HIGH-RESOLUTION CCD SPECTRA OF STARS IN GLOBULAR-CLUSTERS .8. THE SELF-ENRICHMENT HISTORY OF OMEGA-CENTAURI

We have analyzed six stars in Omega Cen whose [Fe/H] values range from - 1.25 to - 2.25. The abundances of ten elements from carbon to iron have been determined. Their wide range in metallicity allow us to trace the cluster's enrichment of various elements as the iron abundance increased from 1/180 solar to 1/20 solar. As in metal-poor field stars, [O/Fe] and [alpha/Fe] congruent-to + 0. 5 over the observed range of metallicity. Contrary to what is seen in field stars, but in agreement with data for other globular clusters, [Na/Fe] and [Al/Fe] congruent-to + 0. 5 over the observed range of [Fe/H]. Thus the odd-even effect in which Na and Al follow iron does not seem to exist in globular cluster stars. Carbon and nitrogen, taken together, show appreciable scatter for [Fe/H] between - 2.2 and - 1. 7 and then rise from about [C + N/Fe] = 0 at [Fe/H] = - 1. 7 to [C + N/Fe] = + 0. 8 at [Fe/H] = - 1. 3. Self-enrichment of omega Cen by WR stars, SN IIs, cool supergiant winds, AGB star winds, and SN la's is evaluated. It appears that most of the oxygen, alpha elements, and iron have been contributed by SN IIs while some C and N may have been contributed by WR stars. It is also likely that much Na and A1 was added by WR stars and M supergiants. Additional N was probably also injected by cool supergiants, which probably caused fluctuations in nitrogen abundance among stars of similar iron abundance in omega Cen (and many other globular clusters). Our study does not reach sufficiently metal-rich stars to investigate the contribution by mass loss from AGB stars, but Lloyd Evans has suggested that the most metal-rich stars in omega Cen were formed with excesses of s-process elements from AGB stars. When stars with [Fe/H] greater than or similar to - 1.0 are analyzed we will be able to tell if Fe from SN Ia's has been retained and incorporated into the most metal-rich cluster stars.