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Physics Astrophysics Solar and Stellar Astrophysics

astro-ph.SR Watch Solar and Stellar Astrophysics

Wednesday 5 August

Solar and Stellar Astrophysics

Tracing expansion-driven star formation in the Perseus molecular cloud using young stellar objects

Chen-Rui Gu, Zhi-Kai Zhu, Zu-Jia Lu et al. (+3 more)

$Context$. Recent studies have linked star formation in the solar neighbourhood to the expansion of the Local Bubble. The Per-Tau Shell (PTS), a vast 3D dust structure hosting the Perseus and Taurus clouds on its surface, is a candidate for such triggered formation. $Aims$. We aim to reconstruct the star formation history of the Perseus molecular cloud and investigate its potential link to the historical expansion of the PTS. $Methods$. We performed a 6D phase-space analysis of 406 young stellar objects (YSOs) of Perseus. Our approach integrates Gaia DR3 kinematics with Bayesian age inference and a heteroscedastic likelihood framework to resolve radial gradients in YSO ages and intrinsic tangential velocity dispersions relative to the PTS centre. We constructed a unified catalogue by integrating existing infrared-based classifications to examine the projected spatial segregation of different evolutionary stages and compared stellar 3D kinematics with $^{12}$CO and $^{13}$CO gas velocity fields to assess kinematic coherence. $Results$. The YSOs exhibit coherent outward motions relative to the centre of the PTS, a negative age-distance gradient with younger populations at larger radii, and a radial decline in tangential velocity dispersions. Younger YSO classes also show a stronger directional concentration towards the PTS centre. Both the coherent proper motions and the line-of-sight velocity gradients of these YSOs align with the gas velocity field, indicating large-scale kinematic coherence. $Conclusions$. The Perseus YSOs exhibit concordant kinematic and temporal signatures consistent with an expansion-driven origin. These results provide robust evidence for a layered, inside-out pattern of star formation, suggesting that the historical expansion of the PTS triggered a sequential progression of star birth and effectively shaped the star formation history of the Perseus complex.
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All authors Chen-Rui Gu, Zhi-Kai Zhu, Zu-Jia Lu, Min Fang, Longhui Yang, Cheng-Yang He

Solar and Stellar Astrophysics

3D simulations of a complete convective silicon shell burning phase

V. Varma, R. Hirschi, F. Rizzuti et al. (+7 more)

We present 3D hydrodynamic simulations of a complete silicon shell burning phase until fuel exhaustion at the end of the evolution of a 14$M_\odot$ core-collapse supernova progenitor, using a reduced 25-isotope nuclear network. We investigate how realistic turbulent convection affects this burning phase, which has a more complicated set of nuclear reactions than previous burning phases. We find broad similarities between the 3D simulation and the 1D MESA model. However, due to more realistic feedback from the very stiff convective boundaries in the hydrodynamic simulations, the 3D simulation experiences lower convective boundary mixing (CMB) compared to 1D, and hence entrains less fresh fuel into the silicon shell. This leads to the silicon shell in the 3D model burning for roughly 800\,s shorter. We find that the nuclear burning timescales for the dominant reactions are faster than the mixing timescale, making this entire process a convective-reactive event. The angular-averaged energy generation profile shows a double-peaked structure, where the region between the positive peaks are close to zero, or are negative. We find that throughout the base of this silicon shell, many $(\alpha, p)$ and their inverse reactions are important. The forward and reverse rates are very similar, so slight fluctuations in the temperature cause regions to be either exoergic or endoergic, leading to a complicated energy generation evolution. This study presents an exploration using a single reduced nuclear network, however, due to the sensitivity of this burning phase future studies should investigate the impact of more complete nuclear networks.
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All authors V. Varma, R. Hirschi, F. Rizzuti, T. Rauscher, A. St.J. Murphy, M. Mocák, C. Meakin, K. Goodman, C. Georgy, W.D. Arnett

Solar and Stellar Astrophysics

Properties of granular light bridges

Michal Sobotka, Jan Jurcak and Marta Garcia-Rivas

Granular light bridges in sunspots are a manifestation of magnetoconvection in an approximately vertical magnetic field that is weaker than the field of the umbra. We study the relations between observed properties of granules and magnetic field in five strong granular light bridges located in a complex sunspot. Thermal parameters, magnetic field vector, and line-of-sight velocity were obtained from an inversion of the GREGOR spectropolarimetric scan in the lines Si I 1082.71 nm and Ca I 1083.90 nm. Sizes and lifetimes of granules were measured in a simultaneous sequence of images in the TiO band. The light bridges under study are comprised of granules spanning 0.2"-1.4" with approximately 6-minute lifetimes. The largest granular structures (> 1") are spatially correlated with 1 km/s upflows and weak, inclined magnetic fields (300 G) near the equipartition threshold. Statistical comparisons demonstrate an inverse relationship between magnetic field strength and continuum intensities, line-of-sight velocity dispersion, and effective granular diameters. These relationships vary among individual light bridges, depending on the large-scale magnetic field topology. Large expanding granules similar to the photospheric ones appear in light bridges only when the local magnetic field strength approaches the equipartition value.
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Solar and Stellar Astrophysics

Asymmetry in the protostellar system HOPS 198: Evidence for the evolution of outflow opening angle driven by density of the surrounding core

Donglin Wu, Héctor Arce and Cheng-Han Hsieh

Protostellar outflows are thought to be responsible for the low star formation efficiency of protostellar cores. However, whether outflows can disperse a significant fraction of the gas in the core depends on the outflow opening angle. It is established that the outflow opening angle increases during the early stages of the protostellar evolution, but the underlying mechanism is poorly understood. Observations of HOPS 198, a Class 0 protostar in the Orion A molecular cloud, provide insights into this question. HOPS 198 exhibits a strong east-west asymmetry in its outflow and its core. The opening angle of the eastern lobe ($\sim80^{\circ}$) is more than twice wider than that of the western lobe ($\sim30^{\circ}$), while the surface density of the west side of the core is $1.5-2.8$ times higher than the east side. Using an analytical model in which the molecular outflow morphology is shaped by interactions between the wide-angle protostellar wind ($\gtrsim 80^{\circ}$) and surrounding material in the core, we find that the difference in opening angle for the two lobes can be explained by the difference in core density on the two sides. This result supports the hypothesis that the evolution of the outflow opening angle is driven by the evolution in the density of the protostellar core.
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Solar and Stellar Astrophysics

Cepheid metallicities from low-resolution near-infrared spectra: validation against the optical reference scale

