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Physics Astrophysics Solar and Stellar Astrophysics

astro-ph.SR Watch Solar and Stellar Astrophysics

Monday 12 October

Solar and Stellar Astrophysics

SiO Maser Line-width Diagnostics of AGB Stars and Red Supergiants: The RSG-adjacent Region in the FWZI–Luminosity Plane

Qiu-Yu Tao, Jun-ichi Nakashima, Yong Zhang et al. (+3 more)

We investigate whether SiO maser line profiles can distinguish asymptotic giant branch (AGB) stars from red supergiants (RSGs) and identify unusual AGB-classified sources near the RSG regime. We compiled 2,956 detected SiO maser spectra for 1,551 objects and constructed reference samples of 1,068 O-rich AGB stars and 36 RSGs. The full width at zero intensity (FWZI) was reliably measured for 1,020 AGB stars and 32 RSGs. AGB stars are concentrated at narrow line widths: 81.5% have FWZI <10 km s^-1, whereas 81.2% of RSGs have FWZI >15 km s^-1 and none has FWZI <10 km s^-1. Infrared two-color diagrams provide only partial separation. Combining FWZI with luminosities based primarily on quality-filtered Gaia/Bailer-Jones distances, with independent literature distances for several RSGs, reveals a weak tendency for wider SiO maser lines at higher luminosities but no clean boundary. We therefore define an empirical RSG-adjacent region in the FWZI-luminosity plane, 15 <= FWZI <= 26.6 km s^-1 and 3.9 <= log(L/L_sun) <= 4.9, to characterize the high-luminosity, broad-line tail of the AGB-classified sample. Ten sources fall within this region. Using infrared SEDs, DUSTY fits, OH/H2O maser information, and variability, we find that six are most naturally interpreted as luminous AGB-like objects, two warrant follow-up to assess possible high-initial-mass AGB, RSG-like, or SAGB-related interpretations, and two are less securely constrained because of poor DUSTY fits. The RSG-adjacent region is therefore a physically mixed screening space, not a definitive classification box, for selecting high-priority follow-up targets.
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All authors Qiu-Yu Tao, Jun-ichi Nakashima, Yong Zhang, Zehao Zhang, Jia-Yong Xie, Chang Liu

Solar and Stellar Astrophysics

From the QBO to Rieger-type periods: Explaining extreme solar events by tidally triggered magneto-Rossby waves

F. Stefani, G.M. Horstmann, G. Mamatsashvili et al. (+1 more)

It has recently been demonstrated that the occurrence of extreme solar events is governed by a phase-stable period close to 1.723 years (629.29 days). According to our previously developed tidally-synchronized solar dynamo model, this is one of the periods resulting from two-planet spring tides exerted by the tidally dominant planets Venus, Earth and Jupiter, which trigger magneto-Rossby waves in the solar tachocline. Overall, the squared sum of these three waves incorporates nine periods ranging from 59 to 629 days. Using a standard Rayleigh test, we demonstrate that all nine of these periods can be identified in the data on extreme solar events. For the latter, we use a combination of 72 ground-level enhancement (GLE) events with various X-flare data, the number of which increases when the threshold is successively lowered from X10 to X1. To identify the spectral peaks, it is necessary to analyze the sidebands that emerge due to the modulation with the longer-term periods such as the 11-year Schwabe cycle and the third harmonics of the Hale cycle. In this framework, the seminal Rieger period of 154 days is re-interpreted as the lower sideband of 146 days which is the quarter of the synodic period of Venus and Earth.
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All authors F. Stefani, G.M. Horstmann, G. Mamatsashvili, T. Weier

Solar and Stellar Astrophysics

Grand minima and intermittency in near-critical convection-driven dynamos

P. J. Käpylä

Context: Solar magnetism exhibits long-term variations on timescales of decades to centuries. Aims: The main aim is to study how the supercriticality of the dynamo affects the variability of magnetic fields on timescales relevant to the Sun. Methods: Three-dimensional rotating magnetohydrodynamic simulations in a spherical wedge geometry are used to produce self-consistent convection-driven dynamos. The supercriticality of the dynamo is controlled by varying the magnetic diffusivity. Results: The main result is that long-term variability of the dynamo increases with decreasing supercriticality of the dynamo and that cases nearest to the onset of dynamo action show extended grand minima-like periods of low activity. The simulations fall in three classes where the least supercritical cases show the largest variations of magnetic activity and where the flow including differential rotation is almost unaffected by the magnetic field. In the cases with the highest supercriticality the magnetic field is permanently strong and differential rotation permanently quenched, both having relatively little temporal variation. In an intermediate regime both the magnetic fields and differential rotation exhibit strong anticorrelated fluctuations. Conclusions: The occurrence of grand minima in the Sun has been conjectured to be due to a weakly supercritical dynamo. The current results support this idea and suggest that the absence of grand minima in earlier global dynamo simulations is because their large-scale dynamos were too supercritical. Furthermore, the current results suggest that the effect of the magnetic field on differential rotation is negligible if the average rms-field is less than about 2~kG.
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Solar and Stellar Astrophysics

Integration of a CME Flux Rope Model with Magnetic Field Draping in Heliospheric Simulations

Ranadeep Sarkar, Jens Pomoell, Emilia Kilpua et al. (+1 more)

Accurate prediction of the magnetic structure of interplanetary coronal mass ejections (ICMEs) remains a key challenge in space weather forecasting. Despite several efforts using global MHD modeling, it remains difficult to deduce a flux-rope solution that can accurately and reliably reproduce the magnetic structure of a CME. We present FROST (Flux Rope embedded in the Solar Magnetic Field), a new flux-rope model implementation within the European Heliospheric Forecast Information Asset (EUHFORIA), featuring an initially force-free toroidal flux rope embedded in the low-coronal magnetic field. FROST employs a novel embedding technique that self-consistently generates a draping magnetic field around the flux-rope, ensuring continuity of the normal magnetic field component at the CME boundary. The dynamics of the flux-rope in the low and middle corona is solved by a non-uniform advection constrained by the observed kinematics of the event. This results in a global, non-toroidal, stretched loop-like magnetic structure. At heliospheric distances, the evolution is modeled as an MHD process using EUHFORIA. We validate the FROST model implementation by simulating the CME event of 2013 April 11 and comparing the results with in situ magnetic field measurements at 1 au. The modeled magnetic field profiles of the flux-rope exhibit strong agreement with the observations, with correlation coefficients ranging from 0.64-0.93 for Bx, 0.92-0.95 for By, and 0.82-0.87 for Bz component of the magnetic field. These results demonstrate the capability of the proposed flux-rope model to significantly improve ICME magnetic field prediction, highlighting its strong potential for advancing space-weather forecasting capabilities.
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All authors Ranadeep Sarkar, Jens Pomoell, Emilia Kilpua, Eleanna Asvestari

