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Astron. Astrophys. 346, 883-891 (1999)


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Coronal emission and dynamo saturation

M. Jardine 1 and Y.C. Unruh 1,2

1 Department of Physics and Astronomy, North Haugh, St. Andrews KY16 9SS, Scotland, UK
2 Institut für Astronomy, Universität Wien, Türkenschanzstrasse 17, A-1180 Vienna, Austria

Received 31 October 1998 / Accepted 5 March 1999

Abstract

We show that the observed saturation of X-ray emission with increasing rotation rate in solar-type stars does not necessarily imply saturation of the underlying dynamo. This effect can be caused by the centrifugal stripping of the corona. As the stellar rotation rate is increased, centrifugal forces cause a rise in the pressure and density in the outer parts of the largest magnetic loops. This may explain the observed formation of large prominences close to the co-rotation radius where centrifugal forces begin to dominate over gravity. In order for these massive prominences to be contained against centrifugal ejection, the coronal magnetic field must be highly stressed and must be evolving on a timescale of a few days as determined by the observed prominence lifetime. We postulate that the co-rotation radius marks the position at which the corona becomes distorted by centrifugal forces. As the stellar rotation rate is increased, the co-rotation radius moves closer to the surface, reducing the extent of the "undisturbed" corona. We take as illustrative examples two simple field topologies and show that if the X-ray emission comes only from this undisturbed volume, then a saturation of the X-ray emission with increasing rotation rate occurs naturally. This effect may mimic true dynamo saturation or disguise its onset.

Key words: stars: activity – stars: coronae – stars: late-type – stars: magnetic fields – stars: starspots


© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999

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