Studies of the Multiple Stellar Populations in Globular Clusters

球状星团（GCs）是近乎球形的聚集体，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，根据哈勃太空望远镜获得的大部分数据，观测者已经确定，GCs拥有多个恒星群，其中一个或多个年轻一代是由前一代恒星排出的气体形成的。对其光谱的分析表明，年轻的恒星可能是由快速旋转的大质量恒星在其核心氢燃烧阶段的喷出物形成的，或者是由在薄壳中燃烧氢和氦的中等质量恒星（所谓的“渐近巨星分支恒星”）的风形成的——但我们对这些现象的理解仍然非常粗糙。提出研究的主要目标是建立一个令人满意的模型来解释GCs中多个恒星群的起源。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，使它们喷出物的化学成分与观测到的下一代(s)金属丰度一致。混合可能是由对流、旋转或其他因素引起的，它很重要，因为它重新分配了参与核反应的元素同位素的相对丰度。在可能的情况下，我们的模拟中采用的混合处方将基于多维水动力模型的结果。FG恒星喷出物的化学成分将成为GCs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，特别是不同恒星群中的氦丰度，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).