower, more skewed, and less populated. With the glitch jump and its two exponential recovery terms included in the timing solution, the mean profile is stable to 1% across all eight days (width change $-0.5\pm0.9$% from pre- to post-glitch); omitting the recovery terms would mimic a post-glitch broadening of up to 55% through a folding-frequency error at the $10^{-7}$ level. The clusters of PSR~J1644$-$4559 differ almost exclusively in amplitude, as expected for a scattering-dominated profile. For PSR~J0437$-$4715, retaining the 10% of pulses with the highest peak-dominance score doubles the signal-to-noise ratio, and a five-cluster decomposition yields narrow, phase-ordered groups a factor $3.6\pm0.4$ narrower than the average profile, suggesting a $\sim$3.5-fold improvement in cluster-based timing precision for this pulsar-timing-array target, to be confirmed in a follow-up paper.
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All authors Carlos O. Lousto, Ruby Shrestha, Ezequiel Zubieta, Susana B. Araujo Furlan, Guillermo Gancio, Federico García, Santiago del Palacio, Ryan Missel, Linwei Wang

Solar and Stellar Astrophysics

The scarcity of white dwarf-brown dwarf binaries in the solar neighbourhood: A population synthesis study

Alejandro Santos-García, Santiago Torres, Alberto Rebassa-Mansergas et al. (+3 more)

White dwarf–brown dwarf (WD–BD) binaries are intrinsically rare systems that provide a unique opportunity to study binary evolution and the formation of substellar companions. Despite recent observational progress, only a small number are known in the solar neighbourhood. We investigate whether their scarcity can be explained by binary formation and evolution together with observational selection effects. We extend the MRBIN binary population synthesis code into the substellar regime by incorporating brown dwarf evolutionary models and an extended initial mass function. We simulate the stellar population within 100 pc of the Sun and compare the predicted WD–BD population with the observed sample. Our simulations predict 13+-5 WD–BD systems within 100 pc, consistent with observations. These systems represent only 0.1% of the local white dwarf population, confirming their rarity. The models reproduce the overall distributions of white dwarf masses, temperatures, and orbital periods, although the simulated brown dwarfs are systematically hotter than observed. Close WD–BD binaries are underproduced with the standard common-envelope efficiency 0.3, suggesting that higher efficiencies may be required to reproduce the observed short-period systems. The simulations also reproduce the observed deficit of systems at intermediate orbital periods between post-common-envelope binaries and wide non-interacting systems, as well as a weakened brown dwarf desert in main-sequence–brown dwarf binaries.
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All authors Alejandro Santos-García, Santiago Torres, Alberto Rebassa-Mansergas, Sarah L. Casewell, Zhangliang Chen, Hongwei Ge

Solar and Stellar Astrophysics

Benchmarking Machine Learning Emulators of Stellar Evolution for Precision Asteroseismology

Naomi Gluck, Earl P. Bellinger, Yan Liang et al. (+5 more)

Fast and accurate stellar evolution emulators—surrogate models that approximate expensive simulation outputs with machine learning (ML)—are powerful tools for modern stellar characterization, hierarchical inference, and population synthesis. We analyze the grid density required for reliable emulation by training ML algorithms on main-sequence models with masses M=[0.7,1.2] solar masses. This range is challenging to emulate due to rapidly varying evolutionary behavior caused by the radiative-to-convective core transition, as well as the requirement to match the part-per-thousand seismic precision that has been delivered for such stars from the NASA Kepler mission. Generating grids from analytical models, as well as MESA, YREC, MIST, and ASTEC, we compare linear interpolation, k-nearest neighbors, random forests, and neural networks (NNs) in interpolating the stellar observables: T_eff, L, Delta nu, and nu_max. While NNs outperform other methods, sparse grids induce localized failures in the core-transition region, resulting in unstable derivatives, ensemble disagreement, and fragmented posterior distributions during inference. Performance gains from denser grids are non-uniform, suggesting that adaptive grid generation should be favored over uniform refinement. Finally, we show that NN ensembles allow for localized uncertainty propagation, more accurately reflecting emulator reliability across parameter space than global uncertainty estimates. As we consider only the two-dimensional case of varying only stellar mass and age along the main sequence, these results represent a lower bound on the challenge in emulating stellar evolution simulations for precision asteroseismology.
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All authors Naomi Gluck, Earl P. Bellinger, Yan Liang, Ebraheem Farag, Nicholas Saunders, Selim Kalici, Christopher J. Lindsay, Sarbani Basu

