2006A&A...456..659N

Recent calculations and analyses of O star spectra have revealed discrepancies between theory and observations, and between different theoretical calculations, for the strength of optical HeI singlet transitions. We investigate the source of these discrepancies. Using a non-LTE radiative transfer code we have undertaken detailed test calculations for a range of O star properties. Our principal test model has parameters similar to those of the O9V star, 10 Lac. We show that the discrepancies arise from uncertainties in the radiation field in the HeI 1s²¹S-1s2p¹P° transition near 584Å. The radiation field at 584Å is influenced by model assumptions, such as the treatment of line-blanketing and the adopted turbulent velocity, and by the FeIV atomic data. It is shown that two FeIV transitions near 584Å can have a substantial influence on the strength of the HeI singlet transitions. Because of the difficulty of modeling the HeI singlet lines, particularly in stars with solar metalicity, the HeI triplet lines should be preferred in spectral analyses. These lines are much less sensitive to model assumptions.