Non-LTE line formation for iron group elements. III. Fe IV.

A non-LTE line formation calculation for FeIV is introduced. The data for radiative bound-bound transitions which have been derived as part of this work are described and their accuracy is evaluated. The theoretical non-LTE spectrum of FeIV is compared to the HST spectrum of BD+75 325 and the IUE spectrum of Feige 67 both of which are classified as sdO stars. It is shown that the FeIV spectrum is consistent with the spectra of FeV and FeVI. Background opacities in the Lyman continuum derived from the full line list of Kurucz are described and their influence on the spectra of the iron ions is discussed. It is shown that they are crucial to the line formation of FeIV.