Sunday 11 October

Solar and Stellar Astrophysics

An asteroseismic age estimate for the Pleiades using a neural-network emulator of rotating $\delta$ Scuti pulsation models

Anuj Gautam and Simon J. Murphy

Asteroseismology of $\delta$ Sct stars has strong potential for determining the ages of young stars through individual mode fitting. We present a neural-network emulator trained on approximately 46 million rotating pulsation models. From stellar mass, metallicity, age, and equatorial rotation velocity, it predicts 12 classical observables, two asteroseismic scalars, and the p mode, f mode and low-order g mode frequencies for degrees $\ell=0$-3, including the individual $m$ components of rotationally split multiplets. We couple the emulator to nested sampling through a multivariate likelihood that propagates correlated emulator residuals, making full Bayesian posterior exploration practical. Applied to five $\delta$ Sct stars in the Pleiades observed by TESS, the framework gives mutually consistent individual asteroseismic ages of 107-117 Myr. Combining their age posteriors gives a cluster age of $113.3\pm4.5$ Myr (statistical), consistent with the $112 \pm 5$ Myr lithium-depletion-boundary age and the 110-160 Myr range from rotating-isochrone fitting. Together with spectroscopic $v\sin i$, the inferred equatorial velocities also provide post-fit inclination estimates. Separate perturbed-grid tests using 100 matched synthetic pairs per input-physics variation give median absolute age shifts of $2.1$, $2.3$, and $5.2$ Myr for $|\Delta f_{\rm ov}|=0.005$, $|\Delta\alpha_{\rm MLT}|=0.1$, and $|\Delta Y|=0.02$, respectively; the corresponding 90th percentile shifts are $6.5$, $7.6$, and $16.2$ Myr. These tests describe conditional population sensitivities, not a systematic uncertainty on the Pleiades age. Together, the cluster-age agreement and the scale of these input-physics sensitivities support $\delta$ Sct asteroseismology as a complementary age estimation method for young stars.
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Solar and Stellar Astrophysics

Assessing the impact of background subtraction on a dust-free corona

Erika Palmerio, Cooper Downs, Phillip Hess et al. (+4 more)

Context. White-light observations of the solar corona contain contributions from both the electron-scattered K-corona and the dust-scattered F-corona. Because the diffuse F-corona dominates the measured intensity at altitudes of approximately 4 $R_{\odot}$ and above, background subtraction techniques are routinely employed to isolate the electron corona in coronagraph observations. Aims. Since no direct observational "ground truth" for the K-corona exists, the amount of coronal signal that is inadvertently removed by background subtraction methods remains unknown. We aim to quantify, for the first time, the impact of background subtraction on the inferred large-scale structure and brightness of the solar corona. Methods. We use a 32-day simulation based on a novel near-real-time, data-assimilative, time-evolving magnetohydrodynamic model to generate synthetic white-light observables. Because these synthetic observations are free from dust and instrumental stray-light contributions, they provide direct access to the "true" K-corona and enable a quantitative assessment of the effects of background subtraction. Results. We find that background subtraction generally preserves the large-scale morphology of the electron corona, including the locations and shapes of major coronal structures. However, the procedure systematically modifies the relative brightness of different features and removes a non-negligible fraction of the K-coronal signal. Conclusions. Although standard background subtraction techniques successfully recover the overall appearance of the electron corona, they can alter its quantitative photometric properties. These findings should be considered when interpreting coronal brightness measurements and deriving physical properties from processed white-light observations.
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All authors Erika Palmerio, Cooper Downs, Phillip Hess, Benjamin J. Lynch, Ryder Davidson, Ronald M. Caplan, Jon A. Linker

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

Scaled three-dimensional simulations of giant-planet accretion and disk clearing

Ignacio Mosqueira

We develop accelerated three-dimensional simulations of growing planet pairs that draw on a finite gas reservoir. An annulus compression (Scaling A) shortens the predicted clearing time and shrinks the computational domain at equal grid resolution while preserving key physical parameters. Planetary seeds start with Hill radii equal to the disk scale height, retaining three-dimensional accretion. We demonstrate the method in scaled PDS 70 and Solar-System configurations, assuming perfect Hill capture by growing planets on circular orbits. We compare Scaling A at radius-compression factor \(\Gamma=1.75\) with the unscaled PDS system at matched resolution, through a 1% reduction of the interplanetary gas inventory. The scaled run is nearly an order of magnitude faster, with similar accreted mass and a comparable partition of gas between the planets. For the Solar-System configuration at \(\Gamma=3\), the scaling forecasts a speedup of nearly two orders of magnitude at equal local resolution. The scaling is complementary to orbital advection. On eight CPU cores of a desktop computer, the scaled PDS 70 and Solar-System runs reduce the gas inventory between the orbiting planets by roughly half in less than a day. Accretion-driven orbital convergence narrows the shared feeding zone and, together with gas clearing, limits the planetary growth budget. In our scaled PDS 70 and Solar-System runs, the angular momentum carried by meridional inflow directly constrains delivery theory. These results establish a practical route to testing accretion theory against simulations at matched scaled parameters, before applying it to unscaled planetary systems.
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Solar and Stellar Astrophysics

Radial increase of the compensated interplanetary magnetic field magnitude as a result of the growth of spherically polarized Alfvén waves

Stuart D. Bale, Timothy S. Horbury and Lorenzo Matteini

Measurements from the NASA Parker Solar Probe (PSP) mission demonstrate that inside $\sim20$ solar radii ($\Rs$) the magnitude of the interplanetary magnetic field does not fall off simply as $1/R^2$, with $\avg{|B|}\,R^2$ 15–20% below its value at $\sim60\,\Rs$. Here we use PSP magnetic field measurements over $10.6$–$56.8\,\Rs$ to show that the outward rise of $\avg{|B|}\,R^2$ is not due to the global structure of the heliospheric magnetic field or proximity to the heliospheric current sheet but is the direct result of the growth of large-amplitude, spherically polarized Alfvén waves (so-called `switchbacks'). Over the same distance range, $\avg{B_R}\,R^2$ stays constant, so that the sector-signed radial flux is conserved. The wave amplitude $\epsilon = B_{\perp,{\rm rms}}/\avg{B_\parallel}$ grows from $0.26$ to $0.57$ as $R^{1/2}\cos\alpha$, and $\avg{|B|}/\avg{B_R}$ rises by $20\%$. A simple model parametrized by the broadening of the distribution of deflection angles reproduces the effect to high accuracy. These results demonstrate that as the relative wave amplitudes grow with increasing solar distance, a dynamo-like process operates to enhance the field magnitude by up to $20\%$. By enhancing magnetic pressure this has implications for the large scale solar wind structure in the low-beta inner heliosphere, as well as partially resolving the long-standing discrepancy between remote measurements of the solar magnetic field and those made in situ by earlier missions farther from the Sun.
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Saturday 10 October

