Zeeman-Doppler imaging of active stars. V. Sensitivity of maximum entropy magnetic maps to field orientation.

In 1989, Semel proposed a new technique (dubbed Zeeman-Doppler imaging or ZDI) for measuring and mapping magnetic fields at the surface of cool active stars from sets of rotationally modulated circularly polarised (Stokes V) profiles of spectral lines. Two years later, an automatic tool (based on maximum entropy image reconstruction techniques) was developed and checked on a few test cases (Brown et al., 1991A&A...250..463B). In particular, this paper established that surface magnetic distributions consisting in circular regions of radially oriented field can be very well recovered with ZDI. In the present paper, we show new simulations demonstrating that ZDI is good at reconstructing the field orientation within magnetic regions. In particular, it can always unambiguously identify azimuthal from radial/meridional field features (where radial, azimuthal and meridional refer to the three vector field components in spherical geometry). For high latitude magnetic spots, all three field components can be recovered. For low latitude regions however, ZDI (from circular polarimetry alone) suffers some crosstalk from radial to meridional field components (or vice versa), especially at low stellar inclination angles.