M. A. Urbaneja, R. P. Kudritzki and N. Przybilla

Classical Cepheid metallicities trace recent chemical enrichment and help assess systematics in Cepheid-based distance measurements. They are derived mainly from high-resolution optical spectroscopy, which provides the benchmark abundance scale but limits measurements to nearby systems. Extending such studies to more distant Cepheids with extremely large telescopes requires complementary low-resolution near-infrared methods and careful treatment of model–data residuals. We test whether low-resolution SpeX/IRTF Y+J-band spectra can yield metallicities compatible with homogenised optical abundances. We analyse 14 Galactic classical Cepheids with periods of approximately 13–69 d, supplemented by two shorter-period stars from the IRTF Spectral Library. Using Bayesian full-spectrum fitting based on MARCS/TURBOSPECTRUM spectra and a regularised covariance likelihood, we infer [M/H] and compare it with homogenised optical [Fe/H]. For the primary 14-star sample, the inferred metallicities reproduce the optical reference scale with a mean offset of -0.05 dex, a scatter of 0.09 dex, and a robust scatter of 0.12 dex; the median posterior uncertainty is 0.22 dex. Including the two additional stars changes these values only slightly. Compared with a diagonal likelihood, the covariance-aware treatment reduces the mean offset and improves the stability of the uncertainty calibration. Low-resolution Y+J spectra therefore contain usable metallicity information for classical Cepheids when short-range correlated residuals and localised modelling imperfections are treated explicitly. The resulting near-infrared metallicity scale provides a practical foundation for extending Cepheid metallicity work to larger and more distant samples.
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Solar and Stellar Astrophysics

Characterization of the variability of the blue supergiant HD 14134

Suryani Guha, Michaela Kraus, Julieta P. Sanchez Arias et al. (+2 more)

The post-main sequence (MS) evolution of massive stars encompasses phases in which the stars display high variability. One such class of objects are the blue supergiants which may be in either the pre- or post-red supergiant phase of their evolution. Their variability patterns might provide constraints for a proper classification of the objects. We aim to characterise the observed variability of the B supergiant HD14134 and to investigate the imprint of a time-variable wind on the brightness variation of the star and its impact on the detectability of pulsation signals. Spectroscopic data were collected over a 5-month period and combined with photometry from TESS. Stellar parameters were derived from modelling of the time-averaged spectrum with CMFGEN and the SED and were confirmed with stellar evolution models computed with MESA. The light curves and radial velocity curves of selected lines were analysed to determine pulsation signals. The wind variability and its imprint on the stellar brightness were investigated from an analysis of the Halpha line. Predictions of mode excitations were computed with the GYRE pulsation code and compared to the frequencies determined from the observations. A g-mode with a period of ~19.2 d and its harmonics are consistently detected in all data sets. The spectra unveil strong, non-periodic wind variability and 3 frequency signals were identified as due to this wind variability. The stellar parameters and age derived for HD14134 together with the absence of radial pulsations classify the star as a post-MS object evolving towards the red-supergiant stage, questioning its classification as alpha Cyg variable. The results reinforce that simultaneous long-term spectroscopic and photometric monitoring is indispensable for reliable frequency detections and for disentangling of variabilities imprinted by a time-variable wind from those imposed by pulsations.
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All authors Suryani Guha, Michaela Kraus, Julieta P. Sanchez Arias, Peter Nemeth, Krzysztof Kaminski

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

Type I X-Ray Burst Models With Rotation

David Martin and Jordi Jose

Type I X-ray bursts are powered by unstable thermonuclear burning on the surface of accreting neutron stars in close binary systems. These brief X-ray flashes, with light curves featuring rise times of $1-10$ s, durations of $10-100$ s, and recurrence periods of hours to days, represent the most frequent stellar explosions in our Galaxy. With typical energies of $\sim 10^{39}-10^{40}$ erg, they rank among the most powerful astrophysical transients after supernovae and classical novae. To date, roughly 120 bursting X-ray binaries have been identified in the Milky Way. Several studies have been conducted to characterize the dynamics of these events, with emphasis on reproducing the observed recurrence periods and light curve shapes. In this paper we show, for the first time, that rotation is a key factor shaping the properties of Type I X-ray bursts in rapidly spinning systems. The inclusion of centrifugal forces, together with a suite of rotationally-induced mixing mechanisms, such as meridional circulation and shear-induced turbulent diffusion, reduce surface gravity, shortening the recurrence times and lowering burst energies. Rotation also modifies the extent of the nuclear activity during these events and affects the morphology of their light curves, which are distinctly broader for rapidly rotating neutron stars.
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Solar and Stellar Astrophysics

A matter of class and flare Flare productivity in solar active regions and power-law constraints on the flare frequency distribution

A.G.M. Pietrow

Solar active regions are the source of virtually all significant space weather events, therefore the relationship between their observable properties and flare production is important to characterize. The flare productivity of solar active regions is analyzed across solar cycles 23–25 as a function of magnetic complexity, sunspot area, and solar-cycle phase. Additionally, the power-law index of the peak flux frequency distribution is estimated. The GOES soft X-ray event reports matched with daily Solar Region Summaries (SRS) are used to create a robust active region catalog. This catalog is supplemented with the 50-year HEK flare catalog and the newly released STIX flare catalog from Solar Orbiter to estimate the power-law index. Magnetically complex active regions ($\beta\gamma$, $\beta\delta$, $\beta\gamma\delta$) produce the majority of M- and X-class flares while comprising only $\sim$13% of all daily active-region observations, showing that larger flares disproportionately occur in larger area sunspots. All three catalogs can be represented with a consistent power-law index of $\alpha = 2.02 \pm 0.01$ for the peak flux frequency distribution. However, a class-by-class analysis of the C/X and M/X flare ratios reveals that the distribution steepens significantly towards higher energies. Fitting this steepening as a lognormal tail and extrapolating two decades beyond the X-class threshold predicts a recurrence time of $370^{+292}_{-159}$~yr for an X100-class (`Carrington-type') superflare, compared to only $\sim$26~yr from a naive single power-law extrapolation. While $\alpha \approx 2$ provides a useful global description of the flare frequency distribution, the steepening area-productivity relation and the divergence between C/X and M/X derived slopes both point to a deficit of the most energetic flares ...
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA

