Self-organization of Dust Forming Media

Multi-dimensional, time-dependent simulations of the dust formation process around late type stars have confirmed the existence of a thermal/radiative instability in dust forming gases. This instability may lead to a clumpy rather than a smooth dust distribution, i. e. a self-organization of irradiated, dust forming matter. The models combine time-dependent dust formation theory with detailed, frequency-dependent radiative transfer calculations by means of a Monte Carlo method, which has been carefully adapted to the special case of clumpy media and tested against deterministic codes. The models show that dust formation behind already existing dust clouds, which shield the stellar radition, is strongly favoured. In the shadow of these clouds, the temperature decreases by several hundered Kelvin which faciliates subsequent dust formation and results in radially aligned dust structures. Theses structures are similar to those revealed by high resolution observations of planetary nebulae and star formation regions.