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

TL;DR

This thesis models the evolution and nucleosynthesis of extremely metal-poor and metal-free stars, revealing unique violent episodes called 'Dual Flashes' that significantly influence surface composition and match observed abundance patterns.

Contribution

It provides the first detailed evolutionary and nucleosynthetic models for low-metallicity stars including the novel 'Dual Flashes' phenomenon and their impact on surface pollution and chemical yields.

Findings

Dual Flashes cause significant surface pollution in EMP stars.

Models predict more carbon-rich stars at lower metallicities.

Comparison with observed CEMP stars shows mixed agreement.

Abstract

In this PhD thesis we investigate stellar evolution and nucleosynthesis in the low- and extremely-low metallicity regime - including models of stars with a pure Big Bang composition (i.e. $\rm{Z} = 0$). The metallicity range of the extremely metal-poor (EMP) models calculated is $-6.5 < \rm{[Fe/H]} < -3.0$, with a mass range $0.85 < \rm{M} < 3.0~\rm{M}_{\odot}$. We have also calculated a series of models with a metallicity of $\rm{[Fe/H]} = -1.4$, to compare with observations of abundance patterns in Galactic globular cluster stars. Many of the extremely metal-poor (EMP) and $\rm{Z} = 0$ models experience violent evolutionary episodes not seen at higher metallicities. We refer to these events as `Dual Flashes' (DF) since they are characterised by peaks in the hydrogen and helium burning luminosities occurring at the same time. Some of the material processed by these events is later dredged up by the convective envelope, causing very significant surface pollution. We have calculated the entire evolution of the $\rm{Z} = 0$ and EMP models, including detailed nucleosynthesis and yields. 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 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. 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. Fluid dynamics calculations are likely needed to model the violent DF episodes. [Abridged]