Star Cluster Evolution with Primordial Binaries. I. A Comparative Study

The evolution of equal-mass star clusters containing a mass fraction of about 20 percent binaries has been followed using direct integration, making one run each for a total number of stars of N = 282 and N = 563, and four runs for N = 1126. For comparison the evolution of an equivalent star system where the binaries were replaced by stars twice as heavy as the other stars was followed. The pre-core-collapse evolution is driven by mass segregation between the equal-mass single stars and the binaries, which are twice as heavy. After core collapse, the cluster shows, on average, a smooth reexpansion driven by a steady rate of burning (hardening) of primordial binaries. With so much primordial fuel present, the postcollapse cluster core is significantly larger than is the case in comparison runs without primordial binaries.