Numerical Experiments on the Escape from Non-Isolated Clusters and the Formation of Multiple Stars

Abstract

Isolated and non isolated clusters with a mass distribution have been studied by numerical techniques. The rates of escape of stars and of kinetic energy are compared with Hénon’s theoretical expressions. Multiple encounters play a very important role in the escape phenomenon, at least for clusters with a small number of stars. This leads to a theoretical underestimate of the rates of escape when the stars have equal masses and to an overestimate when masses are unequal.

For non isolated clusters, the tidal field of the Galaxy is responsible for one half of the rate of escape of the stars. The energy of a star escaping because of the tidal effect grows slowly while that of a star escaping after an encounter increases very rapidly. The stars escaping because of the tidal effect leave the cluster in the vicinity of the equilibrium points.

Encounters and the tidal field are not efficient enough to explain why very old open clusters are not observed. Other escape mechanisms have to be considered.

Very stable subsystems are formed which are not destroyed under the influence of the galactic tide. Separation between stars can be as low as 1000 UA.