Hubble Space TelescopeObservations of New Horizontal‐Branch Structures in the Globular Cluster ω Centauri

Abstract

The globular cluster ω 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 ω Cen. We find that over 30% of the HB objects are "extreme" HB or hot post-HB stars. The hot HB 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 HB 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-HB 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 ω 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.