LP 790-29 - A magnetic degenerate with a heavily blanketed energy distribution

Spectrophotometry and circular spectropolarimetry are presented for the strongly magnetic white dwarf LP 790-29. This star is unique among the magnetic white dwarfs because it exhibits a very broad and deep absorption feature as well as strong circular polarization over most of the optical spectrum. Observations of the strange energy distribution obtained with three different instruments are analyzed. It is found that the star's spectral energy distribution fits a blackbody of approximately 8600 K except for an extremely broad and deep absorption trough, which covers the region from 4300 to 5700 A, has clear structure and an equivalent width of about 870 A, and is 75% deep at maximum. The polarimetry shows that the circular polarization is clearly correlated with the optical feature, being essentially zero in the trough and increasing to 7.5% and 9% on either side. It is suggested that the Swan bands of C2 are likely to be the cause of the unusual absorption feature.