ATOMS IN VERY STRONG MAGNETIC-FIELDS

The X-ray spectra of neutron stars are expected to be determined by the opacities of atoms with atomic number Z > 2 in strong magnetic fields. We calculate the energy levels, wavefunctions and transition rates of hydrogen, helium, carbon, nitrogen and silicon in the very strong (B > 4.7 X 10(9) Z2 G) magnetic fields expected in neutron stars. The wavefunctions are represented in terms of Landau states, and are calculated with a high-field multiconfigurational Hartree-Fock code. We compare our results for hydrogen with previous work and use our wavefunctions to compute bound-bound and bound-free oscillator strengths for heavier elements. The accuracy of our method is sufficient for the applications we make of it in a companion paper (Miller 1991), where we compute neutron star thermal X-ray spectra. The low fluxes expected from such objects (< 10(-11) erg cm-2 s-1 from thermal X-rays) imply that the appropriate wavefunctions and energy levels need only be calculated with an accuracy of a few per cent. There are other methods in the literature which give a higher accuracy (which we do not need in the present context) for hydrogen. However, unlike these other methods, our method can be readily extended, with calibrated accuracy, to elements of higher Z, as we show in the present paper.