On the reduction of the model of highly ionized atoms

In numerous studies, in astrophysics for instance, the large number of energy levels belonging to a highly ionized atom becomes rapidly unwieldy; systems are hard to deal with and simplifying assumptions vary from one author to another, since it is difficult to know a priori which levels can be left out of consideration in the study. This often impairs the accuracy of the results. A technique is proposed for reducing the ion model, in terms of its configurations, without neglecting any level. This procedure is applied to the case of Fe +10 ; the populations of the five lower levels are obtained by solving a system of 10 equations although 89 levels are taken into consideration. [Translated by the Journal]