Rapid eccentricity oscillations and the mergers of compact objects in hierarchical triples

Abstract

Kozai-Lidov (KL) oscillations can accelerate compact object mergers via gravitational wave radiation by driving the inner binaries of hierarchical triples to high eccentricities. We perform direct three-body integrations of high-mass-ratio compact object triple systems using fewbody including post-Newtonian terms. We find that the inner binary undergoes rapid eccentricity oscillations (REOs) on the time-scale of the outer orbital period which drive it to higher eccentricities than secular theory would otherwise predict, resulting in substantially reduced merger times. For a uniform distribution of tertiary eccentricity (e2), ∼40 per cent of systems merge within ∼1-2 eccentric KL time-scales whereas secular theory predicts that only ∼20 per cent of such systems merge that rapidly. This discrepancy becomes especially pronounced at low e2, with secular theory overpredicting the merger time by many orders of magnitude. We show that a non-negligible fraction of systems have eccentricity >0.8 when they merge, in contrast to predictions from secular theory. Our results are applicable to high-mass-ratio triple systems containing black holes or neutron stars. In objects in which tidal effects are important, such as white dwarfs, stars, and planets, REOs can reduce the tidal circularization time-scale by an order of magnitude and bring the components of the inner binary into closer orbits than would be possible in the secular approximation. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.