Tracing the evolution of the galactic halo through asymptotic giant branch stars

The discovery of the ongoing accretion of the Sagittarius dwarf spheroidal galaxy confirms that mergers with satellite dwarf galaxies are contributing to the formation of the Galactic halo. Additionally, a number of satellite dwarf galaxies show evidence of pollution by asymptotic giant branch (AGB) stars at around [Fe/H] of -1 as a result of a slower star formation rate than the Galactic halo. However, finding stars that were accreted during the earliest times of the formation of the Galactic halo has been difficult. The study by Nissen & Schuster (2010) uncovered convincing evidence that the Galactic halo hosts two populations that are separated in [a/Fe]. They argued that the low-alpha stars have kinematics consistent with accretion from a satellite galaxy with a slower star formation rate than the high-alpha population which most likely formed in-situ in the Galaxy. The aim of this thesis is to investigate the role AGB stars have had in the chemical enrichment of halo stars that were either accreted from dwarf galaxies or formed in-situ in the Galaxy. Through an analysis of chemical abundances we seek to determine the extent to which the chemical properties of the surviving galaxies are similar to the satellite galaxies that were accreted long ago. With this analysis, we hope to reveal how environmental differences such as the star formation rate have influenced the observed abundance ratios. The sample compiled by Nissen & Schuster (2010) presents a unique opportunity to further investigate the chemical evolution of neutron-capture elements in dwarf galaxies and the Galactic halo through a comparison of the low- and high-alpha populations. Theoretical models play a key role in furthering our understanding of the origin of the elements. In order to understand the influence of AGB stars on the chemical enrichment of the low-alpha stars, we calculate detailed theoretical models of AGB stars at the mean metallicity of the low- and high-alpha populations. We present stellar evolutionary tracks and nucleosynthesis predictions for a grid of stellar models of low- and intermediate-mass AGB stars at Z = 0.001 ([Fe/H] = -1.2). We present the first study of neutron-capture nucleosynthesis in intermediate-mass AGB models at [Fe/H] = -1.2. We also present the first full s-process predictions and yields for a super-AGB star at any metallicity. Neutron-capture elemental abundances have been measured, using high-resolution spectra obtained from Magellan, for a sub-sample of the low- and high-alpha stars compiled by Nissen & Schuster (2010). We find the low-alpha stars display the same chemical signature that is observed in present-day dwarf spheroidal galaxies with low [a/Fe] and [Y/Eu] and high [Ba/Y]. This confirms that the Galactic halo contains a population of stars accreted from dwarf galaxies and that AGB stars were able to contribute to the chemical evolution of the earliest dwarf galaxies