Massive circumbinary discs and the formation of multiple systems

Abstract

The fragmentation of a circumbinary disc due to the interaction with the embedded binary is presented. Infall on to the disc is ongoing throughout this process, with the matter falling to a radius where the time-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 that neither the centre of mass of the binary nor that of the surrounding 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 unstable condensation which subsequently collapses to form an additional companion. Two such companions are formed within three binary orbits. This process can form companions in circumbinary discs of any size. It can account for the formation of coplanar triple and multiple systems when an initial binary is present. It also suggests that, if massive circumbinary discs are a common occurrence in protostellar evolution, most binary systems will be members of higher order systems. Furthermore, the short time-scale for fragmentation of a circumbinary disc implies that such discs will be short-lived and hence hard to detect. We speculate on the relevance of this process for stellar core collapse in super-novae. The repetitive formation of binary neutron stars would be a significant source for gravitational radiation.