Binary Evolution in Stellar Dynamics

Abstract

Numerical experiments have demonstrated something of the importance of binaries in N-body systems, and this paper aims to provide a comprehensive theoretical picture of their behaviour. It begins by testing possible ‘ equilibrium ’ distributions for binaries against the results of computational experiments, but is mainly concerned with the dynamics of encounters between binaries and other members of the system. Using an impulsive approximation, it is shown that pairs with low binding energies, i.e. much less than the average kinetic energy of single cluster members, tend to be disrupted by encounters. The theory for energetic pairs is complicated by the considerably richer variety of possible phenomena, including distant encounters in which changes in eccentricity much exceed changes in energy, and exchange events in which an incoming star may replace one component of the binary. Especially difficult to treat are close encounters resulting in the formation of temporarily bound triple systems; a new hypothesis on the three-body problem deals successfully with this important case. The result of all types of encounter is that energetic pairs tend to become more energetic, at an average rate which is approximately independent of their binding energy.