The late stages of the evolution of intermediate-mass primordial stars: the effects of overshooting

Abstract

Aims. We compute and analyze the evolution of primordial stars of masses at the ZAMS between 5 M and 10 M , with and without overshooting. Our main goals are to determine the nature of the remnants of massive intermediate-mass primordial stars and to check the influence of overshooting in their evolution. Methods. Our calculations cover stellar evolution from the main sequence phase until the formation of the degenerate cores and the thermally pulsing phase. Results. We have obtained the values for the limiting masses of Population III progenitor stars leading to carbon-oxygen and oxygenneon compact cores. Moreover, we have also obtained the limiting mass for which isolated primordial stars would lead to core-collapse supernovae after the end of the main central burning phases. Considering a moderate amount of overshooting, the mass thresholds at the ZAMS for the formation of carbon-oxygen and oxygen-neon degenerate cores shift to smaller values by about 2 M . As a by-product of our calculations, we have also obtained the structure and composition profiles of the resulting compact remnants. Conclusions.As opposed to what happens with solar metallicity objects, the final fate of primordial stars is not straightforwardly determined from the mass of the compact cores at the end of carbon burning. Instead, the small mass-loss rates typically associated with stellar winds of low metallicity stars might allow the growth of the resulting degenerate cores up to the Chandrasekhar mass, on time scales one or two orders of magnitude shorter than the time required to lose the envelope. This would lead to the formation of supernovae for initial masses as small as ∼5 M .

The end of super AGB and massive AGB stars

“…This would prevent the occurrence of electron-captured supernovae. Compared to the result of Gil-Pons et al (2007), Siess (2007) and Poelarends et al (2008), which deduce the possibility of electron-captured supernovae based on the assumption of regular thermal pulses and extrapolation on core growth rate and mass-loss rate, the actual ranges for the occurrence of electron-captured supernovae could be much narrower in terms of both mass and metallicity. For example, our models show that electron-capture supernovae is very unlikely in solar metallicity.…”

Evolution of massive AGB stars

“…The differences between these simulations come from the treatment of mixing and the difficulty of current models for determining the extent of the convective core, especially during the central helium-burning phase. For example, applying a moderate overshooting at the edge of the convective core shifts the entire SAGB mass range down by 2 M (Gil-Pons et al 2007;Siess 2007). Our conservative treatment of convective boundaries (application of the strict Schwarzschild criterion and absence of numerical diffusion at the convective boundaries) provides an upper limit to the SAGB stars, which in our case range between 8 and 11 M .…”

“…Garcia-Berro et al 1997;Siess 2006), the effects of overshooting (Gil-Pons et al 2007), semiconvection (Poelarends et al 2008), or thermohaline mixing (Siess 2009). The start of electron captures reactions in the oxygen-neon (ONe) core and the URCA process have also been investigated (Ritossa et al 1999), and non-solar models were computed and their main properties analysed (e.g.…”

“…The main reasons are ascribed to our poor knowledge of the mass loss rate and third-dredge-up properties that control how much mass can be removed during the post-carbon burning evolution (Gil-Pons et al 2007;Siess 2007;Poelarends et al 2008). If the envelope is lost before electron capture reactions are activated in the core, an ONe white dwarf forms, otherwise the core collapses and a neutron star forms.…”

On the Fate of Extremely Low Metallicity Stars

“…Instead, inputs from multidimensional hydrodynamic simulations might be required [4,5]. Moreover, recent calculations of the evolution of intermediate-mass primordial stars [6,7,8], point out to the possibility that these stars might end their lives as SNeI1/2 [9], since the initial lack of metals of the envelope and the inefficiency of the dredge-up process favour very weak winds, that are unable to eject the envelope before the core reaches the Chandrasekhar mass.…”