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The chemical feedback from stellar winds in low metallicity (Z) environments is key to understanding the evolution of globular clusters and the early Universe. With a disproportionate amount of mass lost from the most massive stars (M > 100 M_⊙) and an excess of such stars expected at the lowest metallicities, their contribution to the enrichment of the early pristine clusters could be significant. In this work, we examine the effect of mass loss at low metallicity on the nucleosynthesis and wind yields of (very) massive stars. We calculated stellar models with initial masses ranging from 30 to 500 M_⊙during core hydrogen and helium burning phases at four metallicities ranging from 20% Z_⊙down to 1% Z_⊙. We provide the ejected masses and net yields for each grid of models. While mass-loss rates decrease withZ, we find that not only are wind yields significant, but the nucleosynthesis is also altered due to the change in central temperatures, and therefore it also plays a role. We find that 80–300 M_⊙models can produce large quantities of Na-rich and O-poor material, which is relevant for the observed Na-O anti-correlation in globular clusters. 

ABSTRACT Strong metallicity-dependent winds dominate the evolution of core He-burning, classical Wolf–Rayet (cWR) stars, which eject both H and He-fusion products such as $$^{14}$$N, $$^{12}$$C, $$^{16}$$O, $$^{19}$$F, $$^{22}$$Ne, and $$^{23}$$Na during their evolution. The chemical enrichment from cWRs can be significant. cWR stars are also key sources for neutron production relevant for the weak s-process. We calculate stellar models of cWRs at solar metallicity for a range of initial Helium star masses (12–50 $$\rm M_{\odot }$$), adopting recent hydrodynamical wind rates. Stellar wind yields are provided for the entire post-main sequence evolution until core O-exhaustion. While literature has previously considered cWRs as a viable source of the radioisotope $$^{26}$$Al, we confirm that negligible $$^{26}$$Al is ejected by cWRs since it has decayed to $$^{26}$$Mg or proton-captured to $$^{27}$$Al. However, in Paper I, we showed that very massive stars eject substantial quantities of $$^{26}$$Al, among other elements including N, Ne, and Na, already from the zero-age-main-sequence. Here, we examine the production of $$^{19}$$F and find that even with lower mass-loss rates than previous studies, our cWR models still eject substantial amounts of $$^{19}$$F. We provide central neutron densities (N$$_{n}$$) of a 30 $$\rm M_{\odot }$$ cWR compared with a 32 $$\rm M_{\odot }$$ post-VMS WR and confirm that during core He-burning, cWRs produce a significant number of neutrons for the weak s-process via the $$^{22}$$Ne($$\alpha$$,n)$$^{25}$$Mg reaction. Finally, we compare our cWR models with observed [Ne/He], [C/He], and [O/He] ratios of Galactic WC and WO stars. 

Context.Stars with initial mass above roughly 8M_⊙will evolve to form a core made of iron group elements, at which point no further exothermic nuclear reactions between charged nuclei may prevent the core collapse. Electron capture, neutrino losses, and the photo-disintegration of heavy nuclei trigger the collapse of these stars. Models at the brink of core collapse are produced using stellar evolution codes, and these pre-collapse models may be used in the study of the subsequent dynamical evolution (including their explosion as supernovae and the formation of compact remnants such as neutron stars or black holes). Aims.We upgraded the physical ingredients employed by the GENeva stellar Evolution Code, GENEC, so that it covers the regime of high-temperatures and high-densities required to produce the progenitors of core-collapse. Our ultimate goal is producing pre-supernova models with GENEC, not only right before collapse, but also during the late phases (silicon and oxygen burning). Methods.We have improved GENEC in three directions: equation of state, the nuclear reaction network, and the radiative and conductive opacities adapted for the computation of the advanced phases of evolution. We produce a small grid of pre-supernova models of stars with zero age main sequence masses of 15 M_⊙, 20 M_⊙, and 25 M_⊙at solar and less than half solar metallicities. The results are compared with analogous models produced with the MESA code. Results.The global properties of our new models, particularly of their inner cores, are comparable to models computed with MESA and pre-existing progenitors in the literature. Between codes the exact shell structure varies, and impacts explosion predictions. Conclusions.Using GENEC with state-of-the-art physics, we have produced massive stellar progenitors prior to collapse. These progenitors are suitable for follow-up studies, including the dynamical collapse and supernova phases. Larger grids of supernova progenitors are now feasible, with the potential for further dynamical evolution. 

