ON THE SPECTRUM AND FIELD-STRENGTH OF THE MAGNETIC WHITE-DWARF GD 229

Despite recent breakthroughs in our understanding of hydrogen in strong magnetic fields, the remarkable spectrum of the strongly polarized magnetic white dwarf GD 229 is still a mystery. A new high-quality, broad-coverage optical spectrum presented here reveals additional as well as apparent changes in the locations of known absorption features, the latter presumably is due to rotation of the star during the decade since the last published spectrum. Inspection as well as detailed modeling discloses that the spectrum of magnetized hydrogen is not consistent with the observations for any assumed field strength up to at least 2000 MG. Similarly, cyclotron transitions cannot account for the complexity and spacing of the optical features. Nevertheless, a very large surface field is indicated by the observed levels of optical linear and circular polarization of GD 229, and it is quite possible that GD 229 possesses the strongest surface field yet identified on a white dwarf. The most likely origin for the observed spectral features is neutral helium (or possibly carbon), whose existence is consistent with the photospheric temperature of the star. However, until Zeeman calculations of He I transitions are carried beyond the present limit of perturbation theory, this explanation will be impossible to test.