STUDY OF THE UV-BRIGHT STARS IN OMEGA-CEN .2. ABUNDANCES AND EVIDENCE FOR EVOLUTIONARY MIXING

Spectroscopic observations of UV-bright stars were acquired and analyzed in the framework of recent theories concerning post-horizontal branch stellar evolution. The abundances of 26 elements were calculated for 5 UV-bright stars in the omega Cen from high resolution CCD spectra. In contrast to the giants, the most luminous UV-bright stars in omega Cen display significant enhancements of the CNO and s-process elements. Among the UV-bright stars, the CNO enhancements appear abruptly at about log L/L(circle dot)=2.7. The CNO and s-process enhancements are correlated in general in omega Cen, implying that these two sets of elements are mixed into the atmosphere together. The CNO and s-process element abundances of the most luminous UV-bright stars are about 1 dex greater than those of the typical red giant in omega Cen. The UV-bright stars also show an apparent deficiency of Al; the most likely cause of this is mass loss combined with a small NLTE effect. An analysis of the s-process element abundances demonstrates that the average time integrated neutron flux, tau(0), for the most luminous UV-bright stars in omega Cen, V1 and ROA 24, is about 0.6 mb(-1). The amount of s-process element enhancement observed in V1 and ROA 24 requires that about 0.1% of their atmospheres be composed of material mixed to the surface from the s-processed region. The observed s-process abundance patterns are also consistent with theoretical thermally pulsing AGB models.