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

Validation of Space Weather Forecasting at Leonid Space

Scott Shambaugh

We validate Leonid Space's space weather forecasts through comprehensive cross-validation of retroforecasts scored against the entire F10.7 and Ap historical record. We built a unified forecast model that jointly forecasts F10.7 and Ap on all timescales from 3 hours up to multi-decade chains of future solar cycles. This was used to generate an archive of historical 15-year retroforecasts, after performing a leave-one-out fit of the model on held-out data and conditioning only on historical data available at the time. The accuracy of this archive was scored against measured space weather indices at all lookahead horizons from 1 day to 15 years, using the continuous ranked probability score (CRPS) as the primary quality metric. All other publicly available F10.7 and Ap forecast archives were scored using the same approach, including NASA's MSAFE and Schatten forecasts, SWPC's 27-day, 45-day, and Solar Cycle predictions, SWPC/USAF's RSGA, SIDC's Ursigram, CLS, Athens, KSO, BGS, SEPC's 3-day and 27-day forecasts, AFRL/NSO's SIFT, and BoM. Taken together, these 15 flux forecasts and 12 Ap forecasts compared on common lookahead metrics make up the most comprehensive cross-comparison of space weather forecasts we are aware of. Over a common forecast interval since 2000, Leonid had the lowest CRPS prediction error of any F10.7 or Ap prediction model: strictly lowest for Ap at all lookaheads and for F10.7 up to 5 years, and parity with MSAFE after 5 years. Much of this performance comes from our well-calibrated uncertainty bands, and we urge space weather forecasters to include calibrated uncertainties with their predictions and to issue retroforecast archives that can be used to rigorously evaluate their generating models.
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Solar and Stellar Astrophysics

The $R$-Process Alliance: On J0010$-$1735, an Accreted, Chemically Unusual, Metal-poor Star with $R$-process Enhancement

Zoe Hackshaw, Rana Ezzeddine, Shivani P. Shah et al. (+11 more)

We present a detailed chemo-dynamical analysis of the chemically peculiar very metal-poor ([Fe/H]$=-2.22\pm 0.11$) Milky Way (MW) halo star, 2MASS J00101758$-$1735387 (hereafter J0010-1735), discovered by the $R$-Process Alliance. We derive abundances for 45 elements, including 28 neutron-capture elements, and confirm that \thestar\ is highly $r$-process-enhanced, with [Eu/Fe]=$+1.30$. The light, $\alpha$, and Fe-peak element abundances ratios relative to Fe are all sub-solar, aside from C, Si, and most extremely, Ca. We investigate three possible formation scenarios based on the star's chemical abundances and orbital kinematics: whether it formed in the in-situ MW halo, in a MW dwarf galaxy satellite, or in a satellite of the Magellanic Cloud system. Using the position of \thestar\ in the [Mg/Mn]–[Al/Fe] plane, the abundance pattern of both the light and heavy elements, and the kinematics, we conclude that J0010-1735 most plausibly originated in a now-disrupted ultra-faint dwarf galaxy that was accreted into the MW halo. We examine potential chemical enrichment scenarios of J0010-1735, including one similar to the $r$-process-enhanced UFD Reticulum II, or enrichment via a sub-Chandrasekhar Type Ia SNe, potentially supplemented by a Ca-rich gap transient and by a separate neutron-rich site that facilitated $r$-process enrichment. A nuclear cosmochronometric age of $12.5\pm4.2$ Gyr is derived from the [Th/Eu] ratio, consistent with an ancient origin in one of the earliest star-forming systems in the universe.
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All authors Zoe Hackshaw, Rana Ezzeddine, Shivani P. Shah, Terese T. Hansen, Ruijie Shi, Yangyang Li, Timothy C. Beers, Anna Frebel, Erika Holmbeck, Alexander P. Ji, Jeremy Kowkabany, Vinicius M. Placco, Ian U. Roederer, Charli M. Sakari

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

Reawakening of GS 1843+00: A Multiwavelength Study of its 2026 Giant X-ray Outburst

Gaurava K. Jaisawal, Georgios Vasilopoulos, Francesco Carotenuto et al. (+1 more)

GS 1843+00 (Ginga 1843+009) is a poorly studied Be/X-ray binary system that underwent a major X-ray outburst in 2026 February-May after nearly sixteen years of quiescence. We present an X-ray, optical, and radio study of GS 1843+00 during this outburst, focusing on the evolution of the neutron star emission geometry, Be-disk connection, and possible outflow activity. We analyse three NuSTAR observations obtained at different stages of the outburst, complemented by archival RXTE and Chandra data, optical spectroscopy and imaging with the Nordic Optical Telescope, and a MeerKAT radio observation. NuSTAR observations reveal coherent X-ray pulsations around 29.7 s with a spin-up rate of $(-5.1\pm0.1)\times10^{-8}$ s s$^{-1}$ during the outburst. The pulse profiles show strong evolution with energy and outburst phase, with distinct morphologies even at comparable luminosities. The pulsed fraction varies between 1 and 16% and exhibits a complex reversal in energy dependence above 7 keV, increasing with energy during the first two observations and decreasing during the third. The 3-79 keV broadband spectra are described by thermal Comptonization with a blackbody and an Fe K$\alpha$ emission line, with no evidence for a cyclotron absorption feature. Phase-resolved spectroscopy reveals variations in the Comptonization and blackbody parameters, indicating changes in the accretion-column structure. Optical spectroscopy confirms the presence of a Be-circumstellar disk through H$\alpha$ and Paschen-series emission lines. No radio counterpart is detected, with a $3\sigma$ upper limit of 36 $\mu$Jy beam$^{-1}$ at 1.28 GHz. Weaker H$\alpha$ emission compared with previous epochs indicates that the Be-circumstellar disk was in a different state during the current episode, while the radio non-detection suggests that any associated outflow activity was faint or below the detection limit.
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All authors Gaurava K. Jaisawal, Georgios Vasilopoulos, Francesco Carotenuto, Hannah C. I. Wichern

Friday 9 October

Solar and Stellar Astrophysics

TESS Pulsation Timing Reveals the JWST Calibration Star HD 158485 as a Double-pulsator Binary

