Astronomy & Astrophysics最新文献

{"title":"A triple-induced merger in solves the proplyd lifetime problem and the curiously young shell of M42","authors":"M. J. C. Wilhelm,A. A. Trani,S. Portegies Zwart","doi":"10.1051/0004-6361/202662018","DOIUrl":"https://doi.org/10.1051/0004-6361/202662018","url":null,"abstract":"A longstanding tension exists between the stellar ages in the Orion Nebula Cluster (ONC) and the rapid external photoevaporation of its protoplanetary discs. More recently, the shell of the Orion Nebula has also been found to be dynamically younger than its stars. We propose a new scenario aimed at resolving both age issues. In our scenario, the most massive star of the ONC, formed from the merger between two binary stars induced by von Zeipel-Lidov-Kozai oscillations due to the present-day binary companion This merger, taking place between the last episode of star formation and today, resulted in an increased luminosity, leading to stronger external photoevaporation and accelerated expansion of the shell. We used Monte Carlo simulations to explore both whether a sudden brightening of could reconcile the stellar ages and shell expansion. We also considered whether the assumption of triple progenitors for could result in stellar mergers on the relevant timescale. A sudden brightening of a few hundred thousand years ago can be effective in reproducing today's shell, while maintaining a system age close to the stellar age. However, the required increase in the stellar wind luminosity would be greater than a simple merger would be equipped to explain. At a lower luminosity, the mixing of a relatively greater amount of cool photoevaporative wind material into the hot bubble could result in the required increase, and we identified a lower limit on the required strength of this effect. Additionally, we find that progenitor triples merge readily on timescales that are even shorter than the scenario would require. These mergers could be deferred by increased pre-main sequence tidal effects, rotation, and perturbing encounters with other cluster members. Ultimately, our proposed scenario with a merger driven by von Zeipel-Lidov-Kozai evolution successfully resolves two issues: the Orion Nebula's proplyd problem and the shell lifetime problems.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"11 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149150153","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Coupling between stellar and HI lopsidedness in Milky Way--type galaxies from the Auriga Superstars cosmological simulations","authors":"Arianna Dolfi,Facundo A. Gómez,Rebekka Bieri,Francesca Fragkoudi,Robert J. J. Grand,Antonela Monachesi,Ruediger Pakmor,Freeke van de Voort","doi":"10.1051/0004-6361/202660650","DOIUrl":"https://doi.org/10.1051/0004-6361/202660650","url":null,"abstract":"Lopsidedness is a common feature in disk galaxies, but its origin and evolution remain poorly understood. Previous works have generally focused on stellar or gas lopsidedness separately. However, a combined analysis can help identify the mechanisms driving lopsidedness and constrain recent galactic evolution and environmental interactions. We study lopsidedness in the density and kinematics of the stellar and atomic hydrogen (HI) components of nine Milky Way-type galaxies from the Auriga Superstars cosmological zoom-in simulations. The high stellar mass resolution (mathrm m_ * =800, M_⊙ ) of the simulations improves the visibility of features in the stellar disk and halo, reduces the effect of noise on the measurement of lopsidedness, and provides a refined sampling of the gravitational potential. We quantified morphological and kinematical lopsidedness through the amplitude of the first Fourier mode (m=1) of the face-on projected mass distribution and radial velocity maps consistently for stars and gas and over 0.5-1, opt , where R_ opt is the stellar optical radius. At z=0, the morphological lopsidedness of the old stellar component ($>0.5$ Gyr old) strongly correlates with that of HI, tracing distortions in the global gravitational potential, while the young stellar population ($<0.5$ Gyr old) primarily traces asymmetric star formation along strong spiral arms. Morphological and kinematical lopsidedness are strongly correlated in stars but weakly correlated in HI, which typically exhibits stronger kinematical asymmetries. We also find that stellar disk lopsidedness is anticorrelated with bar strength, with strongly barred galaxies hosting more symmetric disks and higher central stellar mass densities. Based on our simulations of lopsidedness evolution over the past 7 Gyr, we observed that tidal interactions with massive satellites (mass ratio $>1$:50) produce coherent lopsided responses in stars and HI. In contrast, smooth gas accretion primarily drives lopsidedness in HI and young stars, while the total stellar component remains largely symmetric. Individual galaxies often experience multiple mechanisms, producing complex coupling and decoupling among distinct tracers.