  This paper reports on computational evidence for the formation of cloud-like
dust structures around C-rich AGB stars. This spatio-temporal structure
formation process is caused by a radiative/thermal instability of dust forming
gases as identified by Woitke et al.(2000). Our 2D (axisymmetric) models
combine a time-dependent description of the dust formation process according to
Gail & Sedlmayr (1988) with detailed, frequency-dependent continuum radiative
transfer by means of a Monte Carlo method (Niccolini et al.2003) in an
otherwise static medium (v=0). These models show that the formation of dust
behind already condensed regions, which shield the stellar radiation field, is
strongly favoured. In the shadow of these clouds, the temperature decreases by
several hundred Kelvin which triggers the subsequent formation of dust and
ensures its thermal stability. Considering an initially dust-free gas with
small density inhomogeneities, we find that finger-like dust structures develop
which are cooler than the surroundings and point towards the centre of the
radiant emission, similar to the cometary knots observed in planetary nebulae
and star formation regions. Compared to a spherical symmetric reference model,
the clumpy dust distribution has little effect on the spectral energy
distribution, but dominates the optical appearance in near IR monochromatic
images.
