Cool Stars 14 - Submitted Abstract # 266 This version created on 05 October 2006 Properties of the Coolest Dwarfs Sandy Leggett, Gemini Observatory Eleven years after the discovery of the first T dwarf, we have a population of ultracool L and T dwarfs that is large enough to show a range of atmospheric properties, as well as model atmospheres advanced enough to study these properties in detail. Since the last Cool Stars meeting, there have been observational developments which aid in these studies. In this talk I will present recent mid-infrared imaging and spectroscopy from the Spitzer Space Telescope, and initial results from the UKIRT Infrared Deep Sky Survey search for ultracool dwarfs. The mid-infrared data confirms the prevalence of vertical mixing in the atmospheres of L and T dwarfs, which has a significant impact on chemical abundances and on the spectral energy distribution. The 700K to 2200K L and T dwarf photospheres require a large number of parameters for successful modelling: effective temperature, gravity, metallicity, grain sedimentation and vertical mixing efficiency. I will explore the impact of these parameters on infrared colors and spectra, and demonstrate methods for measuring, or constraining, these parameters. The coolest known T dwarfs have effective temperatures of 700-800K. The properties of these half-dozen objects will be explored in detail, and synthetic spectra of even cooler objects shown. The definition and detection of the next class of object, the Y dwarf, will be briefly discussed. The author acknowledges major and multiple contributions from her collaborators; full references will be given during the talk and in the proceedings. ----------------------------------