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We revisit the problem of the split main sequence (MS) of the globularcluster omega Centauri, and report the results of two-epoch Hubble SpaceTelescope observations of an outer field, for which proper motions give us apure sample of cluster members, and an improved separation of the two branchesof the main sequence. Using a new set of stellar models covering a grid ofvalues of helium and metallicity, we find that the best possible estimate ofthe helium abundance of the bluer branch of the MS is Y = 0.39 +/- 0.02. Forthe cluster center we apply new techniques to old observations: we use indicesof photometric quality to select a high-quality sample of stars, which we alsocorrect for differential reddening. We then superpose the color-magnitudediagram of the outer field on that of the cluster center, and suggest aconnection of the bluer branch of the MS with one of the more prominent amongthe many sequences in the subgiant region. We also report a group of undoubtedcluster members that are well to the red of the lower MS.

Hubble Space Telescope Observations of an Outer Field in Omega Centauri: A Definitive Helium Abundance