Polar Fields for AB Dor

Polar spots are often observed on rapidly-rotating cool stars, but the nature of the magnetic field in these spots is as yet unknown. While Zeeman-Doppler imaging can provide surface magnetic field maps over much of the observed stellar surface, the Zeeman signature is suppressed in the dark polar regions. We have determined the effect on the global coronal structure of three current models for this polar field: that it is composed (a) of unipolar field, (b) of bipolar regions or (c) of nested rings of opposite polarity. We take as an example the young, rapid rotator AB Dor (rotation period = 0.514 days). By adding these model polar fields into the surface field maps determined from Zeeman-Doppler imaging, we have compared the resulting coronal structure with the observable properties of the corona - the magnitude and rotational modulation of the X-ray emission measure and the presence of slingshot prominences trapped in the corona around the Keplerian co-rotation radius. We find that only the presence of a unipolar spot has any significant effect on the overall coronal structure, forcing much of the polar field to be open.Comment: 10 pages, 21 figure

Prominence support in potential field configurations around rotating stars

We present a general method for determining the locations of stable gravitational centrifugal potential minima within arbitrary potential magnetic field configurations surrounding rotating single stars. We show that stable locations exist both inside and outside the corotation radius of a single star, for even the simplest field configurations. We show that for prominence formation to occur at significant distances above the equatorial plane, as observed in stars such as AB Dor and PZ Tel, the field must show significant departures from a simple dipole to distances as great as four or more stellar radii from the star. We demonstrate that the radial-field distribution derived from a Zeeman-Doppler image of AB Dor yields sufficient field complexity at large distances for prominence sheets to form outside the corotation radius at latitudes that transit the stellar disc, assuming an axial inclination of 60 degrees to the line of sight.</p

Prominence support in potential field configurations around rotating stars

We present a general method for determining the locations of stable gravitational centrifugal potential minima within arbitrary potential magnetic field configurations surrounding rotating single stars. We show that stable locations exist both inside and outside the corotation radius of a single star, for even the simplest field configurations. We show that for prominence formation to occur at significant distances above the equatorial plane, as observed in stars such as AB Dor and PZ Tel, the field must show significant departures from a simple dipole to distances as great as four or more stellar radii from the star. We demonstrate that the radial-field distribution derived from a Zeeman-Doppler image of AB Dor yields sufficient field complexity at large distances for prominence sheets to form outside the corotation radius at latitudes that transit the stellar disc, assuming an axial inclination of 60 degrees to the line of sight.</p