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

Vortex pinning and the elastic response of neutron-star crusts II. Non-axisymmetric loading and Magnus mountains

Elia Giliberti

Pinned superfluid vortices transmit a Magnus force to the neutron-star crust. A non-axisymmetric component of the superfluid-lattice lag can therefore support a persistent mass quadrupole. We calculate the l=m=2 response of self-gravitating, radially stratified spherical stars with SLy4 and BSk21 backgrounds, using a local microscopic pinning cap and a density-dependent Coulomb-lattice shear modulus. For a 1.4 M_sun star at 100 Hz, the equilibrium-compression maxima are epsilon=2.67x10^-9 (SLy4) and 2.44x10^-9 (BSk21). The global quadrupole is strikingly insensitive to the shear prescription: replacing mu=0.01P by the Coulomb profile and strongly varying uncertain edge layers changes the result only at the percent level. The dominant internal sensitivity is instead compressional. A density-resolved compressional kernel increases through the inner crust and steepens close to the crust-core interface, particularly for BSk21, showing that the deep inner crust controls the EoS dependence of the mountain. A fixed-composition calculation is used only as a non-relaxed sensitivity diagnostic, not as the secular prediction. At a common true local velocity benchmark, our fiducial ellipticities remain more than an order of magnitude below the complementary two-component cylindrical calculation of Gangwar & Jones (2026). We argue that the comparison points to the relative superfluid-lattice degree of freedom as the natural next ingredient for a common spherical model. The main result is therefore physical rather than numerical: the Magnus-mountain scale is robust to shear microphysics, while the deep-crust compressional response sets the leading uncertainty of the one-displacement model.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

Revisiting orbital recurrence in the dimmings of Boyajian's star (KIC~8462852)

Hector Socas-Navarro

KIC~8462852 (Boyajian's star) shows deep, asymmetric, and highly irregular dimming events with no simple repetition pattern. A symmetric 1.1% transit detected by TESS in 2019 has been interpreted as a possible giant planet or brown dwarf, while the irregular dips are generally attributed to transiting dust from exocomets or planetesimal fragments. We test whether the broader dip families may nevertheless recur orbitally. Our scenario assumes a major disruption that produced fragments with slightly different periods, causing successive returns to spread progressively in orbital phase. We compare this recurrent model with a null hypothesis of chronologically independent family centers, accounting for the observing window, and marginalize over the recurrence period $P$ and dispersion scale $\sigma_P$. We obtain $B_{10}=44.9$ in favor of recurrence, corresponding to “very strong” evidence on the Jeffreys scale. The maximum-likelihood solution is $P_{\rm ML}=781.6$~d and $\sigma_{P,\rm ML}=80.0$~d, close to the 776.1-d timescale obtained if D800 and the 2019 TESS event are separated by four revolutions. Independently of the timing fit, this period predicts a transit duration of about 20.5~h for the published stellar and transit parameters, in close agreement with the observed $\sim21$~h. The result is conditional on the adopted topology, priors, family definition, and observing selection function, and does not prove orbital recurrence. Nevertheless, the family timings strongly favor this specific recurrent-fragment model over independent occurrence, making progressive orbital phase dispersion a viable and physically motivated interpretation.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.IM

3.5-meter Segmented-Mirror Robotic Space Telescope Mission White Paper III. Key Scientific Mission: Exoplanet Science with a Coronagraph

Juhan Kim, Sang Hyun Lee, Yong-Woo Kang et al. (+22 more)

