WD 1953-011: a magnetic white dwarf with peculiar field structure

We present H alpha spectra of the magnetic white dwarf star WD 1953-011 whi ch confirm the presence of the broad Zeeman components corresponding to a f ield strength of similar to 500 kG found by Maxted & Marsh. We also find th at the line profile is variable over a time-scale of a day or less. The cor e of the H alpha line also shows a narrow Zeeman triplet corresponding to a field strength of similar to 100 kG which appears to be almost constant in shape. These observations suggest that the magnetic field on WD 1953-011 h as 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 is sufficient to explain our observations. Instead, we propose a two-component model consist ing of a high-field region of magnetic field strength similar to 500 kG cov ering about 10 per cent of the surface area of the star, superimposed on an underlying dipolar field of mean field strength similar to 70 kG. Radial v elocity measurements of the narrow Zeeman triplet show that the radial velo city is constant to within a few km s(-1), so this star is unlikely to be a close binary.