Microphysics of condensed carbon in white dwarf atmospheres

The microphysical model of a carbon condensate layer in a white dwarf atmosphere is proposed. It is based on the stationary solutions of an appropriate kinetic equation and takes into account the nucleation, growth-evaporation and sedimentation of condensate particles. The effects of coagulation and diffusion of particles through a condensate layer are shown to be negligibly small. The exact solutions of the kinetic equation have been found for a few probable cases of the condensate particle growth and dynamics. The mu m-sized particles predominate in the condensate layer. The solid condensate particles seem to consist of amorphous carbon rather than of any crystalline form, e.g., graphite or carbyne. The estimates show that a condensate layer may become optically thick in the visual wavelength range if the condensed carbon mass fraction exceeds 1 per cent.