This volume defines the exoplanet science program enabled by the dedicated high-contrast coronagraph in the baseline science payload of the 3.5-meter Segmented-Mirror Robotic Space Telescope. The observatory architecture incorporates the optical interfaces, wavefront sensing and control, pointing stability, and operations software required for coronagraphic observations from the outset. The observing strategy gives priority to the nearest stellar systems because they provide the most accessible laboratories for planetary exploration and the most likely destinations of future interstellar missions. The diffraction limit sets a reflected-light horizon of roughly 10–15 pc for planets at 1 AU and roughly 50–80 pc for Jupiter analogs. Within those horizons, the telescope can image nearby giant planets, obtain reflected-light spectra of their atmospheres, survey young systems and circumstellar disks, and support the habitability and biosignature programs that larger future missions will pursue. The wide-field imager complements the coronagraph through transit photometry, occurrence-rate statistics, and long-term monitoring of stellar magnetic activity. A systematic census of the nearest stellar neighbors provides a lasting reference for exoplanet science and future space exploration.
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All authors Juhan Kim, Sang Hyun Lee, Yong-Woo Kang, Jeong-Yeol Han, Sungwook E. Hong, Bongkon Moon, Donguk Song, Juhyung Kang, Myeong-Gu Park, Sang Chul Kim, Chung-Uk Lee, Sangmo Tony Sohn, Arman Shafieloo, David Parkinson, Hong Soo Park, Dohyeong Kim, Chan Park, Jungjoo Sohn, Young-Beom Jeon, Jong-Hak Woo, Hyung Mok Lee, Hong Bae Ann, Myungkook James Jee, Mansoo Choi, Changbom Park

Solar and Stellar Astrophysics

Solar Soft X-ray Coronal Dimming in a Failed Eruption Associated with Plasma Cooling

Xinyue Wang, Astrid M. Veronig, Hechao Chen et al. (+1 more)

Coronal dimmings are observed as sudden and localized reductions in the extreme-ultraviolet and X-ray emission of the solar corona. Traditionally, significant dimmings of spectral lines formed at temperatures of 1-2 MK are regarded as indicators of coronal mass ejections (CMEs), reflecting the density depletion caused by plasma escaping into interplanetary space. In this Letter, we report a peculiar deep coronal dimming event predominantly observed in high-temperature spectral lines following an M8.8-class confined solar flare associated with a failed filament eruption. Sun-as-a-star measurements from the Geostationary Operational Environmental Satellite and the Extreme Ultraviolet Variability Experiment reveal intensity reductions exceeding 30% in soft X-ray (SXR) and measurable decreases in Fe XVIII (6.5 MK) and Fe XX (9.3 MK). Spatially resolved observations from the Atmospheric Imaging Assembly demonstrate that the dimming originates from the active region core, while the Solar Terrestrial Relations Observatory-A shows no evidence for CME-driven mass loss. Differential emission measure analysis reveals plasma at temperatures >5 MK cooling into lower temperatures, supporting plasma cooling as the dominant contributor to the hot-band dimming rather than CME-associated plasma escape. This event demonstrates that deep hot SXR dimmings can occur without substantial CME-driven mass loss, suggesting that alternative physical mechanisms may also account for unresolved stellar dimmings in addition to the commonly inferred CME signatures.
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All authors Xinyue Wang, Astrid M. Veronig, Hechao Chen, Hui Tian

Solar and Stellar Astrophysics

Visual Discovery of Fading Events in the Wolf-Rayet Star WR 80

Rod Stubbings

Long-term visual and CCD V-band monitoring of the heavily reddened Wolf-Rayet star WR 80 from 2017-2026 reveals optical fading events that depart from its long-term stable baseline. On 26 April 2017 (JD 2457870), I discovered a deep minimum using a 22-inch (0.56-m) telescope, with the source plunging from a baseline of magnitude 14.6 down to magnitude 15.8. This 6-day event was subsequently verified via archival ASAS-SN V-band records. In July 2018, my visual observations identified a short duration fade to magnitude 15.4 that went undetected in concurrent electronic V band data streams, indicating a masking effect likely caused by near-infrared red leak contamination in the CCD filters. Following a stable phase recorded by continuous CCD V-band tracking from 2023 through early 2026, a mid-2026 visual detection caught a full 16-day fading event reaching a minimum magnitude of 15.3. These combined datasets establish that WR 80 undergoes highly variable optical obscuration events.
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Solar and Stellar Astrophysics

