Stellar Evolution Code Research Articles

Mixing by internal gravity waves in stars: assessing numerical simulations against theory

ABSTRACT Here we present a study of radial chemical mixing in non-rotating massive main-sequence stars driven by internal gravity waves (IGWs), based on multidimensional hydrodynamical simulations with the fully compressible code MUSIC. We examine two proposed mechanisms of material mixing in stars by IGWs that are commonly quoted, relating to thermal diffusion and sub-wavelength shearing. Thermal diffusion provides a non-restorative effect to the waves, leaving material displaced from its previous equilibrium, while shearing arising within the waves drives weak localized flows, mixing the fluid there. Using IGW spectra from the simulations, we evaluate theoretical predictions of mixing rates due to these mechanisms. We show, for $20\, \mathrm{M}_\odot$ main-sequence stars, that neither of these mechanisms are likely to create mixing sufficient to correct inaccuracies in current stellar evolution models. Furthermore, we compare these predictions to results obtained from Lagrangian tracer particles, following a method recently used for global simulations of stellar interiors to measure mixing by IGWs in their radiative zones. We demonstrate that tracer particle methods face significant numerical challenges in measuring the small diffusion coefficients predicted by the aforementioned theories, for which they are prone to yielding artificially enhanced coefficients. Diffusion coefficients based on such methods are currently used with stellar evolution codes for asteroseismic studies, but should be viewed with caution. Finally, in a case where tracer particles do not suffer from numerical artefacts, we suggest that a diffusion model is not suitable for time-scales typically considered by 2D numerical simulations.

Stellar Pulsation and Evolution: A Combined Theoretical Renewal and Updated Models (SPECTRUM). I. Updating Radiative Opacities for Pulsation Models of Classical Cepheid and RR-Lyrae

Pulsating stars are universally recognized as precise distance indicators and tracers of stellar populations. Their variability, combined with well-defined relationships between pulsation properties and intrinsic evolutionary parameters such as luminosity, mass, and age, makes them essential for understanding galactic evolution and retrieving star formation histories. Therefore, accurate modeling of pulsating stars is crucial for using them as standard candles and stellar population tracers. This is the first paper in the “Stellar Pulsation and Evolution: a Combined Theoretical Renewal and Updated Models” project, which aims to present an update of Stellingwerf’s hydrodynamical pulsation code, by adopting the latest radiative opacity tables commonly used in stellar evolution community. We assess the impact of this update on pulsation properties, such as periods, instability strip topology, and light-curve shapes, as well as on period–Wesenheit and period–luminosity relations for Classical Cepheids and RR Lyrae stars, comparing the results with those derived using older opacity data. Our results indicate that the opacity update introduces only minor changes: instability strip boundary locations shift by no more than 100 K in effective temperature, and pulsation periods vary within 1σ compared to previous evaluations. Light curves exhibit slight differences in shape and amplitude. Consequently, the theoretical calibration of the Cepheid- or RRL-based extragalactic distance scale remains largely unaffected by the opacity changes. However, achieving consistency in opacity tables between stellar evolution and pulsation codes is a significant step toward a homogeneous and self-consistent stellar evolution and pulsation framework.

The impact of disk-locking on convective turnover times of low-mass pre-main sequence and main sequence stars

The impact of disk-locking on the stellar properties related to magnetic activity from the theoretical point of view is investigated. We use the ATON stellar evolution code to calculate theoretical values of convective turnover times ($ c $) and Rossby numbers ($Ro$, the ratio between rotation periods and $ c $) for pre-main sequence (pre-MS) and main sequence (MS) stars. We investigate how $ c $ varies with the initial rotation period and with the disk lifetime, using angular momentum conserving models and models simulating the disk-locking mechanism. In the latter case, the angular velocity is kept constant during a given locking time to mimic the magnetic locking effects of a circumstellar disk. The local convective turnover times generated with disk-locking models are shorter than those obtained with angular momentum conserving models. The differences are smaller in the early pre-MS, increase with stellar age, and become more accentuated for stars with $M$\,geq M odot $ and ages greater than 100\,Myr. Our new values of $ c $ are used to estimate $Ro$ for a sample of stars selected from the literature in order to investigate the rotation-activity relationship. We fit the data with a two-part power-law function and find the best fitting parameters of this relation. The differences found between both sets of models suggest that the star's disk-locking phase properties affect its Rossby number and its position in the rotation-activity diagram. Our results indicate that the dynamo efficiency is lower for stars that had undergone longer disk-locking phases.