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"78 4 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149150083","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The oldest low-α thin disc stars","authors":"Riano E. Giribaldi,Laura Magrini,Marco Palla,Carlos Viscasillas Vázquez","doi":"10.1051/0004-6361/202660524","DOIUrl":"https://doi.org/10.1051/0004-6361/202660524","url":null,"abstract":"The formation pathway of the α-poor Milky Way (MW) thin disc remains under debate. The latest observational data indicates the presence of comparable numbers of old stars in both the low- and high-α disc components, suggesting either coeval or overlapping formation epochs. Using GALAH DR4, we provide evidence that α-poor stars have coexisted with α-rich stars since ∼12.5 Gyr ago. Stellar ages of 0.5 Gyr precision were re-derived for nearly 30,000 subgiant stars using accurate effective temperatures ( and solar-like metallicities (-0.5 łeq łeq +0.5 dex). These stars exhibit kinematics consistent with those of near solar-age α-poor stars in the thin disc. These stars can be evolved Sun-like stars, i.e. old solar analogues, whose from the infrared flux method (IRFM) together with distances based on Gaia parallaxes. About 22% of the sample constitutes a population peaking at 9.2 Gyr, with evidence for an extended old-age tail reaching 12.5 Gyr. Among stars older than 10 and 11 Gyr, between 20 and 30% have relative low α/Fe Fe/H and gravity modified with time. Comparison with a well-tested Galactic disc chemical evolution model supports the presence of a significant fraction of old, α-poor disc stars, confirming the plausibility of our selection criteria. We provide a catalogue with the selected sample to facilitate spectroscopic follow-ups at high resolution.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"26 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149149909","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Unlocking new information in stellar circular polarisation with second-order least-squares deconvolution","authors":"Oleg Kochukhov","doi":"10.1051/0004-6361/202662504","DOIUrl":"https://doi.org/10.1051/0004-6361/202662504","url":null,"abstract":"Least-squares deconvolution (LSD) is the standard technique used in measuring stellar magnetic fields from circular polarisation spectra. It relies on the weak-field approximation and yields the longitudinal field alone. The field modulus is accessible only through a methodologically distinct analysis of intensity spectra, limited to strongly magnetic stars or slow rotators. We aim to extend LSD beyond the weak-field limit and interpret departures from a linear field response in terms of the field modulus. We expand the Stokes V absorption coefficient in odd wavelength derivatives of the intensity profile, with coefficients given by the Zeeman pattern moments. Truncation after two terms gives every line a second weight, formed from the third moment of its splitting pattern, and two mean Stokes V profiles follow from multiprofile LSD. In this work, this procedure was tested with polarised spectrum synthesis and applied to archival spectra of two Ap stars and two active M dwarfs. The second mean profile is proportional to the second derivative of the first, with a coefficient measuring the mean square field modulus weighted by the longitudinal flux. This observable separates field strength from the area it covers. Simulations at realistic signal-to-noise ratios show that the second component is reliably detected from about 500 G and constrains the field modulus up to a few kG. Applying this methodology to the Ap stars, we found the modulus consistent with their known field strengths, whereas for the M dwarfs it was close to the Zeeman-broadening measurements and far above the large-scale fields from Zeeman-Doppler imaging. Second-order LSD turns the breakdown of the weak-field approximation from a source of error into a new magnetic observable, revealing for the first time the field strength in the regions emitting circular polarisation. It applies to any star with a detectable Stokes V signature and bridges the gap between polarimetry and Zeeman broadening, most promisingly for cool active stars in the infrared.