测定天琴RR变星金属丰度的方法

The Methods of Determining [Fe/H] for RR Lyrae Stars

银晕中天琴RR变星金属丰度的测定对于研究银河系晕的形成和演化具有重要的意义．在当前的技术条件下,高分辨光谱的方法难以测量银晕中较暗的天琴RR变星的金属丰度．高分辨光谱测定金属丰度的方法对恒星大气模型的依赖性较高,而恒星脉动引起的复杂大气状况对于建立正确的大气模型本身就是挑战．△S光谱方法、Caby测光方法和光变曲线的参数方法则弥补了高分辨光谱方法的不足,将能测量更远距离上的天琴RR变星的金属丰度．着重介绍了这3种方法发展的历史、具体的观测流程以及需要注意的问题。通过比较这3种方法的优劣,为实际观测时方法的选用提供借鉴。

The fact, that the absolute light of RR Lyrae star is affected by its metal abundance, requires us to determine its metal abundance when it is used as accurate indicator for distance. While in principle the most fundamental way to determine the abundance of RR Lyrae star is via high dispersion spectroscopy, in practice this method is limited to the brightest stars in the nearest distance to the Sun. This restriction has led over the years to the development of a large number of techniques that measure in various ways such as AS method, Caby photometry and Fourier parameters of light curve. This paper primarily reviewed the above three methods, i.e. △S, Caby and Fourier parameters. The AS method is using the △S metallicitiy index, which is calculated from equivalent width of lines Hδ, Hγ Hβand Ca II K in low resolution spectrum, to determine the abundance of RR Lyrae stars. The photometric method, Caby, employs the correlation between hk index and metal abundance to calculate metal abundance for RR Lyrae stars. Here hk index is defined as hk = （Ca - b） - （b - y）. The method of Fourier parameters is using the experiential relation between the parameters of light curve of RR Lyrae star and metal abundance to determine its metallicity. Although these three methods are all using the experiential relation between their spectral indices and metal abundance, they have different accuracy and condition for usage. The detailed discussion and comparison on these methods are also given in this paper to provide reference for the observer to make a choice based on their advantages and disadvantages.