Mixed-mode coupling in the red clump

The investigation of global, resonant oscillation modes in red giant stars offers valuable insights into their internal structures. In this study, we investigate in detail the information we can recover on the structural properties of core-helium burning (CHeB) stars by examining how the coupling between gravity- and pressure-mode cavities depends on several stellar properties, including mass, chemical composition, and evolutionary state. Using the structure of models computed with the stellar evolution code MESA, we calculate the coupling coefficient implementing analytical expressions, which are appropriate for the strong coupling regime and the structure of the evanescent region in CHeB stars. Our analysis reveals a notable anti-correlation between the coupling coefficient and both the mass and metallicity of stars in the regime M ≲ 1.8 M⊙, in agreement with Kepler data. We attribute this correlation primarily to variations in the density contrast between the stellar envelope and core. The strongest coupling is expected thus for red-horizontal branch stars, partially stripped stars, and stars in the higher-mass range exhibiting solar-like oscillations (M ≳ 1.8 M⊙). While our investigation emphasises some limitations of current analytical expressions, it also presents promising avenues. The frequency dependence of the coupling coefficient emerges as a potential tool for reconstructing the detailed stratification of the evanescent region.

Proton ingestion in asymptotic giant branch stars as a possible explanation for J-type stars and AB2 grains

Context. J-type stars are a subclass of carbon stars that are generally Li-rich, not enriched in s-elements, and have low 12C/13C ratios. They were suggested to be the manufacturers of the pre-solar grains of type AB2 (having low 12C/13C and supersolar 14N/15N). Aims. In this Letter, we investigate the possibility that J-type stars are early asymptotic giant branch (AGB) stars that experienced a proton ingestion event (PIE). Methods. We used the stellar evolution code STAREVOL to compute AGB stellar models with initial masses of 1, 2, and 3M⊙ and metallicities [Fe/H] = − 0.5 and 0.0. We included overshooting above the thermal pulse and used a network of 1160 nuclei coupled to the transport equations. The outputs of these models were compared to observations of J-type stars and AB2 grains. Results. In solar-metallicity AGB stars, PIEs can be triggered if a sufficiently high overshoot is considered. These events lead to low 12C/13C ratios, high Li abundances, and no enrichment in s-elements. We find that the 2 − 3 M⊙ AGB models experiencing a PIE can account for most of the observational features of J-type stars and AB2 grains. The remaining tensions between models and observations are (1) the low 14N/15N ratio of some AB2 grains and of 2 out of 13 J-type stars, (2) the high 26Al/27Al of some AB2 grains, and (3) the J-type stars with A(Li) < 2. Extra mixing mechanisms can alleviate some of these tensions, such as thermohaline or rotation. Conclusions. This work highlights a possible match between AGB stellar models that undergo a PIE and J-type stars and AB2 grains. To account for other types of carbon stars, such as N-type stars, PIEs should only develop in a fraction of solar-metallicity AGB stars. Additional work is needed to assess how the occurrence of PIEs depends on mixing parameters and initial conditions, and therefore to further confirm or exclude the proposed scenario.

A Possible Formation Scenario of the Gaia BH1: Inner Binary Merger in Triple Systems

Based on astrometric measurements and spectral analysis from Gaia DR3, two quiescent black hole (BH) binaries, Gaia BH1 and BH2, have been identified. Their origins remain controversial, particularly for Gaia BH1. By considering a rapidly rotating (ω/ω crit = 0.8) and strongly magnetized (B 0 = 5000 G) merger product, we find that, at typical Galactic metallicity, the merger product can undergo efficient chemically homogeneous evolution. This results in the merger product having a significantly smaller radius during its evolution compared to that of a normally evolving massive star. Under the condition that the initial triple stability is satisfied, we use the Multiple Stellar Evolution code and the MESA code to identify an initial hierarchical triple that can evolve into Gaia BH1. It initially consists of three stars with masses of 9.03 M ⊙, 3.12 M ⊙, and 1 M ⊙, with inner and outer orbital periods of 2.21 days and 121.92 days, and inner and outer eccentricities of 0.41 and 0.45, respectively. This triple initially experiences triple evolution dynamics instability (TEDI) followed by Roche lobe overflow (RLOF). During RLOF, the inner orbit shrinks, and tidal effects gradually suppress the TEDI. Eventually, the inner binary undergoes a merger through contact (or collision). Finally, using models of rapidly rotating and strongly magnetic stars, along with standard core-collapse supernova (SN) or failed supernova (FSN) models, we find that a postmerger binary (PMB) consisting of an 12.11 M ⊙ merger product and a 1 M ⊙ companion star (originally an outer tertiary) can avoid RLOF. After an SN or FSN with a low ejected mass of ∼0.22 M ⊙ and a low kick velocity ( 46−33+25kms−1 or 9−8+16kms−1 ), the PMB can form Gaia BH1 in the Galactic disk.

