arXane › Physics › Astrophysics
Wednesday, 4 November 2026: 13 new papers, 4 cross-lists and 6 replacements in Solar and Stellar Astrophysics (astro-ph.SR), in arXiv's order, with full abstracts. The same papers as arXiv's own listing, set as a newspaper; every title links to arXiv.
The strong nitrogen (N) features observed in high-redshift galaxies, such as GN-z11, with JWST point to an efficient source of N production in the early Universe. Very massive stars (VMSs) are promising candidates owing to their strong winds and large convective cores of CNO-processed material. Whether massive-star surfaces in general become N-enriched depends on the interplay of convective-boundary mixing (overshooting), rotation, and mass loss, which remains poorly constrained. Using a grid of MESA evolutionary models, we isolate the overshooting and rotation channels at $40$, $100$, and $200\,M_\odot$ and investigate how these channels bring N-rich material to the surface of massive and very massive stars as a function of initial mass and metallicity. We also construct a synthetic Large Magellanic Cloud (LMC) population based on a larger initial mass grid ranging from $20-300\,M_\odot$ for comparison with the observed N-enriched fraction. VMS surface enrichment is remarkably robust across cosmic metallicity. At lower metallicities, wind stripping is weaker and the overshooting channel shuts off entirely, yet a modest amount of rotation of $\sim90\,\mathrm{km\,s^{-1}}$ alone still enriches the surface. At higher metallicity, stronger winds dominate and uncover the core almost independently of the adopted mixing. The two channels operate through physically distinct mechanisms: overshooting enriches the surface only once winds strip the envelope down into the N-rich layers left by the receding core, giving an abrupt jump. Rotation, by contrast, mixes N outward continuously through the layers above the core, producing a gradual rise. Applied to the LMC, a population with modest rotation drawn from the observed velocity distribution and modest overshooting reproduces the observed N-enriched fraction without large core boundary mixing efficiencies otherwise required by non-rotating models.
Quasi-stars, massive envelopes surrounding rapidly accreting black hole (BH) seeds, are promising candidates for explaining the rapid formation of supermassive black holes in the early Universe and may account for some of the JWST-discovered Little Red Dots (LRDs). We present a self-consistent framework combining stellar evolution calculations with non-local thermodynamic equilibrium (NLTE) spectral synthesis to test this connection. Our models drop the polytropic assumptions of previous work: the envelope is described entirely by mixing-length theory, with saturated convection and radiative energy setting the luminosity at an inner boundary a few hundred Schwarzschild radii from the BH. This removes the BH-mass limits of earlier models, and we follow the evolution up to BH-to-total mass ratios approaching unity. We then use TLUSTY to compute the first converged NLTE atmospheric models for the surface conditions and primordial hydrogen-helium composition expected in quasi-stars. Our models produce cool photospheres, prominent Balmer breaks, and Balmer and Paschen absorption features. NLTE effects substantially modify line strengths, increasing Balmer absorption depths by $\sim 35-55\%$ relative to LTE models, while having a minor effect on the continuum. Comparison with observed LRD spectral energy distributions shows broad agreement in continuum shape, although our models do not reproduce their commonly observed strong emission lines. Under the quasi-star interpretation, comparisons with individual LRDs suggest characteristic quasi-star masses of approximately $10^5-10^7 M_\odot$. These results establish a physically motivated connection between quasi-star evolution and observable spectra, providing a framework for testing quasi-stars as progenitors of LRDs while identifying limitations of current atmospheric models.
