Evolution, nucleosynthesis and final fates of super- and massive asymptotic giant branch stars

Abstract

In this thesis we explore the evolution, nucleosynthesis and final fates of super- and massive asymptotic giant branch (AGB) stars. Super-AGB stars bridge the divide between low and high mass stars and are characterised by carbon burning within their cores prior to the thermally pulsing phase. We test our implementation of a single composite carbon burning reaction within our stellar evolutionary code by running a series of models, with standardized input physics to compare to previous studies in the literature. In this benchmarking work, which includes phases of evolution up to the cessation of carbon burning, we find excellent agreement between different code results over a large range of metallicities. Due to the fine initial mass resolution grid used to probe the lowest mass models which ignite carbon, a new type of white dwarf is reported, a hybrid CO(Ne) white dwarf, comprising of a CO core surrounded by an ONe shell. Super-AGB stars have long been the ``missing'' mass range in galactic chemical evolution studies due to the lack of stellar yield calculations. To remedy this problem, and assess the impact of the super-AGB star contribution to the galactic chemical inventory of nuclides, an extensive grid of nucleosynthetic yields has been produced for elements up to iron. Since the evolution and element production within stars are strongly dependent on their initial metallicity, we have performed calculations over a wide range of metallicities from Z=0.02 to 0.0001 ([Fe/H]~ 0 to -2.3). We examine the role that the nucleosynthetic processes of first, second, and third dredge-up, as well as hot bottom burning have on the surface composition within super-AGB stars. Stellar yield calculations are subject to a wide range of uncertainties, in particular the wind mass-loss rate, nuclear reaction rate uncertainties, the theory of convective mixing, and efficiency of third dredge-up. We investigate the impact that these uncertainties have on yield predictions. Our results are compared to other studies in the literature, with the major difference being the occurrence of third dredge-up in our calculations. We apply our nucleosynthetic yield predictions of metallicity Z=0.001 to examine the possible role of super-AGB stars as the polluters of the anomalous stars in the globular cluster NGC 2808. Lastly, we examine the final fates of super-AGB and massive-AGB stars in the mass range 5 to 10 Msun. We produce an extensive grid of detailed evolution calculations along the majority of the thermally pulsing AGB phase. These models are computationally demanding due to the necessity of following a vast number of thermal pulses with very fine temporal and spacial resolution. We provide a theoretical initial to final mass relation for massive and ultra-massive white dwarfs.