The two faces of tides: gravitational instability during galaxy mergers

Trisha Khan and Ayush Hazarika

Gravitational instability is a fundamental mechanism driving collapse of interstellar gas, yet in dynamically evolving environments such as galaxy mergers, external tidal fields can significantly alter the conditions for collapse. In this work, we develop an analytical framework to investigate how the tidal field of a companion galaxy modifies the classical Jeans instability by incorporating its anisotropic and time-dependent nature into the dispersion relation. We find that the tidal field introduces a critical angle between the cloud's position vector and the merger axis, $\theta_c \approx 54.7^\circ$, separating disruptive and compressive regimes of the radial tidal component. Disruptive tides suppress instability by increasing the characteristic length scale and restricting the range of unstable modes, whereas compressive tides enhance collapse by extending the unstable spectrum and increasing the growth rate of perturbations. The impact of tidal fields depends sensitively on gas density, being significant in diffuse media but negligible in dense molecular environments where the classical Jeans limit is recovered. All tidal effects peak near pericentric passage due to the strong dependence on galaxy separation. Since the free-fall time of diffuse gas is comparable to the duration of the close passage, tidally assisted collapse in the diffuse medium lags pericentre by roughly a free-fall time, whereas dense gas responds essentially instantaneously. These results demonstrate that companion-induced tidal fields play a key role in regulating the scale and efficiency of gravitational instability of diffuse interstellar gas in galaxy mergers. The code underlying this work is publicly available.
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Solar and Stellar Astrophysics

Doppler-shifted Signatures in the Chromospheric Line Profiles of II Pegasi

Cao Dongtao and Gu shenghong

Stellar flares are frequently accompanied by prominence eruptions, coronal mass ejections (CMEs), and other forms of plasma motion. Based on the long-term high-resolution spectroscopic monitoring data set of the RS CVn-type star II Pegasi (II Peg), we search for stellar prominence eruptions and potential CMEs by investigating asymmetric signatures in the H$_{\alpha}$ and H$_{\beta}$ line profiles during the flare events. We observed seven optical flares with asymmetric spectral profiles, releasing low limit energies of $10^{33}$–$10^{34}$ erg in the H$_{\alpha}$ line. Five of these flares were associated with six potential CMEs, identified through the Doppler-shifted emission signatures in the chromospheric line profiles, particularly the H$_{\alpha}$ line. The mass estimates for the CME candidates range from $10^{19}$ to $10^{20}$ g, while the kinetic energies are calculated to be around $10^{33}$–$10^{34}$ erg. The most massive CME candidate was simultaneously recorded in the H$_{\alpha}$, H$_{\beta}$, and He I D$_{3}$ lines. For II Peg, the possible flare-CME association rate is about 30%. There were also some redshifted excess absorption signatures in the H$_{\alpha}$ line profiles. These signatures could not only be observed in the flaring spectra but also in the spectra without flare, which could be interpreted as features resulting from flare-driven coronal rain and quiescent rain, respectively.
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Solar and Stellar Astrophysics

Further Investigation on Chromospheric Activity Patterns of the RS Canum Venaticorum Star V711 Tauri

Cao Dongtao and Gu Shenghong

We present new long-term spectroscopic observations of the very active RS Canum Venaticorum (RS CVn) star V711 Tauri (V711 Tau), obtained during several observing runs from 2005 to 2021. By means of spectral subtraction technique, several optical chromospheric activity indicators $\mbox{Ca {\sc ii}}$ infrared triplet, H$_{\alpha}$, $\mbox{Na {\sc i}}$ D$_{1}$, D$_{2}$ doublet, $\mbox{He {\sc i}}$ D$_{3}$, and H$_{\beta}$ lines, are analyzed. V711 Tau shows strong excess emission in the chromospheric activity lines, and the emission is mainly associated with the K1 IV primary component. The G5 V secondary component also shows weak emission features. There are four optical flare events during our observations, characterized by the $\mbox{He {\sc i}}$ D$_{3}$ line emission features and stronger emission in other activity lines, which have similar order of magnitude as the strong flares of other active RS CVn-type stars. We separate our long-term observations to twelve observing groups, and find that V711 Tau has two active longitudes around phases 0.25 and 0.75 to dominate the activity in most of time. The chromospheric activity level shows a long-term variation, indicating possible chromospheric activity cycle. Moreover, a close spatial connection between photospheric spots and chromospheric active regions in short and long timescale is found for V711 Tau.
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Tuesday 4 August

Solar and Stellar Astrophysics

Custom Colors: A Module for Computing Synthetic Photometry On-the-Fly in Stellar Evolution Calculations, Integrated with MESA and Usable with Other Stellar Evolution Codes

Niall Miller, Meridith Joyce and Philip Mocz

Stellar evolution simulations predict physical quantities such as luminosity, whereas broadband photometric observations measure flux in specific bandpasses; converting the predicted stellar state to magnitudes requires interpolating a stellar atmosphere model at the surface parameters, convolving the resulting spectral energy distribution (SED) with filter transmission curves, and applying a photometric zero-point correction. We introduce Custom Colors, which performs this conversion from the stellar model. It is integrated into MESA as MESA Custom Colors, and available as the Python package SED_Model for external applications. At each timestep, the module interpolates a user-specified atmosphere grid at the current T_eff, log g, [M/H], applies geometric dilution at a user-specified distance, convolves the SED with each requested filter transmission curve, and appends observer-frame magnitudes to standard MESA output files. We demonstrate the module across six diverse use cases: TP-AGB evolution, nonlinear RR Lyrae pulsation, starspot-modified photospheres, white dwarf cooling, blue-loop evolution, and rotational spot modulation based on an external YREC SPOTS grid. These cases draw on Kurucz/ATLAS9, BT-Settl, and Koester DA atmosphere grids and filter systems including Roman WFI, LSST ugrizy, Gaia, and extended Johnson UBVRIJHKLMN. Two of these demonstrations compare the module directly with observational data: a 0.6M_dot DA white dwarf cooling track follows the broad Gaia DR3 cooling locus from M_G ~ 8.5 to 15mag without intermediate color transformation, and an RSP model recovers the Kepler light curve of FN Lyr (KIC6936115) to 1.1% in period and 0.2% in amplitude.
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Solar and Stellar Astrophysics

A comparison of best fits obtained in white dwarf asteroseismology using the WDEC and the LPCODE

Agnès Bischoff-Kim

We perform the asteroseismic fitting of 4 DAVs using a grid of WDEC models with chemical pro- files that closely mimic those of the LPCODE models and compare them with published asteroseis- mic fitting results using the LPCODE (Romero et al. 2017). These 4 objects are KIC 11911480, J113655.17+040952.6, KIC 4552982, and GD1212. The similarities in the results in those controlled experiments point to a consistency in the models. Given similar input, the LPCODE and the WDEC make similar models and calculate similar periods. We further perform the asteroseismic fitting of the same 4 DAVs by relaxing the constraints on the chemical profiles. We explore the effects of different methods for weighing the modes when calculating the goodness of fit of the models, as well as the effect of only including a subset of the known period spectrum. Such numerical experiments can help place recent and future efforts in pipeline fitting of numerous DAVs and DBVs using the WDEC on firmer footing (Hall et al. 2023).
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA

Phosphorous in the moderately metal-poor bulge globular clusters NGC 6539 and NGC 6569

Morgan S. Camargo, Beatriz Barbuy, Heitor Ernandes et al. (+10 more)

The distinct stellar populations of the Galactic bulge can be disentangled through detailed analysis of their chemical abundances and kinematical properties. Recent studies have suggested that globular clusters located in the Galactic bulge with metallicities around $\rm [Fe/H] \approx -0.7$, may represent some of the oldest systems in the Milky Way, potentially tracing the early spheroidal bulge. The coincidence of a metallicity peak at $\rm [Fe/H] \approx -0.7$ in both field stars and globular clusters, together with the presence of phosphorus-rich (P-rich) stars, may provide important clues to the nature of the first generations of stars formed in the Galaxy. In this work, we investigate the odd-Z elements Na, Al and particularly P in the bulge globular clusters NGC~6539 ($\rm[Fe/H] \sim -0.75$) and NGC~6569 ($\rm [Fe/H] \sim -0.85$) using APOGEE spectra. We also examine the clusters Tonantzintla-1 and NGC~6316, which exhibit evidence of phosphorus enhancement. Our analysis confirms that NGC~6539 is a cluster of interest, with one clearly P-rich star, whereas NGC~6569 shows a lower level of P-enhancement. This again suggests that there might have been an early bulge building block with the metallicity of $\rm[Fe/H] \sim-0.75$, of which NGC~6539 would be part of. The observed abundance patterns indicate that the production of Na and Al is consistent with nucleosynthesis in massive stars. However, the origin of the phosphorus enrichment remains uncertain, suggesting that additional nucleosynthetic channels may be required to explain the observed abundances. These findings provide new constraints on the chemical evolution of the Galactic bulge and the nature of its earliest stellar populations.
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All authors Morgan S. Camargo, Beatriz Barbuy, Heitor Ernandes, Amâncio C. S. Friaça, Cristina Chiappini, Bernardo P. L. Ferreira, Stefano O. Souza, Sergio Ortolani, Domenico Nardiello, Eduardo Bica, José G. Fernández-Trincado, Nicolas Barrera, Doug Geisler

Solar and Stellar Astrophysics

A survey of ultra-compact high-state AM CVn binaries with ZTF and Gaia: New discoveries and observational constraints on Galactic space density

Ilkham Galiullin, Antonio C. Rodriguez, Kareem El-Badry et al. (+9 more)

Ultra-compact AM CVn binaries in the high-state of stable mass transfer ($\dot{M}\gtrsim10^{-9}M_\odot/\rm{yr}$) are expected to be among the loudest persistent sources for upcoming space-based GW observatories. These systems typically have orbital periods $P_{\rm{orb}}\lesssim30$ minutes. We present a systematic search for high-state AM CVn binaries in the Milky Way by targeting a region within the Gaia color-magnitude diagram, combined with ZTF time-domain photometry. We focus on variable targets within that sample with 5-30 minute periods. Our survey discovered three new high-state AM CVn binaries (ZTF J1840-1742, ZTF J2007-0527 and ZTF J2111+3158) with orbital periods of 16.86, 18.63, and 16.71 minutes, and recovered three known systems. We obtain high-speed photometry and phase-resolved spectroscopy to confirm their nature. Their spectra show helium emission lines with no detectable hydrogen. In two targets, the lines are double-peaked, and Doppler tomograms confirm an accretion disk. We estimate mass accretion rates of $\sim10^{-9}M_\odot/\rm{yr}$, and $3\sigma$ X-ray luminosity upper limits of $\sim10^{33}\rm{erg/s}$. Based on this sample, we infer a local space density of high-state AM CVn population in the Milky Way of $\sim(1.0-2.7)\times10^{-8}\rm{pc}^{-3}$ (for disk scale heights $h_z=300-200$ pc), representing 2-5% of the total Galactic AM CVn density. Their birth rate, $\sim(2.4-4.3)\times10^{-4}\rm{yr}^{-1}$ (for $h_z=300-200$ pc), is consistent with the total AM CVn birth rate, implying most systems entering the high-state phase survive and evolve to longer orbital periods. This local space density suggests LISA and TianQin will detect about 36% and 19% of the total Galactic population, respectively, during their nominal 4-yr missions ($S/N\ge5$). The Vera C. Rubin Observatory's LSST will detect about 34-44% of these systems over its 10-yr survey ($m_r\lesssim26.9$ mag).
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All authors Ilkham Galiullin, Antonio C. Rodriguez, Kareem El-Badry, Valery Suleimanov, Vladislav Dodon, Askar Sibgatullin, Warren R. Brown, Kevin Burdge, Jan van Roestel, Edo Berger, V. Ashley Villar, Ilaria Caiazzo

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

Chemo-dynamical stratification of the Galactic disc using Gaia-ESO open clusters

Casmir O. Obasi, M. Gómez, J. G. Fernández-Trincado et al. (+5 more)

Understanding how the Milky Way disc assembled and evolved requires tracing the coupled evolution of stellar chemistry and orbital structure over time. Open clusters, as coeval stellar populations with well-constrained ages, distances, and chemical properties, provide powerful benchmarks for this purpose. We aim to characterise the age-dependent chemo-dynamical structure of the Galactic disc and to investigate how open clusters populate dynamical phase space as a function of age. We analysed a sample of Galactic open clusters using homogeneous chemical abundances from the Gaia-ESO Survey together with Gaia DR3 phase-space information. Combining cluster ages with orbital parameters and actions, we examined how clusters populate dynamical phase space over the last ~4 Gyr. We find that clusters with similar chemical properties occupy coherent regions of action space, while their orbital structure shows a clear dependence on age. Radial excursions and orbital eccentricities broaden systematically toward older ages, indicating increasingly diverse orbital configurations in the older cluster population. The dispersion in the [Fe/H]-Rg relation also increases with age, consistent with a combination of secular redistribution, survival bias, and dynamically distinct outer-disc populations. In addition, older clusters reach larger vertical amplitudes and vertical actions, revealing a coupled but anisotropic evolution between in-plane and vertical orbital structure. Open clusters provide precise, age-resolved tracers of the chemo-dynamical structure of the Galactic disc. Our results reveal systematic age-dependent variations in the orbital and chemical properties of the cluster population, consistent with the combined effects of secular evolution, orbital redistribution, and environmentally dependent cluster survival over the last few Gyr.
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All authors Casmir O. Obasi, M. Gómez, J. G. Fernández-Trincado, D. Minniti, B. Dias, B. Barbuy, M. V. Alonso, L. D. Baravalle