Chenglong Lv, Ali Esamdin, Shuguo Ma et al. (+5 more)

HD~158485 is an A-type $\delta$~Scuti star in the CALSPEC network and the JWST absolute-flux calibration program. Gaia DR3 assigns it a binary classification probability close to unity but provides no orbital solution. Using TESS observations from Sectors 14–26, 40–41, 47–60, and 73–86, we measure the pulsation phases of six $\delta$~Scuti modes. The frequencies form two groups with opposite light-travel-time delays, revealing two pulsating components. A direct light-curve forward model yields an orbital period of $P_{\rm orb}=382.80^{+0.80}_{-0.76}$~d, eccentricity $e=0.242^{+0.045}_{-0.046}$, projected light-time amplitudes of $195.8^{+9.2}_{-8.9}$ and $239.7^{+6.8}_{-6.8}$~s, and a dynamical mass ratio of $q=M_B/M_A=0.818^{+0.046}_{-0.045}$. A blind prediction based only on the first two long TESS campaigns reproduces the third campaign without updating the orbital parameters, with a Pearson correlation coefficient of $r=0.983$. Its held-out rms is 83.8~s, compared with 289.9~s when the period is fixed at 365.25~d. A 5.15-$\sigma$ Hipparcos–Gaia EDR3 proper-motion anomaly independently indicates unresolved astrometric motion. The unresolved companion can also affect spectrophotometric predictions: a conditional PB2-constrained two-atmosphere model differs from the corresponding single-star atmosphere prediction by 1.6% over 1–5~$\mu$m and 2.5% over 5–28~$\mu$m. HD~158485 therefore shows that a calibration-relevant binary can lack a catalogued orbital solution while two independent pulsation clocks reveal and measure its orbit.
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All authors Chenglong Lv, Ali Esamdin, Shuguo Ma, Shahidin Yaqup, Haozhi Wang, Ayzada Jmahali, Jianwei Yao, Kedieryemu Kamili

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

The effect of triples on the Chemically Homogeneous Evolution Channel for producing Gravitational Wave Sources

Silvia A. Popa, Selma E. de Mink, Stephen Justham et al. (+3 more)

Despite the growing number of gravitational-wave (GW) detections, the formation of merging binary black holes (BBHs) is still debated. One of the proposed formation channels is Chemically Homogeneous Evolution (CHE), which makes distinct predictions for the properties of the resulting black holes (BHs): high masses ($\gtrsim35\, M_\odot$) and large, orbitally-aligned spins. These predictions are based on studies that consider this channel only in isolated binaries. However, most massive stars reside in triples. We investigate how a tertiary affects the CHE channel. We use detailed binary models computed with MESA for the inner CHE systems coupled with analytical prescriptions for the hierarchical triple dynamics. Specifically, we study the von Zeipel-Lidov-Kozai mechanism that can accelerate the merger of the inner BBHs through high-eccentricity excitations. We show that a tertiary companion enables mergers at higher metallicities, where the classical binary CHE channel does not operate. At $Z_\odot/3$, triples produce BHs with lower masses (around $15\,M_\odot$) and lower spins (around 0.25) compared to the low-metallicity classical scenario. We further expect triple dynamics to lead to spin-orbit misalignment and potential residual eccentricities at merger. In triples at $Z_\odot/3$, half of the mergers occur within 1 Gyr. Moreover, 10% of the population leads to prompt mergers within 1 Myr (1% within 1 kyr) that are of potential interest for electromagnetic counterparts. We conclude that the triple CHE channel is promising as (i) it may contribute more substantially to the local merger rate than had been expected for the classical CHE channel due to the shorter delay times and effectiveness at higher metallicities and (ii) produces mergers with properties that more closely resemble the lower masses and lower spins inferred for most of the observed GW merger population.
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All authors Silvia A. Popa, Selma E. de Mink, Stephen Justham, Jakob Stegmann, Alejandro Vigna-Gomez, Lieke A. C. van Son

Solar and Stellar Astrophysics

Mapping Wolf-Rayet winds with spectro-interferometry I. Detection of multiple co-rotating interaction regions in WR 6

K. Deshmukh, A.-N. Chené, A. J. Frost et al. (+8 more)

WR 6 is the prototype for periodic variability due to large-scale wind structures in Wolf-Rayet (WR) stars, showing a consistent periodicity of 3.76 d in photometry, spectroscopy, and polarimetry. However, the origin of this variability is debated, attributed either to corotating interaction regions (CIRs) or a low-mass companion. We use time-resolved interferometry to investigate the origin of periodic variability in WR 6. We aim to detect and characterize structures in the stellar winds of WR 6 and discriminate between the CIR and low-mass companion scenarios. Using the extended configuration of the Auxiliary Telescopes at VLTI, we obtained six GRAVITY epochs over six months to search for the presence and variability of wind-structures in WR 6. We also obtained complementary spectroscopic observations during one of the epochs to monitor contemporaneous spectral line variability. For all six epochs of GRAVITY observations: (i) WR 6 is consistent with a single star and has no spatially resolved companion detected above $M\gtrsim6\,M_\odot$; (ii) The He I line-emitting region is spatially resolved, while the He II lines remain unresolved, consistent with atmosphere models; (iii) Around 2.19 $\mu$m, discrete line-emitting features offset from the center of the star are detected. The number, positions, and wavelengths of these features vary across epochs. Spectroscopic monitoring reveals spectral line residuals that broadly correspond to the two features seen in the GRAVITY data of the fifth epoch. Finally, the first three epochs, observed on three consecutive nights, show significant variability in these features but cannot be directly linked across epochs. We thus report the detection of multiple large-scale structures in the wind of WR 6 with VLTI/GRAVITY. We favor CIRs as their likely origin owing to temporal and spatial variability that cannot be reconciled with a low-mass companion.
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All authors K. Deshmukh, A.-N. Chené, A. J. Frost, T. De Prins, L. Fabiani, H. Sana, R. R. Lefever, A. Mérand, N. D. Richardson, A. A. C. Sander, T. Shenar

Solar and Stellar Astrophysics

What does self-supervision add to engineered features for stellar light curves?

