1994PASP..106..835V

The abundances of the iron-peak elements Fe, Ni and Ti, the light metals Mg, Al and K and the s-process elements Rb, Y and Zr are determined for the heavy-element stars ROA 371, ROA 5293 and ROA 3812 in the globular cluster omega Centauri. ROA 3812 and ROA 5293 are classified as S stars while ROA 371 is classified as a K5 barium star. The metallicities of ROA 3812, 5293 and 371 are [Fe/H] = -0.7, -0.8 and -1.0 respectively. Thus, ROA 371 has a metallicity slightly higher than the red giants analyzed by Brown et al., 1991 and ROA 3812 and 5293 have metallicities near the upper end of the range for the cluster. All three stars show an excess of Al, which is common in red giants of omega Cen, and a mean excess of the s-process elements Rb, Y, and Zr of 1.4 dex. In addition, all three stars are too faint to by asymptotic giant branch (AGB) stars according to current theory. Hence, they were either formed with their present composition, as suggested by Lloyd Evans (1983a) or are binaries that have had their atmospheres polluted by a now defunct companion.

Based upon the dependence of [s/Fe] on [Fe/H] for various s-process elements over a range from [Fe/H] = -2.0 to [Fe/H] = -0.7, we show that the Lloyd Evans hypothesis is probably correct. This shows that mass-loss from AGB stars was contributing s-process elements to the intracluster gas for as much as 10^9 years before Type Ia SNe swept the cluster clear of gas and terminated star-formation.