The Coronal Topology of the Rapidly Rotating K0 Dwarf AB Doradus. I. Using Surface Magnetic Field Maps to Model the Structure of the Stellar Corona

We reanalyze spectropolarimetric data of AB Dor taken in 1996 December using a surface imaging code that can model the magnetic field of the star as a nonpotential current-carrying magnetic field. We find that a nonpotential field needs to be introduced in order to fit the data set at this epoch. This nonpotential component takes the form of a strong unidirectional azimuthal field of a similar strength to the radial field. This azimuthal field is concentrated around the boundary of the dark polar spot recovered at the surface of the star using Doppler imaging. As polarization signatures from the center of starspots are suppressed, it is unclear whether or not this nonpotential component genuinely represents electric current at the unspotted surface or whether it results from the preferred detection of horizontal field in starspot penumbrae. This model contains 20% more energy than the corresponding potential field model at the surface. This amount of free energy drops to under 1% about 1R* above the photosphere. We use these surface maps to model the coronal structure of the star. The mixed radial polarities at the pole in the surface maps support closed coronal loops in the high-latitude regions, indicating that a component of the X-ray emission may originate in this area. Assuming that the field remains closed out to 5R*, we find stable surfaces where prominences may form out to the observed distances using this coronal model.