Program: GN-2009B-Q-62

Our UKIDSS/LAS programme is revealing a substantial population of very cool T dwarfs, un-probed by previously world leading surveys and providing unprecedented sensitivity to the form of the substellar initial mass function (IMF), with critical implications for understanding star and planet formation. We are revealing the physics of atmospheres with temperatures well into the warm panetary regime, where ammonia and/or water-cloud condensation is anticipated, potentially necessitating a new spectral type. We have already discovered six T8+ dwarfs (Teff=500-700K), and several show possible near-infrared ammonia absorption. We request 33.8hrs in 09B to continue our NIRI ToO programme to spectroscopically confirm and examine ~40 late-T dwarfs including ~3 T8+ objects, from ~550 sq degs of new LAS sky. Growing our sample in this parameter space is crucial if we are to unambiguously measure the substellar IMF, and reveal the full spectroscopic diversity due to different surface gravities and metallicities.