Discovery of four white dwarfs with strong magnetic fields by the Hamburg/ESO Survey

Four magnetic white dwarfs have been found in the course of the Hamburg/ESO Survey for bright QSOs. The objects have been selected as QSO candidate on the basis of its blue continuum and the apparent absence of strong hydrogen or helium lines. One star, HE1211-1707, shows a rather fast spectral variability: both the strength and the position of the shallow absorption features change on a time scale of 20 minutes. We interpret this variability as being due to a magnetic field on the surface of a rotating white dwarf, having a relatively uniform magnetic field on one hemisphere and a much larger spread of field strengths visible during other phases of the rotational period. All attempts to determine the magnetic field structure in detail with the help of synthetic spectra have failed so far because the star must have a rather complicated field geometry. However, both the optical and the UV spectra indicate that a significant part of the surface is dominated by a magnetic field strength of about 80 MG. The spectrum of HE 0127-3110 is also rotationally modulated. This star (approximate range of magnetic fields: 85-345 MG) as well as HE 2201-2250 (a spectroscopic twin of HE 0127-3110) and HE 0000-3430 (43-118 MG) could be reasonably well reproduced with the help of theoretical spectra calculated assuming magnetic dipoles which are offset by 0.1 and 0.2 stellar radii along the magnetic axis. This result is in agreement with the assumption that Ap stars, also showing significant deviations from a centered dipole, are the progenitors of magnetic white dwarfs.