Evolution of N-body open clusters

Abstract

Realistic n-body simulations of dynamical evolution of open clusters are discussed and compared with observations. Most of the models have 1000 bodies with initial masses following a power-law mass function of slope α = – 2.75 and mean mass 0.5 M⊙. Stellar evolution and mass loss are considered in detail. 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 one to follow binary evolution in detail. Two types of tidal perturbation are considered: a smooth linearized galactic tidal field, simulated assuming that the clusters move in a circular orbit at 10 kpc from the galactic centre; and transient shocks, simulated by encounters with extended interstellar clouds of different mass spectrum, density and space concentration. Excellent agreement is found between the distribution of lifetime for galactic clusters and the life of the models. The combined action of evolutionary stellar mass loss and binaries (for clusters with 1000 bodies and a realistic mass function) is enough to arrest the core collapse. Tidal heating shapes the halo of the cluster and produces a distinctive density and velocity distribution. Encounters with standard clouds do not alter the life-time of the clusters; only giant molecular clouds produce a catastrophic disruption. Effects of mass segregation and preferential escape of light stars can account for the observed depletion of low luminosity stars in the better studied central region of clusters.