A multiwavelength study of the high-field magnetic white dwarf EUVE J0317-85.5 (=RE J0317-853)

We present optical and ultraviolet photometry and spectroscopy and optical circular spectropolarimetry of the ultramassive, rapidly rotating, high-field magnetic white dwarf EUVE J0317-85.5. From the combined data sets, we establish an ephemeris for the photometric, spectroscopic, and polarimetric variations with a spin period of 725.7277 s, the fastest yet measured for an isolated white dwarf. We build a series of far-ultraviolet (FUV) and optical flux spectra as well as optical polarization spectra that describe the atmospheric properties as a function of rotation phase. Twelve components of the Lyman line series are identified in the FUV: seven of them are forbidden lines enforced by the electric field that permeates the strongly magnetized photosphere. Attempts at modeling the FUV and optical data with two classes of models indicate an unusual field structure on the star. Both offset-dipole and multipolar expansion models fail to adequately reproduce the spectroscopic and polarimetric properties simultaneously, and we postulate that the surface of EUVE J0317-85.5 may be punctuated by a high-field magnetic spot.