Combined Ultraviolet-Optical Spectropolarimetry of the Magnetic White Dwarf GD 229

Abstract

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. However, hydrogen does not appear to be represented in the spectrum. Thus, the many peculiar features still elude identification, and the magnetic field strength on the star remains a mystery. Nevertheless, there is compelling evidence in the overall amount and wavelength dependence of linear and circular polarization that GD 229 may be the most strongly magnetic white dwarf known, with B > 1O⁹ G. Symmetry arguments and the detailed behavior of polarization through the spectral features further imply that the field morphology is not a simple centered dipole. It is likely that the mixed composition, magnetic component B of the double-degenerate binary LB 11146 provides a glimpse at B ∼670 MG of the unidentified atmospheric constituent in GD 229.

Lacking accurate model spectral energy distributions, the enormously structured UV spectrum prevents an accurate measurement of the surface temperature: 16000 ≲ T_eff ≲ 23000 K. Regardless, the radius implied by the measured parallax and apparent brightness implies a massive white dwarf, M &#8819 1 M_sun. If, as some scenarios suggest, there is a close relationship between the progenitors of magnetic white dwarfs and pulsars, GD 229 is probably our best example of a "near miss" pulsar.