Evidence of Cloud Disruption in the L/T Dwarf Transition

Clouds of metal-bearing condensates play a critical role in shaping theemergent spectral energy distributions of the coolest classes of low-mass starsand brown dwarfs, L and T dwarfs. Because condensate clouds in planetaryatmospheres show distinct horizontal structure, we have explored a model forpartly cloudy atmospheres in brown dwarfs. Our model successfully reproducesthe colors and magnitudes of both L and T dwarfs for the first time, includingthe unexpected brightning of the early- and mid-type T dwarfs at J-band,provided that clouds are rapidly removed from the photosphere at T_eff ~ 1200K. The clearing of cloud layers also explains the surprising persistence andstrengthening of gaseous FeH bands in early- and mid-type T dwarfs. The breakupof cloud layers is likely driven by convection in the troposphere, analogous tophenomena observed on Jupiter. Our results demonstrate that planetary-likeatmospheric dynamics must be considered when examining the evolution offree-floating brown dwarfs.Comment: 12 pages, 3 figures, accepted to ApJ Letters for June 200