Hydrodynamics of Circumbinary Accretion: Angular Momentum Transfer and Binary Orbital Evolution

We carry out 2D viscous hydrodynamical simulations of circumbinary accretion using the moving-mesh code AREPO. We self-consistently compute the accretion flow over a wide range of spatial scales, from the circumbinary disk (CBD) far from the central binary, through accretion streamers, to the disks around individual binary components, resolving the flow down to 2% of the binary separation. We focus on equal-mass binaries with arbitrary eccentricities. We evolve the flow over long (viscous) timescales until a quasi-steady state is reached, in which the mass supply rate at large distances (M) over dot(0) (assumed constant) equals the time-averaged mass transfer rate across the disk and the total mass accretion rate onto the binary components. This quasi-steady state allows us to compute the secular angular momentum transfer rate onto the binary, <(J) over dot (b)>, and the resulting orbital evolution. Through direct computation of the gravitational and accretional torques on the binary, we find that <(J) over dot (b)> is consistently positive (i.e., the binary gains angular momentum), with l(0) equivalent to <(J) over dot (b)>/(M) over dot(0) in the range of (0.4 - 0.8) a(b)(2)Omega(b), depending on the binary eccentricity (where a(b), Omega(b) are the binary semimajor axis and angular frequency); we also find that this <(J) over dot (b)> is equal to the net angular momentum current across the CBD, indicating that global angular momentum balance is achieved in our simulations. In addition, we compute the time-averaged rate of change of the binary orbital energy for eccentric binaries and thus obtain the secular rates <(a) over dot (b)> and <(e) over dot (b)>. In all cases, <(a) over dot (b)> is positive; that is, the binary expands while accreting. We discuss the implications of our results for the merger of supermassive binary black holes and for the formation of close stellar binaries.