Structural and Nucleosynthetic Evolution of Metal-poor and Metal-free Low and Intermediate Mass Stars

Abstract

We report on an investigation into 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 have calculated is -6.5 < [Fe/H] < -3.0, whilst our models are in the mass range 0.85 < M < 3.0 Msun. Many of the 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 signifcant surface pollution. These events have been reported by previous studies, so our results reaffirm their occurrence -- at least in 1D 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 TPAGB, including detailed nucleosynthesis. We have also calculated the nucleosynthetic yields, which will soon be available in electronic format. Although subject to many uncertainties these are, as far as we are aware, the only yields available in this mass and metallicity range. In this paper we briefy describe some of the results in the context of abundance observations of EMP halo stars. This work formed part of SWC's PhD thesis (completed in March 2007) and a series of subsequent papers will describe the results of the study in more detail.