Neutron-Capture Nucleosynthesis and the Chemical Evolution of Globular Clusters

Abstract

Elements heavier than iron are almost entirely produced in stars through neutron captures and radioactive decays. Of these heavy elements, roughly half are produced by the slow neutron-capture process (s-process), which takes place under extended exposure to low neutron densities. Most of the s-process production occurs in stars with initial masses between roughly 0.8 and 8 M , which evolve through the Asymptotic Giant Branch (AGB) phase. This thesis explores several topics related to AGB stars and the s-process, with a focus on comparing theoretical models to observations in the literature on planetary nebulae, post- AGB stars, and globular cluster stars. A recurring theme is the uncertainty of 13C-pocket formation, which is crucial for building accurate models of s-process nucleosynthesis. We first investigated whether neutron-capture reactions in AGB stars are the cause of the low sulphur abundances in planetary nebulae and post-AGB stars relative to the interstellar medium. Accounting for uncertainties in the size of the partial mixing zone that forms 13C pockets and the rates of neutron-capture and neutron-producing reactions, our models failed to reproduce the observed levels of sulphur destruction. From this, we concluded that AGB nucleosynthesis is not the cause of the sulphur anomaly. We also discovered a new method to constrain the extent of the partial mixing zone using neon abundances in planetary nebulae. We next aimed to discover the stellar sites of the s-process enrichment in globular clusters that have inter- and intra-cluster variation, with the examples of M4 (relative to M5) and M22, respectively. Using a new chemical evolution code developed by the candidate, we tested models with stellar yields from rotating massive stars and AGB stars. We compared our model predictions for the production of s-process elements with abundances from s-poor and s-rich populations. We found that rotating massive stars alone do not explain the pattern of abundance variations in either cluster, and that a contribution from AGB stars with 13C pockets is required. We derived a minimum enrichment timescale from our best-fitting chemical evolution models and, although the value depends on the assumptions made about the formation of 13C pockets, our estimate of 240–360 Myr for M22 is consistent with the upper limit of 300 Myr inferred by isochrone fitting. Lastly, there is accumulating evidence that some stars (e.g., in ! Centauri) have been born with helium mass fractions as high as 40%. This motivated us to explore the impact of helium-rich abundances on the evolution and nucleosynthesis of intermediate-mass (3–6 M ) AGB models. We found that the stellar yields of s-process elements are substantially lower in He-rich models, largely as a result of less intershell material being mixed into the envelope. We also found evidence that high He abundances could restrict the s-process production by 13C pockets to stars with lower initial masses.