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arXiv · 1205.3760

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

I. R. King·L. R. Bedin·S. Cassisi·A. P. Milone·A. Bellini·G. Piotto·J. Anderson·A. Pietrinferni·D. Cordier
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Abstract

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.

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I. R. King, L. R. Bedin, S. Cassisi, A. P. Milone, A. Bellini, G. Piotto, J. Anderson, A. Pietrinferni, D. Cordier. 2012-05-16. Hubble Space Telescope Observations of an Outer Field in Omega Centauri: A Definitive Helium Abundance. https://doi.org/10.1088/0004-6256%2F144%2F1%2F5

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