Rotational Brownian Motion of a Massive Binary

Abstract

The orientation of a massive binary undergoes a random walk due to gravitational encounters with field stars. The rotational diffusion coefficient for a circular-orbit binary is derived via scattering experiments. The binary is shown to reorient itself by an angle of the order of (m/M)1/2 during the time that its semimajor axis shrinks appreciably, where M is the binary mass and m the perturber mass. Implications for the orientations of rotating black holes are discussed.