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.