Abstract

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 < [Fe/H] < -0.7 dex). In addition, we studied the α-element and CN/CH abundances. Stars of different metallicity groups not only show distinct ages (Hilker et al. 2004, A&A, 422, L9), but also different behaviours in their relative abundances. The α abundances increase smoothly with increasing metallicity resulting in a flat [α/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 ω Centauri within a more massive progenitor. © ESO 2006.