Study on a formation and chemical evolution of the Galaxy based on spectroscopic observations of metal-poor stars

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1. Our spectroscopic studies of alpha- elements, O, Si, S, and Ca, in metal-poor stars were carried out to investigate their behaviors against metalicity [Fe/H]. As for O, the abundance derived from the triplet lines was found to show a flat trend. The behaviors of Si and Ca show a correlated trend and were found to be classified into three groups based on dynamical properties. As for S, our result of an increasing trend with decreasing [Fe/H] in a paper of 2002 was first derived from a large sample, and inspired other groups as well as us to make further studies of S behavior. Our result obtained in a paper of 2005 confirmed that S shows a flat trend in -4 <[Fe/H]<-1, which may be explained by chemical evolution models based on standard supernovae nucleosynthesis.2. Other light elements of Li, Na, and K were also studied and found to provide the new results being different from the previous ones.3. The behavior of Zinc in very metal-poor stars was found to show an increasing trend below [Fe/H]<-3, which gives an important constraints to supernova models.4. The behaviors of r-process elements were found to show a large star-to-star scatter and to be explained by r-process nucleosynthesis. However, an unknown process was needed for an explanation of behaviors of Sr and Ba.5. The most iron-deficient star was discovered. The star may be a population III, or even if not, it gives strong constraints to a nature of the first generation stars in the beginning of the Galaxy.