利用星震学方法研究红巨星演化及其Bump处的特征

The red giant branch "Bump" is an intrinsic feature of the red giant branch (RGB) luminosity function of globular clusters. The Bump originates when the H-burning shell crosses the chemical discontinuity left over by the convective envelope soon after the first dredge-up. The identification of the RGB Bump location, as a standard candle, gives us a new and independent measure of the cluster and galaxy distance and age. At present, the determination of Bump location use the luminosity function of globular cluster red giants, which depend on the observation of a large sample of clusters. During the RGB Bump phase, a low-mass star crosses the same luminosity interval three times. Although they have different internal structures, we cannot distinguish them based on their surface properties, i.e., with classical methods like spectroscopy. For the stars on the Bump, they have the same luminosity and very similar temperatures. Asteroseismology is a powerful tool to probe the internal structure of stars, by making use of its natural eigenfrequencies, which depend on the physical properties of its interior. Detection of solar-like oscillations in red giants is also helpful in studying the features of internal structure and evolution for RGB Bump stars. So, using the asteroseismic frequencies and parameters, we will do the research as follows: (1)Detect the differences of internal structure for RGB Bump stars. We will use the periodicity in the small separations between modes of l = 0, 1 to probe the different positions of the HeII ionisation layer and the base of the convective envelope; (2) Determination of the RGB Bump location and comparison between results for asteroseismology and results for luminosity function of globular cluster red giants; (3)Determine the age of the RGB Bump stars, which is helpful to test cluster-membership of the stars and identify non-members in cluster; (4) It has been demonstrated that with the Kepler data the pure g-mode period spacing can be determined from the observed frequencies in data of sufficient quality. It allows us to finely study the features of internal structure and evolution for RGB Bump stars combining the Kepler asteroseismic data and LAMOST observations.

红巨星演化阶段经过第一次挖掘后光度会出现Bump现象。确定Bump处的位置并作为标准烛光，为我们确定遥远的星团乃至星系的距离以及年龄提供了途径。目前确定Bump处位置的方法是利用星团光度函数法，该方法依赖于对星团的大量样本的观测。近年来在红巨星中发现的类太阳振动，为探索红巨星的内部结构及其演化提供了一种有效工具。同时也为我们利用星震学方法详细研究红巨星Bump处的结构特征提供了前所未有的帮助。据此我们提出了以星震观测数据为模型限制，利用星震参量：（1）探测Bump处不同恒星的结构差异；（2）确定Bump处的位置并与星团光度函数方法得到的结果进行对比；（3）确定Bump处的恒星年龄，从而帮助Kepler卫星剔除所观测到的星团非成员星；（4）结合Kepler卫星观测到的红巨星振动模式和LAMOST观测到的海量恒星光谱数据，研究Bump处红巨星的内部精细结构及其演化特征。

红巨星分支阶段所出现的bump现象是恒星星团光度函数的显著特征。同时也是低质量红巨星演化阶段的主要演化特征。利用星震学方法，通过构造不同质量和不同金属丰度恒星模型来研究红巨星分支的bump特征及其红巨星内部结构，我们发现对于给定金属丰度的红巨星bump处的光度会随着恒星的质量增加近似单调增加，同时对于固定质量的红巨星bump处的光度会随着恒星的金属丰度的增加近似单调递减。最终可以判定红巨星bump处的特性与其内部氢元素的不连续性紧密相关。另外，综合利用星震学参量比如小的频率间隔等探测了红巨星内部结构，我们发现在氦II电离区会导致声速有个突变。因此对流包层底部可以被声速半径相对精确确定。并且我们发现另外一个星震学参量g模式周期间隔随着红巨星的演化的单调递减，因此利用其特性来研究红巨星bump，可以作为一个重要的物理量来判断红巨星的演化状态。