Gravitational torque in circumbinary discs: global radial oscillations

TL;DR

This paper investigates the origin and structure of gravitational torque oscillations in circumbinary discs, revealing they are driven by density waves launched near the inner cavity, with implications for various astrophysical systems.

Contribution

The study combines simulations and analytical models to show that torque oscillations are caused by density waves, not local resonances, and identifies key factors influencing their radial structure.

Findings

Torque oscillations are due to density waves launched near the inner cavity.

Disc sound speed and spiral arm multiplicity determine oscillation periodicity.

Resonant Lindblad torques do not directly set the oscillation structure.

Abstract

Circumbinary discs (CBDs) arise in many astrophysical settings, including young stellar binaries and supermassive black hole binaries. Their structure is mediated by gravitational torques exerted on the disc by the central binary. The spatial distribution of the binary torque density (so-called excitation torque density) in CBDs is known to feature global large-amplitude, quasi-periodic oscillations, which are often interpreted in terms of the local resonant Lindblad torques. Here we investigate the nature of these torque oscillations using 2D, inviscid hydrodynamic simulations and theoretical calculations. We show that torque oscillations arise due to the gravitational coupling of the binary potential to the density waves launched near the inner cavity and freely propagating out in the disc. We provide analytical predictions for the radial periodicity of the torque density oscillations and verify them with simulations, showing that disc sound speed and the multiplicity of the density wave spiral arms are the key factors setting the radial structure of the oscillations. Resonant Lindblad torques play no direct role in determining the radial structure and periodicity of the torque oscillations and manifest themselves only by driving the density waves in the disc. We also find that vortices forming at the inner edge of the disc can provide a non-trivial contribution to the angular momentum transport in the CBD. Our results can be applied to understanding torque behaviour in other settings, e.g. discs in cataclysmic variables and X-ray binaries.