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Session 80 - Star Clusters in the Milky Way.
Display session, Friday, January 09
Previously, using HST observations of NGC 6752, we have determined that the fraction of stars in the core of this Globular Cluster which are binaries is roughly 20-30% (Rubenstein amp; Bailyn 1997). Here, we investigate both the distribution of binary fraction as a function of magnitude and the effect that this binary population has on the luminosity function (LF) of this cluster. We find that although Yale Isochrones (Chaboyer et al. 1995) fit the observations quite well, it is clear that at fainter magnitudes there is a systematic deviation of the stars brightward and redward from the isochrone's locus. This suggests a shift from observing single stars on the upper main sequence to observing, almost exclusively, binaries on the lower main sequence. Such a trend is to be expected due to the effects of mass segregation. To investigate the possibility that the binary fraction is increasing at fainter magnitudes, we perform Monte Carlo tests similar to the ones we performed previously (Rubenstein amp; Bailyn 1997). We find that the binary fraction increases by \approx 10% over the 1.5 magnitude interval below the turn-off. To examine the effect of the binaries on the LF, we statistically removed star counts from the appropriate magnitude bins. Before making this correction, the V LF in the inner core region is flat for 5 magnitudes below the main-sequence turn-off before sharply turning over. When we correct for the increasing binary fraction at faint magnitudes, the LF begins to fall immediately below the turn-off. It is clear that future studies of globular cluster LFs must take binaries into consideration. Accidental inclusion of binaries in the single star LF has the effect of over estimating the number of low mass stars and under estimating the degree of dynamical evolution. Chaboyer, B., Demarque, P., Guenther, D.B., Pinsonneault, M.H. amp; Pinsonneault, L.L. 1995, in The Formation of the Milky Way, eds. E.J. Alfaro and G. Tenorio-Tagle (Cambridge: Cambridge U.P.) Rubenstein, E.P. amp; Bailyn, C.D. 1997, Ap.J., 474, 701
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