Gerard T. van Belle, ed. Properties of the Coolest Dwarfs

Abstract. 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 ad-vanced to study these properties in detail. Since the last Cool Stars meeting, several observational developments have aided in these studies. We present re-cent 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. 1