Simulating young star clusters with primordial binaries

Abstract

We simulate a cluster of 144179 stars including 13107 primordial hard binaries (10% of the total number of single stars and binary centers of mass), using direct integration of the equations of motion of all stars and binaries and incorporating the effects of stellar and binary evolution. The initial conditions are representative of young dense star clusters in the Local Group and other nearby galaxies like the Antennae and M82. We find that the early phase of core collapse, driven by mass segregation, is not appreciably delayed by the presence of a large number of hard binaries. By the end of the simulation, at an age of 115Myr, the cluster radius has expanded by about a factor of two. This may be explained as adiabatic expansion driven by the loss (via stellar evolution) of about 40% of the initial total mass. Binary dynamics apparently has little effect on the early cluster expansion. During the evolution, the total binary fraction drops at a roughly constant rate of about 0.01% per Myr. The fraction of very hard binaries, however increases at about 0.025% per Myr. By the end of the simulation the cluster contains 37 binaries containing at least one black hole; roughly half (17) of these contain two black holes.