Multiplicity of Cool Stars and their Evolution

Matthew I. Swayne, Leen Decin, Theo Khouri et al. (+12 more)

Making up a sizeable portion of the galactic census, stellar multiples are experiencing a renaissance. Enabling the study of multiple strands of the study of cool stars, stellar multiples have been found and characterised in great numbers by the missions of the last decade, allowing the exploration of stellar parameters and populations, observation of stellar interactions, studies into stellar formation and evolution, and characterisation of circumbinary systems. This exciting explosion of science is only set to continue, with future missions set to offer even further insights into the topic. Within these proceedings we will summarise the presentations and discussions on cool stellar multiplicity within our splinter sessions at the 23rd Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, as we examine the present state of the field and look to what the future may bring.
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All authors Matthew I. Swayne, Leen Decin, Theo Khouri, Ayush Moharana, Léa Planquart, John Southworth, Nikki J. Miller, José A. Caballero, Orsolo De Marco, Krzysztof G. Hełminiak, Swetlana Hubrig, Valentin D. Ivanov, Pierre Kervella, Thibault Merle, Dorota M. Skowron

Solar and Stellar Astrophysics

Surface brightness inhomogeneity on the helium-rich hot subdwarf in the X-ray binary HD 49798

C. Simon Jeffery and Gavin Ramsay

HD 49798 is an X-ray binary consisting of a hot helium-rich subdwarf primary and an accreting compact companion with an orbital period of 1.55 d. TESS observations of its optical light curve suggest short-period brightenings superimposed on a sinusoidal variation. The latter can be attributed to the tidal deformation of the primary, while frequency analysis of the additional variations shows up to eleven frequencies which form an harmonic series based on a period of 1.4 d, in excellent agreement with the stellar rotation period inferred independently from spectroscopy. These brightenings may be explained by a slowly changing and inhomogeneous temperature distribution across the surface of the hot subdwarf, such as might be associated with a magnetic field or with the presence of Rossby waves in the stellar envelope.
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Solar and Stellar Astrophysics · Cross-listed from physics.plasm-ph

Instabilities in Cylindrical Geometry Using the Minimalist Approach: Formalism and Rotational Instabilities

Nektarios Vlahakis

The minimalist approach for linear stability analysis is applied to fluids and magnetized ideal plasmas in cylindrical geometry. In this approach, the dispersion relation is obtained by integrating a single first-order differential equation - referred to as the principal equation - subject to appropriate boundary conditions. We first derive the principal equation for a general unperturbed state with radially varying density and pressure, axial and azimuthal components of both the velocity and magnetic field, and a radially directed gravitational field. We then use this formulation to analyze rotating flows with axial magnetic fields, addressing both wall-bounded and interface-driven axisymmetric instabilities. In addition to exact results for selected unperturbed states, we obtain approximate dispersion relations using the WKBJ method in the incompressible and compressible limits. The analysis encompasses centrifugal, magnetorotational, and buoyancy-driven instabilities as special cases, and it clarifies how compressibility modifies their stability properties.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.IM

Demonstrating the Time-Domain Capabilities of the 4-m International Liquid Mirror Telescope: An Early Census of Transient and Variable Detections

Kumar Pranshu, Kuntal Misra, Bhavya Ailawadhi et al. (+11 more)

The PyLMT transient detection pipeline has been operational since November 2023, detecting transient and variable objects in the ILMT images in almost real time. Using the image subtraction technique, nearly 3700 CCD frames have been analyzed by the automated pipeline, generating nearly 23,000 alerts for the detection of verified transient candidates and cataloged variable sources. Around 21,000 of the alerts correspond to known MPC asteroids, nearly 2000 correspond to variable stars (including eclipsing binaries, RR Lyrae, Delta Scuti, T-Tauri, etc.), 509 correspond to variable AGNs (including QSOs, Seyfert galaxies, and blazars), 21 supernova candidates, and several other interesting candidates. We provide a concise overview of the detections and their significance, emphasizing the surveys potential contributions to a broad class of astrophysical and scientific cases. A transient detection dashboard called DART was developed using Streamlit to visualize and categorize candidates based on PyLMT and SIMBAD classifications. It includes cone-search functionality and displays key metadata, offering an intuitive interface that is publicly accessible. Our results demonstrate the viability of liquid mirror telescopes such as the ILMT for time-domain astronomy, emphasizing the important role that small-field survey facilities can play in systematic transient science programs.
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All authors Kumar Pranshu, Kuntal Misra, Bhavya Ailawadhi, Monalisa Dubey, Naveen Dukiya, Sara Filali, Paul Hickson, Priyanshi Kumari, Gokul Singh Mehra, Vibhore Negi, Jeewan C. Pandey, Anna Pospieszalska-Surdej, Jean Surdej, Sarvesh Kumar Yadav

