Ammonia as a Tracer of Chemical Equilibrium in the T7.5 Dwarf Gliese 570D

We present the first analysis of an optical to mid-infrared spectrum of the T7.5 dwarf Gliese 570D with model atmospheres, synthetic spectra, and brown dwarf evolution sequences. We obtain precise values for the basic parameters of Gl 570D: Teff = 800-820 K, log g (cm s-2) = 5.09-5.23, and log = -5.525 to -5.551. The Spitzer Infrared Spectrograph (IRS) spectrum shows prominent features of ammonia (NH3) that can only be fitted by reducing the abundance of NH3 by about 1 order of magnitude from the value obtained with chemical-equilibrium models. We model departures from chemical equilibrium in the atmosphere of Gl 570D by considering the kinetics of nitrogen and carbon chemistry in the presence of vertical mixing. The resulting model spectrum reproduces the data very well.