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"32 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149149912","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The Treble Clef radio phoenix and its old nonthermal filaments","authors":"A. Botteon,M. Brienza,K. Rajpurohit,N. Lyskova,E. Churazov,I. Khabibullin,T. Pasini,E. O'Sullivan,G. Brunetti,F. De Gasperin,E. De Rubeis,F. Gastaldello,D. N. Hoang,R. Kraft,G. Schellenberger,R. Sunyaev,R. J. van Weeren,F. Vazza","doi":"10.1051/0004-6361/202661864","DOIUrl":"https://doi.org/10.1051/0004-6361/202661864","url":null,"abstract":"We identified a peculiar bright and filamentary radio source at a low Galactic latitude (b ≈ 0.5 in łotssE data. This source, also detected in previous radio observations, was originally believed to be a pulsar, but was later suggested to lie in a heavily obscured galaxy cluster behind the Galactic plane. In this paper, we characterize the main properties of the host cluster (redshift, mass, temperature, X-ray luminosity, and dynamical status) for the first time, using X-ray observations from ̧handra and SRG/ In addition, we combined new ğmrt follow-up data with observations from the łolssE to perform a multifrequency, spatially resolved spectral analysis of the filamentary radio source ( hereafter referred to as the `` due to its morphology). We conclude that this source is a radio phoenix belonging to a massive, merging galaxy cluster in the zone of avoidance. We speculate that its complex morphology is shaped by gas motions generated in the intracluster medium during the ongoing merger, which are also likely responsible for the candidate radio halo tentatively observed in the cluster center. Owing to its highly filamentary morphology, brightness at łesssim1 GHz, and extremely steep spectrum, reaching values of α > 4 between 400 and 650 MHz, this source represents an ideal target for high-resolution, very-low-frequency follow-up observations with łofar2.0.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"10 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149149990","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The baryonic Tully-Fisher relation as an independent direct probe of cosmology and of the nature of dark matter","authors":"Francesco Sinigaglia","doi":"10.1051/0004-6361/202661408","DOIUrl":"https://doi.org/10.1051/0004-6361/202661408","url":null,"abstract":"The baryonic Tully-Fisher relation (BTFR), a well-established galaxy scaling relation that links the dynamical mass of rotation-supported galaxies through their maximum circular velocity to the baryonic luminous mass, has emerged over the decades as a fundamental scaling relation and a robust calibrated distance indicator. It provides a robust benchmark to test galaxy formation and evolution theories as well as an independent probe of the expansion of the Universe. In this Letter, we show for the first time that the BTFR is also simultaneously directly sensitive to the cosmological parameters Ω_m and σ_8, the astrophysical feedback from supernovae (SNe) and active galactic nuclei (AGNs), and the mass of dark matter particles (M_̊m wdm) and can therefore be used as novel, direct probe of cosmology and fundamental physics. We performed simulation-based inference on the large cosmological magneto-hydrodynamic simulations suite and trained deep neural networks in the form of normalizing flows to estimate the posterior distributions of Ω_m, σ_8, M_̊m wdm, and the three astrophysical free parameters, given a BTFR measurement. DREAMS Our framework is able to recover unbiased values for Ω_m and σ_8 with sub-percent deviation accuracy and a ∼ 2.6% and ∼ 3.9% median precision, respectively, and can therefore capture the warm dark matter particle mass (M_̊m wdm) with a ∼ 30-35% precision, as well as constrain the SN feedback parameters (but not the one regulating AGN feedback). We conclude that, beyond its usage as a distance indicator and for constraining the baryon cycle and the feedback mechanisms that shape galaxy formation and evolution, the BTFR constitutes a direct independent probe of cosmology and fundamental physics. It opens new promising avenues, such as constraining the cosmic growth rate and testing gravity theories, to be explored with the future Square Kilometer Array.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"14 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149149914","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The ALMaQUEST Survey. XVI. The Balmer decrement as a probe of molecular gas in nearby kiloparsec-resolved galaxies","authors":"Alice Concas,Sara L. Ellison,Lihwai Lin,Mallory D. Thorp,Hsi-An Pan,Simone Bianchi,Edvige Corbelli,Leslie Hunt","doi":"10.1051/0004-6361/202662208","DOIUrl":"https://doi.org/10.1051/0004-6361/202662208","url":null,"abstract":"We investigate the relation between molecular gas and dust extinction on kiloparsec scales. We analysed 15,253 spaxels across 51 nearby galaxies with CO(1--0) measurements and optical spectroscopy from the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey. Dust extinction was traced by the Balmer decrement (Hα/Hβ), while molecular gas was traced by the CO(1--0) line intensity (I_ CO). Focusing on the 12,173 spaxels classified as