Program: GN-2011B-Q-43

This is an ongoing programme to confirm and characterise local field brown dwarfs found in the UKIDSS Large Area Survey. We have found that the space density of T6-T9 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 which will have crucial implications for our understanding of brown dwarf formation and evolution. 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 cooling models for brown dwarfs, or the possibility that the bulk of the sub-stellar field population originates from a different environment to that of the well studied young clusters. We also expect to extend the brown dwarf temperature sequence to even cooler temperatures. In previous semesters this program has demonstrated that dwarfs cooler than 500K still lie undiscovered in the solar neighbourhood.