Dust Formation in Radioactive Enironments

P. Woitke

in: Astronomy with Radioactivities, workshop of the Max Gesellschaft at the Ringberg castle, Tegernsee, Germany, eds. R. Diehl et al.

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Abstract:

Basic conditions for the formation of dust grains in stellar environments are summarized. Special emphasis is put upon the proper definition of the "stability" of a dust grain under the prevailing conditions in the ambient medium, including the possibility of thermal non-equilibrium (different dust and gas temperatures) and chemical non-equilibrium in the gas phase. The dust temperatures of pure, crystalline grains with glassy extinction properties (e.g. corundum Al2O3, rutile TiO2 or diamonds) are thereby found to be substantially lower than others (up to 1000 K), which might favor their formation. Additional criteria for dust formation are deduced from the formation of seed particles out of the gas phase (nucleation). Present results of the application of nucleation theory to oxygen-rich gases are reviewed, considering the species (SiO)N, (MgO)N,
(Fe)N, AlxOy and (TiO2)N, where only the latter is found to be capable to provide reasonable nucleation rates at high temperatures.