N-Body Simulations of Realistic Open Clusters

Abstract

N-body simulations of dynamical evolution of open clusters have been computed with the purpose of comparing them with observations. Special effort has been put into reproducing conditions present in galactic clusters. Most of the models contain 1000 bodies with masses following a power-law mass function of slope a = −2.75 and mean mass 0.5M⊙. Neutron stars or white dwarfs (depending on the initial stellar mass) are generated by instantaneous changes in individual masses, when stars reach the end of their main sequence life. Close approaches between particles are treated by a two-body regularization technique that allows to follow binary evolution in detail. Two types of tidal perturbation are considered: a smooth linearized galactic tidal field is simulated assuming that the clusters move in a circular orbit at 10kpc from the galactic centre; transient shocks are simulated by encounters with extended interstellar clouds of different mass-spectrum, densities and space concentration. It is found that the combined action of evolutionary mass loss and binaries (when the cluster has a realistic mass function) is enough to arrest the core collapse. Tidal heating shapes the halo of the cluster. There is good agreement with the observed density and velocity distribution of open clusters and with reported changes in their mass function.