Detailed chemical abundance analysis of metal-poor turn-off stars: One with [Fe/H] < –4 and three with [Fe/H] < –3

Abstract 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 $[{\rm Fe/H}] = -3.3$ to $-4.4$ dex. For one of the stars, we measure the lithium abundance $\log ({\rm Li/H})$ = 2.1 $\pm$ 0.2. Two stars of the sample have low $[\alpha /{\rm Fe}]$ abundances. The most metal-poor star of the sample with $[{\rm Fe/H}] = -4.42$ dex has a high $[{\rm Sr/Fe}]$ abundance ratio $[{\rm Sr/Fe}]$ = 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.