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

The Most Dangerous Seed: Nonlinear Optimal Perturbations in Rayleigh-Taylor Instability

Suoqing Ji and Bin Shi

Long-term instabilities in astrophysical fluids are inherently nonlinear, where even small-amplitude perturbations can trigger dramatic instability. However, owing to the complex interactions among non-normal modes, the perturbation structures responsible for the greatest growth remain poorly understood. In this paper, we employ the nonlinear optimization method known as the conditional nonlinear optimal perturbation (CNOP) to identify the most dangerous initial velocity perturbation, i.e., the perturbation that maximizes the kinetic energy growth in the two-dimensional compressible Rayleigh-Taylor instability in astrophysical hydrodynamics. Compared with random perturbations, the optimal perturbation forms a coherent wave-packet structure localized around the density interface. We investigate its dependence on spatial resolution and optimization time horizon through two sets of numerical experiments. For a fixed optimization time, increasing the spatial resolution produces a more sharply localized wave packet, whereas for a fixed spatial resolution, increasing the optimization time causes the wave packet to become progressively more dispersed. Furthermore, we analyze the optimal perturbations in Fourier space using the fast Fourier transform (FFT), which provides a clearer characterization of the spectral distribution. Higher-resolution simulations concentrate most of the perturbation energy into only a few dominant modes, while longer optimization times distribute the energy over a broader range of modes. These results indicate that short optimization time horizons involve relatively weak modal interactions and remain closer to the linear regime, whereas longer optimization times enhance nonlinear modal interactions, broaden the spectral distribution, and reduce the predictive capability of linear stability theory.
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Solar and Stellar Astrophysics

Expanding magnetic canopy structure and parasitic polarities in a quiet-Sun network element observed by Sunrise III/SCIP

Ryan J. Campbell, Carlos Quintero Noda, Mihalis Mathioudakis et al. (+35 more)

We present high-resolution multi-line spectropolarimetric observations of a quiet-Sun network element obtained with the Sunrise~iii Chromospheric Infrared SpectroPolarimeter. The observations combine photospheric, upper-photospheric, and chromospheric diagnostics at a spatial resolution and polarimetric sensitivity that allow the transverse magnetic structure of the network boundary to be examined directly. We find that the strongest linear polarisation is concentrated in a narrow ridge around the edge of the magnetic element, co-spatial with enhanced transverse magnetic field inferred from multiline inversions. The magnetic azimuth exhibits a coherent, predominantly radial organisation around a more vertical core, consistent with an expanding magnetic canopy. An azimuth proxy derived directly from the observed Fe~i and K~i linear polarisation reproduces the same large-scale organisation, showing that this structure is encoded in the Stokes profiles rather than imposed by the inversion. Response functions and a MURaM-based forward-synthesis test indicate that the Fe~i~8468~Å\ linear polarisation is sensitive to magnetic azimuth in the upper photosphere, with the closest proxy agreement occurring near $\log\tau\approx-3$. We find no evidence for strong azimuthal shear between the Fe- and K-sensitive diagnostics. At the network boundary, we also identify localised parasitic-polarity patches associated with complex, multi-lobed Stokes $V$ profiles, and one case in which the Stokes $V$ polarity reverses between photospheric Fe~i and chromospheric Ca~ii lines. These results demonstrate that quiet-Sun network boundaries contain organised upper-photospheric canopy fields together with small-scale mixed-polarity structure, providing new constraints on the three-dimensional magnetic structure of network elements.
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All authors Ryan J. Campbell, Carlos Quintero Noda, Mihalis Mathioudakis, Manuel Collados, Jack McFall, David B. Jess, Sami K. Solanki, Andreas Lagg, Achim Gandorfer, Jose Carlos del Toro Iniesta, Yukio Katsukawa, Pietro Bernasconi, Thomas Berkefeld, Alex Feller, Tino L. Riethmüller, Alberto Álvarez-Herrero, Masahito Kubo, H. N. Smitha, David Orozco Suárez, Bianca Grauf, Michael Carpenter, Alexander Bell, Valentín Martínez Pillet, Francisco Javier Bailén, Julian Blanco Rodríguez, Juan Sebastián Castellanos Durán, Edvarda Harnes, Johannes Hölken, Francisco A. Iglesias, Ryohtaroh T. Ishikawa, Yusuke Kawabata, Takuma Matsumoto, Takayoshi Oba, Azaymi L. Siu-Tapia, Hanna Strecker, Dušan Vukadinović, Hirohisa Hara, Toshifumi Shimizu

Solar and Stellar Astrophysics

Horizontal Magnetic Fields Dominate the Quiet Sun Internetwork in Sunrise III Observations. Evidence from Traditional and Transformer-Based Inversions

Ryan J. Campbell, Carlos Quintero Noda, Manuel Collados et al. (+32 more)

The relative prevalence of horizontal and vertical magnetic fields in the quiet-Sun internetwork remains debated, owing to the weak linear polarisation signals that make the inferred magnetic inclination distributions sensitive to observational quality and inversion methodology. We investigate the magnetic topology of the quiet-Sun internetwork in Sunrise~iii observations by independently applying transformer-based and traditional inversion techniques to the same spectropolarimetric dataset. We analyse observations obtained with the Sunrise Chromospheric Infrared spectroPolarimeter (SCIP). Transformer-based inversions (SINN), trained exclusively on synthetic Stokes profiles generated from a large radiative magnetohydrodynamic simulation, are compared with independent inversions performed using the DeSIRe code. Both inversion methods recover a quiet-Sun magnetic topology in which internetwork fields are strongly dominated by the horizontal component, whereas network fields exhibit a substantially more balanced topology. At $\log\tau=-1.2$, the median horizontal and unsigned line-of-sight field components in the internetwork are 64 and 7G with DeSIRe, and 71 and 8G with SINN, respectively, corresponding to a horizontal-to-line-of-sight ratio of approximately nine for both methods. The agreement between the two fundamentally different inversion methodologies indicates that the quiet-Sun internetwork is characterised by a substantially stronger horizontal than vertical magnetic field component. These results demonstrate that transformer-based inversions can be successfully transferred from synthetic training data to real spectropolarimetric observations, while providing inference orders of magnitude faster than traditional inversion techniques.
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All authors Ryan J. Campbell, Carlos Quintero Noda, Manuel Collados, Mihalis Mathioudakis, Andreas Lagg, Achim Gandorfer, Jose Carlos del Toro Iniesta, Yukio Katsukawa, Pietro Bernasconi, Thomas Berkefeld, Alex Feller, Tino L. Riethmüller, Alberto Álvarez-Herrero, Masahito Kubo, H. N. Smitha, David Orozco Suárez, Bianca Grauf, Michael Carpenter, Alexander Bell, Valentín Martínez Pillet, Francisco Javier Bailén, Julian Blanco Rodríguez, Juan Sebastián Castellanos Durán, Edvarda Harnes, Johannes Hölken, Francisco A. Iglesias, Ryohtaroh T. Ishikawa, Yusuke Kawabata, Takuma Matsumoto, Takayoshi Oba, Azaymi L. Siu-Tapia, Hanna Strecker, Dušan Vukadinović, Hirohisa Hara, Toshifumi Shimizu

