WD 1953 − 011 – a magnetic white dwarf with peculiar field structure

We present Hα spectra of the magnetic white dwarf star WD1953−011 which confirm the presence of the broad Zeeman components corresponding to a field strength ∼ 500 kG found by Maxted & Marsh (1999). We also find that the line profile is variable over a timescale of a day or less. The core of the Hα line also shows a narrow Zeeman triplet corresponding to a field strength of ∼ 100 kG which appears to be almost constant in shape. These observations suggest that the magnetic field on WD1953−011 has a complex structure and that the star has a rotational period of hours or days which causes the observed variability of the spectra. We argue that neither an offset dipole model nor a double-dipole model are sufficient to explain our observations. Instead, we propose a two component model consisting of a high field region of magnetic field strength ∼ 500kG covering about 10% of the surface area of the star superimposed on an underlying dipolar field of mean field strength ∼ 70 kG. Radial velocity measurements of the narrow Zeeman triplet show that the radial velocity is constant to within a few km/s so this star is unlikely to be a close binary.