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Session 70 - Planetary Systems Near & Far.
Display session, Friday, January 09
Exhibit Hall,
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 resolution element (\gtrsim 30), spans the wavelength range from 0.837 \mum to 5.0 \mum. We identify a total of four different major methane absorption features, including the fundamental band at 3.3 \mum, at least four steam bands, and two neutral cesium features. We confirm the recent detection of carbon monoxide (CO) in excess of what is predicted by thermochemical equilibrium calculations. Carbon is synthesized into methane in the outer layers of the atmosphere where the temperature is low. However, in the deeper, hotter layers it is oxidized into CO. The presence of CO in the observable part of the atmosphere is therefore a sensitive indicator of vertical flows. The high signal-to-noise ratio in the 1 \mum to 2.5 \mum region permits us to place constraints on the quantity of dust in the atmosphere of the brown dwarf. We are unable to reconcile the spectrum with the presence 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 region, we estimate that the boundary between convective and inner radiative layers is between 1250 K and 1600 K, in agreement with recent models.
