Modeling the formation of clouds in brown dwarf atmospheres

Because the opacity of clouds in substellar mass object (SMO) atmospheres depends on the composition and distribution of particle sizes within the cloud, a credible cloud model is essential for accurately modeling SMO spectra and colors. We present a one-dimensional model of cloud particle formation and subsequent growth based on a consideration of basic cloud microphysics. We apply this microphysical cloud model to a set of synthetic brown dwarf atmospheres spanning a broad range of surface gravities and effective temperatures (g_surf = 1.78 × 10³-3 × 10⁵ cm s^-2 and T_eff = 600-1600 K) to obtain plausible particle sizes for several abundant species (Fe, Mg₂SiO₄ and Ca₂Al₂SiO₇). At the base of the clouds, where the particles are largest, the particle sizes thus computed range from ∼5 to over 300 μm in radius over the full range of atmospheric conditions considered. We show that average particle sizes decrease significantly with increasing brown dwarf surface gravity. We also find that brown dwarfs with higher effective temperatures have characteristically larger cloud particles than those with lower effective temperatures. We therefore conclude that it is unrealistic when modeling SMO spectra to apply a single particle size distribution to the entire class of objects.