The subtle Doppler radial velocity (RV) signature of an Earth-mass exoplanet in the Habitable Zone around a nearby Sun-like star is difficult to detect due to stellar variability. While multiple techniques aim to mitigate rotationally-modulated stellar activity, granulation, introducing 0.1-1 m s$^{-1}$ RV shifts, currently sets the floor for detecting Earth-mass RV signals for low-activity stars. Observations of the Rossiter-McLaughlin (RM) effect during a solar eclipse, with a peak-to-trough amplitude of $\sim$1.4 km s$^{-1}$, are a rigorous test of granulation and convective blueshift in the solar spectrum. Using NEID solar feed data of the October 2023 solar eclipse and GRASS-E (a geometry-modified version of GRASS), we compare the observed RVs for 22 lines with the simulated RVs from the synthetic granulation-driven spectra. Residuals of the GRASS-E Fe I 5434 angstrom model show an RMS of 13.47 m s$^{-1}$ over the RM signal. We find that a model with an optimized convective blueshift polynomial for Fe I 5434 significantly outperforms GRASS-E with an RMS of only 2.90 m s$^{-1}$, suggesting that GRASS-E is currently limited by sparse center-to-limb observations. By combining a set of spectral lines, we achieve a weighted RMS of 5.2 m s$^{-1}$ over the RM signal ($\sim$0.37\%), a level of accuracy well beyond the reach of current exoplanet transit observations. In all, we demonstrate that solar eclipse observations allow us to recover the center-to-limb variation of the convective blueshift for individual spectral lines.
The detection of strong magnetic fields in radiative stars, particularly chemically peculiar Ap/Bp stars, has motivated various scenarios regarding their origin, including the fossil field hypothesis. Because field survival depends strongly on stellar internal structure, determining when protostars transition from convective to radiative states is essential to assess whether fossil fields persist. We determine the characteristic mass at which intermediate-mass protostars transition from convective to radiative interiors, exploring how this transition favors magnetic field survival. Using the 1D stellar evolution code MESA, we model the pre-main-sequence evolution of intermediate-mass protostars from convective initial seeds across different metallicities, average accretion rates, and accretion histories. The transition mass from predominantly convective to radiative states depends on both accretion rate and metallicity, occurring between $\sim 1.3\,M_\odot$ and $2.8\,M_\odot$ for $1\,M_\odot$ reference models (and up to $\sim 4\,M_\odot$ for other initial masses at high accretion rates). Accretion history details play a secondary role compared to the average rate and metallicity. Lower metallicities shift the transition to lower masses. For $Z \ge 0.01$ and accretion rates above $4 \times 10^{-6}\,M_\odot\,\mathrm{yr}^{-1}$, models retain convective envelopes up to the end of accretion, potentially dissipating accreted magnetic flux. Because this transition typically occurs above $\sim 1.3\,M_\odot$, our results help explain the preferential survival of fossil fields in intermediate-mass stars, provided the radiative structure persists post-accretion. We also predict an increased incidence of strongly magnetized radiative stars at metallicities below $Z=0.01$.
(abridged abstract) Wolf-Rayet stars have high mass-loss rates and fast stellar winds. Their interaction with circumstellar material from previous evolutionary phases can produce complex nebular morphologies. Hydrodynamic instabilities can fragment wind-swept material, producing condensations, filaments, and gaps that alter the confinement of stellar wind and ionizing radiation. The role of fragmentation in the late evolution and dispersal of WR nebulae remains poorly constrained. PCG 11, with its fragmented morphology and regular scalloped rim, offers an opportunity to investigate these processes. We aim to characterize the structure of PCG 11 and investigate its ring-like morphology and scalloped rim. We constrain its three-dimensional geometry and determine whether the optical fragmentation pattern is also traced by ionized gas. We obtained high-resolution radio continuum images with MeerKAT at 2.6 and 3.1 GHz and ATCA at 5.5 and 9.0 GHz. The radio emission reveals a hollow, equatorially enhanced structure viewed close to its symmetry axis. The images resolve the rim into condensations, low-brightness gaps, and finger-like protrusions. We quantify the fragmentation scale and relate it to the nebular geometry. We also detect an unresolved source coincident with WR 75ab, whose rising radio spectrum is consistent with thermal wind emission. The nebula processes only a small fraction of the stellar ionizing radiation, indicating substantial photon leakage. PCG 11 captures a poorly observed stage in massive-star bubble evolution: the swept-up shell has developed a coherent instability pattern and become fragmented and porous, while remaining dynamically coupled to the stellar wind. It provides a clear view of how fragmentation and erosion reshape the circumstellar environment during a transitional evolutionary phase.
