Coronal emission and dynamo saturation

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.