Solar and Stellar Astrophysics

The relationship between solar and stellar tachoclines, dynamos, and spin-down

Loren I. Matilsky, Lydia Korre and Nicholas Brummell

The solar tachocline, a thin shear layer separating the differentially rotating convective zone (CZ) from the underlying rigidly rotating radiative zone (RZ), remains a dynamical mystery. Under the influence of "radiative spreading" (the process by which meridional circulation, baroclinicity, and differential rotation burrow through a stably stratified fluid), the tachocline should have spread significantly by the current age of the Sun. Some unknown torque must therefore rigidify the whole solar interior below the CZ and keep the tachocline confined to a thin layer. At the same time, the spin-down process (through which cool stars shed angular momentum via surface magnetic torques) must extract angular momentum from the solar RZ, presenting a second mystery: solar spin-down must be communicated from the near-surface layers to the deep interior, all the while leaving the tachocline intact. In this work, we explicitly analyze the dynamics of radiative spreading in two 3D, spherical-shell, fully nonlinear fluid simulations of a solar-like CZ–RZ system, one without a magnetic field (hydrodynamic—HD) and one with a small random seed magnetic field (magnetohydrodynamic—MHD). We find that radiative spreading is unmitigated in the HD case (as expected), but in the MHD case, a self-excited dynamo not only confines the tachocline, but also extracts angular momentum from the RZ, thereby communicating the spin-down downward. We thus speculate that solar and stellar tachoclines may be intimately linked to both the spin-down process and global dynamo.
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Solar and Stellar Astrophysics

Electron acceleration by turbulent reconnection in solar flares

Zining Ren, Xin Cheng, Yulei Wang et al. (+1 more)

Solar flares can release magnetic energy explosively in the corona and produce high-energy particles on short timescales. However, how and where these particles are accelerated remains an open question. Here, we investigate the acceleration and transport of electrons during self-developed three-dimensional turbulent reconnection of solar flares by solving Parker's transport equation in the framework of high-resolution MHD simulations. We find that thermal electrons at both the flare current sheet and loop top are rapidly accelerated up to ~90keV, with energy spectra exhibiting a power-law feature. Although the population of accelerated electrons at the flare loop top is larger than that at the current sheet, their spectral indices are similar, close to the values usually observed. More importantly, the acceleration is achieved by turbulence-driven compression structures of various scales rather than the supposed termination shock, particularly at the flare loop top. A portion of compression structures even forms shocks. These results highlight the critical role of turbulent reconnection in accelerating electrons, thereby shedding new light on the acceleration and transport of particles in other high-energy phenomena.
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All authors Zining Ren, Xin Cheng, Yulei Wang, Mingde Ding

Tuesday 1 September

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

EWOCS-IX: JWST/NIRCam observations of Westerlund 2 - Identification of candidate substellar members

