Journal Article

The stability of accretion discs with inflow driven purely by magnetic winds

C. G. Campbell

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 392, issue 1, pages 271-280
Published in print January 2009 | ISSN: 0035-8711
Published online December 2008 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.14035.x
The stability of accretion discs with inflow driven purely by magnetic winds

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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, Np, measures the ratio of turbulent viscosity to turbulent magnetic diffusivity. Formally, the hypothetical limit Np→ 0 corresponds to the magnetic wind torque dominating the viscous torque, but in practice the inflow is magnetically controlled for Np≲ 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, η, to be assessed. For a standard parametrized form of η, 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 η, due to the temperature perturbation, but this does not relieve field bending sufficiently to prevent the instability.

Keywords: accretion, accretion discs; instabilities; magnetic fields

Journal Article.  4714 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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