Diploma Thesis
Staubdiffusion in protoplanetaren Akkretionsscheiben

Michael Wehrstedt

IR-spectra of many comets show clear evidence of crystalline silicate (predominantly olivine) in notable quantities, beside the known features of amorphous silicate dust. It is likely that the matter in comets represents unaltered pristine material from the time of the solar nebula, which has been "frozen" since its integration in cometesimals. Since the dust in the interstellar medium is exclusively amorphous, the protoplanetary accretion disc must be the origin of crystalline silicate. A possible formation process is annealing of amorphous silicate in the warm inner zone of the accretion disc. Due to turbulence in the optically thick disc, a fraction of the annealed silicate dust reaches the cooler outer region of the disc by being carried along with the gas, where it may be built into larger bodies, e.g. cometesimals. In the matrix of some chondrites are also indications of silicate dust, which has experienced an annealing-event under the conditions of the solar nebula. This work investigates the diffusion and crystallisation of dust in protoplanetary discs. For this purpose the diffusion equation of the dust (including annealing) is linked to the equations for the global evolution of an alpha-disc within an one-dimensional, time-dependent numerical model. The results suggest that the diffusive transport spreads the dust globally throughout the disc, and therefore provides an explanation for crystalline silicate within the primordial bodies of the solar system.

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