A systematic study of extremely metal-poor stars with SDSS/Subaru

Abstract

We have conducted follow-up high-resolution spectroscopy with the Subaru Telescope High Dispersion Spectrograph (HDS) of extremely metal-poor (EMP) star candidates discovered by the Sloan Digital Sky Survey (SDSS), in particular the sub-survey SEGUE, in order to determine accurate metallicities and elemental abundance ratios. Here we report first results for 115 objects, among which 70 stars have [Fe/H]< -3, doubling the known samples of such stars. We clearly confirm the rapid decrease of the number of stars below [Fe/H]= -3.5, although the low-metallicity cutoff is not as sharp as that found by previous studies based on medium-resolution spectra. We identified two cool (5000 K) main-sequence stars with [Fe/H]< -3, the first example of this class of stars studied based on high-resolution spectra. One of them exhibits an extremely large enhancement of r-process elements (e.g., [Eu/Fe]= +1.9). The metallicity of this object ([Fe/H] = -3.4) is the lowest yet found for such r-process-enhanced stars. Show more