Stationary components of He I in strong magnetic fields - a tool to identify magnetic DB white dwarfs

¹ Institut für Theoretische Physik und Astrophysik, 24098 Kiel, Germany
² Institut für Physikalische Chemie, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany e-mail: peter@tc.pci.uni-heidelberg.de

Corresponding author: S. Jordan, jordan@astrophysik.uni-kiel.de

Received: 16 May 2001
Accepted: 4 July 2001

Abstract

In only three of the 61 known magnetic white dwarfs helium has been identified unambiguously while about 20% of all non-magnetic stars of this class are known to contain HeI or HeII. One reason for this discrepancy is that the identification of peculiar objects as magnetic white dwarfs is based either on the presence of hydrogen line components in strong magnetic fields -for which atomic data exist since 1984 -or the polarization of the corresponding radiation which has not been measured for many objects. Until recently, data for HeI data were available only for magnetic fields below 20 MG. This changed with the publication of extensive data by the group in Heidelberg. The corresponding calculations have now been completed for the energetically lowest five states of singlet and triplet symmetry for the subspaces with ; selected calculations have been performed for even higher excitations. In strongly magnetized white dwarfs only line components are visible whose wavelengths vary slowly with respect to the magnetic field, particularly stationary components which have a wavelength minimum or maximum in the range of the magnetic fields strengths on the stellar surface. In view of the many ongoing surveys finding white dwarfs we want to provide the astronomical community with a tool to identify helium in white dwarfs for fields up to 5.3 GG. To this end we present all calculated helium line components whose wavelengths in the UV, optical, and near IR vary slowly enough with respect to the field strength to produce visible absorption features. We also list all stationary line components in this spectral range. Finally, we find series of minima and maxima which occur as a result of series of extremal transitions to increasingly higher excitations. We estimated the limits for 8 series which can possibly give rise to additional absorption in white dwarf spectra; one strong absorption feature in GD229 which is yet unexplained by stationary components is very close to two estimated series limits.