Surveying DC White Dwarfs for Magnetic Fields

A survey of 52 DC white dwarf stars was conducted in circular spectropolarimetry to search for magnetic fields ≳30 kG. Four DC stars were discovered with magnetic fields above 30 kG: G111-49 with B e ~ -220 MG, G183-35 with B e = +6.8 ± 0.5 MG, G256-7 with B e = +4.9 ± 0.5 MG, and G234-4 with B e = +39.6 ± 11.6 kG. A new magnetic DB white dwarf was also discovered, LB 8827 with B e = 1.0 ± 0.5 MG. A total of 15% of the white dwarfs in the survey have a magnetic field >30 kG. This value is far larger than the 2% of DA stars, but more than half of the DC stars were originally misclassified. Only 5% of the reclassified DC stars have magnetic fields above 30 kG. Non-white dwarfs were also observed: the two BL Lacs are unpolarized in circular polarization and an extreme carbon star was discovered. Three magnetic stars from the DC white dwarf survey were reobserved to investigate the possibility of rotation. Two are definitely rotating: LHS 1734 with 16 minutes ≲ P < 1 yr and G158-45 (=LHS 1044) with a probable period P ~ 11 hr but a definite period P ≤ 1 day or P ~ a few days. G183-35 might be rotating with 50 minutes ≲ P < a few yr.