The coronal structure of AB Dor determined from contemporaneous Doppler imaging and X-ray spectroscopy
Résumé
We obtain contemporaneous observations of the surface and corona of AB Dor using ground-based circularly polarised spectra from the Anglo-Australian Telescope and X-ray data from the Chandra satellite. The ground-based data are used to construct surface magnetic field maps, which are extrapolated to produce detailed models of the quiescent corona. The X-ray data serve as a new test for the validity of these coronal models. The high coronal density and complex multi-polar magnetic field indicate a compact X-ray corona, which is concentrated close to the surface, with a height, H~0.3-0.4R*. There is also significant correlation between the surface and coronal active region locations. At this epoch AB Dor appears to possess one very large active longitude region; displaying enhanced activity in the form of large dark spots, strong magnetic fields and chromospheric emission. Finally, the level of rotational modulation and shape of the X-ray lightcurve depend on the distribution of magnetic field in the obscured hemisphere. The models that best reproduce the rotational modulation observed in the contemporaneous Chandra X-ray lightcurve and spectra require the magnetic field in the obscured hemisphere to be of the same polarity as that in the observed hemisphere. The Sun shows different behaviour, with the leading polarity reversed in the opposite hemisphere. The X-ray observations provide a unique constraint on the magnetic structure in the obscured hemisphere.