Studies of the Multiple Stellar Populations in Globular Clusters

球状星团（GC）是一种近似球形的聚集体，通常包含数百万颗恒星。 直到最近，人们还认为单个GC中的所有恒星都是在同一时间（超过100亿年前）形成的，具有相同的初始化学成分。 然而，主要基于哈勃太空望远镜获得的数据，观察者已经确定GC拥有多个恒星种群，其中一个或多个年轻的一代是由前一代排出的气体形成的。 对它们光谱的分析表明，年轻的恒星可能是由快速旋转的大质量恒星在核心氢燃烧阶段的喷出物形成的，或者是由中等质量恒星在薄壳中燃烧氢和氦的恒星风形成的（所谓的“渐近巨星分支星”）--但是我们对这些现象的理解仍然非常粗略。 该研究的主要目标是建立一个令人满意的模型来解释GC中多个恒星种群的起源。 为了实现这一目标，我们将生成数值模型来跟踪第一代（FG）大质量恒星的演化，并尝试调整混合参数，使其喷出物的化学成分与观测到的后续世代的金属丰度一致。 混合，这可能是由对流，旋转或其他因素引起的，是很重要的，因为它重新分配了参与核反应的元素的同位素的相对丰度。 只要有可能，在我们的模拟中采用的混合处方将基于多维流体动力学模型的结果。 FG星喷出物的化学成分将是GC化学和动力学演化模型的起点，这些模型充分考虑了大质量和中等质量恒星喷出的物质所产生的冷却流。 在开发这个模型的同时，我们将继续对预测和观测到的色星等图进行详细的比较，以限制不同恒星群体中的氦丰度，以及最重要的重元素（包括O，Mg和Si）的丰度变化。

Globular clusters (GCs) are nearly spherical aggregates that typically contain a few million stars. Until recently, it was thought that all of the stars in an individual GC formed at the same time (more than 10 billion years ago), with the same initial chemical composition. However, based largely on data obtained with the Hubble Space Telescope, observers have established that GCs harbour multiple stellar populations, with one or more younger generations formed from gas that was expelled by the preceding generation(s). Analyses of their spectra indicates that the younger stars probably formed from the ejecta of rapidly rotating massive stars during their core hydrogen-burning phase or from the winds of intermediate-mass stars that burn hydrogen and helium in thin shells (so-called "asymptotic-giant-branch stars") - but our understanding of these phenomena is still very crude. The main goal of the proposed research is to develop a satisfactory model to explain the origin of multiple stellar populations in GCs. To accomplish this, we will generate numerical models to follow the evolution of first-generation (FG) massive stars and try to adjust mixing parameters so that the chemical composition of their ejecta agrees with the observed metal abundances of the subsequent generation(s). Mixing, which can be caused by convection, rotation, or other factors, is important because it redistributes the relative abundances of the isotopes of elements that participate in nuclear reactions. Wherever possible, the adopted mixing prescriptions in our simulations will be based on the results of multi-dimensional hydrodynamical models. The chemical composition of the ejecta from FG stars will be the starting point for models of the chemical and dynamical evolution of GCs that fully take into account the cooling flows that are produced by the material that is ejected by massive and intermediate-mass stars. While developing this model, we will continue to make detailed comparisons of predicted and observed colour-magnitude diagrams in order to constrain, in particular, the helium abundances in the different stellar populations, but also the abundance variations of the most important heavy elements (including O, Mg, and Si).