(Proposal ID) S13A-027

(PI) Aoki, Wako

(Proposal Title)
Search for ultra metal-poor stars from SDSS/SEGUE with Subaru/HDS

(Abstract)
We propose to obtain R~ 30000 spectra of candidate ultra metal-poor (UMP) 
stars ([Fe/H] <-4) from SDSS/SEGUE using HDS, to investigate the formation of 
low-mass stars in such low-metallicity environments. While all three stars 
previously known to have [Fe/H]  < -4 are carbon-rich (and oxygen-rich) 
objects, and the total metallicity is not as low as the iron abundance, the 
recently discovered UMP star SDSS J102915+172927 ([Fe/H] ~ -5) exhibits no 
excess of carbon (Caffau et al. 2011), suggesting that low-mass stars can be 
formed at lower metallicity than previously thought. Based on our previous 
experience with follow-up high-resolution spectroscopy of SDSS targets using 
Subaru/HDS, we are now able to efficiently select UMP star candidates from 
this database.  We extend our follow-up spectroscopy to fainter objects to 
estimate the metallicity and abundances of important elements (e.g. C, Mg, Ca) 
and to constrain the frequency of low-mass stars with [Fe/H] < -4 (and with 
normal C abundance ratios), which provides a unique opportunity to examine 
models of low-mass star formation depending on metallicity.