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 4AU and 0.6 < σ 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% and overall frequency of 32–45%. 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. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)