Numerical simulations of protostellar encounters — III. Non-coplanar disc–disc encounters

It is expected that an average protostar will undergo at least one impulsive interaction with a neighbouring protostar whilst a large fraction of its mass is still in a massive, extended disc. If protostars are formed individually within a cluster before falling together and interacting, there should be no preferred orientation for such interactions. As star formation within clusters is believed to be coeval, it is probable that, during interactions, both protostars possess massive, extended discs.   We have used an SPH code to carry out a series of simulations of non-coplanar disc–disc interactions. We find that non-coplanar interactions trigger gravitational instabilities in the discs, which may then fragment to form new companions to the existing stars. (This is different from coplanar interactions, in which most of the new companion stars form after material in the discs has been swept up into a shock layer, and this then fragments.) The original stars may also capture each other, leading to the formation of a small- N cluster. If every star undergoes a randomly oriented disc–disc interaction, then the outcome will be the birth of many new stars and substellar objects. Approximately two-thirds of the stars will end up in multiple systems.