Evaluating the gravitational wave detectability of globular clusters and the Magellanic Clouds for LISA

We used the stellar evolution code bpass and the gravitational wave (GW) simulation code legwork to simulate populations of compact binaries that may be detected by the future space-based GW detector LISA. Specifically, we simulate the Magellanic Clouds and binary populations mimicking several globular clusters, neglecting dynamical effects. We find that a handful of sources should be detectable in each of the Magellanic Clouds, but for globular clusters the amount of detectable sources will likely be less than one each. We compared our results to earlier research and find that our predicted numbers are several dozen times lower than both the results from calculations that used the stellar evolution code bse and take dynamical effects into account, and results from calculations that used the stellar evolution code SeBa for the Magellanic Clouds. Earlier research that compared bpass models for GW sources in the Galactic disk with bse models found a similarly sized discrepancy. We determine that this discrepancy is caused by differences between the stellar evolution codes, particularly in the treatment of mass transfer and common-envelope events in binaries: in bpass mass transfer is more likely to be stable and tends to lead to less orbital shrinkage in the common-envelope phase than in other codes. This difference results in fewer compact binaries with periods short enough to be detected by LISA existing in the bpass population. For globular clusters, we conclude that the impact of dynamical effects is uncertain based on the literature, but the differences in stellar evolution have an effect of a factor of 20 to 40 on the number of detectable binaries.

Long-term evolution of spin and other properties of neutron star low-mass X-ray binaries: implications for millisecond X-ray pulsars

ABSTRACT A neutron star (NS) accreting matter from a companion star in a low-mass X-ray binary (LMXB) system can spin up to become a millisecond pulsar (MSP). Properties of many such MSP systems are known, which is excellent for probing fundamental aspects of NS physics when modelled using the theoretical computation of NS LMXB evolution. Here, we systematically compute the long-term evolution of NS, binary, and companion parameters for NS LMXBs using the stellar evolution code mesa. We consider the baryonic to gravitational mass conversion to calculate the NS mass evolution and show its cruciality for the realistic computation of some parameters. With computations using many combinations of parameter values, we find the general nature of the complex NS spin frequency ($\nu$) evolution, which depends on various parameters, including accretion rate, fractional mass-loss from the system, and companion star magnetic braking. Further, we utilize our results to precisely match some main observed parameters, such as $\nu$, orbital period ($P_{\rm orb}$), etc., of four accreting millisecond X-ray pulsars (AMXPs). By providing the $\nu$, $P_{\rm orb}$, and the companion mass spaces for NS LMXB evolution, we indicate the distribution and plausible evolution of a few other AMXPs. We also discuss the current challenges in explaining the parameters of AMXP sources with brown dwarf companions and indicate the importance of modelling the transient accretion in LMXBs as a possible solution.

Ultraluminous X-ray sources with He star companions

ABSTRACT Ultraluminous X-ray sources (ULXs) are non-nuclear point-like objects observed with extremely high X-ray luminosity that exceeds the Eddington limit of a $\rm 10\, M_\odot$ black hole. A fraction of ULXs has been confirmed to contain neutron star (NS) accretors due to the discovery of their X-ray pulsations. The donors detected in NS ULXs are usually luminous massive stars because of the observational biases. Recently, the He donor star in NGC 247 ULX-1 has been identified, which is the first evidence of a He donor star in ULXs. In this paper, we employed the stellar evolution code mesa (Modules for Experiments in Stellar Astrophysics) to investigate the formation of ULXs through the NS+He star channel, in which a He star transfers its He-rich material onto the surface of an NS via Roche lobe overflow. We evolved a large number of NS+He star systems and provided the parameter space for the production of ULXs. We found that the initial NS+He star systems should have $\sim 0.7\!-\!2.6 \, \mathrm{M}_\odot$ He star and $\sim 0.1\!-\!2500\, \mathrm{d}$ orbital period for producing ULXs, eventually evolving into intermediate-mass binary pulsars. According to binary population synthesis calculations, we estimated that the Galactic rate of NS ULXs with He donor stars is in the range of $\sim 1.6\!-\!4.0\times 10^{-4}\, {\rm yr}^{-1}$, and that there exist $\sim 7-20$ detectable NS ULXs with He donor stars in the Galaxy.

