Program: GN-2016A-Q-17

We propose to obtain with GRACES high-resolution (R = 40,000) spectra of candidate ultra metal-poor (UMP) stars ([Fe/H] < -4) selected from Sloan Digital Sky Survey (SDSS) stellar spectra, in order to derive detailed abundances of 12 elements. The estimated elemental abundances will be used to investigate the formation of low-mass stars in such low-metallicity environments. There have been only 20 UMP stars identified thus far. Considering the ~ 200 Extremely Metal-Poor (EMP, [Fe/H] < -3.0) stars discovered, the lack of UMP objects is very remarkable, suggesting the existence of some critical metallicity for low-mass star formation. Five Hyper Metal-Poor (HMP, [Fe/H] < -5) stars found are carbon-rich (and oxygen-rich) objects, so the total metallicity is not as low as the iron abundance. Interestingly, the recently discovered star SDSS J102915+172927 ([Fe/H] ~ -5) exhibits no excess of carbon, implying that low-mass stars can be formed at lower metallicity than previously thought. This suggests the need to increase the sample of known UMP/HMP stars. Since SDSS provides a large sample (about 2500) of candidate EMP stars, we propose to carry out follow-up spectroscopy with higher resolution to identify many new UMP/HMP stars.