The Spectrum of the Brown Dwarf Gliese 229B

Abstract

We present a spectrum of the cool K) brown dwarf Gliese 229B. This spectrum, with a (T eff 900 relatively high signal-to-noise ratio per spectral resolution element spans the wavelength range (Z30), from 0.837 k mt o 5.0 km. We identify a total of four di†erent major methane absorption features, includ- ing the fundamental band at 3.3 km, at least four steam bands, and two neutral cesium features. We con-rm the recent detection of carbon monoxide (CO) in excess of what is predicted by thermochemical equilibrium calculations. Carbon is primarily involved in a chemical balance between methane and CO at the temperatures and pressures present in the outer parts of a brown dwarf. At lower temperatures, the balance favors methane, while in the deeper, hotter regions, the reaction reverses to convert methane into CO. The presence of CO in the observable part of the atmosphere is therefore a sensitive indicator of vertical Nows. The high signal-to-noise ratio in the 1 k mt o 2.5 km region permits us to place con- straints on the quantity of dust in the atmosphere of the brown dwarf. We are unable to reconcile the observed spectrum with synthetic spectra that 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 atmo- sphere: 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 region, we estimate that the boundary between outer convective and inner radiative layers is between 1250 K and 1600 K, in agreement with recent models. Subject headings: circumstellar matter E line: identi-cation E stars: individual (Gliese 229B) E stars: low-mass, brown dwarfs