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 forming media due to multi-dimensional radiative transfer effects. Starting from a solution of a grey 1D or 2D radiative transfer problem, we introduce small harmonic 3D-perturbations of the opacity structure boldsymbol kappa(boldsymbol r) and show that, in the linear approximation, the mean intensity J(boldsymbol r) is modified in the same manner, resulting in spatial variations of the temperature structure. If the medium possesses a strongly temperature-dependent opacity - as expected 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 the dust formation region (r ga 2.5 R_*), which may result in the formation of dust clouds.