31st Annual Meeting of the DPS, October 1999
Session 2. Extra-solar Planets: Dwarfs and Disks
Contributed Oral Parallel Session, Monday, October 11, 1999, 9:00-10:00am, Sala Kursaal

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[2.02] The Chemistry and Dynamics of a Brown Dwarf's Atmosphere

C. A. Griffith, R. V. Yelle (Northern Arizona University)

The brown dwarf, Gliese 229B, exemplifies characteristics native to the giant planets. Optical and near-IR spectra reveal a reduced chemistry largely in thermochemical equilibrium. Disequilibrium processes such as condensation and vertical mixing are also suggested by the CO feature and the continuum of Gl229B's spectrum.

Yet, Gl229B's spectra show that brown dwarfs are also exotic, having properties unique to their genre. The temperature of Gl229B's upper atmosphere is too high for the condensation of ices seen in planets, and too low for the condensation of metals and silicates of stars. The resulting clarity of this atmosphere affords a view to exceptionally high pressures. Gl229B displays features of an element (Cs) present only at temperatures of brown dwarf atmospheres, which is sensitive to the equilibrium chemistry, the heavy element abundance, and condensation processes. In addition, the atmospheric structure appears special: Gl229B's high gravity causes the stratosphere to extend down to a pressure of ~20 bars, in contrast to the ~0.5 bar radiative-convective boundary observed for planets.

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