V. Almendros-Abad, M.G. Guarcello, K. Muzic et al. (+26 more)

[Abridged] Investigating substellar populations in young massive clusters offers crucial insights into the formation of brown dwarfs (BDs) and the role of environmental conditions in shaping their properties. Westerlund 2 (Wd2), as one of the nearest dense and massive clusters in the Milky Way, represents an ideal laboratory for studying the effects of high stellar density and ionizing radiation from massive stars on BD formation. This paper presents deep JWST/NIRCam observations of Wd2 in a large number of filters between 1.15 and 4.1 $\mu m$. Our analysis focuses on identifying and characterizing the BD population within the cluster. We carried out PSF photometry on deep JWST/NIRCam data obtained in 4 wide and 6 medium-band filters, using DOLPHOT. The resulting catalog was used to identify BD candidates in Wd2 through spectral energy distribution (SED) fitting with atmospheric models. The 50% detection limit of our NIRCam catalog is $\sim$0.015-0.02 $M_\odot$, at the distance, age, and extinction of Wd2, providing the deepest view of a supermassive star cluster to date. We identify 353 substellar candidates. Most candidates (301) lie above the 10 Myr isochrone in the Hertzsprung–Russell diagram consistent with cluster membership, which are defined as strong candidates. Comparison with a control field indicates a contamination level of $\lesssim$5% for the strong candidate sample down to masses of $\sim$0.01–0.015~$M_\odot$. We identify 73 candidates exhibiting infrared excess indicative of circumstellar disks. These objects occupy the expected infrared-excess locus in de-reddened color diagrams and represent $27.7^{+3.4}_{-3.2}$% of the candidates detected in F410M. The resulting catalog provides a well-characterized sample for future spectroscopic confirmation and subsequent studies of the substellar population of Wd2.
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All authors V. Almendros-Abad, M.G. Guarcello, K. Muzic, A. Scholz, M. Andersen, A. Bayo, W. Best, D. Capela, M. Gennaro, A. Ginsburg, J.B. Lovell, K. Monsch, E. Moraux, L. Prisinzano, T. Rom, E. Sabbi, P. Zeidler, C. Argiroffi, R. Bonito, D. Calzetti, V. Cusimano, F. Damiani, J.J. Drake, T.J. Haworth, N.D. Richardson, M. Robberto, S. Sciortino, N.J. Wright, T. Ziliotto

Solar and Stellar Astrophysics

Mass-dependent multiplicity fraction in low mass stars revealed by Gaia astrometry

Ben Pennell, Hans-Walter Rix, Kareem El-Badry et al. (+3 more)

Stellar multiplicity at different orbital periods is probed by different techniques: radial velocities at the shortest periods, direct imaging at the longest, and astrometry in between. Gaia DR3 provides unprecedented astrometric information to constrain binary populations with periods of tens to thousands of days. Beyond the small fraction of direct orbit solutions, Gaia publishes coarser astrometric diagnostics for all objects, such as on-sky accelerations, jerks, or the single-star goodness-of-fit measure, RUWE. We show that these diagnostics together are sensitive to a far wider range of orbital periods than orbit solutions alone. Built on the Gaiamock emulator, we develop a forward-modelling framework to constrain rates of multiplicity from Gaia data, using all astrometric solution types. We apply this framework to $366~027$ primaries of $0.2$–$0.75\,M_\odot$ between $50$ and $200$ pc, including vastly more M-dwarfs than previous multiplicity studies. For fiducial models of the period and mass-ratio distributions, the inferred multiplicity fraction is approximately constant at ${\sim}45\%$ from the solar-type regime down to $0.4\,M_\odot$, only then dropping to ${\sim}15\%$ by $0.2\,M_\odot$. This mass-multiplicity relationship revises the conventional picture of a monotonically rising mass–multiplicity relation, and aligns the upper M-dwarf regime with solar-type stars rather than placing it on a continuous decline.
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All authors Ben Pennell, Hans-Walter Rix, Kareem El-Badry, Sahar Shahaf, Johanna Müller-Horn, Jiadong Li

