Near-infrared spectra of 21 giants in \omega Centauri , 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 in the infrared , aimed at studying the complex stellar population of \omega Centauri 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 ] \approx -1.6 , \approx -1.2 and [ Fe/H ] \leq - 0.5 has been confirmed .
The most metal-rich stars in our sample show a lower ( if any ) \alpha -enhancement when compared to 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 ^ { 13 } C/ ^ { 12 } C \simeq 4 , which clearly indicates that extra-mixing processes occurred in the stellar interiors during the ascent on the Red Giant Branch .