Gas meets Kozai: the influence of a gas-rich accretion disc on hierarchical triples undergoing von Zeipel-Lidov-Kozai oscillations

TL;DR

This paper investigates how a gas-rich accretion disc influences the dynamics of hierarchical triples, especially binary systems near supermassive black holes, affecting their evolution, merger rates, and potential as gravitational wave sources.

Contribution

It introduces a model for binary evolution under combined ZLK oscillations and gas drag, revealing conditions for binary expansion or contraction in AGN environments.

Findings

Binary evolution transitions from hardening to softening as inclination decreases.

Minimum pericentre distances during ZLK oscillations decrease with wider binaries.

Gas interactions can enhance stellar merger rates and gravitational wave event likelihood.

Abstract

Active galactic nuclei (AGNs) consist of a central supermassive black hole (SMBH) embedded in a region with both high gas and stellar densities: the gas is present as a thin accretion disc that fuels the central SMBH, while the stars form a dense, roughly isotropic nuclear star cluster. The binaries present in such a cluster could be considered naturally as triples, with the SMBH as a third object, and their dynamics also depend on the interaction with the gas-rich disc. In this paper, we study the evolution of such a binary on an inclined orbit with respect to the disc. The binary experiences both eccentricity excitation via the von Zeipel-Lidov-Kozai (ZLK) effect and drag forces from each time it penetrates the disc. We find that, as the outer orbital inclination decreases, the evolution of inner orbital separation can transition from a regime of gradual hardening to a regime of rapid softening. As such binaries grow wider, their minimum pericentre distances (during ZLK oscillations) decrease. We show that a simple geometric condition, modulated by the complex ZLK evolution, dictates whether a binary expands or contracts due to the interactions with the AGN disc. Our results suggest that the interaction with gas-rich accretion disc could enhance the rate of stellar mergers and formation of gravitational wave sources, as well as other transients. The treatment introduced here is general and could apply, with the proper modifications, to hierarchical triples in other gas-rich systems.