Detailed Chemical Abundance Analysis of Metal-Poor Turn-Off Stars: One with [Fe/H] < -4 and Three with [Fe/H] < -3

We present the analysis of four new extremely metal poor turn-off stars thanks to high-resolution spectra obtained with the Subaru/HDS spectrograph. We determined the abundances and upper limits of Li, C, Mg, Ca, Sr, and Ba. Metallicities range from [Fe/H] = -3.3 to -4.4 dex. For one of the stars, we measure the lithium abundance Log(Li/H) = 2.1 {\pm} 0.2. Two stars of the sample have low [α/Fe] abundances. The most metal-poor star of the sample with [Fe/H] = -4.42 dex has a high [Sr/Fe] abundance ratio [Sr/Fe] = 0.9 dex, a high value also found in HE 1327-2326. This star is the second-most iron-poor star observed with Subaru telescope, suggesting that more ultra metal-poor stars could be discovered using high-resolution spectrographs in the Northern Hemisphere.