A Dynamo from star-disk collisions: a mean field approach
Abstract
It is shown that a dynamo can operate in an AGN accretion disk due to the Keplerian shear and due to the helical motions of expanding and twisting plumes of plasma heated by many star passages through the disk. Each plume rotates a fraction of toroidal flux into poloidal flux, always in the same direction, and proportional to its diameter. The rate of star-disk passages is high and the distance between the plumes produced by such collisions is small compared to the radius of the disk. One can average over many plumes and develop a theory for large scale magnetic fields on scales larger than the size of the plumes. The equations for the mean field are found to be similar to the Galactic dynamo mean field equations. The primary parameter determining the growth rate of the mean field is the disk area coverage factor by plumes, q. The dynamo is excited for q < 0.04 and has a maximum growth rate (quadrupole mode) of 0.1 of the Keplerian frequency for q = 0.01. Because of tidal disruption of stars close to the black hole, the maximum growth rate of quadrupole field, occurs at a radius of about 100 gravitational radii from the central object. Saturated field will be advected inwards leading to a different field distribution at nonlinear stage of the dynamo. Qualitative agreement with numerical simulations of the dynamo is found.
- Publication:
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APS April Meeting Abstracts
- Pub Date:
- April 2002
- Bibcode:
- 2002APS..APRD11010P