Application of the dipole model to selected magnetic white dwarfs

Various authors have reported observations of the flux and circular polarization for the three stars PG 1658+441, PG 1533-057 and K 813-14. On the basis of the observational data, the stars were classified as magnetic white dwarfs. To place constraints on the magnetic field strengths and geometries of these stars, the relevant authors qualitatively compared the data with available theory and, in two cases, used a model of optically thin hydrogen threaded by a magnetic field. In this paper a more detailed model for magnetic white dwarfs is used to assess the results previously obtained for these three stars. In two cases, the observed spectra can be explained by the Zeeman splitting of hydrogen lines in a stellar magnetic field which takes the form of a dipole situated at the centre of the star. The circular polarization data for PG 1658+441, however, may indicate a field geometry for this star which is significantly different from that of a centred dipole. 12 refs., 8 figs