Dust cloud formation in stellar environments I. A radiative/thermal instability of dust forming gases

In this paper, we report on an instability possibly inherent in dust formin g media due to multi-dimensional radiative transfer effects. Starting from a solution of a grey 1D or 2D radiative transfer problem, we introduce smal l harmonic 3D-perturbations of the opacity structure kappa(r) and show that , in the linear approximation, the mean intensity J(r) is modified in the s ame manner, resulting in spatial variations of the temperature structure. I f the medium possesses a strongly temperature-dependent opacity - as expect ed in dust forming environments - the perturbations can be self-amplifying. We apply this concept to a spherically symmetric, stationary model for the dust-driven wind of IRC+10216 and find that large-scale perturbation modes perpendicular to the radial direction are unstable in the outer parts of t he dust formation region (r greater than or similar to 2.5 R-*), which may result in the formation of dust clouds.