利用星震学方法研究星团的年龄

Determining the ages of clusters is crucial for studies of stellar and galactic evolution. Open clusters provide unique opportunities in astrophysics. Asteroseismology of stars in clusters has been a long-sought goal because the assumption of a common age, distance, and initial chemical composition allows strong tests of the theory of stellar evolution. There are many ways to determine the ages of clusters. The way this is normally done is to fit theoretical isochrones. Isochrones fitting works with the stellar models over the full extent of the HRD but has only the simple surface descriptors (Lbol and Teff ) to work with. Using this method, the ages of errors are large. Astereoseismology, the study of stellar oscillations, enable us to probe the interiors of stars with great precision. It allows stringent tests of stellar models and can provide accurate fundamental properties of individual stars, including the ages of stars. When the signal-to-noise ratios in the seismic data are insufficient to allow robust extraction of individual oscillation frequencies, the initial seismic data also provide the seismic parameters, i.e. the so-called large frequency separation, the small frequency separation and the frequency of maximum oscillations power. In 2009 March, NASA launched the Kepler satellite. The Kepler field-of-view contains of four open clusters, including NGC6791,NGC6811,NGC6866 and NGC6819. Kepler is detecting the solar-like oscillations in the cluster stars, which provide lots of high precision asteroseismic data to study clusters ages. These data, combined with conventional observations of effective temperature, Teff, and metallicity, [Fe/H], are being used to determine the ages of the stars. So in this item, we will provide two methods to get precise value of cluster ages. The first method, we will use combinations of seismic parameters and conventional observations to fit theoretical isochrones again, not just use conventional observations. The second method is Yale-Birmingham Pipeline. We will study the ages of the individual stars which has different evolutionary states, and study which parameters are most important for age. We improve the precision of results within 10%, which is important for study the evolution of the chemical gradients in the Milky Way and the age-metallicity relation for the thin disc stars of the Galaxy.

星团的年龄对理解恒星与星系的演化至关重要。传统等时线拟合方法求解星团年龄，仅依赖于表征恒星表面属性的物理参量，导致星团年龄具有很大的误差。星震学作为有效探测恒星内部的工具，能利用观测频率限制恒星模型来获得较精确的恒星年龄，并提供星震参量作为探测工具。目前，Kepler卫星正对其视场中四个恒星星团：NGC6791、NGC6811、NGC6866、NGC6819进行星震观测，并提供了大量高精度的观测数据。据此，本项目拟采用两种方法求解星团年龄：（1）把星震参量与非星震参量相结合，改进传统等时线拟合方法；（2）利用以最大似然函数为技术核心的Yale-Birmingham Pipeline对星团中各个演化阶段的成员星年龄进行精确求解，并确定影响年龄的重要物理参量，把误差降低到10%以内。精确获得星团年龄对研究银河系元素丰度梯度的演化、银盘恒星的年龄- - 金属丰度关系等问题具有重要的科学意义。

恒星年龄是天体物理研究中最重要的一个物理量，它经常用于研究星团、星系和宇宙学等领域。我们结合以前求解恒星年龄的传统方法，利用最新的星震学方法来研究恒星年龄，并依靠KEPLER卫星对星团恒星的星震观测数据做限制，求解了大量恒星模型从而获得了恒星年龄。从中我们发现最大振动能谱频率这个物理量和恒星年龄有着密切的联系，可以作为恒星年龄的探针。我们求解了最大振动能谱频率和恒星年龄从主序星阶段到红巨星演化阶段两者的关系。该关系可以依靠星震观测来快速求解恒星和星团年龄，为理论研究和观测研究提供了另外一个可比较的平台。