Predicting the properties of binary stellar systems: the evolution of accreting protobinary systems

We investigate the formation of binary stellar systems. We consider a model where a 'seed' protobinary system forms, via fragmentation, within a colla psing molecular cloud core and evolves to its final mass by accreting mater ial from an infalling gaseous envelope. This accretion alters the mass rati o and orbit of the binary, and is largely responsible for forming the circu mstellar and/or circumbinary discs. Given this model for binary formation, we predict the properties of binary systems and how they depend on the initial conditions within the molecular cloud core. We predict that there should be a continuous trend such that cl oser binaries are more likely to have equal-mass components and are more li kely to have circumbinary discs than wider systems. Comparing our results w ith observations, we find that the observed mass-ratio distributions of bin aries and the frequency of circumbinary discs as a function of separation a re most easily reproduced if the progenitor molecular cloud cores have radi al density profiles between uniform and 1/r (e.g., Gaussian) with near-unif orm rotation. This is in good agreement with the observed properties of pre -stellar cores. Conversely, we find that the observed properties of binarie s cannot be reproduced if the cloud cores are in solid-body rotation and ha ve initial density profiles which are strongly centrally condensed. Finally , in agreement with the radial-velocity searches for extrasolar planets, we find that it is very difficult to form a brown dwarf companion to a solar- type star with a separation less than or similar to 10 au, but that the fre quency of brown dwarf companions should increase with larger separations or lower mass primaries.