A search for binary systems among the nearest L dwarfs

We have used the NICMOS NIC1 camera on the Hubble Space Telescope to obtain high angular resolution images of 51 ultracool dwarfs in the immediate Solar Neighbourhood. Nine systems are resolved as binary, with component separations from 1.5 and 15 AU. All of the systems have components with similar luminosities, and, consequently, high mass ratios, q > 0.8. Limiting analysis to L dwarfs within 20 parsecs, the observed binary fraction is 12(+7/-3). Applying Bayesian analysis to our dataset, we derive a mass-ratio distribution that peaks strongly at unity. Modelling the semi-major axis distribution as a logarithmic Gaussian, the best fit is centered at log(a_0) = 0.8 AU (~6.3 AU), with a (logarithmic) width of 0.3. The current data are consistent with an overall binary frequency of ~24%.