Cool Stars 14 - Submitted Abstract # 89 This version created on 05 October 2006 Observable micro-physics in brown dwarfs -- non-equilibrium dust cloud formation Christiane Helling, University of St Andrews, UK Peter Woitke, University Leiden, NL Matthias Dehn, University Hamburg, D Wing-Fai Thi, Royal Observatory Edinburgh, UK Brown dwarfs are the only stellar objects where dust formation takes place inside the atmosphere. Brown dwarf atmospheres are convectively dynamic. Non-equilibrium dust formation takes place and influences their spectral appearance. By applying various dust parameterizations, static model atmospheres were successful in explaining for instance the different appearance of the L and the T dwarfs. Our dust model is based on the observation, that seed formation and subsequent growth are governed by considerable differences in their characteristic time-scales which justifies the separate treatment of the two processes. The formation of the seed particles requires large supersaturation ratios. Many solid compounds are already thermodynamically stable at such low temperatures, and condense simultaneously on these seeds. On an even longer time-scale these dirty grains can gravitationally settle and continue to grow until they evaporate. This dust formation model enables us to investigate the physical and chemical structure of cloud layers in the convective atmosphere of brown dwarfs. Resultant quantities like grain size distribution and chemical composition change with atmospheric height. The upper layers are populated by small ~0.1 mum silicate grains while the deeper layers are populated by big ~100 mum grains composed of high-temperature condensates like Fe, Al2O3 and traces of CaTiO3. We will furthermore discuss the spectral appearance of such a chemically heterogeneous brown dwarf cloud layer and results on the dust abundance profiles as function of spectral type. ----------------------------------