We investigate the electronic structure of the helium atom in a magnetic field between B = 0 and B = 100a.u.. The atom is treated as a nonrelativistic system with two interacting electrons and a fixed nucleus. Scaling laws are provided connecting the fixed-nucleus Hamiltonian to the one for the case of finite nuclear mass. Respecting the symmetries of the electronic Hamiltonian in the presence of a magnetic field, we represent this Hamiltonian as a matrix with respect to a two-particle basis composed of one-particle states of a Gaussian basis set. The corresponding generalized eigenvalue problem is solved numerically, providing in the present paper results for vanishing magnetic quantum number M = 0 and even or odd z-parity, each for both singlet and triplet spin symmetry. Total electronic energies of the ground state and the first few excitations in each subspace as well as their one-electron ionization energies are presented as a function of the magnetic field, and their behaviour is discussed. Energy values for electromagnetic transitions within the M = 0 subspace are shown, and a complete table of wavelengths at all the detected stationary points with respect to their field dependence is given, thereby providing a basis for a comparison with observed absorption spectra of magnetic white dwarfs.

The electronic structure of the helium atom in the magnetic field regime B = 0−100a.u. is investigated, using a full configuration interaction approach which is based on a nonlinearly optimized anisotropic Gaussian basis set of one-particle functions. The corresponding generalized eigenvalue problem is solved for the magnetic quantum number M = −1 and for both even and odd z-parity as well as singlet and triplet spin symmetry. Accurate total electronic energies of the ground state and the first four excitations in each subspace as well as their one-electron ionization energies are presented as a function of the magnetic field. Additionally we present energies for electromagnetic transitions within the M = −1 subspace and between the M = −1 subspace and the M = 0 subspace treated in a previous work. A complete table of wavelengths and field strengths for the detected stationary points is given.

The electronic structure of the helium atom in the magnetic field regime B = 0−100a.u. is investigated, using a full configuration interaction approach which is based on a nonlinearly optimized anisotropic Gaussian basis set of one-particle functions. The corresponding generalized eigenvalue problem is solved for the magnetic quantum number M = −1 and for both even and odd z-parity as well as singlet and triplet spin symmetry. Accurate total electronic energies of the ground state and the first four excitations in each subspace as well as their one-electron ionization energies are presented as a function of the magnetic field. Additionally we present energies for electromagnetic transitions within the M = −1 subspace and between the M = −1 subspace and the M = 0 subspace treated in a previous work. A complete table of wavelengths and field strengths for the detected stationary points is given.

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 He I or He II. 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 He I 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 |m| ≤ 3; 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.

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