Zijun Liu, Yue Ma and Christopher A. Theissen

TESS light curves encode a star's pulsation, rotation, binarity and oscillations, among other physical properties, but the survey produces far more light curves than can be labelled by hand. Astronomers have engineered a few dozen physically motivated features from them, which together remain a strong baseline. Previous work has trained encoders on light curves and judged them by their own downstream scores, but has not asked what they add to the engineered features. We pre-train a convolutional encoder on unlabelled TESS light curves to predict each window's latent representation from those of its neighbours. We then freeze it, add its features to the 25 engineered features, and read the combination out with one fixed linear probe on ten tasks: five variability classes, four asteroseismic targets and rotation period. Adding the learned features improves the readout on 7 of 10 tasks over the engineered features alone, where an untrained encoder does not. The combination also outperforms two networks trained end-to-end on the same labelled stars on most tasks. An encoder trained without labels therefore gives astronomers new features that complement the ones they already use rather than replace them.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA

Stellar discs and intermediate-mass black holes in galactic nuclei II. Fragmenting the disc and forming the S-stars in a live nuclear star cluster

Taras Panamarev and Bence Kocsis

We present direct $N$-body simulations of a stellar disc around the supermassive black hole Sgr~A*, perturbed by an intermediate-mass black hole (IMBH) and embedded in a live nuclear star cluster. A retrograde IMBH with $m_\bullet=2000\,M_\odot$, relative inclination $\theta_\bullet=150^\circ$, and initial semi-major axis $a_\bullet=0.15$~pc breaks the disc into three disc-like components and an angular momentum filament. The stochastic torques of the cluster, together with the IMBH's inward migration, increase the angular separation between these substructures. By ${\sim}5$~Myr, their separations become comparable to those among the observed Galactic-centre young star structures. The innermost ($a\le0.04$~pc) disc stars evolve into an S-star-like population. Relaxation driven by the cluster disperses their orbital planes, excites their eccentricities and depletes the `zone of avoidance' within ${\sim}10$~Myr even without an IMBH. The IMBH further accelerates their orbital-plane reorientation. However, the most eccentric S-stars may require an additional formation channel, such as the Hills mechanism. The distinctive signatures of the IMBH are the well-separated disc components and the eccentricity excitation confined to the radial range swept by its orbit. Dynamical friction from the cluster reduces the orbital size of a $2000\,M_\odot$ IMBH initially at $0.15$~pc by more than a factor of two within $10$~Myr. Prograde IMBHs tend to align with the stellar disc, whereas a $2000\,M_\odot$ retrograde IMBH can anti-align with the radially overlapping disc stars. We also quantify how the IMBH affects the supply of stars to the loss cone and the rates of stellar tidal disruptions and binary separations.
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Solar and Stellar Astrophysics

A Spectroscopic Census of Cluster White Dwarfs from DESI, LAMOST, and SDSS

David R. Miller, Jeremy Heyl, Huahui Yan et al. (+2 more)

Gaia has substantially expanded the sample of candidate white dwarfs (WDs) in open clusters, but many remain without spectroscopic characterization. We leverage the increasingly extensive archives of large public spectroscopic surveys to identify 62 candidate cluster WDs with available spectra from DESI, LAMOST, and SDSS. After discarding sources with very low S/N and unreliable host clusters, 44 candidates remain, including 37 non-magnetic DA WDs, of which 15 had not previously been characterized in the literature for cluster studies. We find that nine are consistent with single-star origins in their proposed host clusters, increasing the combined cluster DA initial–final mass relation (IFMR) sample to 132 WDs. All lie within $2\sigma$ of the best-fit IFMR from the most comprehensive recent cluster WD sample, suggesting these additions do not require recalibrating the empirical IFMR. One is a likely Hyades tidal-tail WD, providing further evidence that dynamical dispersal helps explain the long-standing WD deficit in the nearest cluster. We further compare WDs with spectra from multiple surveys. Among seven Praesepe WDs observed by both DESI and SDSS, DESI gives higher WD masses in six cases, with a mean difference of $0.067\,\mathrm{M}_\odot$, while the inferred progenitor masses differ by as much as $0.459\,\mathrm{M}_\odot$. More broadly, nine of ten single-star cluster DAs with DESI spectra lie above the empirical IFMR, with a mean final-mass residual of $0.06\pm0.04\,\mathrm{M}_\odot$. These results show that survey-dependent differences can strongly affect cluster WD parameters, and should be considered when combining heterogeneous spectroscopy in empirical IFMR studies.
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All authors David R. Miller, Jeremy Heyl, Huahui Yan, Chengyuan Li, Pier-Emmanuel Tremblay

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

OGLE-2016-BLG-0523: A Low-mass Brown Dwarf or super-Jupiter in a $>100$ au Orbit

Hongjing Yang, Weicheng Zang, Yoon-Hyun Ryu et al. (+37 more)

To search for very wide-orbit binary-lens events, we cross-match microlensing events detected by OGLE-IV and KMTNet between 2011 and 2024. After removing contaminants from variable stars, we identify OGLE-2016-BLG-0523 as a microlensing event showing two short-lived bumps in its light curve, with the two separated by 8.36 yr. We investigate three scenarios to explain the two bumps, including a binary-lens single-source (2L1S), a single-lens binary-source (1L2S), and two unrelated single-lens single-source events (2$\times$1L1S). The 1L2S interpretation is excluded due to its poorer fit to the data, while the 2$\times$1L1S scenario is disfavored by the low event rate, unphysical total source flux, and unphysical color for one of the two putative sources. For the surviving 2L1S interpretation, the Einstein timescales of the two bumps are $8.343 \pm 0.076$ days and $3.231 \pm 0.047$ days, respectively, implying a binary mass ratio of $q = 0.15$. A Bayesian analysis indicates that the primary lens is most likely a low-mass M dwarf, while the companion is probably a low-mass brown dwarf or super-Jupiter. The inferred projected separation is about 450 au, making the companion the widest low-mass object discovered via microlensing to date. Our discovery highlights both the promise and challenges of identifying very wide, low-mass companions through microlensing. Although upcoming facilities such as Roman and Earth 2.0 space telescopes will improve sensitivity to such events, overcoming degeneracies and distinguishing them from variable phenomena will remain essential for building a robust statistical sample.
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All authors Hongjing Yang, Weicheng Zang, Yoon-Hyun Ryu, Andrzej Udalski, Yuchen Tang, Michael D. Albrow, Sun-Ju Chung, Andrew Gould, Cheongho Han, Kyu-Ha Hwang, Youn Kil Jung, In-Gu Shin, Yossi Shvartzvald, Jennifer C. Yee, Sang-Mok Cha, Dong-Jin Kim, Hyoun-Woo Kim, Seung-Lee Kim, Chung-Uk Lee, Dong-Joo Lee, Yongseok Lee, Byeong-Gon Park, Richard W. Pogge, Przemek Mróz, Michał K. Szymański, Jan Skowron, Radoslaw Poleski, Igor Soszyński, Paweł Pietrukowicz, Szymon Kozłowski, Krzysztof A. Rybicki, Patryk Iwanek, Krzysztof Ulaczyk, Marcin Wrona, Mariusz Gromadzki, Mateusz J. Mróz, Jiyuan Zhang, Shude Mao, Hongyu Li, Qiyue Qian