The effects of ${}^{12}\mathrm{C}+{}^{12}\mathrm{C}$ fusion and ${}^{12}\mathrm{C}(\alpha,\gamma){}^{16}\mathrm{O}$ capture on stellar heating, composition, and black-hole masses are examined. Rates from a published Bayesian analysis of carbon fusion are used in calculations with MESA. In an accreting carbon--oxygen white dwarf, the Bayesian rates move the point at which nuclear heating equals combined neutrino and surface photon losses to slightly larger stellar masses and higher central temperatures and densities. At an accretion rate of $10^{-7}\,M_\odot\,\mathrm{yr}^{-1}$, the Bayesian median gives a central temperature approximately $2.57\%$ higher and a central density $18.9\%$ higher than the calculation using the standard CF88 rate. A separate calculation follows carbon burning in an initially $20\,M_\odot$, $Z=0.0142$ star. Relative to the CF88 rate with MESA's standard division between proton and alpha channels, the Bayesian-median rate with the proton-to-alpha ratio of Yang et al. increases the neon-20 mass remaining at central carbon depletion by $10.8\%$ and decreases the sodium-23 mass by $28.7\%$. For helium capture on carbon, the nuclear constraint $S_{\rm tot}(300\,\mathrm{keV})=118\pm17\,\mathrm{keV\,b}$ is applied to the published $Z=10^{-5}$ stellar models of Farag et al. Linear interpolation gives a maximum black-hole mass below the pair-instability gap of approximately $61.5$--$77.4\,M_\odot$ within the assumptions of those models. New MESA calculations at $Z=0.00142$ give smaller bound masses before collapse. Differences in the stellar assumptions prevent attributing the difference between the two curves to metallicity alone.
Earth's surface and deep mantle are strongly depleted in carbon compared to the Sun. Sublimation of solid carbonaceous material in the inner midplanes of young disks could cause this depletion. We study such thermal processing in HOPS-68, a Class I protostar with a rich hydrocarbon spectrum. A companion paper has shown that it is recondensing refractory dust from a hot inner disk region, and that oxygen-bearing species are tracing a warm (300-500 K) molecular layer entrained in a disk-wind. In this work, we analyze the carbon species in the JWST spectrum of this source using local thermal equilibrium slab models. We detect warm (~400 K) HCN and C2H2 at the base of a disk wind, as well as colder (~40 K) C4H2, CH3, and CS in the outer disk or outflow cavity wall. Additionally, 13CCH2, CH4, and C6H2 are detected, while ALMA observations reveal C2H tracing the outflow cavity walls. We suggest that in a "refractory carbon destruction zone" in the inner disk, carbon-bearing species are released from the grains, possibly contributing to the observed carbon depletion of Earth. The disk wind then redistributes carbon carried by HCN and C2H2 from the inner disk to the colder outer disk, where UV-induced gas-phase reactions with C2H in the outflow cavity walls could produce the aliphatic carbon chains seen. This result implies that the Warm Carbon Chain Chemistry (WCCC, e.g. N. Sakai & S. Yamamoto 2013) might trace outflows rather than infall. The formation of aliphatic chains in the disk wind could promote the abundant aliphatic organics found in meteorites and asteroids.
Type II radio bursts, produced by magnetohydrodynamic shocks propagating through the solar corona and interplanetary medium, serve as powerful diagnostics of shock properties and ambient coronal magnetic fields. While circular polarization in type II bursts has been extensively studied, most investigations rely on dynamic spectra (DS), providing only spatially integrated measurements. DS obscures spatial variations of the polarization sign and magnitude, potentially causing cancellation effects and misrepresenting true polarization levels. We present a full-polarimetric imaging study of a metric type II burst observed with the Murchison Widefield Array at 78--133 MHz. Remarkably, we report the first detection of linear polarization in a metric type II burst, reaching a polarization fraction of $\sim12\%$ with significant spectro-temporal variability across observing bands. The spectro-temporal, spatial, and morphological properties found in this study establish linear polarization as a new diagnostic of large-scale coronal and interplanetary magnetic field structure through full-Stokes imaging of metric solar radio bursts -- challenging conventional paradigms predicting negligible linear polarization from coherent plasma emission processes.
