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M. Marley (New Mexico State University), A. Ackerman (NASA/Ames Research Center), S. Seager (Inst. for Advanced Study)
The infrared spectra and colors of L- and T-dwarfs are strongly influenced by the vertical distribution of condensates in their atmospheres. The appearance and later disappearance of iron and silicate grains with falling effective temperature control the vertical temperature structure and emergent spectra of these objects. However simplistic chemical equilibrium models in which the condensates do not fall out of the atmosphere fail to reproduce the available observations.
We have developed a a new method for calculating vertical profiles of particle size distributions in condensation clouds in giant planet and brown dwarf atmospheres. By assuming a balance between turbulent diffusion and precipitation by large drops or grains, we are able to include rainfall for the first time in a brown dwarf atmosphere model. By accounting for downward transport by grains larger than the mean particle size, the model produces clouds which are thinner in vertical extent than in previous treatments. This method reproduces the particle size and thickness of Jupiter's ammonia clouds and accounts for the variation in infrared colors among the L- and T-dwarfs. The model also predicts that water clouds, when they appear in objects cooler than T\rm eff ~ 500\,\rm K, will produce objects which appear red in J-K, unlike slightly warmer T-dwarfs which appear blue.
We will present comparisons of model results to brown dwarf colors and spectra compiled by the SDSS and 2MASS surveys. A new effective temperature scale based on the appearance and disappearance of silicate, iron, and water clouds will also be discussed.
Research supported by NSF grants AST-0086288 and AST-9624878.