Greatly enhanced eccentricity oscillations in quadruple systems composed of two binaries: implications for stars, planets and transients

We study the orbital evolution of hierarchical quadruple systems composed of two binaries on a long mutual orbit, where each binary acts as a Kozai-Lidov (KL) perturber on the other. We find that the coupling between the two binaries qualitatively changes the behaviour of their KL cycles. The binaries can experience coherent eccentricity oscillations as well as excursions to very high eccentricity that occur over a much larger fraction of the parameter space than in triple systems. For a ratio of outer to inner semimajor axes of 10-20, about 30-50 per cent of equal-mass quadruples reach eccentricity 1 - e < 10^-3 in one of the binaries. This is about 4-12 times more than for triples with equivalent parameters. Orbital `flips' and collisions without previous tidal interaction are similarly enhanced in quadruples relative to triples. We argue that the frequency of evolutionary paths influenced by KL cycles is comparable in the triple and quadruple populations even though field quadruples are a factor of ∼5 less frequent than triples. Additionally, quadruples might be a non-negligible source of triples and provide fundamentally new evolutionary outcomes involving close binaries, mergers, collisions and associated transients, which occur without any fine tuning of parameters. Finally, we study the perturbations to a planetary orbit due to a distant binary and we find that the fraction of orbital flips is a factor of 3-4 higher than for the corresponding triple system given our fiducial parameters with implications for hot Jupiters and star-planet collisions.