Program: GN-2014A-Q-64

Our group has developed complex model atmospheres for L-, T- and Y-type brown dwarfs, starting with the discovery of the first T dwarf in 1995. For each discovery we have combined high quality photometry and spectroscopy with the models to make progress in understanding the physics and chemistry of these cool atmospheres. Here we propose to obtain near-IR photometry for the two apparently coolest Y dwarfs known with T_eff 300-350 K, and near-IR spectroscopy for two Y0 dwarfs with T_eff 400-450 K. These two pairs each seem to have similar T_eff but quite different near-IR colors. This may be an indication of different metallicity or gravity. The data will be compared against models which we are updating to include water clouds, non-solar metallicity and mixing. These four observation sets will greatly clarify the properties of this cold solar-neighbourhood population of ~10 Jupiter-mass objects.