Solar and Stellar Astrophysics

Chemical evolution of Na, Mg, and Al in the Galactic bulge from UVES data

R. P. Nunes, B. Barbuy, C. Chiappini et al. (+9 more)

The formation of the Galactic bulge remains incompletely understood, with evidence pointing to different stellar populations, including a bar-driven component, an inner-disk population, and an older spheroidal component. Chemical abundances provide critical constraints on the origin of these populations, particularly for odd-Z elements such as Na and Al, as well as Mg, whose behaviour at high metallicity is still a matter of debate. We aim to investigate the presence of overabundances of Na, Mg, and Al in Galactic bulge stars, with particular emphasis on the metal-rich regime, and to evaluate their consistency with predictions from chemical evolution models. We re-derived the abundances of Na, Mg, and Al for a sample of 55 bulge red giants previously analysed in the literature. Our study is based on high-resolution UVES spectra obtained with the ESO Very Large Telescope and employs spectrum synthesis using the Turbospectrum code, with updated atomic and molecular line lists. We find somewhat lower abundances of Mg and Al at the metal-rich end than previous studies, while a fraction of the metal-rich stars still exhibit significant Na enhancements. These enhancements persist when different sets of stellar parameters are adopted, indicating that they are robust. The presence of Na-enhanced stars at high metallicity is difficult to reconcile with standard chemical evolution models and suggests additional enrichment processes in the bulge, or a particular behaviour of stellar yields with metallicity. The Na enhancement could be due to metallicity-dependent yields from massive stars, not taken into account in available models, and/or enrichment by asymptotic giant branch stars, or due to second-generation stars evaporated from globular clusters, the latter option arising because for the metal-rich ([Fe/H]>0) stars a Na-O anti-correlation appears to occur.
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All authors R. P. Nunes, B. Barbuy, C. Chiappini, A. Friaça, M. C. Jelin, V. Hill, M. Zoccali, A. Renzini, A. Pérez-Villegas, H. Ernandes, S. O. Souza, M. J. Castro

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

Precise Stellar Age Constraints and Habitable Zone Evolution in Exoplanet Systems

Grace Henry, Ted von Hippel and Anna Childs

Accurate stellar ages are fundamental to interpreting the evolutionary histories of habitable zone (HZ) planets. Because stellar luminosity evolves over time, HZ boundaries migrate outward, meaning that present-day HZ planets may have experienced different irradiation environments earlier in their evolution. Accurate ages are therefore required to estimate HZ residence times and place planetary habitability in an evolutionary context. We use the Bayesian Analysis of Stellar Evolution with nine parameters (BASE-9) code to constrain ages and masses for exoplanet host stars with planets located in the HZ. Our sample includes single stars with effective temperatures of 4475-7200 K that host planets within their calculated HZ boundaries. We fit isochrones to broadband photometry from Gaia DR3, Pan-STARRS, and 2MASS jointly with Gaia parallaxes, spectroscopic metallicities, and extinction estimates, and assess the resulting age posterior distributions. Using these age constraints, we track HZ migration along stellar evolutionary sequences to estimate continuous HZ residence times, allowing us to identify ideal targets for followup high resolution characterization. Because isochrones separate most strongly near and beyond the main-sequence turnoff, our analysis emphasizes evolved and turnoff hosts, where BASE-9 produces the tightest constraints. We quantify age precision across the sample, yielding highly precise ages for subgiants and evolved main-sequence or turnoff stars, and less precise but measurable ages for lower main-sequence hosts. The final catalog provides our age constraints for 149 HZ planets around 146 host stars and supports evolutionary interpretations of planetary histories.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

The Mass-Ratio Distribution of the Low-Mass Binary Black Hole Subpopulation: A Natural Outcome of Isolated Binary Evolution

Monica Gallegos-Garcia, Anarya Ray, Vicky Kalogera et al. (+7 more)

The observed merging binary black hole population is increasingly consistent with being composed of a mixture of subpopulations, each likely the result of different formation mechanisms. In particular, the low-mass subpopulation, with primary black hole masses below $\approxeq 15\,M_{\odot}$, has been attributed to mergers formed through isolated binary evolution. We use the binary population synthesis code POSYDON to study the mass ratio $(q)$ distribution of binary black hole mergers from isolated binary evolution and compare to the observed low-mass subpopulation. We explore variations in supernova remnant prescriptions, common-envelope efficiency, and black hole accretion efficiency. We find that our models have a preference for asymmetric $q$, most with a broad peak near $q\approxeq0.5-0.7$, and a near-equal-mass component whose relative strength varies across models. This resultant $q$ distribution is consistent with the $q$ distribution of the low-mass subpopulation observed with gravitational waves. We find that for the majority of models these features arise from physically distinct formation subchannels: the asymmetric peak reflects contributions from common-envelope and stable mass-transfer systems, while the near-equal-mass component traces double-core common envelope and contact systems. We conclude that the features in the $q$ distribution of the observed low-mass subpopulation emerge naturally from isolated binary evolution across a range of model assumptions. As the gravitational-wave catalog continues to grow, the relative strength of these $q$ features will provide an increasingly powerful diagnostic of isolated binary evolution and its formation subchannels.
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All authors Monica Gallegos-Garcia, Anarya Ray, Vicky Kalogera, Max Briel, Michael Zevin, Abhishek Chattaraj, Zepei Xing, Jeff J. Andrews, Seth Gossage, Philipp M. Srivastava

