Recovery of the global magnetic field configuration of 78 Virginis from Stokes IQUV line profiles

Abstract

The surface magnetic field configuration of the Ap star HD 118022 (78 Vir) has been reconstructed in the framework of the magnetic charge distribution (MCD) method from the analysis of Stokes $IQUV$ spectra obtained using the MuSiCoS spectropolarimeter at Pic du Midi Observatory. Magnetically-sensitive Fe~{\sc ii} lines were primarily employed in the analysis, supposing that iron is evenly distributed over the stellar surface. We show that the Stokes $IQUV$ profile shapes and variations of 78 Vir can be approximately fit assuming a global magnetic field configuration described by a slightly decentered, inclined magnetic dipole of polar surface intensity approximately 3.3~kG. The derived inclinations of the stellar rotational axis to the line of sight $i=24\pm 5\degr$ as well as to the magnetic dipole axis $\beta=124\pm5\degr$ are in good agreement with previous estimations by other authors, whereas the sky-projected position angle\thanks{$\Omega$ increases clockwise from the axis to the North Celestial Pole and relates to the azimuth angle $\Theta$ specified by Landolfi at al.~(\cite{Landolfi+93}) as $\Omega=360\degr-\Theta$.} of the stellar rotation axis $\Omega\sim110\degr$ is reported here for the first time. In addition, several lines of Cr~{\sc ii} and Ti~{\sc ii} were studied, yielding evidence for non-uniform surface distributions of these elements, and magnetic field results similar to those derived from Fe.