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Received ; accepted Abstract. An explanation of the observed saturation of X-ray emission with increasing rotation rate of late-type stars is suggested. The suggestion is based on a simple model for the closed field line region of the magnetized stellar corona (stellar magnetosphere). The model assumes a magnetic dipole field aligned with the rotation axis confining a co-rotating, isothermal plasma. The e ects of a stellar wind and of toroidal fields are neglected in the model. The closed field line region in the model is defined by those field lines on which the ratio of plasma pressure to magnetic pressure (plasma ) does not exceed one. Using scaling relations of surface magnetic field strength, surface density and coronal temperature with angular velocity, the model is used to calculate the stellar X-ray emission measure as a function of rotation rate. The results are in good agreement with the saturation in X-ray emission with rotation that is observed in G-K stars. The dependence of the results on the parameters of the model will be discussed.

A simple model for the saturation of coronal X-ray emission of rapidly rotating late-type stars