Program: GN-2010B-Q-41

Recent results from this programme's UKIDSS Large Area Survey census of cool brown dwarfs in the local field have found that the space density of T6 and later dwarfs disagrees with the value predicted from the observed initial mass function (IMF) in nearby young clusters. Our observed local space density of late-T dwarfs appears most consistent with a steeply declining underlying substellar mass function (given a flat birthrate), compared to a rising IMF in young clusters. The origin of this disagreement is unknown, and here we propose a well defined expansion of our programme to solve this important discrepancy. We will build up the statistical robustness of our late-T spectral type distribution to isolate the cause as one of two possibilities: shortcomings of the cooling models for brown dwarfs, or the possibility that the bulk of the field population originates from a different environment to that of the well studied nearby young clusters. We also expect to continue to extending the brown dwarf temperature sequence to ever cooler temperatures. In 10A NIRI spectroscopy has demonstrated that dwarfs cooler than 500K still lie undiscovered in the solar neighbourhood.