Dynamical formation and scattering of hierarchical triples: cross-sections, Kozai–Lidov oscillations, and collisions

Abstract

Dynamical scattering of binaries and triple systems of stars, planets, and compact objects may produce highly inclined triple systems subject to Kozai–Lidov (KL) oscillations, potentially leading to collisions, mergers, Type Ia supernovae, and other phenomena. We present the results of more than 400 million gravitational scattering experiments of binary–binary, triple–single, and triple–binary scattering. We compute the cross-sections for all possible outcomes and explore their dependences on incoming velocity, mass, semimajor axis, and eccentricity, including analytic fits and discussion of the velocity dependence. For the production of new triple systems by scattering we find that compact triples are preferred, with ratios of outer to inner semimajor axes of ∼few–100, flat or quasi-thermal eccentricity distributions, and flat distributions in cosine of the mutual inclination. Dynamically formed triples are thus subject to strong KL oscillations, the ‘eccentric Kozai mechanism’, and non-secular effects. For single and binary flyby encounters with triple systems, we compute the cumulative cross-section for changes to the mutual inclination, eccentricity, and semimajor axis ratio. We apply these results to scattering events in the field, open clusters, and globular clusters, and explore the implications for Type Ia supernovae via collisions and mergers, stellar collisions, and the lifetime and dynamical isolation of triple systems undergoing KL oscillations. An appendix provides an analysis of the velocity dependence of the collision cross-section in binary–single scattering.