Hubble Space Telescope observations of new horizontal-branch structures in the globular cluster ω Centauri

The globular cluster omega Centauri contains the largest known population of very hot horizontal-branch (HB) stars. We have used the Hubble Space Telescope to obtain a far-UV/optical color-magnitude diagram of three fields in omega Cen. We find that over 30% of the HE objects are extreme HE or hot post-MB stars. The hot HE stars are not concentrated toward the cluster center, which argues against a dynamical origin for them. A wide gap in the color distribution of the hot HE stars appears to correspond to gaps found earlier in several other clusters. This suggests a common mechanism, probably related to giant branch mass loss. The diagram contains a significant population of hot sub-MB stars, which we interpret as the blue-hook objects previously predicted by D'Cruz et al. These are produced by late He flashes in stars which have undergone unusually large giant branch mass loss. The cluster omega Cen has a well-known spread of metal abundance, and our observations are consistent with a giant branch mass-loss efficiency which increases with metallicity.