Medium resolution spectroscopy in ω Centauri: abundances of 400 subgiant and turn-off region stars

Medium resolution spectra of more than 400 subgiant and turn-off region stars in ω Centauri were analysed. The observations were performed at the VLT/Paranal with FORS2/MXU. In order to determine the metallicities of the sample stars, we defined a set of line indices (mostly iron) adjusted to the resolution of our spectra. The indices as determined for ω Cen were then compared to line indices from stars in the chemically homogeneous globular cluster M 55, in addition to standard stars and synthetic spectra. The uncertainties in the derived metallicities are of the order of ±$0.2 {dex}$. Our study confirms the large variations in iron abundances found on the giant branch in earlier studies ($-2.2