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

King, I. R.; Bedin, L. R.; Cassisi, S.; Milone, A. P.; Bellini, A.; Piotto, G.; Anderson, J.; Pietrinferni, A.; Cordier, D.
Referencia bibliográfica

The Astronomical Journal, Volume 144, Issue 1, article id. 5 (2012).

Fecha de publicación:
7
2012
Número de autores
9
Número de autores del IAC
1
Número de citas
73
Número de citas referidas
67
Descripción
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. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.
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