Solar and Stellar Astrophysics

Statistical Study of Solar Prominence Plumes Based on NVST H$\alpha$ Observations

Yangrui Chen, Yijun Hou, Jincheng Wang et al. (+9 more)

Plumes are one of the most representative dynamic features observed in prominences and play a key role in mass and magnetic transport within them. However, their physical nature and triggering processes remain actively debated. Based on limb H$\alpha$ observations from the New Vacuum Solar Telescope (NVST) during 2013–2025, we statistically investigated 34 plumes with clear and complete evolutions by developing an automated image-processing pipeline. It is revealed that plume lifetimes mainly range from 300 s to 700 s, with vertical displacements between 3–7 Mm. The mean widths and velocities are concentrated in the range of 0.5–1.5 Mm and 10–20 km s$^{-1}$, respectively. Besides wide distribution ranges, plume parameters exhibit irregular evolution fluctuations, indicating that the formation and evolution of various plumes may exhibit different physical patterns. Correlation analysis among the parameters further reveals that: (1) Positive correlations were found among lifetime, vertical displacement, and mean width, indicating an intrinsic coupling between the temporal and spatial scales of plumes. (2) Trajectory curvature is negatively correlated with lifetime, vertical displacement, and velocity. Accelerating and width-contracting plumes typically have lower curvature, suggesting that curvature may reflect environmental influences and the stability of plumes. (3) Plumes with higher initial velocities were more likely to be accompanied by precursor brightening, suggesting that these plumes may be triggered by magnetic reconnection. Furthermore, we infer that some plumes in non-bubble regions may be inherently driven by mini-filament eruptions. These results establish a statistical framework for prominence plumes and reveal diversity in their dynamical evolution and triggering mechanisms.
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All authors Yangrui Chen, Yijun Hou, Jincheng Wang, Ting Li, Shuo Yang, Yilin Guo, Junyi Zhang, Haitang Li, Feiyang Sha, Qing Zhou, Yu Liu, Xiaoli Yan

Solar and Stellar Astrophysics

Hot water emission during an outburst in a classical T Tauri star

Zhen Guo, Javier Osses, Vitor Fermiano et al. (+34 more)

In this paper, we present observations of an eruptive young star in the Rosette Nebula, identified by the Gaia Science Alerts system using Gaia time series data. We aim to investigate the evolution of the brightness and mass accretion rate of V557 Mon throughout its outburst and subsequent decline. In addition, we trace the evolution of the inner accretion disk during the outburst by monitoring molecular emission features. We compiled multi-band photometric time series from Gaia, ZTF, and several 1 m-class ground-based telescopes and obtained optical and near-infrared spectra at multiple epochs covering the outburst and fading phases. Stellar parameters were derived from quiescent colour/spectra and spectral energy distribution (SED) fitting. We also measured the mass accretion rate and fit models to molecular emission bands. Since late 2024, V557 Mon has undergone a year-long outburst consistent with EXor variability. Based on quiescent photometry, V557 Mon has a spectral type of M1 with an extinction of AV = 1.8+_0.3 mag, consistent with a 0.4-0.5 M_sol star at an age of 2 Myr. Our multi-epoch spectra and u-band photometry indicate a peak accretion rate of 6.3x10^(-7)M_sol/yr during the outburst, roughly 70 times higher than in quiescence. We report the detection of hot water vapour emission bands, together with TiO, VO, and CO emission features. Using ExoMol models, we measured the inner-disk temperature changed from 3000 K to 2000 K during the fading phase of the outburst. We report a recent EXor outburst in a low-mass Class II YSO. Our observations reveal the transient formation of a hot molecular inner disk, traced by variable water vapour emission during the EXor event. A positive correlation is found between the molecular excitation temperature and the overall stellar brightness.
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All authors Zhen Guo, Javier Osses, Vitor Fermiano, Yuting Zhou, Min Fang, Gregory Herczeg, Adolfo Carvalho, Vardan Elbakyan, Lynne Hillenbrand, Mutian Wang, Hanpu Liu, Yao Liu, Cesar Briceno, Koshvendra Singh, Valentin D. Ivanov, Joe Ninan, Teresa Giannini, Aldo Aliaga, Calum Morris, Matias Montesinos, He Zhao, Carlos Contreras Pena, Jessy Jose, Tarak Chand, Wen-Ping Chen, Wei-Hao Wang, Yang Huang, Camila Lopez, Pablo Fernandez-Schlosser, Diego Correa-Herrera, Radostin Kurtev, Vania Rodriguez, Carolina Lizana-Vidal, Jura Borissova, Michael Kuhn, Roberto K. Saito, Ram K. Yadav

Solar and Stellar Astrophysics

Wave excitation by a collapsing granule: insights from IFU observations and high-resolution RMHD simulations

J. Buttler, G. Vigeesh, I. Milić et al. (+5 more)

Context. Granular collapse is an ubiquitous process of granular evolution on the solar surface, but it is hard to analyze in sufficient spatial, temporal, and spectral detail. Aims. We analyze the change in physical conditions in the photosphere during a specific granular collapse event and the subsequent atmospheric response. Methods. We contrast a high-resolution radiative magneto-hydrodynamic simulation of a granular collapse performed using the CO5BOLD code with the recent integral field unit observations carried out using the MiHI instrument at the Swedish 1-m Solar Telescope. Results. Our analysis shows that the observed and simulated granular collapse show remarkable similarity. Specifically, they both exhibit the signature of a wave pulse excited in the deep photosphere and visible up to the temperature minimum. This wave is detectable through a blue-wing emission in the observed and synthetic Na i D1 line. We also estimate the acoustic energy flux carried by the wave and analyze its initiation. Conclusions. Combining high-resolution IFU spectropolarimetry and state-of-the-art simulations of the solar lower atmosphere, this study showcases our current capabilities in identifying specific physical processes taking place during the granular collapse and their impact on the atmosphere above.
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All authors J. Buttler, G. Vigeesh, I. Milić, M. van Noort, S. M. Díaz Castillo, C. J. Díaz Baso, F. Riva, O. Steiner

