Interpreting the Hydrogen IR Lines - Impact of Improved Electron Collision Data

Abstract

We evaluate the effect of variations in the electron-impact excitation cross sections on the non-LTE line formation for hydrogen in early-type stars. While the Balmer lines are basically unaffected by the choice of atomic data, the Brackett and Pfund series members allow us to discriminate between the different models. Non-LTE calculations based on the widely-used approximations of Mihalas, Heasley & Auer and of Johnson fail to simultaneously reproduce the observed optical and IR spectra over the entire parameter range. Instead, we recommend a reference model using data from ab-initio calculations up to principal quantum number n<=7 for quantitative work. This model is of general interest due to the ubiquity of the hydrogen spectrum.