The Core Binary Fractions of Star Clusters from Realistic Simulations

Abstract

We investigate the evolution of binary fractions in star clusters using N-body models of up to 100,000 stars. Primordial binary frequencies in these models range from 5% to 50%. Simulations are performed with the NBODY4 code and include a full mass spectrum of stars, stellar evolution, binary evolution, and the tidal field of the Galaxy. We find that the overall binary fraction of a cluster almost always remains close to the primordial value, except at late times when a cluster is near dissolution. A critical exception occurs in the central regions, where we observe a marked increase in binary fraction with time-a simulation starting with 100,000 stars and 5% binaries reached a core binary frequency as high as 40% at the end of the core-collapse phase (occurring at 16 Gyr with ~20,000 stars remaining). Binaries are destroyed in the core by a variety of processes as a cluster evolves, but the combination of mass segregation and creation of new binaries in exchange interactions produces the observed increase in relative number. We also find that binaries are cycled into and out of cluster cores in a manner that is analogous to convection in stars. For models of 100,000 stars we show that the evolution of the core radius up to the end of the initial phase of core collapse is not affected by the exact value of the primordial binary frequency (for frequencies of 10% or less). We discuss the ramifications of our results for the likely primordial binary content of globular clusters.