1994PASP..106..201G

Photometric and spectroscopic observations of UV-bright stars were acquired and analyzed in the framework of recent theories concerning post-horizontal branch stellar evolution. Photometric CCD B and V observations of V1, V9, V43, V48, and V92 in omega Cen and of the field cepheid, ST Pup, with a period of about 19 days, are used to calculate their masses, temperatures, and surface gravities. The resultant masses range from 0.51 to 0.67 solar masses and are consistent with theoretical expectations. The new observations are also combined with historical data to derive more accurate (and precise) period derivatives. The most luminous cepheids are found to experience large and sudden period changes, while the fainter ones exhibit constant period derivatives.

The abundances of 26 elements were calculated for 5 UV-bright stars in omega Cen and ST Pup from high resolution CCD spectra. Compared to the giants, the most luminous UV-bright stars in omega Cen display significant enhancements of the CNO and s-process elements. The CNO enhancements correlate with s-process enhancemenets in general in omega Cen, implying that these two sets of elements are mixed with the atmosphere together. 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, t_0, for the most luminous UV-bright stars in omega Cen, V1, and ROA 24, is about 0.6 mbarn^-1. The observed s-process abundance patterns are also consistent with theoretical AGB models. The amount of s-process element enhancement observed in V1 and ROA 24 requires that about 0.1% of their atmopheres be composed of material mixed to the surface from the s-processed region. The abundance patterns of ST Pup are significantly different from those in any star in omega Cen. While it displays enhanced CNO abundances, its s-process abundances are much less than those of the omega Cen stars. It most closely resembles the field CH stars.