Coronal emission and dynamo saturation

We show that the observed saturation of X-ray emission with increasing rota tion rate in solar-type stars does not necessarily imply saturation of the underlying dynamo. This effect can be caused by the centrifugal stripping o f 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 prominenc es close to the co-rotation radius where centrifugal forces begin to domina te over gravity. In order for these massive prominences to be contained aga inst centrifugal ejection, the coronal magnetic field must be highly stress ed and must be evolving on a timescale of a few days as determined by the o bserved 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 clo ser to the surface, reducing the extent of the "undisturbed" corona. We tak e 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.