Mixed UTA and detailed line treatment for mid-Z opacity and spectral calculations

We developed a method to mix detailed and statistical treatments within the same transition array and for a set of transition arrays entering the spectral or opacity calculations of light and mid-Z elements. By retaining the strongest lines within a given transition array, the method provides a spectral description comparable in accuracy to a detailed treatment approach, where all the lines are explicitly included in the spectral calculation. The remaining weak lines are represented by a UTA-like functional form. Overall, we show that the computational cost approaches the statistical UTA method. The method has been implemented in the Cowan atomic structure code and applied to the calculation of the LTE and non-LTE spectra of two mid-Z elements, xenon and iron.