AAS 201st Meeting, January, 2003
Session 46. Planetary Systems: Observations and Models
Poster, Tuesday, January 7, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[46.07] L'- and M'-band Photometry and Models of Brown Dwarfs: Evidence for Non-Equilibrium Chemistry

D. A. Golimowski (JHU), S. K. Leggett (JAC), D. Saumon (LANL), M. S. Marley (NASA/ARC), X. Fan (Arizona), R. S. Freedman (NASA/ARC), T. R. Geballe (Gemini North), G. R. Knapp (Princeton), K. Lodders (Washington U.)

Most observational and theoretical studies of L and T dwarfs have been conducted in the near-infrared J, H, and K bandpasses. However, models of brown-dwarf atmospheres show that the spectral region around 5~\mum is relatively clear of molecular absorption and reveals the structure of different atmospheric layers. To explore these layers, we have obtained M' (4.69~\mum) photometry of twelve L and T dwarfs and L' (3.77~\mum) photometry of 36 L and T dwarfs using the United Kingdom Infrared Telescope's IRCAM. Our sample of L and T dwarfs spans the range of temperatures in which carbon is transformed from CO to CH4 and in which the role of dust changes rapidly. The data support our earlier conclusion based upon a smaller sample that L and T dwarfs are fainter at M' than expected from published models of either dusty or clear atmospheres. However, these models assume molecular abundances under conditions of chemical equilibrium. We have developed new models that accommodate non-equilibrium abundances resulting from convective or diffusive vertical mixing of slowly reacting species within the atmosphere. The general effect of this mixing is the enhancement in the photosphere of CO and N2, and a corresponding depletion of CH4, H2O, and NH3. By adjusting the diffusion coefficient that parametrizes our models, we accurately reproduce the observed L' and M' colors and magnitudes of our sample of L and T dwarfs, as well as the spectrum of Gl 229B between 3.5 and 5.5~\mum. Our observations and models have important implications for the direct detection of extrasolar giant planets (EGPs) in the M' band. EGPs of certain effective temperatures and ages may be substantially fainter than previous evolutionary models suggest.

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