The chemical composition of omega centauri. I. Heavy-element abundances of seven warm giants

We derive abundances for a range of elements (O, Ca, Sc, Ti, Fe, Ni, Y, Ba, La, and Eu) in seven giant stars in the massive Galactic globular cluster ω Centauri. These giant stars have similar stellar parameters (T_eff=4000-4500 K and log g=0.5-1.5) but they sample metallicities from [Fe/H]=-2.2 up to -1.3. As in other metal-poor populations, we find that the α elements (O, Ca, and Ti) are overabundant relative to Fe, by +0.2-0.3 dex. The Ni/Fe and Sc/Fe ratios are indistinguishable from solar. As found by previous investigators, the s-process elements (Y, Ba, and La) show a dramatic increase in abundance with increasing [Fe/H]: the slope of the trend of [s/Fe] with [Fe/H] is approximately 3. For the first time, Eu abundances are determined in a number of ω Cen stars. We find a remarkable deficiency in [Eu/Fe] in most of the ω Cen stars when compared to field stars. Although Eu is usually identified with the r process, we argue that the Eu abundances in ω Cen can be explained largely as s process in origin. The implications for this type of chemical evolution in ω Cen, relative to most of the metal-poor populations studied previously, are discussed.