  It appears that most stars are born in clusters, and that at birth most stars
have circumstellar discs which are comparable in size to the separations
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 binary orbits. Such
interactions may also cause fragmentation of the discs, thereby triggering the
formation of additional stars.
  We have carried out a series of simulations of disc-star interactions using
an SPH 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, and for truncating
the discs. However, capture during such encounters is unlikely to be an
important binary formation mechanism.
  A more significant consequence of such encounters is that they can trigger
fragmentation of the disc, via tidally and compressionally induced
gravitational instabilities, leading to the formation of additional stars. When
the disc-spins and stellar orbits are randomly oriented, encounters lead to the
formation of new companions to the original star in 20% of encounters. If most
encounters are prograde and coplanar, as suggested by simulations of
dynamically-triggered star formation, then new companions are formed in
approximately 50% of encounters.
