Greatly Enhanced Eccentricity Oscillations in Quadruple Systems Composed of Two Binaries: Implications for Stars, Planets, and Transients

TL;DR

This paper investigates how quadruple star systems with two binaries exhibit significantly enhanced eccentricity oscillations and orbital flips compared to triples, leading to more frequent collisions, mergers, and transients, with implications for stellar and planetary evolution.

Contribution

It demonstrates that coupling two binaries in quadruples greatly amplifies Kozai-Lidov cycles, resulting in higher eccentricities and dynamic interactions than in triple systems.

Findings

30-50% of quadruples reach near-parabolic eccentricities

Quadruples show 4-12 times more high-eccentricity events than triples

Enhanced orbital flips and collisions in quadruples

Abstract

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 behavior 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 semi-major axes of 10 to 20, about 30 to 50% of equal-mass quadruples reach eccentricity 1-e < 10^{-3} in one of the binaries. This is about 4 to 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 to 4 higher than for the corresponding triple system given our fiducial parameters with implications for hot Jupiters and star-planet collisions.