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Lookup NU author(s): Dr Christopher Campbell
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A model is presented for an accretion disc in which the inflow is driven purely by the angular momentum removed in a centrifugally accelerated magnetic wind. Turbulent discs around compact stars are considered, with the required magnetic field being generated in the disc by a simple dynamo. The turbulent magnetic Prandtl number, N-p, measures the ratio of turbulent viscosity to turbulent magnetic diffusivity. Formally, the hypothetical limit N-p -> 0 corresponds to the magnetic wind torque dominating the viscous torque, but in practice the inflow is magnetically controlled for N-p less than or similar to 0.1. The suggestion by previous authors that purely magnetic wind-driven discs may be unstable is investigated. A detailed steady solution is found which allows perturbations to the thermal balance and vertical equilibrium to be calculated, and hence the effect of perturbations to the magnetic diffusivity eta, to be assessed. For a standard parametrized form of eta, the wind-driven angular momentum balance is found to be linearly unstable. An increase in the inflow rate leads to increased bending of the poloidal magnetic field and an enhanced wind mass loss rate. This increases the angular momentum loss rate which drives further inflow. There is a resultant increase in eta, due to the temperature perturbation, but this does not relieve field bending sufficiently to prevent the instability.
Author(s): Campbell CG
Publication type: Article
Publication status: Published
Journal: Monthly Notices of the Royal Astronomical Society
Year: 2009
Volume: 392
Issue: 1
Pages: 271-280
ISSN (print): 0035-8711
ISSN (electronic): 1365-2966
Publisher: Wiley-Blackwell Publishing Ltd.
URL: http://dx.doi.org/10.1111/j.1365-2966.2008.14035.x
DOI: 10.1111/j.1365-2966.2008.14035.x
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