A Research on the Evolution of Population III Stars and on the Method of Their Search

这项研究的目的是在理论上和观测上讨论寻找星族III恒星的可能性，这些恒星在我们的宇宙中最早诞生，但尚未被发现。这些恒星是由经过大爆炸核合成处理的原始物质组成的，因此完全没有碳和较重的金属元素。只有质量不超过0.8个太阳质量的低质量恒星，才能作为核燃烧恒星存活至今。众所周知，这种低质量的星族III恒星会变成含氮量增加的碳星。我们扩展了已有的初始质量和金属丰度参数区域的计算，给出了碳星演化的概貌：金属丰度极低的碳星与富金属对应的碳星的形成机制不同，这与[Fe/H]<-2.5的低金属丰度时碳星数量大幅增加的观测结果一致。在这些理论结果的基础上，我们提出了一种新的寻找星族III星的方法，通过碳和氮的分子谱来探测富氮碳星，并进一步用氮碳比来区分它们。为了验证这种新方法的有效性，我们在东京大学木户天文台使用施密特望远镜和2KCCD进行了测试观测。

The purpose of this research is to discuss theoretically and observationally the possibility of searching for Population III stars, which have been born first in our Universe and are yet to be discovered. These stars have been composed of the primordial matter processed by the Big-Bang nucleosynthesis, and hence, totally devoid of carbon and heavier metal elements. It is only low-mass stars not heavier than 0.8 solar masses that survive to date as nuclear burning stars.It has been known that such low-mass, population III stars become carbon stars with enhanced nitrogen. We have extended the exsistent computation in the parameter region of initial mass and metallicity to present a general picture concerning the evolution to carbon stars: The formation mechanism of carbon stars with extremely low metallicity is different from those of metal-richer correspondences, which gives a result consistent with the observation that a fraction of carbon stars increases greatly for low-metalliclty of [Fe/H]<-2.5.On the basis of these theoretical results, we propose a new method of search for population III stars to detect nitrogen-rich carbon stars by aid of molecular lines of carbon and nitrogen and further to discriminate them with the nitrogen/carbon ratio.In order to check the validity of this new method, we made test observations by using Schmidt telescope with 2KCCD at Kiso observatory, the Uniersity of Tokyo.