Solar and Stellar Astrophysics

Magnetic connectivity from the Sun to the Earth with MHD models II. Impact of input magnetic maps

Barbara Perri, Silke Kennis, Brigitte Schmieder et al. (+1 more)

Connectivity estimation needs to be fast and thus usually relies on semi-empirical models. We previously established that optimised magnetohydrodynamics (MHD) models can also be used to derive magnetic connectivity, yielding good results, especially for complex configurations. However, the modelling is not the only factor that can impact the connectivity estimation. Models' results, both semi-empirical and MHD, depend heavily on the input data used to drive the simulation. In order to quantify this impact on our connectivity estimation, we used 20 different maps by four providers. We selected the period of July 2019, which corresponds to Carrington rotation (CR) number 2219, because it had already been studied in a previous paper. It corresponds to a period of minimum activity, meaning minimal interference from transient structures, but with a high-speed stream and several polarity switches. We have compared our results with in situ data from ACE and developed several visualisation techniques to validate connectivity not only for a specific date but also for a full CR. The choice of the input magnetic map drastically affects the connectivity estimates, with global scores going from 28% global agreement to 63% in the most extreme case. The HMI and GONG maps consistently gave the best connectivity estimations, with more than 50% correct results of the full CR. For this specific period, the GONG-ADAPT maps yielded the least reliable results, with a score below 35%. This is not surprising, as we have shown before that COCONUT required parameter adjustments for these maps. However, WSO maps which showed poor coronal agreement actually provided good connectivity estimates, showing that the Heliospheric Current Sheet (HCS) location is in the end the most crucial parameter for connectivity. Consistent data over a full Carrington rotation are to be favored compared to daily.
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All authors Barbara Perri, Silke Kennis, Brigitte Schmieder, Stefaan Poedts

Solar and Stellar Astrophysics

X-Shooter view of planetary nebulae microstructures I: The case of NGC 7009 knots

L. Konstantinou, S. Akras, K. Bouvis et al. (+4 more)

The properties of NGC 7009 knots are investigated by employing UV-to-near-infrared data from the X-Shooter spectrograph. These structures belong to the category of low-ionization structures (LISs), which are characterized by enhanced emission from low-ionization species and whose origin remains enigmatic. Our main goal is to conduct the first dedicated spectroscopic study of LISs using X-Shooter, that covers a wide spectral range from the UV to the near-IR, simultaneously. As a pilot study, we focus on NGC 7009 to investigate its ionized, atomic, and molecular gas, aiming to better characterize the physical properties of its LISs and explore their largely unexplored molecular content. The spectrum was processed through the ESO Reflex pipeline, and the physical properties of LISs were derived with the latest version of PyNeb. More than 250 emission lines were detected in the spectra of NGC 7009 LISs, including rare, forbidden transitions from low- ionization species such as Fe+ and Ni+, as well as two transitions from P+, identified here for the first time in LISs. The detection of multiple shock tracers, together with the shock excitation of [Fe II] and [O I], as revealed by emission-line diagnostics, supports the presence of shocks at LISs. The richness of the spectrum allowed us to derive electron temperatures and densities at the LISs using both collisionally excited and optical recombination lines. In the near-IR spectrum of the LISs, nine ro-vibrational H2 emission lines were detected, seven of which were reported here for the first time. These lines were used to construct the H2 excitation diagram, providing new insights into the physical conditions of the nebular gas. The excitation of molecular hydrogen is most likely dominated by collisional processes through shocks.
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All authors L. Konstantinou, S. Akras, K. Bouvis, P. Boumis, J. Garcia-Rojas, D.R. Gonçalves, K.N. Gourgouliatos

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

Detailed study of HD 219134: a benchmark K-dwarf with a compact planetary system

Jérôme Bétrisey, Camilla Pezzotti, Gaël Buldgen et al. (+4 more)

Space missions such as CoRoT, Kepler, and TESS have transformed asteroseismology and its synergies with exoplanet science. We revisit the nearby K-dwarf HD 219134 to investigate the tension between asteroseismic and interferometric radii, update its fundamental parameters, and reconstruct the rotational, magnetic, and atmospheric evolution of the system. We combine forward asteroseismic modeling with seismic inversions to assess systematics from non-seismic constraints, radius measurements, and surface modeling. The revised stellar properties are then used for rotational evolution, stellar-wind, and high-energy irradiation calculations. Seismic inversions rule out the most recent interferometric radius, while the asteroseismic and spectroscopic results are compatible with the 2019 interferometric measurement. We obtain $M=0.779\pm0.015_\mathrm{(stat)}\pm0.006_\mathrm{(sys)}\ M_\odot$, $R=0.753\pm0.005_\mathrm{(stat)}\pm0.002_\mathrm{(sys)}\ R_\odot$, and an age of $8.92\pm0.89_\mathrm{(stat)}\pm0.19_\mathrm{(sys)}$ Gyr. These systematics include only non-seismic constraints; literature combined uncertainties from physical inputs and surface effects are $0.007M_\odot$, $0.002R_\odot$, and $0.81$ Gyr. Rotational and wind modelling shows that standard magnetic braking is broadly compatible with the present-day rotation and angular-momentum loss. Weakened braking gives a closer match but challenges standard wind-loss scalings. The inferred X-ray history suggests early stripping of the innermost planets, while the outer planets likely retained part of their primordial envelopes; HD 219134d experienced only moderate erosion. The apparent radius discrepancy likely reflects underestimated uncertainties in previous interferometric analyses rather than an astrophysical inconsistency. We advocate adopting the asteroseismic radius, which is robust to modeling assumptions and optimisation strategies.
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All authors Jérôme Bétrisey, Camilla Pezzotti, Gaël Buldgen, Aline A. Vidotto, Colin P. Folsom, Michaël De Becker, Michaël Gillon

Solar and Stellar Astrophysics

Formation Dynamics of a Rotating Solar Spicule-like Event and Associated Energy-Mass Transport

Liufan Gong, Lei Ni, Guanchong Cheng et al. (+3 more)

