Zeeman Doppler imaging of stars with the AAT

Zeeman Doppler Imaging (ZDI) is a recent technique for measuring magnetic fields on rapidly rotating, active stars. ZDI employs spectropolarimetry taken at different rotational phases, to derive information on the magnetic field distribution over the stellar surface. The Zeeman effect is used to identify the presence of a magnetic field, and variations in Doppler wavelength shifts across the rapidly rotating star allow fields to be resolved on different parts of the visible disc. Analysis of the spectra can be used to produce both thermal and surface magnetic images. ZDI requires very high S/N spectra to be acquired, within a time interval short compared to the stellar rotation period. As a result, a large aperture telescope is needed. Since an initial successful test in 1989, the 3.9m Anglo-Australian Telescope has been used to obtain ZDI spectra of active stars of different evolutionary stages. The observations have concentrated on the K subgiant in the RS CVn system HR 1099, to monitor changes on this bright and active star. With the advent in 1991 of ZDI spectropolarimetry with the AAT 'echelle spectrograph, it has become possible to co-add the polarisation signature from the many magnetically sensitive lines recorded simultaneously. As a result, stellar magnetic field detections of unprecedented quality have been obtained. The aims of this paper are to briefly outline the principles of ZDI, describe the instrumental setup at the AAT and present some preliminary results from recent observations.