2003ApJ...591..916O

Near-infrared spectra of 21 giants in ω Cen spanning the whole range of metallicities observed in this cluster are presented. This work is part of a coordinated photometric and spectroscopic campaign in the optical and the infrared, aimed at studying the complex stellar population of ω Cen and understanding its formation and chemical evolution. By analyzing the several CO and OH molecular bands and atomic lines in the spectra of the selected giants, metal abundances and abundance ratios have been obtained. The existence of three major metallicity regimes at [Fe/H]~-1.6 and -1.2 and [Fe/H]≤-0.5 has been confirmed. The most metal-rich stars in our sample show a lower (if any) α enhancement when compared with the more metal-poor components, suggesting that they should have formed in a medium significantly polluted by Type Ia supernova ejecta. Isotopic carbon abundances have been also inferred, providing an average ¹²C/¹³C≃4, which clearly indicates that extra-mixing processes occurred in the stellar interiors during the ascent on the red giant branch.