We performed three-dimensional radiative magnetohydrodynamic(3D RMHD) simulations in an upward-directed (positive-polarity) open magnetic field with an initial background strength of 10 G. We identified vortex-associated spicule-like events and carried out a detailed analysis of one representative event. Prior to the formation of this spicule-like event, a photospheric vortex appears and twists the magnetic field lines, then develops upward into higher atmospheric layers, forming a magnetic tornado system. This event is generated during this process. At the periphery of this magnetic tornado, twisted magnetic field lines with negative vertical components reconnect with both external positive-polarity background fields and central positive-polarity fields. The magnetic reconnection sites rise and rotate together with the developed magnetic tornado, and many jet-like substructures in this event are closely associated with plasma upward ejections from these reconnection regions. A vertical force analysis shows that, for this event, plasma uplift in the photosphere and lower chromosphere is dominated by the pressure-gradient force, whereas in the upper chromosphere, the Lorentz force gradually becomes dominant. At a height of about 4 Mm above the solar surface, this event produces a mean upward mass flux density on the order of $10^{-7}~{\rm kg~m^{-2}~s^{-1}}$ and a mean upward Poynting flux density of approximately $525~{\rm W~m^{-2}}$. The wave-mode decomposition results show that the energy transport associated with the rotational spicule-like event is dominated by the Alfvén mode. Alfvén waves, fast-mode waves, and slow-mode waves carry energy into the lower corona in an approximate ratio of $10:7:3$.
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All authors Liufan Gong, Lei Ni, Guanchong Cheng, Jiajia Liu, Quan Xie, Robert Erdélyi

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

An Ultraviolet Instrument on the Habitable Worlds Observatory: Considerations and Capability Trades on the Multi-Object, Integral-field, and High-resolution modes of a potential UVI

Brian Fleming, Kevin France, Dmitry Vorobiev et al. (+4 more)

The Habitable Worlds Observatory (HWO) is NASA's next-generation astrophysics platform, delivering transformative capability in the search for life and study of the origins of the universe. Amongst the instruments under consideration for HWO is a multi-purpose ultraviolet spectrograph and imager that provides a deep toolbox for some of the most pressing science investigations of the next several decades. In this paper, we present a basic draft model of a potential UVI, including a multi-object spectrograph, far ultraviolet imager, high resolution spectrograph, and integral-field spectrograph. The draft designs for these channels of the UV instrument serve as a basis for a general discussion of the instrument architecture and design challenges, while also identifying potential trades, enhancements, and other considerations that the HWO project will need to address as it moves into the formulation phase. We conclude with a presentation of several scientific simulations that have helped guide the capabilities of the draft design. This paper is intended to serve as a summary of UV instrument modes, technology, and potential design ahead of planned instrument studies in the late 2020's, and then Phase A in the early 2030's.
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All authors Brian Fleming, Kevin France, Dmitry Vorobiev, Jeanette Donber, Eric Frater, Thomas Delker, Vicki Herde

Thursday 8 October

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

Bridging Theory and Observation: Synthetic Far-infrared Insights into Star Formation Efficiency

Alexander Escamilla, Michael Y. Grudić and Anna L. Rosen

The star formation efficiency per free-fall time ($\epsilon_{\rm ff}$) quantifies how efficiently giant molecular clouds (GMCs) convert gas into stars and is often observed to be orders of magnitude lower than expected for free-fall collapse. Observers typically estimate $\epsilon_{\rm ff}$ by mapping far-infrared (FIR) dust emission to infer gas surface densities ($\Sigma_{\rm gas}$) and counting embedded protostars, assuming simplified lifetimes and masses. Using the fiducial STARFORGE radiation-magnetohydrodynamics simulation of an $M_{\rm cl} = 2\times10^4 M_\odot$ GMC that self-consistently models star cluster formation and stellar feedback, we generate synthetic FIR observations and apply the same methodologies used in GMC surveys to investigate this discrepancy. We present the first $\epsilon_{\rm ff}-\Sigma_{\rm gas}$ analysis in a fully feedback-regulated GMC simulation that resolves the formation of stellar systems. Our synthetic measurements reproduce the low observationally inferred efficiencies, showing that feedback-regulated star formation naturally produces $\epsilon_{\rm ff} \sim 1-3\%$ without requiring extreme initial conditions. We also find that $\epsilon_{\rm ff}$ varies strongly over a GMC's lifetime, suggesting that much of the observed scatter reflects evolutionary sampling rather than intrinsic cloud-to-cloud differences. Finally, by comparing observational and simulation definitions, we show that methodological assumptions introduce systematic biases, with a transition near $\log \Sigma_{\rm gas} \approx 2.3 M_\odot {\rm pc}^{-2}$ that depends on resolution. Above this threshold, the smoothing of dense structure increases both the inferred free-fall time and enclosed gas, with the latter dominating and suppressing $\epsilon_{\rm ff}$. Below the threshold, the discrepancies are primarily driven by star formation rate assumptions.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

The Fast & Fourier: Using neural operators to speed up radiative modeling of protoplanetary disks

Lukas Welzel and Melissa K. McClure

Edge-on protoplanetary disks provide a spatially resolved view of the vertical distribution of icy planetary building blocks, but inferring their relative abundances requires radiative-transfer (RT) modeling with multiple anisotropic scattering. Each model costs $10^{1}$-$10^{3}$ CPU hours, which precludes principled Bayesian inference and homogeneous, population-scale analyses. We develop and validate a fast and reusable surrogate for Bayesian inference with ice-bearing edge-on disk models. We train a Fourier Neural Operator on a grid of RADMC-3D spectra. The network maps disk parameters and analytic physical input fields to the spectrum, using mixed-resolution spectra. We use the surrogate in an explicit-likelihood nested sampler and validate it on held-out simulations and synthetic recovery tests with TARP coverage. Finally, we use it to retrieve the disk structure and ice abundances of ESO-H$\alpha$ 569 from its spectral energy distribution. The surrogate reproduces the RADMC-3D spectra with a median error of 4.84%. We recover synthetic parameters without detectable bias and with nominal coverage. For ESO-H$\alpha$ 569, the posterior favors a massive ($M_{\rm gas}=0.028^{+0.015}_{-0.011}\,M_\odot$), nearly edge-on ($i=79.8^{+5.2}_{-2.4}$ deg) disk consistent with the image-plus-SED model of Wolff et al. (2017). Its ice abundances and cavity size remain weakly constrained by broadband data alone. The marginal spectrum-evaluation speed-up is $\sim10^{9}$-$10^{10}$. Neural-operator emulation makes full posterior inference feasible within the adopted model and training prior. Key posterior points should still be checked with high-fidelity RT models. By drastically cutting evaluation time, this approach enables for the first time homogeneous population studies of ices in disks, which new JWST surveys can capitalize on to map the range of compositions within planetary nurseries.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

