Chemical abundance patterns of extremely metal-poor stars

We have obtained high resolution (R =60,000), high quality (S/N >100) spectroscopy using Subaru/HDS for ∼20 candidate extremely metal-poor stars ([Fe/H]< -3) to determine their chemical abundance patterns.In the observing program we found HE1327-2326, which has [Fe/H]= -5.4, the lowest Fe abundance known in normal stars.This star shows extremely large excesses of C and N, and also excesses of other light elements (e.g.O, Na).The low iron abundance and the peculiar abundance pattern provide a signature of the nucleosynthesis by the first generation stars in the Universe.We also present the preliminary results for other stars: (1) The iron abundances of our sample, except for HE 1327-2326, are higher than [Fe/H]= -4.There is a gap of iron abundance between [Fe/H]= -4 and -5.This would indicate that the nucleosynthesis, or formation processes that are responsible for the stars with [Fe/H]< -5 and others are quite different.(2) Six stars of our sample, including HE1327-2326, show clear excesses of carbon.Only two of them show excesses of the heavy neutron-capture element Ba, suggesting a large contribution of AGB nucleosynthesis.The origin of carbon-excesses in other stars are still unclear, but the existence of these stars is a remarkable feature only found at the extremely low metallicity.