$\beta$ Cephei type stars are massive ($8-25$M$_{\odot}$) and known to have several pulsation modes with periods typically of a few hours. Despite a better understanding of their physics since 1992, the inner structure of these stars involves key stellar physics processes that are still uncertain. Asteroseismology offers insights into these variable stars by analysing pulsation modes, helping to probe their inner layers and understand their structure and mixing processes. We measure the angular diameter of two $\beta$ Cephei stars, $\theta$ Oph and $\beta$ CMa, using the MAGIC Stellar Intensity Interferometer and compare them, together with the Gaia and Hipparcos parallaxes, to the predictions of the asteroseismic model. The latter are based on Levenberg-Marquardt local minimisation of stellar evolution model parameters to fit observed pulsation modes. In the case of $\theta$ Oph, the observed angular diameters are compatible with the asteroseismic model. For $\beta$ CMa, the radius derived from the angular diameter and the Hipparcos parallax is in tension, at the $2.5\sigma$ level, with the asteroseismic prediction. We show that this tension mirrors a pre-existing conflict between independent spectroscopic determinations of the surface gravity of the star, that percent-level systematics of the stellar models are quantitatively insufficient to explain it, and that it most likely points to a bias of the Hipparcos parallax. In the frame of this paper, the statistical uncertainty on the interferometric results is similar to the systematic uncertainty on the parallaxes. With the already implemented addition in early 2024 of the first Large-Sized Telescope (CTAO-North LST-1) to the setup, the uncertainty in the angular diameter of these stars is expected to decrease by an order of magnitude, allowing to further constrain stellar evolution models and parallaxes.
Extreme ultraviolet (EUV) waves are large-scale, tsunami like disturbances on the solar surface associated with eruptive phenomena such as solar flares, filament eruptions, and coronal mass ejections (CMEs). Because their strong correlation with major solar eruptions, we present here a comprehensive statistical study of EUV waves and their connections to Moreton waves, CMEs, solar flares, type II radio bursts, and solar energetic particle (SEP) events during Solar Cycle 25 (December 2019-December 2025). This study utilizes EUV data from the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory and coronagraph data from the Large Angle and Spectrometric Coronagraph onboard Solar and Heliospheric Observatory. Our analysis reveals that 12.3% of the observed EUV waves are associated with Moreton waves, 95% with CMEs, 92.7% associated with flares, 44.3% with metric type II radio bursts, and 11.8% with SEPs. While nearly all EUV waves are linked to CMEs, their association with solar flares varies significantly, ranging from weak B-class events to major X-class flares. Furthermore, the majority of Moreton waves correlate with large- or medium-class flares, with only a small fraction associated with weaker C-class flares. Our findings may contribute to a better understanding of space weather events and, potentially, to their forecasting.
We derive an adjoint formulation of eigenfrequency sensitivity for linear nonadiabatic stellar pulsations, including the frequency dependence of the boundary conditions. Local sensitivity kernels and their cumulative integrals describe the responses of the excitation rate and oscillation frequency to prescribed scaling of the pulsation matrix and its acoustic, thermal and coupling blocks. Applications to selected radial modes of post-AGB envelopes reveal distinct spatial and subsystem sensitivity profiles. In the cool model, the ordinary mode has weak sensitivity near both boundaries for the slow branch outer boundary conditions, whereas the strange mode exhibits pronounced sensitivity in the outer layers. Selected modes of the hotter model also show appreciable outer-layer sensitivity. The block decomposition reveals differing contributions and cancellations in the excitation-rate and oscillation-frequency responses.
