Magnetic topology and prominence patterns on AB Doradus

Abstract

We report new Zeeman--Doppler imaging observations of the rapidly rotating young K0 dwarf AB Doradus, obtained with the Anglo-Australian Telescope in 1996 December. From such observations, simultaneous brightness and magnetic images of the stellar photosphere of AB Dor were reconstructed at three different epochs. The magnetic topology of AB Dor is found to be very complex, with at least 12 different radial field regions of opposite polarities located all around the star. Significant azimuthal field fluxes are also detected in the form of one negative polarity region close to the equator, a series of positive polarity patches at intermediate latitudes and an almost complete ring of negative polarity encircling the rotational pole at high latitudes. In particular, the azimuthal polarities we reconstruct are in very good agreement with those obtained by Donati & Cameron, confirming that this field component is directly related to the dynamo-generated large-scale toroidal magnetic structure. The triple polarity latitudinal pattern observed for this structure in the upper hemisphere of AB Dor indicates that the degree of the underlying large-scale poloidal structure in an axisymmetric spherical harmonics expansion is equal to or greater than five. It also strengthens the idea that the dynamo processes operating in AB Dor feature a non-solar component distributed throughout the convective zone. From the subtle distortion of successive brightness images, we can also confirm the surface differential rotation first measured on this star by Donati & Cameron in both sense and magnitude, with a pole rotating more slowly than the equator by about one part in 220. Finally, the rotation periods we measure for four prominences (from the recurrence rate of their spectral signatures in Balmer lines) confirm the presumption that such clouds are anchored at intermediate to high latitudes. The intrinsic variability of these prominences is not associated with any abrupt changes in the photospheric brightness or magnetic distributions, implying that they essentially result from the reorganization of coronal field lines.