We revisit the problem of the split main sequence (MS) of the globular cluster ω 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 MS. 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.

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

KING, IVAN ROBERT;BEDIN, LUIGI;MILONE, ANTONINO;BELLINI, ANDREA;PIOTTO, GIAMPAOLO;
2012

Abstract

We revisit the problem of the split main sequence (MS) of the globular cluster ω 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 MS. 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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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11577/2498614
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