Sulphur and iron abundances in halo stars

From equivalent widths of the S i lines at 8694 Å, Israelian & Rebolo (2001) and Takada-Hidai et al. (2002) have derived a surprisingly high sulphur-to-iron ratio ([S/Fe] ≃ 0.5 to 0.7) in six halo stars with [Fe/H] ≃ −2.0 suggesting perhaps that hypernovae made a significant contribution to the formation of elements in the early Galaxy. To investigate this problem we have used high-resolution spectra obtained with the ESO VLT/UVES spectrograph to determine the S/Fe ratio in 19 mainsequence and subgiant stars ranging in [Fe/H] from −3.2 to −0.7. The sulphur abundances are determined from S i lines at 8694 Å and 9212 9237 Å, and the iron abundances from about 20 Fe ii lines. S/Fe ratios as derived from 1D model atmospheres are presented and possible 3D effects are discussed. The initial results from our survey do not confirm the high values of [S/Fe] quoted above; instead we find that the ratio [S/Fe] remains constant at about 0.35 dex for metallicities −3 < [Fe/H] < −1.