Context.In addition to being spectacular objects, very massive stars (VMSs) are suspected to have a tremendous impact on their environment and on cosmic evolution in general. The nucleosynthesis both during their advanced stages and their final explosion may contribute greatly to the overall enrichment of the Universe. Their resulting supernovae are candidates for the most superluminous events possible and their extreme conditions also lead to very important radiative and mechanical feedback effects, from local to cosmic scale. Aims.We explore the impact of rotation and metallicity on the evolution of VMSs over cosmic time. Methods.With the recent implementation of an equation of state in the GENEC stellar evolution code, which is appropriate for describing the conditions in the central regions of very massive stars in their advanced phases, we present new results on VMS evolution from Population III to solar metallicity. Results.Low-metallicity VMS models are highly sensitive to rotation, while the evolution of higher-metallicity models is dominated by mass-loss effects. The mass loss strongly affects their surface velocity evolution, breaking quickly at high metallicity while reaching the critical velocity for low-metallicity models. Comparison to observed VMSs in the LMC shows that the mass-loss prescriptions used for these models are compatible with observed mass-loss rates. In our framework for modeling rotation, our models of VMS need a high initial velocity in order to reproduce the observed surface velocities. The surface enrichment of these VMSs is difficult to explain with only one initial composition, and could suggest multiple populations in the R136 cluster. At a metallicity typical of R136, only our non- or slowly rotating VMS models may produce pair-instability supernovae. The most massive black holes that can be formed are less massive than about 60M_⊙. Conclusions.Direct observational constraints on VMS are still scarce. Future observational campaigns will hopefully gather more pieces of information to guide the theoretical modeling of these objects, whose impacts can be very important. VMS tables are available at the CDS. 

ABSTRACT The most massive stars provide an essential source of recycled material for young clusters and galaxies. While very massive stars (VMSs, M>100 $$\rm {\rm M}_{\odot }$$) are relatively rare compared to O stars, they lose disproportionately large amounts of mass already from the onset of core H-burning. VMS have optically thick winds with elevated mass-loss rates in comparison to optically thin standard O-star winds. We compute wind yields and ejected masses on the main sequence, and we compare enhanced mass-loss rates to standard ones. We calculate solar metallicity wind yields from MESA stellar evolution models in the range 50–500 $$\rm {\rm M}_{\odot }$$, including a large nuclear network of 92 isotopes, investigating not only the CNO-cycle, but also the Ne–Na and Mg–Al cycles. VMS with enhanced winds eject 5–10 times more H-processed elements (N, Ne, Na, Al) on the main sequence in comparison to standard winds, with possible consequences for observed anticorrelations, such as C–N and Na–O, in globular clusters. We find that for VMS 95 per cent of the total wind yields is produced on the main sequence, while only ∼ 5 per cent is supplied by the post-main sequence. This implies that VMS with enhanced winds are the primary source of 26Al, contrasting previous works where classical Wolf–Rayet winds had been suggested to be responsible for galactic 26Al enrichment. Finally, 200 $$\rm {\rm M}_{\odot }$$ stars eject 100 times more of each heavy element in their winds than 50 $$\rm {\rm M}_{\odot }$$ stars, and even when weighted by an IMF their wind contribution is still an order of magnitude higher than that of 50 $$\rm {\rm M}_{\odot }$$ stars. 

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 GENECmodels, with a metallicity ofZ = 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 GENECspanning masses between 1.7 and 500M_⊙, with and without rotation, at a metallicity ofZ = 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. 

Context. The 26 Al short-lived radioactive nuclide is the source of the observed galactic diffuse γ -ray emission at 1.8 MeV. While different sources of 26 Al have been explored, such as asymptotic giant branch stars, massive stellar winds, and supernovae, the contribution of very massive stars has not been studied so far. Aims. We study the contribution of the stellar wind of very massive stars, here, stars with initial masses between 150 and 300 M ⊙ , to the enrichment in 26 Al of the galactic interstellar medium. Methods. We studied the production of 26 Al by studying rotating and non-rotating very massive stellar models with initial masses between 150 and 300 M ⊙ for metallicities Z  = 0.006, 0.014, and 0.020. We compared this result to a simple Milky Way model and took the metallicity and the star formation rate gradients into account. Results. We obtain that very massive stars in the Z  = 0.006 − 0.020 metallicity range might be very significant contributors to the 26 Al enrichment of the interstellar medium. Typically, the contribution of the winds of massive stars to the total quantity of 26 Al in the Galaxy increases by 150% when very massive stars are considered. Conclusions. Despite their rarity, very massive stars might be important contributors to 26 Al and might overall be very important actors for nucleosynthesis in the Galaxy. 

