Abstract

We carry out numerical simulations of circumbinary discs, solving the viscous hydrodynamics equations on a polar grid covering an extended disc outside the binary co-orbital region. We use carefully controlled outer boundary conditions and long-term integrations to ensure that the disc reaches a quasi-steady state, in which the time-averaged mass accretion rate on to the binary, (M) over dot >, matches the mass supply rate at the outer disc. We focus on binaries with comparable masses and a wide range of eccentricities (e(B)). For e(B) less than or similar to 0.05, the mass accretion rate of the binary is modulated at about five times the binary period; otherwise, it is modulated at the binary period. The inner part of the circumbinary disc (r less than or similar to 6a(B)) generally becomes coherently eccentric. For low and high e(B), the disc line of apsides precesses around the binary, but for intermediate e(B) (0.2-0.4), it instead becomes locked with that of the binary. By considering the balance of angular momentum transport through the disc by advection, viscous stress and gravitational torque, we determine the time-averaged net angular momentum transfer rate to the binary, (J) over dot >. The specific angular momentum, l(0) = (J) over dot >/(M) over dot >, depends non-monotonically on e(B). Contrary to previous claims, we find that l(0) is positive for most e(B), implying that the binary receives net angular momentum, which may cause its separation to grow with time. The minimum l(0) occurs at intermediate e(B) (0.2-0.4), corresponding to the regime where the inner eccentric disc is apsidally aligned with the binary.