Diploma Thesis
Selbstgravitierende Akkretionsscheiben: Modelle mit alpha- und beta-Viskosität

(Selfgravitating accretion disks: Models with alpha- and beta-viscosity)


Achim Traut

Abstract:
In the standard-theory of accretion disks the viscosity is described by the so-called alpha-ansatz. This description cannot be applied to selfgravitating accretion disks in a consistent way. Duschl, Strittmatter and Biermann [DSB] introduced the so-called beta-viscosity as an alternative, which avoids this inconsistency. In the present thesis the consequences of the application of beta-viscosity on the structure of the solutions were investigated with numerical calculations. Computations were performed for accretion disks in Active Galactic Nuclei with masses of the central Black Hole ranging from 106 to 109 solar masses and accretion rates between 10-2 and 10 solar masses per year. A result of the beta-parametrization is that, in contrast to the alpha-ansatz, selfgravity becomes important only for the highest accretion rates at large distances (about 104 Schwarzschild radii and above) from the central object. A further major difference arises in the column density, which is distinctly smaller in the case of the beta-description. This implicates that the disks are optically thin in wide regions. Whereas the bolometric luminosity is independent of the viscosity, the spectral composition may well be affected. The beta-model explains the observed emission lines in the visible frequency range and the so-called "5µ-bump" better than the alpha-model. These results support the beta-ansatz and motivate further investigations.


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