Grids of stellar models with rotation

Context. Grids of stellar evolution models with rotation using the Geneva stellar evolution code (GENEC) have been published for a wide range of metallicities. Aims. We introduce the last remaining grid of GENEC models, with a metallicity of Z = 10−5. We study the impact of this extremely metal-poor initial composition on various aspects of stellar evolution, and compare it to the results from previous grids at other metallicities. We provide electronic tables that can be used to interpolate between stellar evolution tracks and for population synthesis. Methods. Using the same physics as in the previous papers of this series, we computed a grid of stellar evolution models with GENEC spanning masses between 1.7 and 500 M⊙, with and without rotation, at a metallicity of Z = 10−5. Results. Due to the extremely low metallicity of the models, mass-loss processes are negligible for all except the most massive stars. For most properties (such as evolutionary tracks in the Hertzsprung-Russell diagram, lifetimes, and final fates), the present models fit neatly between those previously computed at surrounding metallicities. However, specific to this metallicity is the very large production of primary nitrogen in moderately rotating stars, which is linked to the interplay between the hydrogen- and helium-burning regions. Conclusions. The stars in the present grid are interesting candidates as sources of nitrogen-enrichment in the early Universe. Indeed, they may have formed very early on from material previously enriched by the massive short-lived Population III stars, and as such constitute a very important piece in the puzzle that is the history of the Universe.

A Model for Eruptive Mass Loss in Massive Stars

Eruptive mass loss in massive stars is known to occur, but the mechanism(s) are not yet well understood. One proposed physical explanation appeals to opacity-driven super-Eddington luminosities in stellar envelopes. Here, we present a 1D model for eruptive mass loss and implement this model in the MESA stellar evolution code. The model identifies regions in the star where the energy associated with a local super-Eddington luminosity exceeds the binding energy of the overlaying envelope. The material above such regions is ejected from the star. Stars with initial masses of 10−100 M ⊙ at solar and SMC metallicities are evolved through core helium burning, with and without this new eruptive mass-loss scheme. We find that eruptive mass loss of up to ∼10−2 M ⊙yr−1 can be driven by this mechanism, and occurs in a vertical band on the H-R diagram between 3.5≲log(Teff/K)≲4.0 . This predicted eruptive mass loss prevents stars of initial masses ≳20 M ⊙ from evolving to become red supergiants (RSGs), with the stars instead ending their lives as blue supergiants, and offers a possible explanation for the observed lack of RSGs in that mass regime.

Giant Eruptions in Massive Stars and their Effect on the Stellar Structure

Giant eruptions (GEs) in luminous blue variables are years-to-decades-long episodes of enhanced mass loss from the outer layers of the star during which the star undergoes major changes in its physical and observed properties. We use the Modules for Experiments in Stellar Astrophysics stellar evolution code to model the evolution of a 70 M ⊙ star that undergoes a GE. We let the star evolve to the termination of the main sequence, and when it reaches T ≃ 19,400 K we emulate a GE by removing mass from its outer layers at a rate of 0.15 M ⊙ yr−1 for 20 yr. As mass is being lost, the star contracts and releases a substantial amount of gravitational energy. The star undergoes an initial ≃3 days of expansion followed by years of contraction. During that time the star tries to reach an equilibrium state, and as a result of loss in gravitational energy, its luminosity drops about 1 order of magnitude. As the GE terminates, we let the star continue to evolve without any further mass loss and track its recovery as it regains its equilibrium by adjusting its internal structure. After ≃87 yr it reaches a state very close to the one where the GE was first initiated. We suggest that at this point another GE or a cycle of GEs may occur.

