High-resolution spectroscopy of metal-rich giants in ω Centauri:: First indication of type Ia supernova enrichment

We have obtained high-resolution, high signal-to-noise ratio spectra for six red giants in omega Centauri: three belong to the recently discovered metal-rich red giant branch (RGB-a; as defined by Pancino et al.) and three to the metal-intermediate population (RGB-MInt). Accurate iron, copper, and alpha-element (Ca and Si) abundances have been derived and discussed. In particular, we have obtained the first direct abundance determination based on high-resolution spectroscopy for the RGB-a population, {Fe/H] = 0.60 +/- 0.15. Although this value is lower than previous estimates based on calcium triplet measurements, we confirm that this population is the most metal-rich in omega Cen. In addition, we have found a significant difference in the alpha-element enhancement of the two populations. The three RGB-MInt stars have the expected overabundance, typical of halo and globular cluster stars: [alpha/Fe] = 0.29 +/- 0.01. The three RGB-a stars show, instead, a significantly lower alpha-enhancement: [alpha/Fe] = 0.10 +/- 0.04. We have also detected an increasing trend of [Cu/Fe] with metallicity, similar to the one observed for field stars by Sneden et al. The observational facts presented in this Letter, if confirmed by larger samples of giants, are the first indication that supernovae Type Ia ejecta have contaminated the medium from which the metal-rich RGB-a stars have formed. The implications for current scenarios on the formation and evolution of omega Cen are briefly discussed.