The properties of external accretion discs

The properties of external accretion discs (discs with a central source of angular momentum) are explored both analytically and numerically. An illustrative example of the effect of a disc of material around a binary star on the stellar separation is considered. A Greens-function-type solution is considered in which an initial ring of matter is put in orbit around the central binary. We find that the solution splits temporally into three parts. First, the disc evolves as a standard accretion disc unaware of the inner boundary condition. Secondly, the disc interacts with the inner boundary and changes its character to become, thirdly, an outflowing disc which is propelled outwards by the source of angular momentum at the center.