We report the discovery, validation, and characterization of 44 new symbiotic stars (SySts) accreting at the lowest detectable levels (Lacc =< 3 Lsun). Candidates were initially selected from the Galah survey among non-pulsating M-type red giants exhibiting Halpha excess. Between 2021 and 2025, these targets were subjected to intensive multi-wavelength campaigns, including ground-based, high-resolution Echelle spectroscopy over 544 nights, multi-epoch flickering searches down to 1 mmag, and UV/X-ray observations with the Swift satellite. Our high-resolution spectra allowed for a detailed investigation of Halpha flux and line-profile variability. Crucially, the radial velocities measured for the M-giant components yielded reliable spectroscopic orbits for 22 objects and preliminary orbits for an additional eight, corroborated by binary-induced modulations in ASAS-SN photometry. Measured accretion levels vary by factors of up to 20X, emphasizing the need for high-sensitivity, long-term monitoring. Swift X-ray data exclude neutron star companions in all targets, consistent with predominance of white-dwarf degenerates in known SySts. Combined with the 33 objects from Paper I, this sample of 77 new SySts significantly exceeds the number of previously known, accreting-only Galactic symbiotic stars, constituting the bulk of the low-accretion (LowAcc) variety discovered to date and representing >10 percent of all known SySts in the Galaxy. As SySts likely spend most of their lifetimes in LowAcc faint states, characterizing this population is essential for estimating the total Galactic SySt population and evaluating the role of SySts as Type Ia supernova progenitors or sources of UV excess in elliptical galaxies.
Gaia22alz (nova Vel 2022) shows characteristic properties in its light curves and spectral evolution similar to slow novae HR Del, V723 Cas, and V5558 Sgr. Their outburst begins in a way of hydrostatic expansion with no mass loss, followed by large amplitude ($\Delta V \sim 2$ mag) multiple peaks around the mean magnitude of $M_V \sim -5.5$, and eventually it decays similarly to normal classical novae. We present a light curve model of Gaia22alz based on our metamorphic nova theory: a nova starts from a static expansion and then metamorphoses to an optically thick wind evolution through a large amplitude multiple peak phase as a violent relaxation process. Our theoretical light curve for the rising phase consistently explains the slow rise and early spectral evolution of Gaia22alz. Gaia22als is a close binary as well as other three novae, which is consistent with our metamorphic nova theory that the companion motion in the nova envelope triggers the metamorphic transition. Our results do not support E. Aydi et al.'s explanation that the slow optical rise of Gaia22alz is owing to the irradiation of X-ray/UV flash.
Meteoritic inclusions indicate that the early Solar System contained a substantial abundance of the short-lived radionuclide 10Be. Whether it was inherited from the parent molecular cloud or requires local irradiation or late injection remains debated. Here we present the first three-dimensional simulations of a Milky Way-like disc that follow the multi-phase interstellar medium, cosmic rays (CRs), and the production and transport of 10Be by galactic CRs (GCRs) and in supernova remnants (SNRs). Nearly all 10Be is produced by GCR interactions; SNRs contribute at most about ten percent of the 10Be inherited by newly formed stars. The 10Be/9Be ratio traces the CR energy density, declining smoothly with galactocentric radius, and because stars form where supernovae inject CRs, their natal gas is CR-enriched roughly fivefold relative to the mean interstellar medium, which one-zone estimates miss. Accounting for a possibly higher Galactic star formation rate 4.6 Gyr ago, inheritance of GCR-produced 10Be can then explain the early Solar System's 10Be/9Be in our models, without late injection or local irradiation. Even when GCR-induced 10Be production in dense gas is reduced tenfold, the meteoritic value is reached in the innermost disc and missed by only a factor of a few elsewhere, so the main conclusion is robust to uncertain low-energy GCR penetration. The inferred birth radius of the Sun depends strongly on the CR diffusivity, ranging between 3 and 8 kpc, suggesting that improved modelling of low-energy CR transport could sharpen constraints on the birthplace of the Solar System.
