  Medium resolution spectra of more than 400 subgiant and turn-off region stars
in Omega 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 Omega Centauri were
then compared to line indices from stars in the chemically homogeneous globular
cluster M55, 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 <[Fe/H]< -0.7 dex). In addition, we studied the
alpha-element and CN/CH abundances. Stars of different metallicity groups not
only show distinct ages, but also different behaviours in their relative
abundances. The alpha abundances increase smoothly with increasing metallicity
resulting in a flat [alpha/Fe] ratio over the whole observed metallicity range.
The combined CN+CH abundance increases smoothly with increasing iron abundance.
The most metal-rich stars are CN-enriched. In a CN vs. CH plot, though, the
individual abundances divide into CN- and CH-rich branches. The large abundance
variations observed in our sample of (unevolved) subgiant branch stars most
probably have their origin in the pre-enriched material rather than in internal
mixing effects. Together with the age spread of the different sub-populations,
our findings favour the formation of Omega Centauri within a more massive
progenitor.
