Asymptotic giant branch star nucleosynthesis and constraints from meteoritic stardust
Creators: name: Buntain, Joelene Francis
Publisher: name: Monash University
Year: 2017
Subjects: Uncategorized

description: Almost 30 years ago, stardust grains were identified in meteorites, which have retained the exotic isotopic signatures of their parent stars and display enormous anomalies (up to four orders of magnitude) with respect to the composition of the solar system. The large majority of stardust grains originated from the winds of asymptotic giant branch (AGB) stars. These are stars of low mass (less massive than roughly 8M) at the end of their evolution, which burn H and He in shells located above a C-O degenerate core. The burning shells are separated by a He-rich intershell, and a convective envelope forms the outer layer of the star. The H- and He-burning shells are activated alternately. The mass of the He intershell increases and results in a massive increase in the He-burning rate for a short time, producing a `thermal pulse' (TP). The star expands and cools and H and He burning ceases. The envelope sinks into the intershell and carries material to the stellar surface by the `third dredge-up' (TDU). This cycle is repeated many times, depending on the initial stellar mass. Very dense winds erode the envelope of an AGB star down to a thin H-rich layer. The star becomes a post-AGB star and evolves at constant luminosity towards hotter temperatures. It may become a planetary nebula (PN) with a planetary nebula nucleus (PNN) at its centre, and will then spend the remainder of its life cooling as a white dwarf (WD). Specific nucleosynthesis processes like the slow (s) neutron-capture process occur in AGB stars and their signatures are imprinted in the stardust grains. This process is responsible for roughly half of the cosmic abundances of the elements heavier than Fe (e.g., Kr, Hf, W and Pb) and operates in AGB stars via two neutron sources: the 13C(α,n)16O and the 22Ne(α,n)25Mg reactions, with the 13C(α,n)16O reaction being the main neutron source. This reaction requires protons to be mixed down from the convective envelope into the He intershell for a sufficient amount of 13C to be produced for the s-process. This 13C-rich region is known as the 13C pocket. This thesis aims to investigate which are the AGB parent stars of stardust grains in terms of their range of masses and metallicities, allowing us to understand which type of stars contributed to the inventory of stardust in the early solar system. We also use the composition of the grains to constrain our models of evolution and nucleosynthesis in AGB stars to understand mixing processes in stars and to estimate nuclear reaction rates. We run stellar nucleosynthesis models based on computed stellar structures of low masses from 1M to 4M, and metallicities of 0.0001, 0.01, 0.014 and 0.02. We then compare model predictions of nuclear abundances to the composition of the grains. We compared the composition of the winds coming from the post-AGB and PNN phases of low mass and solar metallicity AGB models to stardust oxide and silicate grains to test the hypothesis that some oxide grains originated from post-AGB stars and PNN. We find that overall the models do not match most of the grains, unless some of the reaction rates used are different than currently assumed. We tested different proton profiles, which determine the size of the 13C pocket formed during nucleosynthesis calculations. We found that abundances produced in models less massive than 1.8M are not affected by the choice of proton profile. On the other hand, stellar models more massive than 1.8M are sensitive to the choice of proton profile, and this affects their abundances. We studied the rate of the 13C(α,n)16O neutron source and its effect on heavy element production. We investigated a number of evaluations of this rate, however, we cannot conclude which evaluation is the most accurate. We studied the W and Hf isotopic compositions predicted in AGB stars and compared them to stardust grains. We found that there are no stellar models in our range of masses or metallicities that match all of the grain data, specifically the 186W/184W ratios in the models is lower than observed. Finally, we analysed the Kr isotopic compositions in AGB stars using the new determination of the 85Kr(n,γ)86Kr neutron-capture rate from Raut et al. (2013). We investigated whether the Kr isotopes measured in large stardust SiC grains can be explained by the composition of the fast stellar winds of the PNN phase. We compared the composition of this material to SiC grains, and found that models less massive than 1.8M that experience 13C ingestion during the TP may be the source of the largest SiC grains.