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

A study on the contribution of the interplanetary medium in radio occultation experiments

Keshav Aggarwal, R. K. Choudhary, Abhirup Datta et al. (+4 more)

Irregularities in electron density within the interplanetary medium (IPM) can cause fluctuations in the Doppler frequency of spacecraft radio signals. The amplitude of these fluctuations depends on factors such as the carrier frequency, propagation geometry, and link configuration. However, quantitative characterization of these effects across different frequencies in various occultation experiments is currently limited. We analyze five complementary datasets: two-way S-band observations from Chandrayaan-3 outside the lunar ionosphere, two-way S-band data from Chandrayaan-2 during lunar occultation, one-way S/X band measurements from the Venus Express Radio Science (VeRa)/Akatsuki Radio Science (Akatsuki) under IPM-only conditions, and one-way X-band Akatsuki data during solar occultation. The Chandrayaan-3 and Akatsuki IPM observations isolate IPM effects by excluding contributions from planetary atmospheres, the lunar ionosphere, and, except during solar occultation, the solar corona. Chandrayaan-3 data sample dynamically evolving Earth-Moon geometries and exhibit weak, mHz-level Doppler fluctuations, while Chandrayaan-2 observations provide near-lunar plasma benchmarks with higher amplitudes, during quiet time solar and geomagnetic conditions. Akatsuki and VeRa's IPM-only measurements capture long-path interplanetary effects, whereas Akatsuki solar occultation data reveal strong coronal signatures. Power spectral density analysis indicates Kolmogorov-like turbulence for lunar occultation and solar occultation cases, while IPM-only spectra show low-amplitude fluctuations. These results quantify the IPM contribution to Doppler noise, demonstrate the enhanced plasma sensitivity of two-way coherent links, and provide constraints relevant to turbulence modelling, precision spacecraft tracking, and the interpretation of radio occultation experiments.
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All authors Keshav Aggarwal, R. K. Choudhary, Abhirup Datta, Soumyaneal Banerjee, R. Manikantan, Anshuman Sharma, Takeshi Imamura

Solar and Stellar Astrophysics

The symphony of pulsations and binarity among massive stars using HERMES spectroscopy and TESS photometry

K. Thomson-Paressant, D. M. Bowman, F. Nardini et al. (+16 more)

A wide range of variability mechanisms exist among intermediate mass and massive stars, which are not yet fully understood. Using complementary data sources for a large population of B- and O-type stars, we aim to study the prevalence and interplay of different types of variability, including binarity, pulsation, and rotation, to prepare for future modelling. To this end, we analyse high-resolution HERMES spectra and 2-min cadence TESS photometry and characterise the diverse variability observed within a population of 873 O- and B-type stars. The spectroscopic data were normalised using machine-learning techniques, compared to a grid of synthetic TLUSTY spectra to determine stellar parameters, and used to identify radial velocity variability. Photometric time series were analysed using standard frequency analysis methods to detect pulsations and rotational modulation signatures. We find that more than 93 per cent of the sample exhibits photometric variability. Photometric variability caused by pulsations is identified in 82 per cent of the sample, with dominant contributions from $\beta$ Cep and slowly pulsating B-type stars, as well as stochastic low-frequency variability. Based on a limited number of spectroscopic epochs, at least 14 per cent of the stars show evidence of binarity, including both eclipsing and spectroscopic systems. This work represents one of the largest homogeneous surveys of variability for intermediate-mass and massive stars in the Northern hemisphere, and complementing similar efforts in the Southern hemisphere. It provides a statistical framework for future studies of stellar structure and evolution, particularly in the context of asteroseismology.
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All authors K. Thomson-Paressant, D. M. Bowman, F. Nardini, L. J. A. Scott, J. Bodensteiner, T. Shenar, L. Mahy, G. Handler, N. Shitrit, I. Arcavi, M. Abdul-Masih, S. Simon-Diaz, P. Van Daele, A. J. Kalita, L. Dennis, J. Henneco, A. Tkachenko, H. Sana, H. Van Winckel

Solar and Stellar Astrophysics

Unravelling Mass Transfer in Algols from Surface Abundances. I. Z Vulpeculae

Ahmet Dervişoğlu, Simge Adalal{\i}, Ferhat Güney et al. (+7 more)

The photospheric abundance pattern of the components is a sensitive probe of mass and angular momentum transfer in Algols. With the aim to trace the evolutionary history and disclosed efficiency of mass transfer in Algols we collected new high-resolution spectra for about a dozen Algols with the hermes spectrograph at the Mercator Telescope on La Palma, Canary Islands. In the present paper, which is the first in the series, we present the results of a comprehensive analysis of Z Vul, a hot Algol-type system. We derived the absolute stellar quantities and elemental abundances for the components. The carbon-to-nitrogen (C/N) ratio is the most sensitive tracer of thermonuclear processing and mass transfer. It is found to be within expected value for gainer but donor lacks the severe C/N inversion traditionally expected for a deeply stripped star. An extensive grid of over three million evolutionary models was calculated with the Cambridge STARS code which allows determination of the best-fitting and all surviving models. Constraints from the He and CNO abundances indicate that Z Vul underwent an episode of nearly conservative mass transfer during the Main Sequence life-time of the originally more massive component (Case A). Our detailed chemical tagging confirms that the surface abundances of the mass-losing component represent the delicately stripped outer layers still not reaching the CNO-rich core. This underscores the need of coupling high-resolution spectroscopic abundances with interior stellar profiles to accurately reconstruct the evolutionary history of interacting binaries.
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All authors Ahmet Dervişoğlu, Simge Adalal{\i}, Ferhat Güney, Bar{\i}ş Hoyman, Timur Şahin, Ömür Çak{\i}rl{\i}, Kresimir Pavlovski, David Mkrtichian, Peter De Cat, Patricia Lampens

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

Chemical Abundances and Globular Clusters of Milky Way Dwarf Galaxies

Baitian Tang, Shihui Lin, Ruoyun Huang et al. (+5 more)