Irradiation effect and binary radio pulsars with giant companions

Context. Pulsars are neutron stars that rotate rapidly. Most of the pulsars in binary systems tend to spin faster than those in isolation. According to binary evolution theory, radio pulsars in binary systems can have various types of companion stars. However, binary radio pulsars with giant stars, helium stars, and black holes as companions are lacking so far. Aims. We aim to investigate the possible parameter space of binary radio pulsars with giant companions. Methods. We used the MESA stellar evolution code to consider the effect of irradiation in binary evolution and evolved a series of binary models. Results. We present the potential physical properties of binary radio pulsars with giant star companions in the framework of the classical recycling scenario and the irradiation model. We found that the parameter space of binary radio pulsars with giant companions can be greatly expanded by the effect of irradiation. Moreover, when irradiation is strong enough, submillisecond pulsars might be accompanied by giant companion stars. We also demonstrate a correlation between the timescale of the binary system as a low-mass X-ray binary and the irradiation efficiency. Conclusions. The birthrate problem between millisecond pulsars and low-mass X-ray binaries might not be resolved by the irradiation effect. A more detailed binary population synthesis is necessary. Our findings may offer guidance for observers that wish to locate binary radio pulsars with giant companions or submillisecond pulsars.

A study of the electrostatic properties of the interiors of low-mass stars: Possible implications for the observed rotational properties

Context. In the partially ionized material of stellar interiors, the strongest forces acting on electrons and ions are the Coulomb interactions between charges. The dynamics of the plasma as a whole depend on the magnitudes of the average electrostatic interactions and the average kinetic energies of the particles that constitute the stellar material. An important question is how these interactions of real gases are related to the observable stellar properties. Specifically, the relationships between rotation, magnetic activity, and the thermodynamic properties of stellar interiors are still not well understood. These connections are crucial for understanding and interpreting the abundant observational data provided by space-based missions, such as Kepler/K2 and TESS, and the future data from the PLATO mission. Aims. In this study, we investigate the electrostatic effects within the interiors of low-mass main sequence (MS) stars. Specifically, we introduce a global quantity, a global plasma parameter, which allows us to compare the importance of electrostatic interactions across a range of low-mass theoretical models (0.7 − 1.4 M⊙) with varying ages and metallicities. We then correlate the electrostatic properties of the theoretical models with the observable rotational trends on the MS. Methods. We use the open-source 1D stellar evolution code MESA to compute a grid of main-sequence stellar models. Our models span the log g − Teff space of a set of 66 Kepler main-sequence stars. Results. We identify a correlation between the prominence of electrostatic effects in stellar interiors and stellar rotation rates. The variations in the magnitude of electrostatic interactions with age and metallicity further suggest that understanding the underlying physics of the collective effects of plasma can clarify key observational trends related to the rotation of low-mass stars on the MS. These results may also advance our understanding of the physics behind the observed weakened magnetic braking in stars.

Radiation Hydrodynamic Simulations of Massive Stars in Gas-rich Environments: Accretion of AGN Stars Suppressed by Thermal Feedback

Massive stars may form in or be captured into active galactic nuclei (AGN) disks. Recent 1D studies employing stellar-evolution codes have demonstrated the potential for rapid growth of such stars through accretion up to a few hundred solar masses. We perform 3D radiation hydrodynamic simulations of moderately massive stars’ envelopes in order to determine the rate and critical radius R crit of their accretion process in an isotropic gas-rich environment in the absence of luminosity-driven mass loss. We find that in the “fast-diffusion” regime where characteristic radiative diffusion speed c/τ is faster than the gas sound speed c s , the accretion rate is suppressed by feedback from gravitational and radiative advection energy flux, in addition to the stellar luminosity. Alternatively, in the “slow-diffusion” regime where c/τ < c s , due to adiabatic accretion, the stellar envelope expands quickly to become hydrostatic and further net accretion occurs on thermal timescales in the absence of self-gravity. When the radiation entropy of the medium is less than that of the star, however, this hydrostatic envelope can become more massive than the star itself. Within this subregime, the self-gravity of the envelope excites runaway growth. Applying our results to realistic environments, moderately massive stars (≲100M ⊙) embedded in AGN disks typically accrete in the fast-diffusion regime, leading to a reduction of steady-state accretion rate 1–2 orders of magnitudes lower than expected by previous 1D calculations and R crit smaller than the disk scale height, except in the opacity window at temperature T ∼ 2000 K. Accretion in slow diffusion regime occurs in regions with very high density ρ ≳ 10−9 g cm−3, and needs to be treated with caution in 1D long-term calculations.