star-forming, we found a significant and robust correlation between Hα/Hβ and I_̊m CO (Pearson r = 0.61, Spearman ̊ho = 0.59). This correlation becomes even stronger when considering deprojected quantities ( r = 0.65, ̊ho = 0.64), revealing a close connection between the effective V-band optical depth (τ_V) inferred from the Balmer decrement and the molecular gas surface density (Σ_̊m mol), derived assuming a constant CO-to-H_2 conversion factor. The observed Σ_̊m mol--τ_V relation is sub-linear, deviating from the expectation for a simple foreground-screen geometry and suggesting a more complex distribution of dust, molecular gas, and emitting regions, and/or variations in the CO-to-H_2 conversion factor. Existing literature prescriptions fail to reproduce the observed distribution over the full extinction range. We parametrized this relation and derived a new empirical calibration linking Σ_̊m mol and τ_V. The relation is largely independent of galaxy stellar mass, star formation rate, gas-phase metallicity, morphology, and specific star formation rate, but it shows a secondary dependence primarily on inclination, highlighting the role of geometry. The relation becomes less tight in both composite and active galactic nucleus (AGN)-dominated regions, which exhibit a substantially larger scatter than star-forming regions. Using the independent EDGE--CALIFA dataset, we show that the ALMaQUEST calibration predicts Σ_̊m mol with a scatter of ∼ 0.3 dex and no significant systematic offset (-0.01 dex), demonstrating its applicability to independent kiloparsec-scale datasets. Our calibration thus provides a practical empirical method for estimating molecular gas surface density from optical spectroscopy alone, without requiring direct CO observations.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"50 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149149907","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Long-term ultraviolet variability of Taurus T Tauri stars from the IUE archive","authors":"Ana Inés Gómez de Castro","doi":"10.1051/0004-6361/202661978","DOIUrl":"https://doi.org/10.1051/0004-6361/202661978","url":null,"abstract":"Ultraviolet (UV) radiation from classical T Tauri stars is generated primarily by magnetospheric accretion and plays a fundamental role in the evolution of circumstellar disks. Despite its importance, the long-term variability of the UV radiation field remains poorly characterized because homogeneous time-series observations are scarce. We investigate the long-term variability of the near-UV continuum and the Mg,II resonance doublet, and assess whether the two diagnostics trace the same underlying accretion variability. We analyzed the complete archive of long-wavelength International Ultraviolet Explorer (IUE) observations of T Tauri stars in the Taurus--Auriga star-forming region, which comprises 475 low-dispersion spectra of 36 objects accumulated during the 18-year operational lifetime of the mission. The variability of the UV continuum was characterized using three continuum windows between 2600 and 3100 Å. Continuum-subtracted Mg,II line fluxes, UV--V colors, and Lomb--Scargle periodograms were used to investigate the relationship between continuum and line variability and to search for characteristic timescales. The continuum fluxes measured within the 2675--3055 Å interval are strongly correlated, with variability occurring predominantly in the continuum level rather than in the spectral shape. The near-UV and quasi-simultaneous optical fluxes are significantly correlated in six of the seven best-sampled sources, although their relative variability differs among objects. The relation between Mg II and the near-UV continuum shows substantial object-to-object diversity, ranging from correlated variations with different fractional responses to largely decoupled or anticorrelated behavior. Thus, Mg II does not show a universal temporal response to continuum variability. Lomb--Scargle periodograms show formally significant features in some of the best-sampled objects, including features near 10 and 33 d. However, the irregular and clustered temporal sampling provides insufficient cycle coverage to establish these features as periodic or characteristic timescales.