Instability of an accretion disk with a magnetically driven wind

¹ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China, and National Astronomical Observatories, Chinese Academy of Sciences, China
² Max-Planck-Institut für Astrophysik, Postfach 1317, 85741 Garching bei München, Germany

Corresponding author: H. C. Spruit, henk@mpa-garching.mpg.de

Received: 2 August 2000
Accepted: 18 December 2001

Abstract

We present a linear analysis of the stability of accretion disks in which angular momentum is removed by the magnetic torque exerted by a centrifugally driven wind. The effects of the dependence of the wind torque on field strength and inclination, the sub-Keplerian rotation due to magnetic forces, and the compression of the disk by the field are included. A WKB dispersion relation is derived for the stability problem. We find that the disk is always unstable if the wind torque is strong. The growth time scale of the instability can be as short as the orbital time scale. The instability is mainly the result of the sensitivity of the mass flux to changes in the inclination of the field at the disk surface. Magnetic diffusion in the disk stabilizes if the wind torque is small.