Binaries are common in astrophysics. Stars typically form in binary or multiple systems, and when two galaxies merge their supermassive black holes form a binary system in the merged galaxy’s centre. In each case it is possible for gas to accumulate in circumbinary orbits, and the interaction between this disc of gas and the binary is a recurring theme in astrophysics. Such discs have been recently observed directly in protostellar systems. And they are thought to facilitate the mergers of black holes which creates gravitational wave emission.
Following analytical investigations and pioneering numerical simulation work in the 20th century, the standard picture of circumbinary discs is that they drive the binary to shrink over time. This occurs because there are discrete locations in the disc where the binary orbit resonates with the disc orbits, and these resonances transfer energy and angular momentum from the rapidly rotating object (the binary) to the slower rotating object (the disc). As a result the disc expands and the binary contracts. Circumbinary discs are typically expected to be truncated at a radius of a few times the binary semi-major axis with, in some cases, a small amount of accretion on to the binary components via time-dependent streams from the disc inner edge.
In the last few years, this standard picture has been challenged by numerical simulation results from several groups that report that the binary expands with time upon interacting with a circumbinary disc. These simulations were typically performed with large disc viscosity to reduce computational cost and often in 2D. In this work (Heath & Nixon, 2021) we explore 3D hydrodynamical simulations with more realistic values of the disc viscosity to test whether the binary expands or contracts when interacting with a circumbinary disc. We find that when the disc viscosity is sufficiently large as to overwhelm the binary-disc resonances, the binary expands, and we show that this is due to the accretion of material that comes from orbits with a larger specific angular momentum than the binary orbit. However, we also show that when the simulations are performed with realistic disc parameters, the binary shrinks as the binary-disc resonances dominate the evolution over the accretion of matter in this case. We show that the structure and evolution of the system is dependent on several disc and binary parameters (see Fig. 1), and conclude that in most systems we expect the binary orbit to shrink over time.