Hubble Space Telescope Observations of an Outer Field in Omega Centauri:   A Definitive Helium Abundance
  We revisit the problem of the split main sequence (MS) of the globular cluster omega Centauri, and report the results of two-epoch Hubble Space Telescope observations of an outer field, for which proper motions give us a pure sample of cluster members, and an improved separation of the two branches of the main sequence. Using a new set of stellar models covering a grid of values of helium and metallicity, we find that the best possible estimate of the helium abundance of the bluer branch of the MS is Y = 0.39 +/- 0.02. For the cluster center we apply new techniques to old observations: we use indices of photometric quality to select a high-quality sample of stars, which we also correct for differential reddening. We then superpose the color-magnitude diagram of the outer field on that of the cluster center, and suggest a connection of the bluer branch of the MS with one of the more prominent among the many sequences in the subgiant region. We also report a group of undoubted cluster members that are well to the red of the lower MS. 
I. R. King (1), L. R. Bedin (2), S. Cassisi (3), A. P. Milone (4), A.   Bellini (5), G. Piotto (5), J. Anderson (6), A. Piet...
astro-ph.GA