Discovery of a sub-megagauss magnetic white dwarf through spectropolarimetry

A circular polarization survey for weakly magnetized (about 10 kG-1 MG) white dwarfs has been initiated using a newly developed CCD imaging/spectropolarimeter. Among the first two-dozen stars studied, WD 0637 + 477 revealed strong polarization reversals across both H-alpha and H-beta, indicating a mean longitudinal ('effective') field of Be = +349 +/- 19 kG. Corresponding estimates of the mean surface field and dipolar field values are Bs = 950 +/- 100 kG and Bp = 1.2 +/- 0.2 MG. WD 0637 + 477 portrays the weakest field yet found on a magnetic white dwarf by a factor of 2. Given the small survey sample observed to date, statistics on the incidence of magnetic fields in the range 100 kG-1 MG permit a distribution which either increases smoothly with decreasing field strength, or is bimodal, wherein a small percentage of stars possess fields above about 1 MG and the vast majority are essentially nonmagnetized.