The global magnetic field configuration of 78 Virginis: Modeling from line profiles in all 4 Stokes parameters

Abstract

The surface magnetic field configuration at the Ap star 78 Vir is reconstructed in the framework of the magnetic charge description (MCD) method from the analysis of Stokes IQUV spectra obtained using the MuSiCoS spectropolarimeter at Pic du Midi Observatory. Only the magnetically-sensitive Fe ii lines were employed in the analysis, supposing that iron is evenly distributed over the surface of 78 Vir. A stellar atmosphere model with T eff = 9250 K, log( g ) = 4.5 provides the best fit to the majority of lines analysed at the level of χ 2 = 1.68÷6.30. We show that the global magnetic field structure of 78 Vir can be well-described by a slightly decentered magnetic dipole. The derived inclinations of the stellar rotational axis to the line of sight i = 24° as well as to the magnetic dipole axis β = 125° agree reasonably well with previous estimations by other authors, whereas the sky-projected position angle of the stellar rotation axis Ω ~ 110°, increasing clockwise toward the north celestial pole, is determined here for the first time.