. "3 Theoretical Models of SMO Structure and Atmosphere." Failed Stars and Super Planets: A Report Based on the January 1998 Workshop on Substellar-Mass Objects. Washington, DC: The National Academies Press, 1998.
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Failed Stars and Super Planets: A Report Based on the January 1998 Workshop on Substellar-Mass Objects
by A. Burrows in this chapter). At temperatures exceeding ~1000 K, however, N2 and CO dominate. Jupiter's upper troposphere exhibits signatures of CH4 and NH3. This is in contrast to the dominance of CO and N2 in cool stars. Gliese 229B, with an effective temperature of 900 to 1000 K, lies in between these two regimes. It is just cool enough to contain CH4 in thermochemical equilibrium at photospheric levels. Yet Gliese 229B's effective temperature is near that required for equal abundances of N2 and NH3, with best estimates indicating a dominance of NH3 over N2. Spectra of Gliese 229B agree with thermochemical equilibrium calculations, displaying copious features of CH4 and H2O (see Figure 3.1). The weaker signatures of NH3 make it more difficult to observe and, in fact, it has not been detected yet.
Brown dwarfs allow us to examine the nature of atmospheric dynamics from those species that are not in equilibrium. It is possible that NH3 is depleted from the upper atmosphere of Gliese 229B as a result of convection: hot underlying gas transported to the photosphere at rates fast enough to quench in abundances of a deeper atmospheric level. The upper atmosphere would then be in disequilibrium and contain compounds native to this hotter and deeper level. Such a mechanism has been proposed to explain the trace amounts of CO in Jupiter' s upper troposphere—CO rapidly upwells from the level where temperatures are ~1100 K.
If, indeed, further investigation implicates convection as the source of excessive amounts of CO in the observable atmosphere, the NH3, N2, and CO abundances in Gliese 229B will serve as powerful probes of deep atmospheric dynamics. The composition of disequilibrium species may indicate the range of levels from which material is convected and perhaps even the nature of the convection itself.
Gliese 229B's spectrum reveals an extremely clear atmosphere devoid of high cloud decks. The clarity of a planet's or brown dwarf's atmosphere depends on its thermal structure and the saturation temperature of the major constituents. Jupiter's deep atmosphere is largely concealed by an upper deck of NH3 clouds at 0.3 to 0.6 bar, and below that by the condensation of H2O at 2 bar. Gliese 229B's atmosphere is warm enough to prevent these species from condensing and cool enough to keep the next suite of condensables from residing in the upper atmosphere. As a result, Gliese 229B's atmosphere allows visibility to depths at which the pressure is greater than 20 bar, well below the region observable in Jupiter by remote sensing, and the limit of the Galileo probe's in situ exploration. Spectra at visible wavelengths taken by the U.K. Infrared Telescope (UKIRT), Keck, and HST may allow us to identify the structure and composition of Gliese 229B's deep clouds, and hence probe the characteristics of the deep atmospheres of substellar objects.
Detailed atmospheric models of SMOs are also being used to interpret observations at the bottom of the main sequence of old, low-metallicity clusters (see presentation by G. Chabrier in this chapter). The ability of the models to reproduce the colors and luminosities of such objects over a range of metallicities (and hence contributions of molecules to the spectrum) attests to the utility of high-fidelity, but complex, models employing accurate molecular opacities. Information that can be extracted from such models includes mass functions of the clusters, the contribution of SMOs to dark matter in the galactic spheroid and dark halo, and even the validity of distance determinations from various techniques (e.g., Hipparcos versus ground-based data). Such models are time-consuming to develop and computationally demanding (see presentation by P. Hauschildt in this chapter); hence their success in interpreting both individual objects (Gliese 229B) and SMOs in clusters is gratifying.