银河系核纪年研究进展

Progress in Study on the Age-Dating of the Galaxy by Nucleo-Cosmochronology

确定银河系的年龄是现代天体物理学的一项基本任务。其方法之一是核纪年法,即通过恒星中某一长寿命放射性元素的丰度随时间的变化来确定恒星的年龄,并以此作为银河系年龄的下限,其中目前的观测丰度来自恒星的光谱分析,恒星形成时的初始丰度来自理论模型的预言。这种方法最初是利用元素对Th/Nd来确定G矮星的年龄,近年来开始利用元素对Th/Eu和U/Th来确定晕族场星和球状星团内恒星的年龄。简要介绍了核纪年法确定银河系年龄的原理,回顾了恒星中Th和U的观测研究,其中着重介绍了极贫金属星的研究。详细讨论了用核纪年法估计银河系年龄的不确定性。作为与核纪年法的比较,简单介绍了确定银河系年龄的其他方法。提出了今后需要进一步研究的几项工作。

The dating of the Galaxy is a basic task in modern astrophysics. One method of estimating the age of the Galaxy is nucleo-cosmochronology. The idea of such a chronometry is to determine the age of individual stars by comparing the change of the abundance ratio with time between a long-lived radioactive element and a stable element in the oldest stars. The present ratio of the element pairs, derived from the stellar spectrum, is compared with the initial abundance ratio at the time of the star formation, predicted from a theoretical model. At first, this age-dating approach was used to estimate the age of G-dwarf stars with Th/Nd chronometry. Recently, it has been used to evaluate the age of halo field stars and globular cluster stars with the Th/Eu and U/Th chronometers. We first briefly introduce the principle of this approach, and then review the history of Th and U abundance study in stars, with emphasis on the extremely metal-poor halo stars. The reliability of this technique and the uncertainties in the age estimate are well discussed. As comparison, other methods of dating the Galaxy are briefly introduced. Some further investigations are suggested at last.