DISCOVERY OF A SUB-MEGAGAUSS MAGNETIC WHITE-DWARF THROUGH SPECTROPOLARIMETRY

A circular polarization survey for weakly magnetized (approximately 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 Halpha and Hbeta, indicating a mean longitudinal ("effective") field of B(e) = + 349 +/- 19 kG. Corresponding estimates of the mean surface field and dipolar field values are B(s) = 950 +/- 100 kG and B(p) = 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 approximately 1 MG and the vast majority are essentially nonmagnetized.