[论文解读] Effects of Helium Enrichment in Globular Clusters I.Theoretical Plane with PGPUC stellar evolution code

A. A. R. Valcarce, M. Catelan+1 more|Aug 25, 2012

Stellar, planetary, and galactic studies参考文献 143被引用 33

一句话总结

本文使用PGPUC恒星演化代码，系统性地改变氦丰度（Y = 0.230–0.370）和金属度（Z = 1.60×10⁻⁴ 至 1.57×10⁻²），提出了球状星团的理论演化轨迹、等龄线以及零龄水平分支（ZAHB）位置。结果表明，氦丰度的增加会在主序星、次巨星分支和红巨星分支上产生可观测到的分裂现象，对关键演化阶段的光度和温度产生显著影响，为从颜色-星等图中推断初始氦丰度提供了理论框架。

ABSTRACT

Recently, the study of globular cluster (GC) CMDs has shown that some of them harbor multiple populations with different chemical compositions and/or ages. In the first case, the most common candidate is a spread in the initial helium abundance, but this quantity is difficult to determine spectroscopically due to the fact that helium absorption lines are not present in cooler stars, whereas for hotter GC stars gravitational settling of helium becomes important. As a consequence, indirect methods to determine the initial Y among populations are necessary. For that reason, in this series of papers, we investigate the effects of a Y enrichment in populations covering the range of GC metallicities. In this first paper, we present the theoretical evolutionary tracks, isochrones, and ZAHB loci calculated with the Princeton-Goddard-PUC (PGPUC) stellar evolutionary code, which has been updated with the most recent input physics and compared with other theoretical databases. The chemical composition grid covers 9 Z ranging from Z=1.60x10^-4 to 1.57x10^-2, 7 Y from Y=0.230 to 0.370, and an alpha-element enhancement of [alpha/Fe]=0.3. The effects of different helium abundances that can be observed in isochrones are: splits in the MS, differences in the L and Teff of the turn off point, splits in the SGB being more prominent for lower ages or higher metallicities, splits in the lower red giant branch being more prominent for higher ages or higher metallicities, differences in L of the RGB bump (with small changes in Teff), and differences in L at the RGB tip. At the ZAHB, when Y is increased there is an increase of L for low Teff, which is affected in different degrees depending on the age of the GC being studied. Finally, the ZAHB morphology distribution depending on the age explains how for higher GC metallicities a population with higher helium abundance could be hidden at the red ZAHB locus.

研究动机与目标

• 在广泛的金属度和氦丰度范围内，建模氦富集对球状星团中恒星演化的影响。
• 为解释具有多重星族的球状星团观测颜色-星等图（CMD）提供理论基础。
• 解决由于较冷恒星的光谱观测限制，难以直接推断初始氦丰度的挑战。
• 开发一套全面的演化轨迹和等龄线网格，用于星族合成及星团年龄/金属度分析。
• 解释氦丰度变化如何解释水平分支及其他演化阶段观测到的形态差异。

提出的方法

• 采用更新输入物理参数的PGPUC恒星演化代码，包括灰消光表、状态方程和能量生成速率。
• 生成了7个质量（0.5–1.1 M☉）、9个金属度（Z = 1.60×10⁻⁴ 至 1.57×10⁻²）和7个氦丰度（Y = 0.230 至 0.370）的演化轨迹网格，[α/Fe] = 0.3。
• 在特定演化阶段（EEP）之间使用线性插值，构建适用于任意质量、金属度和氦丰度的连续轨迹。
• 应用等效ZAHB点（EZAHBPs）插值ZAHB位置，区分具有和不具有对流包层的恒星。
• 通过组合给定年龄下的插值轨迹计算等龄线，便于与观测CMD进行比较。
• 通过将插值轨迹和ZAHB位置与原始模型对比，验证了插值方法的准确性，确认其在整个参数空间内均有效。

实验结果

研究问题

• RQ1 初始氦丰度增加如何影响球状星团颜色-星等图中主序星的形态？
• RQ2 氦富集对等龄线中主序转折点的光度和有效温度有何可观测影响？
• RQ3 次巨星分支和红巨星分支的分裂如何依赖于氦丰度、金属度和星团年龄？
• RQ4 氦丰度变化如何影响红巨星分支的“驼峰”和“尖端”位置及光度？
• RQ5 氦富集如何改变零龄水平分支（ZAHB）形态，尤其与星团金属度和年龄的关系如何？

主要发现

• 氦富集在主序星上产生清晰的分裂，尤其在较高金属度和较年轻星团中更为显著。
• 主序转折点的光度随氦丰度增加而上升，其有效温度则呈现中等程度的偏移，具体取决于金属度和年龄。
• 次巨星分支的分裂在年龄较低或金属度较高时更为明显，且在较高氦丰度下效应被进一步放大。
• 红巨星分支的分裂在较高金属度或较老年龄下更易观测，且光度差异随氦含量增加而增大。
• 红巨星分支“驼峰”处的光度随氦丰度增加而上升，而其有效温度几乎保持不变，表明“驼峰”位置沿红巨星分支发生移动。
• 在红巨星分支“尖端”处，光度随氦丰度增加而上升，且ZAHB形态发生改变，使得高氦丰度种群更倾向于聚集在ZAHB的红色端，尤其在金属丰富的星团中更为显著。