Toward a Comprehensive Grid of Cepheid Models with MESA. I. Uncertainties of the Evolutionary Tracks of Intermediate-mass Stars

Helium-burning stars, in particular Cepheids, are especially difficult to model, as the choice of free parameters can greatly impact the shape of the blue loops—the part of the evolutionary track at which the instability strip is crossed. Contemporary one-dimensional stellar evolution codes, like Modules for Experiments in Stellar Astrophysics (MESA), come with a large number of free parameters that allow us to model the physical processes in stellar interiors under many assumptions. The uncertainties that arise from this freedom are rarely discussed in the literature despite their impact on the evolution of the model. We calculate a grid of evolutionary models with MESA, varying several controls, like solar mixture of heavy elements, mixing-length theory prescription, nuclear reaction rates, the scheme to determine convective boundaries, atmosphere model, and temporal and spatial resolution, and quantify their impact on age and location of the evolutionary track on the H-R diagram from the main sequence until the end of core helium burning. Our investigation was conducted for a full range of masses and metallicities expected for classical Cepheids. The uncertainties are significant, especially during core helium burning, reaching or exceeding the observational uncertainties of logTeff and logL for detached eclipsing binary systems. For ≥9 M ⊙ models, thin convective shells develop and evolve erratically, not allowing the models to converge. A careful inspection of Kippenhahn diagrams and convergence study is advised for a given mass and metallicity, to assess how severe this problem is and to what extent it may affect the evolution.

Younger age for the oldest magnetic white dwarfs

Abstract Sufficiently old white dwarfs cool down through a convective envelope that directly couples their degenerate cores to the surface. Magnetic fields may inhibit this convection by stiffening the criterion for convective instability. We consistently implemented the modified criterion in the stellar evolution code mesa, and computed the cooling of white dwarfs as a function of their mass and magnetic field B. In contrast to previous estimates, we find that magnetic fields can significantly change the cooling time t even if they are relatively weak $B^2\ll 8\pi P$, where P is the pressure at the edge of the degenerate core. Fields B ≳ 1 MG open a radiative window that decouples the core from the convective envelope, effectively lowering the luminosity to that of a fully radiative white dwarf. We identified a population of observed white dwarfs that are younger by Δt ∼ Gyr than currently thought due to this magnetic inhibition of convective energy transfer – comparable to the cooling delay due to carbon–oxygen phase separation. In volume-limited samples, the frequency and strength of magnetic fields increase with age. Accounting for magnetic inhibition is therefore essential for accurate cooling models for cosmic chronology and for determining the origin of the magnetic fields.

Double periodic variable V4142 Sgr: A key to approaching the stellar dynamo

Aims. In this work we focus on the double periodic variable (DPV) star V4142 Sgr, aiming to provide a deeper understanding of its evolution, the formation of its accretion disk, and the operation of magnetic dynamos within the donor star. We analyze its characteristics in detail as well as the phenomena associated with DPV stars more generally. Methods. The model was implemented using the stellar evolution code MESA r22.11.1. The modeling process starts from the zero age main sequence and incorporates differential rotation to facilitate the creation of a stellar dynamo in the donor star. We adjusted the model by employing a chi-square algorithm, minimizing the deviation between theoretical and observed values based on previously published fundamental parameters for this system. Our analysis includes an evaluation of various parameters, such as initial masses, orbital periods, mixing parameters, the thermohaline parameter, and metallicities. We assessed the algorithm convergence and set the stopping criterion at 20% helium core depletion in the donor star. A comprehensive analysis was conducted at each evolutionary stage, utilizing the Tayler–Spruit formalism to understand the mechanism of magnetic dynamos. Results. The model begins by adjusting fundamental parameters published for this system through a chi-squared optimization algorithm, adopting an initial orbital period of 15.0 days and initial masses for the donor and gainer star of Mi, d = 3.50 M⊙ and Mi, g = 1.50 M⊙, with a metallicity associated with this type of DPV of Z = 0.02. It successfully converges with six degrees of freedom and 5% confidence, resulting in a chi-squared value of 0.007. In addition, the best-fit model for V4142 Sgr shows it is in thermal-timescale mass transfer. Our analysis provides insights into the role of differential rotation in facilitating the formation of a stellar dynamo. Additionally, we have determined that our type-B gainer star is located in a region similar to other type-B DPVs that have undergone rejuvenation due to the transfer of matter. The size of the gainer star shrinks considerably, but it rejuvenates thanks to the material acquired from its donor companion. As for the donor star, the creation and amplification of magnetic fields are influenced by the mixing diffusivity, DST, which is activated by advection outside the overshooting zone.

