Program: GN-2005A-Q-23

Recent studies of late-L and early-T dwarfs (Golimowski et al. 2004) have shown that the effective temperature of these objects is roughly constant at $\sim$1450 K. This implies that the changes observed in near-infrared spectra across the L-T transition are governed by complex cloud dynamics, not effective temperature (Leggett et al. 2004). Therefore a focused study of the late-L and early-T dwarfs is crucial to understanding the agents that govern the dynamic and chemical processes in the atmospheres of these brown dwarfs. To this end we are observing 6 late-L and early-T dwarfs from 1 to 14.5 $\rm \mu m$ so that we can combine these observations with our atmospheric models to explore in detail the parameter space involving condensation, cloud opacity, cloud clearing, metallicity, gravity and non-equilibrium chemistry. For these objects, we have 1-2.5 $\rm \mu m$ spectra and are obtaining 5.2-14.5 $\rm \mu m$ spectra from an approved Spitzer GO program. The observations proposed here will provide the remaining spectral coverage from 3-4 $\rm \mu m$ and measure the fundamental band of $\rm CH_{4}$ at 3.3 $\rm \mu m$ which is needed to constrain the atmospheric temperature scale and vertical extent of the clouds.