Coronal stripping in supersaturated stars

Abstract

A recent unambiguous detection of X-ray rotational modulation of the supersaturated star VXR45 (P = 0.223 days) has shown that its corona has discrete dark and bright X-ray regions. We suggest that due to the rapid rotation, the X-ray emitting corona has been centrifugally stripped away, creating open field regions that are dark in X-rays. This leads naturally both to a significant rotational modulation in X-rays but also to the lower X-ray luminosity of supersaturated stars compared to those rotating more slowly. To demonstrate the effect, we take as an example a more slowly rotating star for which surface magnetograms are available. We extrapolate the potential coronal magnetic field based on these magnetograms and determine for a hydrostatic, isothermal atmosphere the structure of the density and of the optically-thin X-ray emission. We show that if the rotation rate of this star were increased, the magnitude of the X-ray luminosity would decrease while its rotational modulation would increase in a way that is consistent with the recent observations of VXR45.