Hydrogen and helium line formation in OB dwarfs and giants - A hybrid non-LTE approach

Aims. Hydrogen and helium line spectra are crucial diagnostic features for the quantitative analysis of OB stars. Hybrid non-LTE line-formation calculations for these elements have not been discussed thoroughly so far, despite their wide use for analyses of metal line spectra. We compute synthetic spectra based on a hybrid non-LTE approach in order to test the ability of these models to reproduce high-resolution and high-S/N spectra of dwarf and giant stars and also to compare them with published grids of non-LTE (OSTAR2002) and LTE (Padova) models. Methods. Our approach solves the restricted non-LTE problem based on classical line-blanketed LTE model atmospheres. State-of-the-art model atoms and line-broadening theories are employed to model the H and He I/II spectra over the entire optical range and in the near-IR. Results. A comparison with published line-blanketed non-LTE models validates the suitability of the LTE approximation for modelling the atmospheric structure of late O to early B-type dwarf and giant stars at metallicities down to ( at least) 1/5 x solar. Our hybrid non-LTE synthetic spectra simultaneously match almost all measurable hydrogen and helium lines observed in six test stars over a wide spectral range from the Balmer limit to the near-IR, except for only a few well-understood cases. A robust starting point for studies of the metal spectra is thus established. Our approach reproduces other published non-LTE calculations, but avoids inconsistencies in the modelling of the He I singlets. These have recently been discussed in the literature in the context of O-type stars and we show that they persist in the early B-types. Our approach improves on published pure LTE models - widely applied for OB star analyses - in many aspects: non-LTE strengthening and the use of improved line-broadening data result in significant differences in the line profiles and equivalent widths of the Balmer and helium lines. Where possible, systematic effects on the stellar parameter determination are quantified, e.g. gravities derived from the H gamma wings may be overestimated by up to similar to 0.2 dex for dwarfs at our upper temperature boundary of 35 000 K in LTE.