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.

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