Advanced evolutionary stages of lower mass stars

在我职业生涯的大部分时间里，我的研究重点一直是建立尽可能满足观测约束的恒星模型--主要集中在主序星和巨型分支恒星上。事实上，这些阶段的额外工作将在短期内开展。例如，将模型大气用作恒星内部模型的边界条件对预测温度尺度的影响将与B·古斯塔夫松的Uppsala小组合作进行研究。此外，适用于考虑扩散过程的贫金属恒星的等时线将与蒙特利尔大学的同事合作进行计算。在接下来的几年里，我研究的主要目标是将已发表的网格扩展到包括水平分支(HB)和渐近巨大分支(AGB)相，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，试图更好地理解所谓的“第二参数问题”(具有非常相似的金属性的星团包围着HB形态广泛多样性的未知原因)，并改进目前对星团氦丰度的估计。模型的脉动特性将使用G.Fontaine提供的代码来计算，以便利用由簇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(大约是现有程序的5倍)，将建立具有拉格朗日模式的混合代码，以相当详细地跟踪在壳层闪光期间的演化、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，埃格尔顿(P.Eggleton)开发的求解恒星结构方程的方法)，以跟踪在连续壳层闪光之间薄的核燃烧壳层的演化。有了这样的代码，就有可能比迄今可能更充分地探索参数空间，从而提高我们对这一演化阶段的理解，这在星系的化学演化中发挥着特别重要的作用。

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.