The Cosmological Evolution of Dust Clouds inBrown Dwarf Atmospheres

摘要

We present our latest DRIFT-PHOENIX model results for dust cloud formation in coolatmospheres and investigate the influence of the metallicity [MM] on the dust cloud structure onthe example of a typical mid-L dwarf (Teff=2000K, log(g)=5.0). Our new dust model simulates theformation of TiO_2seed particles (nucleation) and their subsequent gravitational settling, accompa-nied by kinetic growth and evaporation of seven solid species on top of these seeds. The gas phasedepletion by each of the considered surface reactions and the element replenishment by convectiveup-mixing are consistently coupled into this mechanism. The result is a stationary cloud structure inphase-non-equilibrium. We observe a comparably low decrease of the dust particle number densityand a weak increase of the mean grain size in the lower dust cloud for decreasing metallicities downto [M/H]=-4.0. For even lower metallicities to [M/H]=-6.0, these trends are reversed.