Star cluster ecology - VII. The evolution of young dense star clusters containing primordial binaries

We study the first ~100 Myr of the evolution of isolated star clusters initially containing 144179 stars, including 13107 (10 per cent) primordial hard binaries. Our calculations include the effects of both stellar and binary evolution. Gravitational interactions among the stars are computed by direct N-body integration using high-precision GRAPE-6 hardware. The evolution of the core radii and central concentrations of our simulated clusters are compared with the observed sample of young (<~100Myr) star clusters in the large Magellanic Cloud. Even though our simulations start with a rich population of primordial binaries, core collapse during the early phase of the cluster evolution is not prevented. Throughout the simulations, the fraction of binaries remains roughly constant (~10 per cent). Due to the effects of mass segregation the mass function of intermediate-mass main-sequence stars becomes as flat as α = -1.8 in the central part of the cluster (where the initial Salpeter mass function had α = -2.35). About 6-12 per cent of the neutron stars were retained in our simulations; the fraction of retained black holes is 40-70 per cent. In each simulation about three neutron stars become members of close binaries with a main-sequence companion. Such a binary will eventually become an X-ray binary, when the main-sequence star starts to fill its Roche lobe. Black holes are found more frequently in binaries; in each simulated cluster we find ~11 potential X-ray binaries containing a black hole. Binaries consisting of two white dwarfs are quite common, but few (20-30) are sufficiently close that they will merge within a Hubble time due to the emission of gravitational radiation. Clusters with shorter relaxation times tend to produce fewer merging white dwarf binaries. The white dwarf binaries that do merge are all sufficiently massive to produce a Type Ia supernova. The densest cluster produces about twice as many blue stragglers as a field population containing the same number of binaries, and these blue stragglers are more massive, bluer and brighter than in less dense clusters.