RSAA Colloquia / Seminars / Feast-of-Facts: Thursday, 05 February 2015, 11:00-12:00; Duffield Lecture Theatre

Cherie Fishlock

"Final Thesis Talk: Why should we care about asymptotic giant branch stars?"

Clues to the formation and evolution of our Galaxy, the Milky Way, are hidden in the chemical abundances and kinematics of its stars. The low-mass stars, with ages comparable to the age of the Universe, retain the chemical composition of the gas cloud from which they formed. Therefore, spectroscopic studies of these ’stellar fossils’ reveal clues about the history of our Galaxy. The discovery of the ongoing accretion of the Sagittarius dwarf galaxy confirmed that mergers with satellite dwarf galaxies have contributed to the formation of the Galactic halo. However, stars that were accreted during the earliest times of the formation of the Galactic halo have been elusive. This thesis aims to better understand how the Galactic halo came to be. In particular, I investigated the accretion of dwarf galaxies by the Galaxy and the role of asymptotic giant branch (AGB) stars in the chemical evolution of these systems. Low- to intermediate-mass stars end their nuclear burning life on the AGB. It is during this time that rich nucleosynthesis occurs in the stellar interior. The newly synthesised elements are then brought to the surface through mixing episodes before being ejected into the interstellar medium via strong mass loss. Therefore, AGB stars are important contributors to the build-up of elements in our Universe. I present stellar evolutionary tracks and nucleosynthetic predictions for a grid of stellar models of low- and intermediate-mass AGB stars at Z = 0.001 ([Fe/H] = -1.2). The models cover an initial mass range from 1 M_sol to 7 M_sol. Final surface abundances and stellar yields are calculated for all elements from hydrogen to bismuth as well as isotopes up to the iron group. We present the first study of neutron-capture nucleosynthesis in intermediate-mass AGB models, including a super-AGB model, of [Fe/H] = -1.2. Neutron-capture elemental abundances have been measured, using high-resolution spectra obtained from Magellan, for the sample of stars compiled by Nissen & Schuster (2010). The sample consists of a population of stars theorised, due to their abundances and kinematics, to have been accreted from dwarf galaxies. The results provide further evidence that the Galactic halo contains a population of stars accreted from dwarf galaxies and we find that AGB stars were able to contribute to the chemical evolution of the earliest dwarf galaxies.