Offset magnetic dipoles in white dwarfs

Various authors have reported observations of the flux and circular polarization for the four stars KPD 0253 + 5052, 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 four stars. For one star (KPD 0253 + 5052), clear evidence is found for deviation from a centered dipole field structure. The observational data for the other stars are not uniquely explained by centered dipole models. The observations can be interpreted, however, through the additional use of dipole field geometries offset from the stellar center.