A transition in circumbinary accretion discs at a binary mass ratio of 1:25

TL;DR

This paper investigates how circumbinary accretion discs change their structure and behavior at a specific binary mass ratio of about 1:25, revealing a phase transition from steady to fluctuating flow regimes.

Contribution

It identifies a critical mass ratio near 0.04 where the disc's morphology and dynamics undergo a significant transition, linking it to orbital stability and disc evolution.

Findings

Disc behavior shifts from steady to fluctuating at q≈0.04

Formation of a hollow cavity replaces a narrow gap above this ratio

The transition is insensitive to disc viscosity and pressure in thin discs

Abstract

We study circumbinary accretion discs in the framework of the restricted three-body problem (R3Bp) and via numerically solving the height-integrated equations of viscous hydrodynamics. Varying the mass ratio of the binary, we find a pronounced change in the behaviour of the disc near mass ratio $q \equiv M_s/M_p \sim 0.04$. For mass ratios above $q=0.04$, solutions for the hydrodynamic flow transition from steady, to strongly-fluctuating; a narrow annular gap in the surface density around the secondary's orbit changes to a hollow central cavity; and a spatial symmetry is lost, resulting in a lopsided disc. This phase transition is coincident with the mass ratio above which stable orbits do not exist around the L4 and L5 equilibrium points of the R3B problem. Using the DISCO code, we find that for thin discs, for which a gap or cavity can remain open, the mass ratio of the transition is relatively insensitive to disc viscosity and pressure. The $q=0.04$ transition has relevance for the evolution of massive black hole binary+disc systems at the centers of galactic nuclei, as well as for young stellar binaries and possibly planets around brown dwarfs.