Advanced evolutionary stages of lower mass stars

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The main thrust of my research over much of my career has been to produce stellar models that satisfy observational constraints as tightly as possible - focusing primarily on main-sequence and giant-branch stars. Indeed, additional work on these phases will be carried out in the near term. For instance, the impact on the predicted temperature scale of using model atmospheres as boundary conditions for the stellar interior models will be examined in collaboration with B. Gustafsson's Uppsala group. In addition, isochrones applicable to metal-poor stars that take diffusive processes into account will be computed in collaboration with colleagues at the U. of Montreal. However, the main goal of my research over the next several years is to extend the published grids to include the horizontal-branch (HB) and asymptotic-giant-branch (AGB) phases, so that University of Victoria stellar models may be used to study complex stellar populations. Initially these models will be applied to globular star clusters to try to obtain an improved understanding of the so-called "second-parameter problem" (the unknown cause of the wide diversity in HB morphology encompassed by clusters having very similar metallicities), and to refine current estimates of the cluster helium abundances. Moreover, the pulsational characteristics of the models will be computed using a code provided by G. Fontaine, in order to make use of the constraints provided by the cluster RR Lyrae variables. To generate AGB models much more efficiently than existing programs (by about a factor of 5), a hybrid code will be set up having a Lagrangian mode to follow the evolution, nucleosynthesis, and mixing during He shell flashes in considerable detail (using techniques developed by F. Herwig), as well as a non-Lagrangian mode (i.e., the method of solving the stellar structure equations developed by P. Eggleton) to follow the evolution of thin nuclear-burning shells between consective shell flashes. With such a code, it will be possible to explore parameter space much more fully than has been possible to date, and thereby to improve our understanding of this evolutionary phase, which plays an especially important role in the chemical evolution of galaxies.