COMBINED ULTRAVIOLET-OPTICAL SPECTROPOLARIMETRY OF THE MAGNETIC WHITE-DWARF GD-229

Detailed flux and polarization spectra of the magnetic white dwarf GD 229 have revealed a host of new absorption features in the ultraviolet and rich detail in the polarization of the strong optical lines. Howe ver, hydrogen does not appear to be represented in the spectrum. Thus, the many peculiar features still elude identification, and the magnet ic field strength on the star remains a mystery. Nevertheless, there i s compelling evidence in the overall amount and wavelength dependence of linear and circular polarization that GD 229 may be the most strong ly magnetic white dwarf known, with B > 10(9) G. Symmetry arguments an d the detailed behavior of polarization through the spectral features further imply that the field morphology is not a simple centered dipol e. It is likely that the mixed composition, magnetic component B of th e double-degenerate binary LB 11146 provides a glimpse at B similar to 670 MG of the unidentified atmospheric constituent in GD 229. Lacking accurate model spectral energy distributions, the enormously structur ed UV spectrum prevents an accurate measurement of the surface tempera ture: 16000 less than or similar to T-eff less than or similar to 2300 0 K. Regardless, the radius implied by the measured parallax and appar ent brightness implies a massive white dwarf, M greater than or simila r to 1 M.. If, as some scenarios suggest, there is a close relationshi p between the progenitors of magnetic white dwarfs and pulsars, GD 229 is probably our best example of a ''near miss'' pulsar.