A Critique of the Hydrogenic Approximation in the Calculation of Stellar Opacity

The validity of the hydrogenic approximation in the evaluation of atomic energy levels and radiative absorption cross-sections used in opacity calculations, is critically examined. It is shown that the approximation can be retained with increased accuracy by a careful choice of the parameters, such as screening constants and effective nuclear charges, so as to take proper account of the various contexts in which they occur. The degree of accuracy is tested by the comparison of results with those obtained for the same problem by more exact but less facile methods. Application to stellar opacity calculations indicate systematic deviations from the results obtained via the usual hydrogenic approximation. Comparisons are also made with the opacity obtained by more elaborate methods.