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Title:
Wide-Field Imaging of Omega Centauri with the Advanced Camera for Surveys
Authors:
Haggard, D.; Dorfman, J. L.; Cool, A. M.; Anderson, J.; Bailyn, C. D.; Edmonds, P. D.; Grindlay, J. E.
Affiliation:
AA(San Francisco State University), AB(San Francisco State University), AC(San Francisco State University), AD(Rice University), AE(Yale University), AF(Harvard-Smithsonian Center for Astrophysics), AG(Harvard-Smithsonian Center for Astrophysics)
Publication:
American Astronomical Society Meeting 203, #52.03; Bulletin of the American Astronomical Society, Vol. 35, p.1289
Publication Date:
12/2003
Origin:
AAS
Bibliographic Code:
2003AAS...203.5203H

Abstract

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.

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