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[52.03] Wide-Field Imaging of Omega Centauri with the Advanced Camera for Surveys

D. Haggard, J.L. Dorfman, A.M. Cool (San Francisco State University), J. Anderson (Rice University), C.D. Bailyn (Yale University), P.D. Edmonds, J.E. Grindlay (Harvard-Smithsonian Center for Astrophysics)

We present initial results of a wide-field imaging study of the globular cluster Omega Cen (NGC 5139) using the Advanced Camera for Surveys (ACS). We have obtained a mosaic of 3x3 pointings of the cluster using the HST/ACS Wide Field Camera covering approximately 10' x 10', roughly out to the cluster's half-mass radius. Using F435W (B₄₃₅), F625W (R₆₂₅) and F658N (H-alpha) filters, we are searching for optical counterparts of Chandra X-ray sources and studying the cluster's stellar populations. Here we report the discovery of an optical counterpart to the X-ray source identified by Rutledge et al. (2002) as a possible quiescent neutron star on the basis of its X-ray spectrum. The star's magnitude and color (R₆₂₅ = 24.4, B₄₃₅-R₆₂₅ = 1.5) place it more than 1.5 magnitudes to the blue side of the main sequence. Through the H-alpha filter it is about 1.3 magnitudes brighter than cluster stars of comparable R₆₂₅ magnitude. The blue color and H-alpha excess suggest the presence of an accretion disk, implying that the neutron star is a member of a quiescent low-mass X-ray binary. The object's faint absolute magnitude (M₆₂₅ ~ 10.6, M₄₃₅ ~ 11.8) implies that the system contains an unusually weak disk and that the companion, if it is a main-sequence star, is of very low mass (< 0.16 solar masses). We also identify ~ 10 probable white dwarfs and three possible BY Draconis stars in a 20'' x 20'' region, suggesting that a large number of white dwarfs and active binaries will be observable in the full ACS study.

This work is supported by NASA grant GO-9442 from the Space Telescope Science Institute.

The author(s) of this abstract have provided an email address for comments about the abstract: dhaggard@astro.washington.edu

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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.