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 \times 10 ^ { 3 } – 3 \times 10 ^ { 5 } cm s ^ { -2 } and T _ { eff } = 600 – 1600 K ) to obtain plausible particle sizes for several abundant species ( Fe , Mg _ { 2 } SiO _ { 4 } , and Ca _ { 2 } Al _ { 2 } SiO _ { 7 } ) .
At the base of the clouds , where the particles are largest , the particle sizes thus computed range from \sim 5 \micron to over 300 \micron 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 .