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D. Saumon (Vanderbilt), T.R. Geballe (Joint Astronomy Center Hilo), S.K. Leggett (UKIRT), M.S. Marley (New Mexico State), R.S. Freedman (NASA Ames), S.K. Sengupta, Y.V. Kalinin (Vanderbilt)
The presence of CH4 in the atmosphere of Gl 229B and the near absence of CO indicate that its effective temperature is less than ~1000K. Atmosphere models reveal that at such a low temperatures, nitrogen should be in the form of N2 and NH3, with the latter favored at lower temperatures and at higher pressures. While N2 is spectroscopically invisible, NH3 has several strong absorption bands, most notably near 2\mum and in the 10\mum region. We present a high-resolution, high signal-to-noise K band spectrum of Gl 229B which we fit with synthetic spectra to constrain the effective temperature, the surface gravity, the metallicity and the NH3 abundance of Gl 229B. We find that the atmosphere of Gl 229B has a subsolar H2O abundance. NH3 is detected in the spectrum but with a NH3/H2O ratio of about 25% of the value expected from chemical equilibrium. This suggests that the NH3 to N2 chemical equilibrium is quenched at a non-equilibrium value by dynamical processes in the atmosphere. Finally, we discuss how observations in the 10\mum region can provide a better constraint on the NH3/H2O ratio.
This work has been supported in part by NASA grant NAG5-4988.