Primordial Binaries in the Globular

In a search for spectroscopic binaries in ! Cen, the radial velocities of 310 giant stars have been monitored with multiple observations over more than ten years. The percentage of binaries with periods from those allowed by the Roche lobe limit and up to the limit set by dynamical disruption is about a factor ve smaller than among G dwarfs in the solar vicinity in the same period range. Assuming a similar period distribution in ! Cen as for the nearby G dwarfs, we estimate the global binary frequency in ! Cen to be as low as 3 { 4%. In contrast to similar stars in the eld, the rich population of chemically peculiar giant stars (Ba, CH, S) in ! Cen seems to have a rather low binary frequency. The most massive of all globular clusters in our galaxy, ! Cen, is of special interest in the search for primordial binaries. With its moderate central concentration, ! Cen has probably preserved a large fraction of primordial bi-naries in its outer regions, since the relaxation time at the half-mass radius ((r 1=2) = 2:610 10 y) is larger than the age of the cluster. However, the present period range for primordial binaries among red giants is limited on the short-period side (P inf) by the onset of Roche-lobe overrow, and on the long-period side (P sup) by dynamical disruption. P inf is a function of the stellar radii and the binary mass ratio, and in our sample, P inf is mostly in the range 100 { 400 days. The upper limit of the period window, P sup , depends on position in the cluster in addition to the mass ratio. In the core P sup , as estimated from Hill's (1984) formalism, can be as short as three years due to the very large central velocity dispersion. In the outer regions, it can be as large as 10 years or more. Therefore, most of the primordial binaries among the giant of ! Cen are expected to have periods from 200 to 4000 days. Among the interesting characteristics of ! Cen are the large range in metal-licity and many chemically peculiar giants (Ba, CH, and S stars). In the eld, most or all of these chemically peculiar giants are known to be binaries with periods in the range P inf ; P sup ] (McClure 1983, Jorissen & Mayor 1992). Since …