ACCRETION DURING BINARY STAR-FORMATION .1. BALLISTIC ACCRETION

We consider the effects of accretion, during binary star formation, on the mass ratio and separation of a 'seed' binary system. Numerical fr agmentation calculations show that 'seed' binary systems may be formed during the gravitational collapse of molecular cloud cores. When init ially formed, however, these protobinaries typically contain only a sm all fraction of the total mass of the cloud core. For the star formati on process to reach completion, a protobinary system must grow in mass via accretion from the remainder of the infalling cloud. The accretio n of, and interaction with, this matter affects the mass ratio, separa tion and eccentricity of the protobinary, and thus the final state of the system cannot be determined unless the effects of the accretion ar e known. In this paper, the effects of accretion on the mass ratio and separation of an initially circular protobinary system are determined as functions of the initial mass ratio of the protobinary and the spe cific angular momentum of the infalling cloud material. The effects ar e studied by modelling the cloud ballistically, with non-interacting p articles which are accreted by the protostars. Qualitatively, it is fo und that the mass ratio and separation decrease for the accretion of m atter with low specific angular momentum, and increase for the accreti on of material of high specific angular momentum. The quantitative res ults, however, are found to depend critically on the assumed sizes of the protostars. As well as the effects on mass ratio and separation, t he spin angular momentum of the accreted material is examined, and evi dence is found that the circumstellar discs formed in a binary system may differ considerably between the two components. The implications f or the formation of binary stellar systems are discussed.