A window to the first stars: An investigation of chemically near-pristine environments.

Abstract

The first stars in the Universe are an elusive stellar population that we know relatively little about. They are the only stellar population to necessarily form in the absence of metals. Thus, they are in principle easy to identify. However, we are yet to detect a metal-free star in the local Universe. The fusion of metals in the cores of these first stars marks the onset of complex chemical evolution.
The eventual collapse of massive (>10M_sun) metal-free stars released the first instances of metals into the surrounding environment. From simulations of stellar evolution, we know that stars with different properties produce different chemical patterns. Thus, to study these stars, we can search for environments that have retained the chemical fingerprint of the first supernovae. It is the environments that have experienced minimal processing through stars that are likely to showcase the chemistry of the first stellar population. These `windows to the first stars' can be identified through their lack of metals.

This thesis presents a novel stochastic chemical enrichment model that uses the chemistry of relic environments to investigate the properties of the enriching stellar population. This model, along with nucleosynthetic yield calculations, allows us to investigate the number of massive stars that have chemically contributed to an environment, the mass distribution of the enriching population, the typical explosion energy of their supernovae, and the degree of mixing between the stellar layers. The utility of this model is shown by searching for the chemical fingerprint of the first stars in both the most metal-poor damped Lyman-alpha systems (DLAs) and the most metal-poor stars in the halo of the Milky Way. We also present and analyse new observational data of near-pristine DLAs. This includes the first bound on the carbon isotope ratio of a chemically near-pristine DLA. This isotope ratio can be used to investigate enrichment from low mass ( ~1M_sun) metal-free stars. The full complement of chemistry available for this DLA suggests that it may have experienced a hiatus in star formation following the epoch of reionisation. Finally, we present the precise [O/Fe] abundance determinations of two near-pristine DLAs. This investigation aims to establish whether [O/Fe] is elevated amongst the most metal-poor DLAs.
The ultimate goal of this thesis is to investigate the properties of the first stars through their surviving chemistry and establish the most useful metals to utilise to this end. There is a focus on the most metal-poor DLAs and their role in early chemical enrichment and galaxy evolution.