Published online by Cambridge University Press: 13 May 2016
We discuss the important roles which impulsive processes seem likely to play in the formation of binary star systems and higher multiples. On the basis of numerical simulations and theoretical considerations, we show (i) that when a dense layer is produced by a cloud/cloud (or clump/clump) collision, or a dense shell is swept up by an expanding nebula (HII Region, Stellar-Wind Bubble or Supernova Remnant), fragmentation leads to the formation of small-N subclusters of massive protostellar discs, including many wide binaries; (ii) that impulsive interactions between these protostellar discs are frequent and can lead to an hierarchical cascade, spawning new protostellar discs in closer binary systems, plus some escapers. This cascade is particularly effective if there is continuing infall replenishing the protostellar discs. The binaries formed have a wide range of separations and orbital eccentricities, and the Mass Function of the new protostars has an exponent ∼ −1.