Accretion during binary star formation .2. Gaseous accretion and disc formation

We consider the effects of accretion during binary star formation. When a 'seed' protobinary system forms within a collapsing molecular cloud core, its final state is determined by the accretion of, or interaction with, the remaining cloud material as it falls on to the system. The mass ratio and orbit of the binary and the formation of circumstellar and circumbinary discs are all dependent on the dynamics of this accretion process. In this paper, we study the effects of accretion on the mass ratio and separation of the binary. The gas is modelled using smoothed particle hydrodynamics (SPH), and the results are compared and contrasted with those obtained using ballistic calculations ill an earlier paper. The inclusion of hydrodynamic effects also allows us to study the disc formation process. We determine what fractions of the infalling gas form circumstellar and circumbinary discs and find that, under some circumstances, a large circumstellar disc forms around the primary while the secondary has only a small circumstellar disc, or indeed no disc at all. The observational implications are discussed.