Search for Population III Stars and study on Stars in Down of the Universe

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This study started after the discovery a giant HE 0107-5240 with the iron abundance as small as 1/200,000 of the solar value and aimed at exploring the characteristics of surface abundances systematically and discussing the search for of the extremely metal-poor stars, formed during the formation of the Galaxy, and in particular, the population III stars that are the survivors of first generation stars made of matter completely devoid of metals. Through the 3 years research, we have made considerable progress in understanding the properties of these relics from the early Universe both theoretically and observationally from the viewpoints of their formation, evolution and nucleosynthesis and from the spectroscopic study.For the theoretical side, we find that in the extremely metal-poor stars, neutron capture reactions occur during the helium shell-flashes invoked by hydrogen mixing and yield not only the light elements from carbon, to sodium, magnesium, and aluminum but also, heavy s-pr … More ocess elements such as strontium as observed form extremely metal-poor stars.We also elucidate the peculiar aspects of their evolution and supernova nucleosynthesis of under the devoid of metals and demonstrate that supernova explosion can produce the yields of various ratio between carbon to iron including such a large ratio as observed if the effect of non-spherical-symmetry is taken into account. In addition it is shown that the enhancement of odd nuclei such as sodium and aluminum can be produced during the carbon-burning in extremely metal-poor massive stars. As for the star formation, we proposed a scenario to give birth to low-mass stars that can survive to date under with the effect of irradiation and supernova explosion of massive stars taken into account in the primordial clouds.For the observational research, we clarify the nature of extremely metal poor-stars as to the carbon-enrichment and also to the enhancement of neutron-capture elements on the basis of the spectroscopic data from Subaru HDS of the projects organized by participants of this research program. In particular, most remarkable is the discovery of sub-giant HE 1327-232 of the metal-poorest star known t data in collaboration with USA and German research teams. This is the second star below metallicity below 1/100,000 of solar value and along the theoretical development, may serve as a probe to stimulate future progress in the study of the early Universe. Less