Stokes imaging, Doppler mapping and Roche tomography of the AM Herculis system V834 Cen

Abstract

We report on new simultaneous phase-resolved spectroscopic and polarimetric observations of the polar (AM Herculis star) V834 Cen during a high state of accretion. Strong emission lines and high levels of variable circular and linear polarization are observed over the orbital period. The polarization data are modelled using the Stokes imaging technique of Potter et al. The spectroscopic emission lines are investigated using the Doppler tomography technique of Marsh and Home and the Roche tomography technique of Dhillon and Watson. Up to now, all three techniques have been used separately to investigate the geometry and accretion dynamics in cataclysmic variables. For the first time, we apply all three techniques to simultaneous data for a single system. This allows us to compare and test each of the techniques against each other and hence to derive a better understanding of the geometry, dynamics and system parameters of V834 Cen. All three techniques are consistent with an interpretation in which a ballistic stream extends to a minimum of ∼40 degrees in azimuth around the white dwarf before becoming threaded by the magnetic field lines. Interestingly, the observed ballistic Doppler velocities do not show a reduced vv component, as found in Doppler imaging of other AM Her systems. Furthermore, the secondary star in V834 Cen shows more He II (4686 Å) emission on its leading inner face, as opposed to the trailing face like in other AM Her systems. We propose that the accretion shock preferentially illuminates the leading face of the secondary star. In addition, the ballistic stream does not obscure the leading face of the secondary from the accretion shock, and, in fact, our Doppler maps show that the ballistic stream is a strong He II (4686 Å) emission source in itself and thus adds to the illumination of the leading face of the secondary.