Advanced evolution in globular clusters. II. The ultraviolet-bright stars in Omega Centauri

The results of a spectrographic and photometric survey of stars which occupy the region above the horizontal branch and to the left of the giant branch (the so-called UV-bright region) in the color-magnitude diagram of the globular cluster omega Centauri are presented. There is no large scatter in the apparent metal abundances of the cool stars ROA 164 and 342 and the red giant ROA 159. Helium lines are present in the spectra of ROA 3596 and 5701. A mean mass of about 0.58 solar mass (internal s.e.) is obtained for the UV-bright stars, indicating that they have the same mass as the blue horizontal-branch stars. The distribution of the UV-bright stars in the theoretical H-R diagram indicates that these objects can occupy any position in the UV-bright region. This implies that if they are to be explained by loops resulting from helium shell flashes and/or by final evolution from the asymptotic branch or the giant branch, the loops must be long and occur at a range in luminosities, and/or the final evolution from the giant branches must occur at a range of luminosities.