High-resolution Spectroscopy of Extremely Metal-poor Stars from SDSS/SEGUE. III. Unevolved Stars with [Fe/H] ? ?3.5*

摘要

We present elemental abundances for eight unevolved extremely metal-poor (EMP) stars with , among which seven have . 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. 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 EMP star G64–12, an average Li abundance for stars with is , with a standard deviation of 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 for Na, Mg, Cr, Ti, Sr, and Ba, and an apparent bimodality in with a separation of , (ii) an absence of a sharp drop in the metallicity distribution, and (iii) the existence of a CEMP-s star at and possibly at , which may provide a constraint on the mixing efficiency of unevolved stars during their main-sequence phase.