Subaru/HDS studies of r-process elements in metal-poor stars from near UV-spectra

We present detailed abundance measurements of neutron-capture elements for very metal-poor stars. We obtained very high quality, near-UV spectra of four bright metal-poor stars using Subaru/HDS, and determined the abundances of many neutron-capture elements. We investigated the detailed abundance patterns of moderately neutron-capture-enhanced-stars and neutron-capture-poor-stars, and clarified the difference in the abundance patterns. We focus on the well-known metal-poor giant HD 122563, in which 19 neutron-capture elements including Nb, Mo, Ru, Pd, Ag, Pr, and Sm have been detected by the present work. Our new results clearly demonstrate that the elemental abundances more steeply decrease with increasing atomic number than those of the r-process component in solar-system material. Especially, the tendency that the abundance continuously decreases in the region of 38 <= Z <= 47 does not agree with any neutron-capture processes known (i.e., main s-process, weak s-process, or main r-process). Since the abundance pattern of such very metal-poor stars, in contrast to that of solar system material, is expected to be formed by a small number of nucleosynthesis events, another process yielding elements with 38 <= Z <= 47 would be required. This result will give a new, strong constraint on the modeling of the process that has provided light neutron-capture elements in the very early Galaxy.