The effect of star-disc interactions on the binary mass-ratio distribution

We investigate the effect of circumstellar discs on the dynamical evolution of a small cluster of protostars formed by ‘prompt initial fragmentation’. In particular, we study how the presence of discs affects the resultant mass components of binaries formed in the cluster. We find that, when the stars are assigned circumstellar discs, the occurrence of lower mass stars in binaries is greatly increased compared with discless simulations. This is due to the fact that discs both increase the number of binaries formed and also randomize the selection of secondary mass companions to each primary. For clusters containing four to ten stars with massive discs the predicted binary fraction is in good agreement with observations. We also find that discs boost the number of triple and quadruple systems formed, and suggest that the eventual disruption of the less hierarchical multiples might account for the excess of binaries among pre-main-sequence stars.