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Session 26 - Neutrinos & Stellar Abundances.
Oral session, Wednesday, January 07

[26.02] Detection of Abundant Carbon Monoxide in the Brown Dwarf Gliese 229B

K. S. Noll (STScI), T. R. Geballe (JACH), M. S. Marley (NMSU)

The spectrum of Gl 229B in the 4.5-5.1 \mum interval shows evidence for CO at mole fractions of qCO > 50 ppm. Molecular line opacity limits the depth to which we can see at these wavelengths to the T \approx 800 K level. At this temperature, the predicted equilibrium abundance of CO (Fegley and Lodders, ApJ 472, L37 [1996]) is more than 1600 times lower than the lower limit we determine. Dynamical quenching of CO-CH_4 equilibrium is one mechanism that can lead to enhanced CO at low temperatures, but this mechanism requires convection in the T \approx 800-1250 K, P \approx 1-8 bar region of Gl 229B's atmosphere, a region in which a detached convection zone is predicted by some models of Gl 229B (Marley et al. Science 272, 1919 [1996]). The presence of disequilibrium abundances of CO in Gl 229B's upper atmosphere reduces the emergent flux in the 4-5 \mum interval and may make searches for new brown dwarfs using this band less sensitive.

Program listing for Wednesday