The moment of core collapse in star clusters with a mass function

Star clusters with multimass components dynamically evolve faster than those modelled with equal-mass components. Using a series of direct N-body simulations, we investigate the dynamical evolution of star clusters with mass functions, especially their core-collapse time. Multimass clusters tend to behave like systems with a smaller number of particles, which we call the effective number of particles (N_eff) and for which N_eff = M/m_max (here M and m_max are the total cluster mass and the mass of the most massive star in the cluster, respectively). We find that the time of core collapse is inversely proportional to the mass of the most massive star in the cluster and analytically confirm that this is because the core collapse of clusters with a mass function proceeds on the dynamical friction time-scale of the most massive stars. As the mass of the most massive star increases, however, the core-collapse time, which is observed as a core bounce of the cluster core from the evolution of the core density or core radius, becomes ambiguous. We find that in that case the total binding energy of the hard binaries gives a good diagnosis for determining the moment of core collapses. Based on the results of our simulations, we argue that the core bounce becomes ambiguous when the mass of the most massive star exceeds 0.1 per cent of the total mass of the cluster.