Formation and dynamical evolution of multiple stellar generations in globular clusters
Formation and dynamical evolution of multiple stellar generations in globular clusters
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DOI：
10.1111/j.1365-2966.2008.13915.x
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发表时间：
2008-09
影响因子：
4.8
通讯作者：
A. D'ercole;E. Vesperini;F. D’Antona;S. McMillan;S. Recchi
中科院分区：
文献类型：
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作者：
A. D'ercole;E. Vesperini;F. D’Antona;S. McMillan;S. Recchi
我们研究了具有多个恒星世代的星系团的形成和动力学演化。观测研究发现，一些球状星团中存在着第二代（SG）恒星，这些恒星显示出化学异常，并且一定是由第一代（FG）星团恒星的包层中含有气体的物质形成的。我们研究SG的形成过程通过一维流体动力学模拟，从FG已经到位，并假设SG是由气体喷射的渐近巨星分支（AGB）的明星。这些气体以冷却流的形式聚集到星团核心，在那里形成SG星。从这个过程中出现的SG子系统最初强烈集中在集群的最里面的区域，其结构特性在很大程度上是独立的FG初始属性。我们还提出了一个模型，其中原始气体有助于SG形成的结果。在这个模型中，一个非常富氦的SG星群和一个中等氦富集的SG星群形成;由此产生的SG双峰氦分布类似于在NGC 2808中观察到的SG星。通过N体模拟，我们研究了两种群团簇的动力学演化和质量损失。在我们的模拟中，大部分FG星失去了早期的集群演化，由于扩展和剥离的集群外层造成早期的质量损失与FG SN喷出物。SG人口，最初集中在最里面的集群区域，在很大程度上是毫发无损的，这种早期的质量损失，这种早期的演变导致SG FG星的数量比的值与观测一致。我们还展示了可能的进化路线，导致大部分FG人口的损失，留下一个SG占主导地位的集群。随着星团的演化和两个种群的混合，SG星与FG星的局部比例，最初是半径的递减函数，在星团的内部趋于一个恒定值。在混合完成之前，该数量比的径向轮廓的特征在于平坦的内部部分和外部簇区域中的倾斜部分。
We study the formation and dynamical evolution of clusters with multiple stellar generations. Observational studies have found that some globular clusters host a population of second generation (SG) stars which show chemical anomalies and must have formed from gas containing matter processed in the envelopes of first generation (FG) cluster stars. We study the SG formation process by means of 1D hydrodynamical simulations, starting from a FG already in place and assuming that the SG is formed by the gas ejected by the Asymptotic Giant Branch (AGB) stars. This gas collects in a cooling flow into the cluster core, where it forms SG stars. The SG subsystem emerging from this process is initially strongly concentrated in the cluster innermost regions and its structural properties are largely independent of the FG initial properties. We also present the results of a model in which pristine gas contributes to the SG formation. In this model a very helium-rich SG population and one with a moderate helium enrichment form; the resulting SG bimodal helium distribution resembles that observed for SG stars in NGC 2808. By means of N-body simulations, we then study the two-population cluster dynamical evolution and mass loss. In our simulations, a large fraction of FG stars are lost early in the cluster evolution due to the expansion and stripping of the cluster outer layers resulting from early mass loss associated with FG SN ejecta. The SG population, initially concentrated in the innermost cluster regions, is largely unscathed by this early mass loss, and this early evolution leads to values of the number ratio of SG to FG stars consistent with observations. We also demonstrate possible evolutionary routes leading to the loss of most of the FG population, leaving an SG-dominated cluster. As the cluster evolves and the two populations mix, the local ratio of SG to FG stars, initially a decreasing function of radius, tends to a constant value in the inner parts of the cluster. Until mixing is complete, the radial profile of this number ratio is characterized by a flat inner part and a declining portion in the outer cluster regions.