ZEEMAN DOPPLER IMAGING OF STARS WITH THE AAT

Zeeman Doppler Imaging (ZDI) is a recent technique for measuring magne tic fields on rapidly rotating, active stars. ZDI employs spectropolar imetry taken at different rotational phases to derive information on t he magnetic field distribution over the stellar surface. The Zeeman ef fect is used to identify the presence of a magnetic field, and variati ons in Doppler wavelength shifts across the rapidly rotating star allo w fields to be resolved on different parts of the visible disk. Analys is of the spectra can be used to produce both thermal and surface magn etic images. ZDI requires very high S/N spectra to be acquired within a time interval short compared to the stellar rotation period. As a re sult, a large-aperture telescope is needed. Since an initial successfu l test in 1989, the 3.9 m Angle-Australian Telescope has been used to obtain ZDI spectra of active stars of different evolutionary stages. T he observations have concentrated on the K subgiant in the RS CVn syst em HR 1099 to monitor changes on this bright and active star. With the advent in 1991 of ZDI spectropolarimetry with the AAT echelle spectro graph, it has become possible to co-add the polarisation signature fro m the many magnetically sensitive lines recorded simultaneously. As a result, stellar magnetic field detections of unprecedented quality hav e been obtained. The aims of this paper are to briefly outline the pri nciples of ZDI, describe the instrumental setup at the AAT and present some preliminary results from recent observations.