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

Amortized Opacity Marginalization Improves C/O Interval Calibration for Brown-Dwarf Retrievals

Kellen Heraty

Molecular line lists disagree at a level that measurably shifts retrieved C/O, so retrievals conditioned on a single list omit a dominant systematic from their error bars. Classical marginalization re-runs a full retrieval under K opacity realizations per object. We move that cost into simulation: while generating the training set we randomize opacities (smooth multiplicative Fourier fields in $\log\nu$ plus a discrete line-list mixture, under a deliberately broad prior informed by measured inter-list differences), so a neural posterior estimator learns an already-marginalized posterior. Conceptually, this is the amortized analogue of repeating a conventional retrieval under many plausible opacity realizations and averaging the resulting posteriors. On perturbed held-out spectra (n=2048) a standard NPE covers nominal 90% C/O intervals 43.3% of the time against 82.4% for a compute-matched marginalized network; three-seed control ensembles reach 93.2% on clean data but 61.4% under perturbation, against 89.4% marginalized, isolating the deficit as opacity-induced (TARP joint expected-coverage deviation 0.005 against 0.035 for a single control network). An axis ablation traces the gain to the continuous field (45.4% for the discrete swaps alone, 79.3% for the field alone). Applying the 105 real bad-pixel masks with median filling then collapsed the ensemble's C/O coverage from 0.901 to 0.479; retraining on the same mask distribution and pooling three such seeds returns a nominal 0.905 C/O and 0.891 overall under the full deployment path, still in simulation (n=1024, same 105 masks), at 0.21 to 0.31 of the prior width. On 105 JWST/G395H spectral products of 13 late-T/Y dwarfs, single marginalized networks bracket the Hood et al. (2024) T8 benchmark C/O where the control excludes it, a consistency check rather than a validation, at 1.6-2.6x wider intervals.
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Solar and Stellar Astrophysics

The quiescent states of V745 Sco and V3890 Sgr: VLT/X-shooter and Swift/XRT+UVOT observations

Bor Jamnik, Ulisse Munari, Nicola Masetti et al. (+2 more)

Symbiotic recurrent novae are symbiotic binaries in which multiple thermonuclear outbursts have been observed. Only four such systems are currently known in the Galaxy: T CrB, RS Oph, V745 Sco, and V3890 Sgr. They provide convenient laboratories for studying accretion and binary interaction in symbiotic novae during quiescence. We characterise the quiescent states of the less-studied systems V745 Sco and V3890 Sgr and compare their properties with those of T CrB and RS Oph. We analyse VLT/X-shooter spectra together with Swift/XRT and UVOT observations, supplemented by ground-based optical photometry and archival data. We determine the spectral types of the red-giant donors and compare the spectral energy distributions of the systems with those of single bulge giants of the same spectral type to investigate the properties of the accretion. We classify the donor stars in V745 Sco and V3890 Sgr as M7III and M6.5III, respectively. We derive reddenings and distances consistent with both systems belonging to the Galactic bulge. Both objects exhibit a significant ultraviolet excess relative to normal late-type giants, indicating ongoing accretion during quiescence. From this excess we estimate mass transfer rates of the order of $\sim 5 \times 10^{-8}\ M_{\odot}\ \mathrm{yr}^{-1}$, broadly consistent with the observed recurrence timescales. These results suggest that recurrent nova eruptions in V745 Sco and V3890 Sgr can be sustained by long-term accretion at approximately the present rate, without requiring an episode of enhanced accretion similar to that observed in T CrB.
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All authors Bor Jamnik, Ulisse Munari, Nicola Masetti, Gregor Traven, Manica Perko