NUMERICAL SIMULATIONS OF PROTOSTELLAR ENCOUNTERS - I - STAR-DISC ENCOUNTERS

It appears that most stars are born in clusters, and that at birth mos t stars have circumstellar discs which are comparable in size to the s eparations between the stars. Interactions between neighbouring stars and discs are therefore likely to play a key role in determining disc lifetimes, stellar masses, and the separations and eccentricities of b inary orbits. Such interactions may also cause fragmentation of the di scs, thereby triggering the formation of additional stars. We have car ried out a series of simulations of star-disc interactions using an SP H code which treats self-gravity, hydrodynamic and viscous forces. We find that interactions between discs and stars provide a mechanism for removing energy from, or adding energy to, the orbits of the stars, a nd for truncating the discs. However, capture during such encounters i s unlikely to be an important binary formation mechanism. A more signi ficant consequence of such encounters is that they can trigger fragmen tation of the disc, via tidally and compressionally induced gravitatio nal instabilities, leading to the formation of additional stars and su bstellar objects. When the disc spins and stellar orbits are randomly oriented, encounters lead to the formation of new companions to the or iginal-star in 20 per cent of encounters. If most encounters are progr ade and coplanar, as suggested by simulations of dynamically triggered star formation, then new companions are formed in approximately 50 pe r cent of encounters.