First stars.: III.: A detailed elemental abundance study of four extremely metal-poor giant stars

This paper reports detailed abundance analyses for four extremely metal-poor (XMP) giant stars with [Fe/H] < -3.8, based on high-resolution, high-S/N spectra from the ESO VLT (Kueyen/UVES) and LTE model atmosphere calculations. The derived [α/Fe] ratios in our sample exhibit a small dispersion, confirming previous findings in the literature, i.e. a constant overabundance of the α-elements with a very small (if any) dependence on [Fe/H]. In particular, the very small scatter we determine for [Si/Fe] suggests that this element shows a constant overabundance at very low metallicity, a conclusion which could not have been derived from the widely scattered [Si/Fe] values reported in the literature for less metal-poor stars. For the iron-peak elements, our precise abundances for the four XMP stars in our sample confirm the decreasing trend of Cr and Mn with decreasing [Fe/H], as well as the increasing trend for Co and the absence of any trend for Sc and Ni. In contrast to the significant spread of the ratios [Sr/Fe] and [Ba/Fe], we find[Sr/Ba] in our sample to be roughly solar, with a much lower dispersion than previously found for stars in the range -3.5 < [Fe/H] < -2.5.