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"40 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149149987","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Exceptional gamma-ray flaring activity of the blazar S4 0954+65 in early 2025","authors":"Zhihao Ouyang,Jingyu Wu,Hubing Xiao,Lili Yang,Shangchun Xie,Shaohua Zhang,Zhijian Luo,Jianzhen Chen,Jianguo Wang,Mingjun Liu,Qinyu Wu,Huaqing Cheng,Wenda Zhang,Junhui Fan","doi":"10.1051/0004-6361/202661371","DOIUrl":"https://doi.org/10.1051/0004-6361/202661371","url":null,"abstract":"S4 0954+65 (4FGL J0958.7+6534) is a TeV-detected blazar at a redshift of z = 0.3694 ± 0.0011, classified as an intermediate-synchrotron-peaked BL Lac object. In early 2025, it entered an exceptional γ-ray high state, exceeding the flux of the 2015 outburst, during which contemporaneous MAGIC TeV observations were obtained. This event offers an opportunity to investigate the radiation mechanisms, particle acceleration, and jet dynamics. We aim to investigate the origin and physical properties of the exceptional 2025 flare, and to constrain the emission processes responsible for this flaring activity. We performed a multiwavelength analysis using γ-ray, X-ray, optical/UV, radio, and 43 GHz Very Long Baseline Array (VLBA) observations. We examined multiband correlations with the z-transformed discrete correlation function (zDCF), analyzed the spectral evolution and parsec-scale jet kinematics, and modeled the broadband spectral energy distributions (SEDs). The γ-ray variations lead the optical and radio emission by 3.22 days and possibly 18.26 days, respectively, while no significant correlation is found between the γ-ray and X-ray emission. The γ-ray spectra show a harder-when-brighter behavior, consistent with enhanced particle acceleration during the active state. A similar spectral trend is also observed in the X-ray band, where the photon index is anticorrelated with the flux. During the major γ-ray flare, VLBA images reveal the emergence of a new superluminal radio knot from the core, whose extrapolated ejection time is consistent with the peak of the γ-ray flare. The temporal and structural evolution during the 2025 flare is consistent with a shock-in-jet scenario, in which a newly emerging disturbance propagates downstream and drives the flare. Broadband SED modeling shows that a leptonic synchrotron self-Compton plus external Compton model with dusty torus seed photons reproduces the multiwavelength observations with physically plausible parameters. A lepto-hadronic interpretation remains possible but requires substantially higher jet power.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"32 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149150019","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A toy model for testing opacity effects on stellar structure computations","authors":"Matthew Raymond Gent,Pierre Houdayer,Michel Rieutord,Lorenzo Valdettaro","doi":"10.1051/0004-6361/202662219","DOIUrl":"https://doi.org/10.1051/0004-6361/202662219","url":null,"abstract":"New multi-dimensional modelling of stellar evolution requires a new formulation of the mathematical problem; instead of the classical Lagrangian approach of 1D stellar models, multi-dimensional ones, such as Evolution STEllaire en Rotation (ESTER) models, use an Eulerian description of fluid flows. In this approach a spectral discretisation of the equations is used. However, ESTER models are presently restricted to early-type stars since numerical difficulties arise when dealing with low-mass or solar-type stars. This study is aimed at analysing the origin of these numerical difficulties and offering new strategies to circumvent them. After showing that the difficulties of ESTER in the low-mass star range are associated with the strong variations of opacities with temperature, we designed a toy model based on a simple plane-parallel fluid layer in hydrostatic equilibrium, equipped with an opacity law that reproduces the temperature dependence of solar opacity well. We took the Sun as the typical example of a low-mass star. The toy model clearly reproduces the difficulties faced by ESTER models when the surface effective temperature drops below 10,000 K. In this case the non-linear differential equation that determines the temperature profile becomes stiff and the number of the Newton solver iterations increases dramatically. As this model allows for the introduction of an adiabatic layer when destabilising temperature gradients are met, we show that one additional difficulty is the determination of the interfaces between this layer and the neighbouring radiative layers. It is shown that convergence towards the solution is efficient if interface positions are updated only when Newton corrections are small enough. The toy model also indicates that for low-mass stars, Newton iterations should be initialised with a fully isentropic layer. Convergence is then faster and more robust than with an initial fully radiative model, as is actually the case for ESTER models. This toy model opens a pathway to 2D or 3D models describing the stellar evolution of low-mass stars. This path has now to be assessed.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"54 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2026-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149150161","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}