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

This paper reports detailed abundance analyses for four extremelymetal-poor (XMP) giant stars with [Fe/H]<-3.8, based onhigh-resolution, high-S/N spectra from the ESO VLT (Kueyen/UVES) and LTEmodel atmosphere calculations. The derived [alpha /Fe] ratios in oursample exhibit a small dispersion, confirming previous findings in theliterature, i.e. a constant overabundance of the alpha -elements with avery small (if any) dependence on [Fe/H]. In particular, the very smallscatter we determine for [Si/Fe] suggests that this element shows aconstant overabundance at very low metallicity, a conclusion which couldnot have been derived from the widely scattered [Si/Fe] values reportedin the literature for less metal-poor stars. For the iron-peak elements,our precise abundances for the four XMP stars in our sample confirm thedecreasing trend of Cr and Mn with decreasing [Fe/H], as well as theincreasing trend for Co and the absence of any trend for Sc and Ni. Incontrast to the significant spread of the ratios [Sr/Fe] and [Ba/Fe], wefind [Sr/Ba] in our sample to be roughly solar, with a much lowerdispersion than previously found for stars in the range -3.5 < [Fe/H]< -2.5.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (Large Programme ID 165.N-0276(A)).The complete version of Table 5 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp:/ /cdsweb.u-strasbg.fr/cgi-bin/qcat?J /A+A/403/1105