The formation of close binary systems

A viable solution to the origin of close binary systems, unaccounted for inrecent theories, is presented. Fragmentation, occurring at the end of thesecondary collapse phase (during which molecular hydrogen is dissociating), canform binary systems with separations less than 1 au. Two fragmentation modesare found to occur after the collapse is halted. The first consists of thefragmentation of a protostellar disc due to rotational instabilities in aprotostellar core, involving both an $m=1$ and an $m=2$ mode. Thisfragmentation mechanism is found to be insensitive to the initial densitydistribution: it can occur in both centrally condensed and uniform initialconditions. The second fragmentation mode involves the formation of a rapidlyrotating core at the end of the collapse phase which is unstable to theaxisymmetric perturbations. This core bounces into a ring which quicklyfragments into several components. The binary systems thus formed contain lessthan 1 per cent of a solar mass and therefore will need to accrete most oftheir final mass if they are to form a binary star system. Their orbitalproperties will thus be determined by the properties of the accreted matter.Comment: 6 pages, uuencoded compressed postscript file (containing 2 figures