Close binary systems among very low mass stars and brown dwarfs

Using Monte Carlo simulations and published radial velocity surveys we have constrained the frequency and separation (a) distribution of very low mass star (VLM) and brown dwarf (BD) binary systems. We find that simple Gaussian extensions of the observed wide binary distribution, with a peak at 4 au and 0.6 < σlog(a/au) < 1.0, correctly reproduce the observed number of close binary systems, implying a close (a < 2.6 au) binary frequency of 17–30 per cent and overall frequency of 32–45 per cent. N-body models of the dynamical decay of unstable protostellar multiple systems are excluded with high confidence because they do not produce enough close binary VLMs/BDs. The large number of close binaries and high overall binary frequency are also completely inconsistent with published smoothed particle hydrodynamical modelling and argue against a dynamical origin for VLMs/BDs.