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 < \sigma _ { \log ( 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 .