ABSTRACT In this work, we investigate the impact of uncertainties due to convective boundary mixing (CBM), commonly called ‘overshoot’, namely the boundary location and the amount of mixing at the convective boundary, on stellar structure and evolution. For this we calculated two grids of stellar evolution models with the MESA code, each with the Ledoux and the Schwarzschild boundary criterion, and vary the amount of CBM. We calculate each grid with the initial masses of 15, 20, and $$25\, \rm {M}_\odot$$. We present the stellar structure of the models during the hydrogen and helium burning phases. In the latter, we examine the impact on the nucleosynthesis. We find a broadening of the main sequence with more CBM, which is more in agreement with observations. Furthermore, during the core hydrogen burning phase there is a convergence of the convective boundary location due to CBM. The uncertainties of the intermediate convective zone remove this convergence. The behaviour of this convective zone strongly affects the surface evolution of the model, i.e. how fast it evolves redwards. The amount of CBM impacts the size of the convective cores and the nucleosynthesis, e.g. the 12C to 16O ratio and the weak s-process. Lastly, we determine the uncertainty that the range of parameter values investigated introduces and we find differences of up to $$70{{\ \rm per\ cent}}$$ for the core masses and the total mass of the star. 

ABSTRACT We present a grid of stellar models at supersolar metallicity (Z = 0.020) extending the previous grids of Geneva models at solar and sub-solar metallicities. A metallicity of Z = 0.020 was chosen to match that of the inner Galactic disc. A modest increase of 43 per cent (= 0.02/0.014) in metallicity compared to solar models means that the models evolve similarly to solar models but with slightly larger mass-loss. Mass-loss limits the final total masses of the supersolar models to 35 M⊙ even for stars with initial masses much larger than 100 M⊙. Mass-loss is strong enough in stars above 20 M⊙ for rotating stars (25 M⊙ for non-rotating stars) to remove the entire hydrogen-rich envelope. Our models thus predict SNII below 20 M⊙ for rotating stars (25 M⊙ for non-rotating stars) and SNIb (possibly SNIc) above that. We computed both isochrones and synthetic clusters to compare our supersolar models to the Westerlund 1 (Wd1) massive young cluster. A synthetic cluster combining rotating and non-rotating models with an age spread between log10(age/yr) = 6.7 and 7.0 is able to reproduce qualitatively the observed populations of WR, RSG, and YSG stars in Wd1, in particular their simultaneous presence at $$\log _{10}(L/\mathit {\mathrm{ L}}_{\odot })$$ = 5–5.5. The quantitative agreement is imperfect and we discuss the likely causes: synthetic cluster parameters, binary interactions, mass-loss and their related uncertainties. In particular, mass-loss in the cool part of the HRD plays a key role. 

Context. Grids of stellar models, computed with the same physical ingredients, allow one to study the impact of a given physics on a broad range of initial conditions and they are a key ingredient for modeling the evolution of galaxies. Aims. We present here a grid of single star models for masses between 0.8 and 120 M ⊙ , with and without rotation for a mass fraction of heavy element Z  = 0.006, representative of the Large Magellanic Cloud (LMC). Methods. We used the GENeva stellar Evolution Code. The evolution was computed until the end of the central carbon-burning phase, the early asymptotic giant branch phase, or the core helium-flash for massive, intermediate, and low mass stars, respectively. Results. The outputs of the present stellar models are well framed by the outputs of the two grids obtained by our group for metallicities above and below the one considered here. The models of the present work provide a good fit to the nitrogen surface enrichments observed during the main sequence for stars in the LMC with initial masses around 15 M ⊙ . They also reproduce the slope of the luminosity function of red supergiants of the LMC well, which is a feature that is sensitive to the time-averaged mass loss rate over the red supergiant phase. The most massive black hole that can be formed from the present models at Z  = 0.006 is around 55 M ⊙ . No model in the range of mass considered will enter into the pair-instability supernova regime, while the minimal mass to enter the region of pair pulsation instability is around 60 M ⊙ for the rotating models and 85 M ⊙ for the nonrotating ones. Conclusions. The present models are of particular interest for comparisons with observations in the LMC and also in the outer regions of the Milky Way. We provide public access to numerical tables that can be used for computing interpolated tracks and for population synthesis studies.