We present elemental abundances for eight unevolved extremely metal-poor stars with T _ { eff } > 5500 \mathrm { K } , among which seven have [ \mathrm { Fe / H } ] < -3.5 .
The sample is selected from the Sloan Digital Sky Survey / Sloan Extension for Galactic Understanding and Exploration ( SDSS/SEGUE ) , and our previous high-resolution spectroscopic follow-up with the Subaru Telescope ( Aoki et al . ) .
Several methods to derive stellar parameters are compared , and no significant offset in the derived parameters is found in most cases .
From an abundance analysis relative to the standard extremely metal-poor star G 64–12 , an average Li abundance for stars with [ \mathrm { Fe / H } ] < -3.5 is A ( \mathrm { Li } ) = 1.90 , with a standard deviation of \sigma = 0.10 dex .
This result confirms that lower Li abundances are found at lower metallicity , as suggested by previous studies , and demonstrates that the star-to-star scatter is small .
The small observed scatter could be a strong constraint on Li-depletion mechanisms proposed for explaining the low Li abundance at lower metallicity .
Our analysis for other elements obtained the following results : i ) A statistically significant scatter in [ \mathrm { X / Fe } ] for Na , Mg , Cr , Ti , Sr , and Ba , and an apparent bimodality in [ \mathrm { Na / Fe } ] with a separation of \sim 0.8 \mathrm { dex } , ii ) an absence of a sharp drop in the metallicity distribution , and iii ) the existence of a CEMP- s star at [ \mathrm { Fe / H } ] \simeq - 3.6 and possibly at [ \mathrm { Fe / H } ] \simeq - 4.0 , which may provide a constraint on the mixing efficiency of unevolved stars during their main-sequence phase .