(Proposal ID) S10A-069 (PI) Lucas, Philip (Proposal Title) Discovering the coolest brown dwarfs and measuring the substellar mass function. (Abstract) Since the discovery of the first cold brown dwarf in 1995, the sample of brown dwarfs has steadily increased and the extension of the conventional spectral sequence from M to T dwarfs has been established. Recent discoveries from UKIDSS and the CFHT Legacy survey have now extended the sequence to brown dwarfs cooler than the coolest T dwarfs that were previously known. We provisionally classify these new objects as T8.5 and T9 dwarfs, although the new Y dwarf spectral type has been suggested. Some of these objects have temperatures T=500-600 K and it is likely that we can find even cooler objects. This new temperature range is interesting in itself because of the new atmospheric physics. It also provides the essential data needed to determine the substellar mass function in the local field by breaking the age-mass degeneracy in brown dwarfs. This requires us to build a statistically useful sample of the coolest brown dwarfs. We propose to observe the best candidate late T and Y dwarfs from UKIDSS DR7, prior to the official data release, to detect the predicted ammonia absorption features and additional features. This is a continuation of past successful proposals, which have resulted in the detection of the first UKIDSS T dwarf and 85 other T dwarfs down to T9.