The incidence of magnetism among white dwarfs: The first stars below 100 kilogauss

A survey for magnetic fields among a magnitude-limited sample of DA white dwarfs has identified two stars with weak circular polarization features across the profiles of Hα and Hβ. WD 1350-090 (LP 907-037) was found to have a disk-averaged longitudinal field component B_e = +85 ± 9 kG at one epoch, while WD 0009 + 501 (G 217-037) has been measured on several occasions at values between B_e ∼ 0 and nearly -100 kG. The latter results imply an oblique rotator with a period between 2 and 20 hr. Magnetism on white dwarfs has now been detected over more than four orders of magnitude in strength. Assuming flux conservation, the new discoveries imply organized field patterns near the end of the main-sequence phase of only ∼ 10 G. However, the overall incidence of magnetism among white dwarfs remains low, with more than 90% of stars having fields below ∼ 10 kG. There is tentative evidence from line profile analysis that WD 1350-090 is a high-mass object (M > 1 M_⊙), but an accurate parallax and more thorough spectroscopic study are required.