Structural and nucleosynthetic evolution of metal-poor & metal-free low and intermediate mass stars
thesisposted on 10.01.2017 by Campbell, Simon Wattana
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In this study we have investigated stellar evolution and nucleosynthesis in the low and extremely low metallicity regime - including models of stars with a pure Big Bang composition (i.e. Z = 0). The metallicity range of the extremely metal-poor (EMP) models we calculated is -6.5 < [Fe/H] < -3.0, whilst most of our models are in the mass range 0.85 < M < 3.0 M. We have also calculated a series of models with a metallicity of [Fe/H] = -1.4 to compare with observations of abundance patterns in Galactic globular cluster stars. Many of the extremely metal-poor (EMP) and Z = 0 models experience violent evolutionary episodes not seen at higher metallicities. We refer to these events as 'Dual Flashes' since they are characterised by peaks in the hydrogen and helium burning luminosities occurring at roughly the same time. Some of the material processed by these events is later dredged up by the convective envelope, causing very significant surface pollution. These events have been reported by previous studies, so our results confirm their occurrence - at least in stellar models. The novelty of this study is that we have calculated the entire evolution of the Z = 0 and EMP models, from the ZAMS to the end of the AGB - including detailed nucleosynthesis. We have also calculated the nucleosynthetic yields, which we make available in the appendices. Although subject to many uncertainties these are, as far as we are aware, the only yields available in this mass and metallicity range. We find that our models predict an increased number of carbon-rich stars at the lowest metallicities. This is mainly due to the extra pollution provided by the Dual Flash (DF) events - which do not occur in higher metallicity models. This concurs well with the observations that show the proportion of carbon-enhanced metal-poor (CEMP) stars in the Galactic halo to be higher at lower metallicities. It is also found that the pollution arising from the DF events is simultaneously C- and N-rich, as also observed in the CEMP stars. This contrasts with the pollution expected from third dredge-up at low mass, which would be C-rich only. Furthermore, the models predict that the proportion of CEMP stars should continue to increase at lower metallicities. We also compare the chemical pollution arising from our models with the detailed abundance patterns available for some of the most metal-poor CEMP stars, and find mixed results. In relation to our globular cluster (GC) models we find that our 'standard' AGB models have fundamental problems in explaining the GC abundance anomalies, mainly due to the occurrence of third dredge-up (3DUP). As an experimental test we also calculate a series of models in which we 'turn off' 3DUP. Here we find that the match with observations is better, but problems still remain. We provide the yields for all these GC models in the appendices. Finally we note that all these calculations contain many uncertainties. These include the unknown mass-loss rates, uncertain nuclear reaction rates, and the treatment of convection. In the case of the Dual Flash events a further uncertainty is the possibility that full fluid dynamics calculations are really needed to model these violent episodes.