Clouds and Chemistry: Probing the Atmospheres of Ultracool Dwarfs with Spitzer

Michael Cushing

In contrast to hot stars, the atmospheres of very low-mass stars and brown dwarfs are exceedingly complex because their low temperatures and high pressures favor the formation of molecules (both gas and dust) from their constituent atoms. As a result, the emergent spectra of these so-called ultracool dwarfs are dominated by broad molecular absorption bands (e.g., TiO, H2O, and CH4).

The launch of the Spitzer Space Telescope has opened a heretofore untapped wavelength range for the study of ultracool dwarfs. The 5.5 to 14.5 micron spectra exhibit not only absorption bands of H2O and CH4, but also NH3. I will describe what we have learned to date about the atmospheres of ultracool dwarfs using Spitzer. In particular, I will show that vertical mixing in the atmospheres of the coolest brown dwarfs reduces the abundance of NH3 by nearly an order of magnitude from that predicted by chemical equilibrium. I will also discuss the tentative direct detection of the silicate cloud decks in the atmospheres of a small sample of brown dwarfs via the classic 10 micron silicate feature.