http://arxiv.org/abs/1910.04763
We carry out 2D viscous hydrodynamics simulations of circumbinary accretion using the moving-mesh code AREPO. Our simulations resolve the accretion flow over a wide range of scales, from the large circumbinary disk (CBD) to accretion streams and disks around individual binary components. Extending our recent studies (Mu\~noz et al 2019), we consider circular binaries with various mass ratios ($0.4\leq q_b\leq 1$) and explore accretion from “infinite”, steady-supply disks and from finite-sized, viscously spreading tori. In the “infinite” disk case, a global steady state can be reached, and we find that the accretion “eigenvalue” $l_0$, defined as the net angular momentum transfer from the disk to the binary per unit accreted mass, is always positive and falls in the range ($0.65$-$0.75)a_{\rm b}^2\Omega_{\rm b}$ (where $a_{\rm b}$, $\Omega_{\rm b}$ are the binary separation and angular frequency), depending weakly on the mass ratio and disk viscosity. This positive $l_0$ leads to expansion of the binary separation. Binary 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 pseudo-stationary phase, during which the torus viscously spreads and accretes onto the binary. We find that, in the stationary viscous phase, the net torque on the binary per unit accreted mass is close to the accretion “eigenvalue” $l_0$ derived for the steady-supply disks. We conclude that in general, binaries accreting from circumbinary disks/tori gain angular momentum and expand over long time scales. This result can significantly impact the evolution and coalescence of supermassive binary black holes and newly formed binary stars. We offer a word of caution when drawing conclusions from simulations of transient accretion onto empty circumbinary cavities.
D. Muñoz, D. Lai, K. Kratter, et. al.
Mon, 14 Oct 19
11/69
Comments: 10 pages, 11 figures, submitted to ApJ, comments welcome