The chemical composition of Omega Centauri .1. 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 omega 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 ct 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 omega Cen stars. We fmd a remarkable deficiency in [Eu/Fe] in most of the omega Cen stars when compared to field stars. Although Eu is usually identified with the r process, we argue that the Eu abundances in omega Cen can be explained largely as s process in origin. The implications for this type of chemical evolution in omega Cen, relative to most of the metal-poor populations studied previously, are discussed. (C) 1995 American Astronomical Society.