High-Resolution Spectroscopy of Metal-rich Giants in ω Centauri: First Indication of Type I[CLC]a[/CLC] Supernova Enrichment

We have obtained high-resolution, high S/N spectra for six red giants inomega Cen: three belonging to the recently discovered, metal-rich Red GiantBranch (RGB-a, Pancino et al. 2000) and three to the metal intermediatepopulation (RGB-MInt). Accurate Iron, Copper and alpha-elements (Ca and Si)abundances have been derived and discussed. In particular, we have obtained thefirst direct abundance determination based on high-resolution spectroscopy forthe RGB-a population, <[Fe/H]>=-0.60+/-0.15. Although this value is lower thanprevious estimates based on Calcium triplet measurements, we confirm that thispopulation is the most metal rich in omega Cen. In addition, we have found asignificant difference in the alpha-elements enhancement of the twopopulations. The three RGB-MInt stars have the expected overabundance, typicalof halo and globular clusters stars: <[alpha/Fe]>=+0.29+/-0.01. The three RGB-astars 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.(1991). The observational facts presented in this letter, if confirmed bylarger samples of giants, are the first indication that supernovae type Iaejecta have contaminated the medium from which the metal rich RGB-a stars haveformed. The implications for current scenarios on the formation and evolutionof omega Cen are briefly discussed.Comment: 7 pages, 1 figure, accepted for publication in ApJ Letter