A search for magnetic fields among DA white dwarfs

A survey for magnetism has been carried out for a magnitude-limited sample of nearly 170 DA white dwarfs utilizing the technique of Zeeman spectropolarimetry. With a mean uncertainty for longitudinal fields 〈σ_Be〉 = 8600 G, the results provide the first statistically useful survey of white dwarfs for magnetic fields weaker than a few million gauss. An additional, very weak field "minisurvey," with 〈σ_Be〉 ∼ 2000 G, is also reported. Because DA spectral types comprise nearly 80% of all white dwarfs, the results are very likely representative of the entire class. Four new magnetic stars, with field strengths between ∼10⁵ and nearly 10⁹ G, were discovered, including the three weakest field white dwarfs yet identified. The complete sample of 42 known magnetic examples is enumerated. The incidence of magnetism among white dwarfs is found to be 4.0% ± 1.5% for fields between ∼3 × 10⁴ and 10⁹ G. When broken down by field strength, the detection rate is consistent with being constant at a value of ∼1% per decade interval over the same range in B. There is no evidence thus far for the rapid increase in number of magnetic stars with declining field strength which must eventually occur. Implications for the origin of magnetic white dwarfs and their relation to stars at other stages of evolution are discussed. Several composite-spectrum binaries (DA + dM) were identified in the course of the survey, including one apparently noninteracting system with a photometric period of ∼4 hr.