Theoretical models for wide binary evolution

Summary Wide binary evolution under the influence of stars, giant molecular clouds, and the galactic tidal field is reviewed. Recent results show that binary lifetimes ata≳0.1 pc are determined by an interplay between relatively gentle evolution due to stellar encounters and catastrophic collisions with cloud subclumps. It is emphasized that binary semimajor axis distributions may not be understood from lifetimes alone. To determine the theoretical distribution binary ‘birthrate functions’ must be convolved with probability distributions for the evolution of an ensemble of binaries from given initial separations. Simple models show no sharp breaks below the tidal cut-off imposed by the Galaxy.