2009PASP..121.1307A

We considered 17 open clusters for which published radial velocities studies have been made to search for spectroscopic binaries among the brighter stars. They have log ages of 6.7 to 8.9, with mostly B stars. Some clusters such as the Pleiades have mostly broad-lined stars, indicating large rotational velocities and few binaries, while clusters such as NGC 6475 have many narrow-lined stars and many binaries. We confirm with this larger sample the previous evidence of an inverse correlation between mean cluster rotational velocity and duplicity. It is not due to measuring errors or to preferred orientations of rotational axes in some clusters. Samples of field stars, such as Am and B2-B5, that have been observed enough to determine their duplicities, also fall on the same correlation. Not only are there large differences in the fractions of binaries, but in the clusters with many binaries, 25% of the stars have orbital periods days while in the clusters with few binaries, only 4% have periods days. We wonder why some clusters developed many binaries and other did not. There is marginal evidence that currently the denser clusters have more binaries and with shorter binary periods, but we do not know the initial cluster densities when the binaries were formed.