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Session 64 - Globular Clusters.
Oral session, Tuesday, January 14
The fraction of stars in Globular Clusters (GCs) which are binaries has important implications for the dynamical evolution of GCs. The energy stored in the orbit of binary systems is the main source of the dynamical ``heating'' which prevents the cores of GCs from collapsing into a single massive singularity (Hut et al. 1992). The binary frequency also helps to determine how many and what sorts of exotic objects are produced via stellar collisions and binary mergers. Many unusual objects such as low mass X-ray binaries, blue stragglers,and cataclysmic variables are formed either as part of a binary system or an interaction with one.
Unfortunately, two factors make detecting binaries in GCs extremely difficult. First, the extreme crowding of stars makes it difficult to observe faint stars. Second, mass segregation ensures that most binaries will be found in the innermost regions of the GCs where the crowding is most severe.
Recent HST observations have enabled us to probe the cores of nearby GCs with a resolution an order of magnitude finer than previous ground-based observations. To constrain the binary fraction in the core-collapse GC NGC 6752 we made 20 hours of HST observations. Over 100 images were obtained in each of 2 filters (F555W\approx V amp; F814W\approx I); these frames were divided among 9 slightly offset pointings. We use a new technique, which we call Ridge Line Color Dispersion to analyze the resulting Color-Magnitude Diagram. We report (Rubenstein amp; Bailyn 1997) on the existence of a large binary population (\approx 20-40% of the stars) in the core of NGC 6752.
By studying the time series data we found an SX Phoenecis pulsator among the blue straggler stars; its pulsation mass is \approx 1-1.2 M_ødot. Two cataclysmic variable candidates were also identified (Bailyn et al. 1996).
Bailyn, C.D., Rubenstein, E.P., Slavin, S.D., Cohn, H., Lugger, P., Cool, A.M. amp; Grindlay, J.E. 1996, to be published in Astrophys. J. (Letters)
Hut, P. et al. 1992, PASP, 104, 981
Rubenstein, E.P. amp; Bailyn, C.D. 1997, ApJ to appear 10 January 1997
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