Circumbinary Accretion from Finite and Infinite Disks

We carry out 2D viscous hydrodynamics simulations of circumbinary disk (CBD) accretion using AREPO. We resolve the accretion flow from a large-scale CBD down to the streamers and disks around individual binary components. Extending our recent studies, we consider circular binaries with various mass ratios (0.1 <= q(b) <= 1) and study accretion from infinite, steady-supply disks and from finite-sized, viscously spreading tori. For infinite disks, a global steady state can be reached, and the accretion variability has a dominant frequency similar to 0.2 omega(b) for q(b) > 0.5 and omega(b) for q(b) < 0.5 (omega(b) is the binary angular frequency). We find that the accretion eigenvalue l(0)-the net angular momentum transfer from the disk to the binary per unit accreted mass-is always positive and falls in the range (0.65-) (with a(b) the binary separation), depending weakly on the mass ratio and viscosity. This leads to binary expansion when q(b) greater than or similar to 0.3. Accretion from a finite torus can be separated into two phases: an initial transient phase, corresponding to the filling of the binary cavity, followed by a viscous pseudostationary phase, during which the torus viscously spreads and accretes onto the binary. In the viscous phase, the net torque on the binary per unit accreted mass is close to l(0), the value derived for infinite disks. We conclude that binaries of similar mass accreting from CBDs gain angular momentum and expand over long timescales. This result significantly impacts the coalescence of supermassive binary black holes and newly formed binary stars. We offer a word of caution against conclusions drawn from simulations of transient accretion onto empty circumbinary cavities.