The impact of asteroseismically calibrated internal mixing on nucleosynthetic wind yields of massive stars

Context. Asteroseismology gives us the opportunity to look inside stars and determine their internal properties, such as the radius and mass of the convective core. Based on these observations, estimations can be made for the amount of the convective boundary mixing and envelope mixing of such stars and for the shape of the mixing profile in the envelope. However, these results are not typically included in stellar evolution models. Aims. We aim to investigate the impact of varying convective boundary mixing and envelope mixing in a range based on asteroseismic modelling in stellar models up to the core collapse, both for the stellar structure and for the nucleosynthetic yields. In this first study, we focus on the pre-explosive evolution and we evolved the models to the final phases of carbon burning. This set of models is the first to implement envelope mixing based on internal gravity waves for the entire evolution of the star. Methods. We used the MESA stellar evolution code to simulate stellar models with an initial mass of 20 M⊙ from zero-age main sequence up to a central core temperature of 109 K, which corresponds to the final phases of carbon burning. We varied the convective boundary mixing, implemented as ‘step-overshoot’, with the overshoot parameter (αov) in the range 0.05−0.4. We varied the amount of envelope mixing (log(Denv/cm2 s−1)) in the range 0−6 with a mixing profile based on internal gravity waves. To study the nucleosynthesis taking place in these stars in great detail, we used a large nuclear network of 212 isotopes from 1H to 66Zn. Results. Enhanced mixing according to the asteroseismology of main-sequence stars, both at the convective core boundary and in the envelope, has significant effects on the nucleosynthetic wind yields. This is especially the case for 36Cl and 41Ca, whose wind yields increase by ten orders of magnitude compared to those of the models without enhance envelope mixing. Our evolutionary models beyond the main sequence diverge in yields from models based on rotational mixing, having longer helium-burning lifetimes and lighter helium-depleted cores. Conclusions. We find that the asteroseismic ranges of internal mixing calibrated from core hydrogen-burning stars lead to similar wind yields as those resulting from the theory of rotational mixing. Adopting the seismic mixing levels beyond the main sequence, we find earlier transitions to radiative carbon burning compared to models based on rotational mixing because they have lower envelope mixing in that phase. This influences the compactness and the occurrence of shell mergers, which may affect the supernova properties and explosive nucleosynthesis.

The evolution of accreting population III stars at 10−6–103 M⊙ yr−1

Context. The first stars formed over five orders of magnitude in mass by accretion in primordial dark matter halos. Aims. We study the evolution of massive, very massive and supermassive primordial (Pop III) stars over nine orders of magnitude in accretion rate. Methods. We use the stellar evolution code GENEC to evolve accreting Pop III stars from 10−6–103 M⊙ yr−1 and study how these rates determine final masses. The stars are evolved until either the end central Si burning or they encounter the general relativistic instability (GRI). We also examine how metallicity affects the evolution of the star at one accretion rate. Results. At rates below ∼2.5 × 10−5 M⊙ yr−1 the final mass of the star falls below that required for pair-instability supernovae. The minimum rate required to produce black holes with masses above 250 M⊙ is ∼5 × 10−5 M⊙ yr−1, well within the range of infall rates found in numerical simulations of halos that cool via H2, ≲10−3 M⊙ yr−1. At rates of 5 × 10−5 M⊙ yr−1 to 4 × 10−2 M⊙ yr−1, like those expected for halos cooling by both H2 and Lyα, the star collapses after Si burning. At higher accretion rates the GRI triggers the collapse of the star during central H burning. Stars that grow at above these rates are cool red hypergiants with effective temperatures log(Teff) = 3.8 and luminosities that can reach 1010.5 L⊙. At accretion rates of 100–1000 M⊙ yr−1 the gas encounters the general relativistic instability prior to the onset of central hydrogen burning and collapses to a black hole with a mass of ∼106 M⊙ without ever having become a star. Conclusions. Our models corroborate previous studies of Pop III stellar evolution with and without hydrodynamics over separate, smaller ranges in accretion rate. They also reveal for the first time the critical transition rate in accretion above which catastrophic baryon collapse, like that which can occur during galaxy collisions in the high-redshift Universe, produces supermassive black holes via dark collapse.