Properties of the Coolest Dwarfs
Eleven years after the discovery of the first T dwarf, a population of ultracool L and T dwarfs has been identified that is large enough to show a range of atmospheric properties. Also, model atmospheres are sufficiently advanced to study these properties in detail. Since the last Cool Stars meeting, several observational developments have aided in these studies. We present recent mid-infrared photometry and spectroscopy from the Spitzer Space Telescope which confirms the prevalence of vertical mixing in the atmospheres of L and T dwarfs. Hence, the 700 K to 2200 K L and T dwarf photospheres require several parameters for successful modelling: effective temperature, gravity, metallicity, grain sedimentation efficiency and vertical mixing efficiency. We also describe initial results of a search for ultracool dwarfs in the UKIRT Infrared Deep Sky Survey, and present the latest T dwarf found to date. We conclude with a discussion of the possible spectral indicators of Y dwarfs.