Possible observational consequences of primordial binaries in globular clusters

The observational consequences of the evolution of a primordial binary population in a globular cluster are investigated. For those binary systems which begin mass transfer at orbital periods ≲1 day, the system is expected to evolve to a contact binary stage and to eventually merge to form a single star. At longer orbital periods (1 day ≲ P ≲ 1 yr), the binary system will enter into a common envelope stage. For periods ≲15 days, the system is likely to merge, and the outcome of the evolution is a single red giant star. On the other hand, for systems characterized by orbital periods ≳15 days, a detached red dwarf-white dwarf binary system is expected to emerge. As a consequence of the mergers, a range in masses for the red giant stars is possible leading to a color spread on the red giant branch of the color magnitude diagram. Furthermore, the luminosity function of stars at high luminosities is expected to steepen as a result of tidal interaction in the binary. These observational properties can potentially be used as probes to determine the abundance of primordial binary stars in globular clusters. The role of binaries in affecting stellar mass loss is also examined in the context of blue horizontal branch stars and anomalous Cepheids. Finally, we suggest that the formation of cataclysmic variables via primordial binaries is expected only in loosely bound clusters of low-velocity dispersion.