On the possibility of a collisional origin for the blue stragglers and contact binaries in the old open clusters M67 and NGC 188

The problem of the origin of the blue stragglers and contact binaries in the old open clusters M67 and NGC 188 is examined. The hypothesis that blue stragglers and contact binaries are formed via physical stellar collisions and tidal captures, respectively, during strong dynamical interactions involving binary stars is considered. This mechanism can naturally account for many of the observed properties of these objects. An attempt is made to test the collisional hypothesis by carrying out binary-binary and binary-single scattering experiments tailored to the old open clusters M67 and NGC 188. The experiments indicate that isolated interactions involving binaries composed of main-sequence stars are unlikely to account for 100 percent of the blue stragglers and contact binaries in the two clusters. However, such interactions can easily produce about 10 percent of the observed objects. The ways in which the production rate of blue stragglers and contact binaries via collisions may be enhanced are described.