Rapid Eccentricity Oscillations and the Mergers of Compact Objects in Hierarchical Triples

TL;DR

This paper demonstrates that rapid eccentricity oscillations in hierarchical triple systems significantly increase merger rates of compact objects, challenging secular theory predictions and impacting gravitational wave event rates.

Contribution

It reveals the importance of rapid eccentricity oscillations in triple systems, showing they lead to faster mergers than secular theory suggests, with implications for gravitational wave sources.

Findings

REOs cause higher eccentricities than secular theory predicts.

Approximately 40% of systems merge within 1-2 KL timescales.

Secular theory overestimates merger times at low tertiary eccentricity.

Abstract

Kozai-Lidov (KL) oscillations can accelerate compact object mergers via gravitational wave (GW) 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 timescale 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 ($e_2$), ~40% of systems merge within ~1-2 eccentric KL timescales whereas secular theory predicts that only ~20% of such systems merge that rapidly. This discrepancy becomes especially pronounced at low $e_2$, 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 timescale by an order of magnitude and bring the components of the inner binary into closer orbits than would be possible in the secular approximation.