Spectroscopic Binaries in Globular Clusters. II. A Search for Long-Period Binaries in M22

Abstract

A catalog of 383 radial velocities for red giants in the globular cluster M22 has been compiled from the literature and from new observations accumulated between 1972 and 1994. This 22-year baseline is the longest available for any sample of globular cluster stars. Using 333 repeat velocities for 109 cluster members, we have carried out a search for spectroscopic binaries with periods in the range 0.2 -- 40 years and with mass ratios between 0.1 and 1.0. Although the velocities for these evolved stars show clear evidence for atmospheric motions, no star is convincingly found to exhibit a velocity variation greater than 7 km/s. By comparing the observed velocity variations to those found in a series of Monte-Carlo simulations, we estimate the cluster binary fraction to be X = 0.01 (circular orbits) and X = 0.03 (thermal orbits). These results are to be compared to the corresponding binary fraction of X = 0.12 for nearby solar-type stars having similar mass ratios and periods. We speculate that both the relative abundances of short- and long-period binaries in globular clusters and the large differences in measured binary fractions for clusters with high binary ionization rates (M22, Omega Cen) compared to those for clusters with low ionization rates (M71, M4, NGC 3201) point to a frequency-period distribution in which soft binaries have been disrupted by stellar encounters. Finally, we note that none of the three CH stars in our survey shows evidence for velocity variations; this is in stark contrast to field CH stars, virtually all of which are binaries. We argue that binaries in M22 which have binding energies similar to field CH stars are unlikely to have been disrupted by stellar encounters and suggest that the cluster CH stars are otherwise normal red giants which lie in the carbon-enriched tail of the cluster metallicity