On the turbulent viscosity prescription in accretion discs

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Abstract
The validity of the standard alpha viscosity prescription in turbulent accretion discs is briefly analysed. It is found that the alpha viscosity prescription is no longer valid when the rotation law of the disc is not Keplerian. The analysis is based on simple scaling arguments for fully developed turbulences in accretion discs. An analytical treatment is carried out for geometrically thin non-Keplerian discs in vertical hydrostatic equilibrium. The analytical expression of the viscosity is compared with standard values of the viscosity in boundary layer regions and with the viscosity prescription derived from a general causal equation for the evolution of the stress tensor.

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