PHASE-RESOLVED HIGH-RESOLUTION SPECTROPHOTOMETRY OF THE ECLIPSING POLAR HU AQUARII

We present phase-resolved spectroscopy of the bright. eclipsing polar HU Aqr obtained with high time (similar to 30 sec) and spectral (1.6 A ngstrom) resolution when the system was in a high accretion state. The trailed spectrograms reveal clearly the presence of three different l ine components with different width and radial velocity variation, By means of Doppler tomography their origin could be located unequivocall y (a) on the secondary star, (b) the ballistic part of the accretion s tream (horizontal stream), and (c) the magnetically funnelled part of the stream (vertical stream). For the first time we were able to deriv e a (near-)complete map of the stream in a polar. We propose to use Do ppler tomography of AM Herculis stars as a new tool for the mass deter mination of these binaries. This method, however, still needs to be ca librated by an independent method. The asymmetric light curve of the n arrow emission line originating on the mass-donating companion star re veals evidence for significant shielding of 60% of the leading hemisph ere by the gas between the two stars.