THE CHEMICAL-COMPOSITION OF OMEGA-CENTAURI .1. HEAVY-ELEMENT ABUNDANCES OF 7 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 globula r cluster omega Centauri. These giant stars have similar stellar param eters (T-eff = 4000-4500 K and log g = 0.5-1.5) but they sample metall icities from [Fe/H] -2.2 up to -1.3. As in other metal-poor population s, we find that the ct elements (O, Ca, and Ti) are overabundant relat ive to Fe, by + 0.2-0.3 dex. The Ni/Fe and Sc/Fe ratios are indistingu ishable from solar. As found by previous investigators, the s-process elements (Y, Ba, and La) show a dramatic increase in abundance with in creasing [Fe/H]: the slope of the trend of [s/Fe] with [Fe/H] is appro ximately 3. For the first time, Eu abundances are determined in a numb er of omega Cen stars. We fmd a remarkable deficiency in [Eu/Fe] in mo st of the omega Cen stars when compared to field stars. Although Eu is usually identified with the r process, we argue that the Eu abundance s in omega Cen can be explained largely as s process in origin. The im plications for this type of chemical evolution in omega Cen, relative to most of the metal-poor populations studied previously, are discusse d. (C) 1995 American Astronomical Society.