Advanced evolutionary stages of lower mass stars

在我职业生涯的大部分时间里，我研究的主要重点是建立尽可能严格地满足观测约束的恒星模型——主要关注主序星和巨支星。事实上，这些阶段的额外工作将在近期内进行。例如，使用模型大气作为恒星内部模型的边界条件对预测温标的影响将与B. Gustafsson的Uppsala小组合作进行研究。此外，将与蒙特利尔大学的同事合作计算适用于考虑扩散过程的贫金属恒星的等时线。然而，我在未来几年研究的主要目标是扩展已发布的网格，以包括水平分支（HB）和渐近巨分支（AGB）阶段，以便维多利亚大学的恒星模型可以用于研究复杂的恒星群。最初，这些模型将应用于球状星团，以试图获得对所谓的“第二参数问题”（金属丰度非常相似的星团所包含的HB形态多样性的未知原因）的更好理解，并改进目前对星团氦丰度的估计。此外，将使用G. Fontaine提供的代码计算模型的脉动特性，以便利用星系团RR Lyrae变量提供的约束。为了比现有程序更有效地生成AGB模型（大约5倍），将建立一个混合代码，该代码具有拉格朗日模式，可以相当详细地跟踪He壳闪光期间的演化，核合成和混合（使用F. Herwig开发的技术），以及非拉格朗日模式(即，（P. Eggleton提出的求解恒星结构方程的方法），以跟踪连续壳层闪光之间薄核燃烧壳层的演化。有了这样一个代码，就有可能比迄今为止更充分地探索参数空间，从而提高我们对这一进化阶段的理解，这一阶段在星系的化学演化中起着特别重要的作用。

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.