We present an overview of our ongoing GASTRONOMI project, which investigates the coevolution of the Milky Way (MW), its satellite dwarf galaxies, and their star clusters through chemo-dynamical analysis. We derive precise chemical abundances for stars in five classical dwarf galaxies, which reveal mass-dependent chemical evolution, particularly in alpha elements, such as [Si/Fe]. A distinct dichotomy in [Al/Fe] is found between metal-rich ([Fe/H]>-1.5) stars formed in-situ in the MW and those originating in dwarf galaxies. Star clusters act as sensitive environmental probes. The presence of multiple populations correlates with galactic evolution, and nitrogen-rich stars in Fornax are likely relics of disrupted globular clusters (GCs). We developed a chemical classification for Galactic GCs, isolating primordial populations by their low [Al/Fe]. This places in-situ and accreted GCs in distinct regions of the [Al/Fe]-[Fe/H] plane, providing a new tool to reconstruct the Galaxy's accretion history.
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All authors Baitian Tang, Shihui Lin, Ruoyun Huang, Cheng Xu, Jose G. Fernandez-Trincado, Doug Geisler, Chengyuan Li, Zhiqiang Yan

Solar and Stellar Astrophysics

The double white dwarf merger model for the progenitors of SN 2021yfj-like events

Dongdong Liu, Chengyuan Wu, Takashi J. Moriya et al. (+4 more)

Context. Recently, a new class of supernovae with prominent narrow emission features of Si, S and Ar has been reported, i.e. SN 2021yfj-like events (SNe Ien). Their progenitor origin is still unknown. It has been suggested that a massive CO WD may evolve into a Si/S-rich WD when accreting He-rich matter at a high mass-transfer rate. If the He companion subsequently evolves into another more massive WD, the merger of this double WD system can generate Si/S-rich CSM through tidal stripping. Following the SN explosion, the interaction between the SN ejecta and the Si/S-rich CSM could produce the Si, S, and Ar emission lines characteristic of SN 2021yfj-like events. Aims. In this work, we aim to determine the initial parameter space of WD+He star systems that can lead to SN 2021yfj-like events via the double WD merger scenario, and to quantify their theoretical Galactic birthrate and delay-time distributions. Methods. We perform detailed binary evolution simulations of a large number of semidetached WD+He star systems to obtain the parameter space that leads to the formation of Si/S-rich WDs and subsequent double WD mergers. We then use binary population synthesis to calculate the Galactic birthrate and delay-time distribution of such events. Results. For the formation of SN 2021yfj-like events, we found that the initial CO WD and He companion masses must lie in the ranges of 1.0-1.2 M_sun and 2.2-2.5 M_sun, respectively. The derived merger rate for this scenario is ~(2.6-4.4)x10^-5 yr^-1 with delay times longer than 450 Myr, which is about 1% of the observed SN Ia rate. Conclusions. We suggest that the double WD merger scenario involving a Si/S-rich WD originating from a CO WD+He star system, represents a significant and competitive model for SN 2021yfj-like events, underscoring the need for further observations of similar events coupled with relevant theoretical investigations.
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All authors Dongdong Liu, Chengyuan Wu, Takashi J. Moriya, Jiawen Ning, Zhengwei Liu, Zhanwen Han, Bo Wang

Solar and Stellar Astrophysics

Electron transport in a radiation-dominated plasma Application to solar corona brightenings

Roland Duclous, Vladimir Tikhonchuk, Marina Leboulanger et al. (+3 more)

Bremsstrahlung scattering of fast electrons on ions can be enhanced by the microwave radiation present in the solar corona. It can account for the electron diffusive transport along magnetic loops and high precipitation rates. This process can also dominate the transport of thermal electrons confined in such loops. The influence of stimulated Bremsstrahlung scattering on the electron transport is studied, with focus on the return current induced by the fast electron population trapped in magnetic loops. Overall, transport coefficients are reevaluated in the radiation-dominated plasma, characterized by the stimulated action of radiation on the Bremsstrahlung electron-ion collision, down to thermal velocities. We develop a theoretical framework for electron transport driven by a large bandwidth, bright low-frequency part of the photon spectrum and compute a set of radiation-enhanced transport coefficients. UV, XEUV and hard X-ray signals from flares, evidencing anomalous resistivity, thermal conduction inhibition and high precipitation rates of fast electrons, are reinterpreted. The anomalous resistivity due to stimulated Bremsstrahlung scattering is found to dominate the classical resistivity in flares. The runaway effect due to Coulomb collisions is suppressed. Thermal conduction is inhibited compared to the Spitzer conduction, in agreement with coronal seismology of slow-mode waves. Stimulated Bremsstrahlung scattering is found to be a key collisional process in flaring events. It can explain the above loop-top hard X-ray signal due to the fast electrons, and the measured electrical conductivity due to the thermal electrons. As a perspective, the corresponding transport coefficients can be used in radiation MHD codes. The radiation model could also be applied to stimulate large-angle electron scattering in the kinetic or hybrid models used to study the solar corona.
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All authors Roland Duclous, Vladimir Tikhonchuk, Marina Leboulanger, Xavier Blanc, Allan-Sacha Brun, Antoine Strugarek

Solar and Stellar Astrophysics

A Possible Stellar CME Event Associated with a Long-duration Optical Flare on the RS CVn-type Star UX Arietis

Cao Dongtao and Gu Shenghong

Coronal mass ejections (CMEs) are large-scale eruptions of magnetized plasma from stellar coronae, and they are always accompanied by flares. In this study, we present high-resolution spectroscopic observations of a long-duration optical flare on the RS CVn-type star UX Arietis (UX Ari). Around the flare peak, we have detected a far-blueshifted emission component in the H$\alpha$ line profile—a signature of an erupting prominence—indicating a possible stellar CME. After correcting for projection effects, the eruption velocity reaches $\sim -432$ km s$^{-1}$ when assuming radial propagation, significantly exceeding the escape velocity of the primary component ($\approx 304$ km s$^{-1}$). This strongly supports the detection of a stellar CME. The mass of this CME is at least $2.7 \times 10^{20}$ g, and its kinetic energy is $2.5 \times 10^{35}$ erg. The mass value is consistent with the solar flare–CME scaling relation, while its kinetic energy lies below the solar trend. The flare itself released an energy of $4.2 \times 10^{36}$ erg in the H$\alpha$, corresponding to a white-light bolometric energy of roughly $2.8 \times 10^{38}$ erg. Throughout the flaring event, evolving asymmetries in the H$\alpha$ profile reveal complex plasma dynamics.
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