The vertical distribution of molecular gas in the Milky Way disk is poorly studied, especially in the 4th quadrant and the far side of the Galaxy. While surveys over 20 years ago suffered from coarse beam and non-Nyquist sampling, most of the recent surveys are limited to $|b|<1^\circ$ and/or focus on the Northern Galactic plane. We examine the molecular gas's vertical distribution and properties in the fourth quadrant to check the impact of Galactic structure on current star formation. We mapped two arm tangent regions ($l=[329^\circ.0,327^\circ.0]$ for the Norma-Outer arm, and $l=[310^\circ.0,308^\circ.5]$ for the Scutum-Centaurus arm) with the $^{12}$CO 3-2 line using the 7-pixel LASMA receiver of the APEX 12 m telescope, covering up to $b=[-2^\circ,+2^\circ]$ (or $|z|\sim200$ pc near the tangent points). The noise level of the $^{12}$CO 3-2 line is about 0.4 K [$T_{MB}$] in velocity channels of 0.5 km s$^{-1}$. The vertical distribution around the tangent regions of the spiral arm is fitted with two Gaussian components, the narrow one with an FWHM thickness of 25 pc and the wide one of about 100 pc. Two explanations are proposed: they are two sub-components of the thin disk (the narrowest tracing the spiral arm and the other the inter arms components), or they trace both thin and thick disks that show a smaller thickness when observed with CO 3-2 than when observed with CO 1-0. The latter indicates that the density distribution of the interstellar medium exhibits a steeper gradient at larger heights above the plane. Clumps in the two tangent regions are searched and divided into groups by relative height. Compared with the low relative height clumps ($|z|<50$ pc), the median relative height clumps ($50~\mathrm{pc}<|z|<100~\mathrm{pc}$) have lower excitation temperature and surface density, with fewer high values in the distribution. A similar tentative trend is found for the mass.
Close-in planets orbiting core-helium-burning red-clump (RC) giants directly challenge tidal-engulfment theory, which predicts that planets within about 1 au of Sun-like stars should be engulfed during red-giant-branch (RGB) expansion. The existence of such survivors would imply missing physical processes in the late stages of stellar and planetary evolution. Yet only one such "forbidden" planet had been identified to date. Here we report three systems-24 Boo b, HD 12648 b, and TYC 4282-605-1 b-whose host stars are confirmed red-clump giants through asteroseismology, showing that each has already undergone dramatic expansion and contraction following helium ignition. Their planets now reside at orbital separations smaller than the maximum stellar radii attained during the RGB phase, implying that they should not have survived. Among them, 24 Boo b orbits at only 0.183 au with a 30-day period, making it the closest planet known around a red-clump giant. These discoveries demonstrate that this paradox is not an isolated anomaly but signals the emergence of a new class of evolved planetary systems. Their survival defies current models of stellar-planetary coevolution and points to unexplored physics shaping planetary architectures around post-RGB stars.
We present the luminous, long-lived Type IIn SN 2019qem and use it to reconstruct how much mass its progenitor lost and when. We combine host-subtracted optical and mid-infrared photometry and 26 optical spectra from -22 to +699 d. We model the bolometric light curve with thin-shell CSM-interaction models in REDBACK, with the shock velocity constrained by the H$\alpha$ line widths, and compare them with an analytic interaction model. SN 2019qem rose in $\approx30$d to $M_I = -20.5$mag and a bolometric peak of $7.0\times10^{43}$ erg s$^{-1}$, and radiated $6.9\times10^{50}$ erg in the optical within 575 d, closely resembling luminous, strongly interacting SNe IIn such as SN 2010jl and SN 2015da. A Heii/Niii flash feature at -22 d and unresolved narrow Balmer lines reveal dense, slow CSM. From +78 to +445 d the H$\alpha$ wings are shaped by electron scattering, and the line stays blueshifted by 400-700 km s$^{-1}$, which points to an optically thick, asymmetric interaction region rather than to newly formed dust. In the preferred model, $\approx7 M_\odot$ of ejecta run into $\approx9 M_\odot$ of gas ejected $\approx2$ yr before explosion and then into a wind of $\approx0.6 M_\odot$ yr$^{-1}$ (for $v_w = 100$ km s$^{-1}$), sweeping up $\approx32 M_\odot$ by +544 d. The mid-infrared excess brightens for two years while the optical fades, tracing dust heated by the ongoing interaction. The host is a low-mass, star-forming galaxy, $\log(M_*/M_\odot) = 9.19$, with a sub-solar metallicity at the explosion site. In its final decades the progenitor of SN 2019qem lost mass far faster than any steady stellar wind, ending with an eruption of $\approx9 M_\odot$ two years before core collapse, consistent with luminous-blue-variable-like eruptive mass loss. Taken at face value, the mass budget implies an initial progenitor mass of $\gtrsim40 M_\odot$.