Program: GN-2010A-Q-44

Our UKIDSS/LAS programme is revealing a substantial population of very cool T dwarfs, cooler than found by 2MASS or SDSS, 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 planetary regime, where ammonia and/or water-cloud condensation is anticipated, potentially necessitating a new spectral type. We have already discovered 7 T8+ dwarfs (Teff=500-700K), and several show possible near-infrared ammonia absorption. We request 36.9hrs in 10A to continue our NIRI ToO programme to spectroscopically confirm and examine ~46 late-T dwarfs including ~3 T8+ objects, from ~400 sq degs of new LAS sky, and completing follow-up of LAS DR5. Increasing 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.