MASSIVE CIRCUMBINARY DISKS AND THE FORMATION OF MULTIPLE SYSTEMS

The fragmentation of a circumbinary disc due to the interaction with t he embedded binary is presented. Infall on to the disc is ongoing thro ughout this process, with the matter falling to a radius where the tim e-dependent binary potential is significant. The binary system drives both an m = 1 and an m = 2 mode into the disc. The m = 1 mode means th at neither the centre of mass of the binary nor that of the surroundin g disc is located at the centre of mass of the system. The dynamics of the combined non-axisymmetric modes allow the spiral arms to interact and collide. This collision results in the formation of a Jeans unsta ble condensation which subsequently collapses to form an additional co mpanion. Two such companions are formed within three binary orbits. Th is process can form companions in circumbinary discs of any size. It c an account for the formation of coplanar triple and multiple systems w hen an initial binary is present. It also suggests that, if massive ci rcumbinary discs are a common occurrence in protostellar evolution, mo st binary systems will be members of higher order systems. Furthermore , the short time-scale for fragmentation of a circumbinary disc implie s that such discs will be short-lived and hence hard to detect. We spe culate on the relevance of this process for stellar core collapse in s upernovae. The repetitive formation of binary neutron stars would be a significant source for gravitational radiation.