"Mega-Earths" are likely an artifact of stellar activity

Casey L. Brinkman and Nicolas Cowan

Empirical relations and planet formation simulations suggest an upper mass limit for rocky planets of $\sim$10M$_{\oplus}$, as cores above this will almost certainly undergo runaway gas accretion. Nevertheless, a small number of planets appear to have densities consistent with rocky Earth-like compositions above this threshold, indicating that unusual formation and evolutionary scenarios are capable of producing extremely massive rocky planets called "Mega-Earths." However, all of these planets orbit active stars with log$R'_{HK}$ $>$-4.75. We find a <1$\%$ probability that all literature ultra-massive rocky planets would coincidentally orbit such active stars, and find no astrophysical reason to expect Mega-Earths to preferentially form around active stars. We therefore conclude that Mega-Earths are artifacts of stellar activity, and we question the reliability of Gaussian Processes to enable mass measurements for rocky planets around active stars.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

The accretionally heated atmospheres of protoplanets and brown dwarfs

Nick Choksi, Peter Gao, Jerry Xuan et al. (+2 more)

The ranks of accreting planets and brown dwarfs are growing. To help interpret their spectra, we study how accretion affects a planet's atmosphere. The atmosphere is irradiated from above by whatever nonthermal radiation from the accretion shock does not directly escape into space. This accretional heating drives a temperature inversion. Even relatively slow accretion, corresponding to mass doubling times longer than system ages, can raise temperatures by many hundreds of Kelvin at ambient pressures of order millibars. The heated atmosphere radiates an optical/infrared excess. For environments that are not too dusty, the excess concentrates in molecular lines that trace the heated layer. Thus an accretionally heated atmosphere may be distinguished by its muted absorption bands. Irradiation from the accretion shock also changes the chemical composition of the atmosphere, further altering spectra. Atmospheric retrievals promise to map out the deposition of accretional energy, helping to decide between ”hot” and ”cold” start modes of planet formation.
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All authors Nick Choksi, Peter Gao, Jerry Xuan, William Balmer, James Mang

Solar and Stellar Astrophysics

Rotation period of the O giant xi Persei: Coexistence of cyclic and periodic variability. A weakly magnetic candidate star

H.F. Henrichs, N.P. Sudnik and A. David-Uraz

OB stars exhibit line profile variability, often associated with rotation; notably, in their UV wind-sensitive lines where discrete absorption components tend to recur cyclically. We searched for periodicity in spectral regions of the O7.5III(n)((f)) star xi Persei formed very close to the star. The NIV 1718 A line was identified as the most uncontaminated spectral with a large and homogeneous available dataset is available: 307 IUE spectra over 12 years and 11 STIS spectra taken 21 years later. We also studied 322 time-resolved HeII 4686 A spectra and MOST, BRITE, and TESS space photometry, covering a time span of 13 years. We also reconsidered X-ray studies with Chandra. A CLEAN analysis and subsequent weighted least-squares fit of the flux in this region resulted in a unique period of 2.040514(20) d, attributed to rotation. The phase of maximum flux in the NIV, SiIV, Ha, and HeII line profiles coincide with the results of X-ray variability studies. This period was also found in the photometric data. Given the coherent periodic behaviour probing the region of the wind nearest the stellar surface, we propose that xi Per has an as-yet undetected weak global surface magnetic field. The sinusoidal behaviour suggested that only one magnetic pole is visible, implying an inclination of i approximately 51 deg, and therefore, 39 deg for the magnetic obliquity. We present a conceptual framework within which weak wind confinement by a global magnetic field can create a small magnetospheric disc, while allowing for the formation of more classic corotating interaction regions at higher magnetic latitudes. The periodic variations are then understood to result from the changing projected area of the magnetospheric disc as a function of rotational phase, both along the line of sight and off the stellar limb.
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Solar and Stellar Astrophysics

EWOCS-XI: Anisotropic expansion and shear in the cluster Westerlund 1. A 2D kinematic analysis with Gaia DR3

L. Prisinzano, F. Damiani, C. Ordenes-Huanca et al. (+14 more)

Westerlund 1 (Wd1) is one of the Milky Way's most massive young clusters, in which heavy extinction and field contamination have historically hampered its kinematic characterization. We aim to assess Wd1's internal kinematics, dynamical state, and formation scenario. Using a robust 5D clustering approach, we identified 559 bona fide members. We modeled their proper motions via a 2D first-order Taylor expansion to derive the velocity gradient tensor, mapping expansion, rotation, and shear. Restricting the kinematic analysis to 524 stars with proper motion uncertainties $<0.5~\mathrm{mas\,yr^{-1}}$, we find at least 103 gravitationally unbound members, including 14 walkaway stars. The 2D analysis reveals a statistically significant divergence (>4$\sigma$) highlighting preferred expansion along the Galactic latitude (position angle $164^\circ\pm10.1^\circ$). A detected cross-gradient ($\partial v_b / \partial l = -0.41 \pm 0.08$ km s$^{-1}$ pc$^{-1}$) provides a clear signature of shear on the plane of the sky. Wd1 does not behave as a single cohesive population, but comprises two decoupled subsystems: a stable, bound core in dynamical equilibrium and an escaping, unbound envelope. This prominent anisotropic expansion perpendicular to the Galactic plane has a kinematic timescale ($\Delta t_{\mathrm{kin}} = 2.08 \pm 0.44$ Myr) matching the independent orbital mid-plane crossing time ($\Delta t_{\mathrm{orbit}} = 2.10 \pm 0.05$ Myr). This temporal alignment indicates the expansion onset coincided with Wd1's Galactic disk passage, triggered either by internal dynamics (e.g., early supernova feedback) or external tidal perturbations. Finally, the robust unbound population demonstrates that active cluster dissolution is underway, driven by discrete dynamical ejection events rather than long-term stellar evaporation.
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All authors L. Prisinzano, F. Damiani, C. Ordenes-Huanca, M. Tarantino, S. Sciortino, N.J. Wright, V. Almendros-Abad, M. Andersen, A. Bayo, R. Bonito, V. Cusimano, M. Haberle, K. Mužic, I. Negueruela, E. Sabbi, A. Winter, M.G. Guarcello

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

An updated model of neutrino emission from proto-neutron stars

Matteo Ballelli, Federica Pompa, Christoph Ternes et al. (+2 more)

We present an improved parametric model for neutrino emission from core-collapse supernovae that directly connects neutrino luminosity and average energy to stellar properties during emission. Our model incorporates two critical physical processes: convection within the proto-neutron star and the evolution of the neutrinosphere radius during the early emission phase. Using simulated datasets of supernova neutrino events in the Super-Kamiokande detector, we assess the sensitivity to astrophysical properties in the event of a future detection. We show that the neutrino mass ordering plays a crucial role in determining which parameters can be constrained by the observed neutrino data.
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All authors Matteo Ballelli, Federica Pompa, Christoph Ternes, Marco Drago, Giulia Pagliaroli