The spectrum of the brown dwarf Gliese 229B

We present a spectrum of the cool (T-eff = 900 K) brown dwarf Gliese 229B. This spectrum, with a relatively high signal-to-noise ratio per spectral re solution element (greater than or similar to 30), spans the wavelength rang e from 0.837 mu m to 5.0 mu m. We identify a total of four different major methane absorption features, including the fundamental band at 3.3 mu m, at least four steam bands, and two neutral cesium features. We confirm the re cent detection of carbon monoxide (CO) in excess of what is predicted by th ermochemical equilibrium calculations. Carbon is primarily involved in a ch emical balance between methane and CO at the temperatures and pressures pre sent in the outer parts of a brown dwarf. At lower temperatures, the balanc e favors methane, while in the deeper, hotter regions, the reaction reverse s to convert methane into CO. The presence of CO in the observable part of the atmosphere is therefore a sensitive indicator of vertical flows. The hi gh signal-to-noise ratio in the 1 mu m to 2.5 mu m region permits us to pla ce constraints on the quantity of dust in the atmosphere of the brown dwarf . We are unable to reconcile the observed spectrum with synthetic spectra t hat include the presences of dust. The presence of CO but lack of dust may be a clue to the location of the boundaries of the outer convective region of the atmosphere: The lack of dust may mean that it is not being conveyed into the photosphere by convection, or that it exists in patchy clouds. If the dust is not in clouds, but rather sits below the outer convective regio n, we estimate that the boundary between outer convective and inner radiati ve layers is between 1250 K and 1600 K, in agreement with recent models.