Nevertheless, one can state that models of this type are needed to gain an understanding of the change in the physical conditions occuring in the central star. When this final analysis is combined with an interpretation of the reported decline of the radio flux, this could result in a unique measurement of the evolutionary speed of the central star of NGC 7027. Using the model of NGC 6543, a rather stringent upper limit could be placed on the He II Lyman continuum emanating from the central star. This is one of the first times that such a constraint could be derived for a stellar spectrum. Cloudy has also been used to conduct a parameter study of the spectral evolution of a typical post-AGB star, with particular emphasis on the evolution of the infrared colors.
Several assumptions have to be made in order to calculate evolutionary tracks for post-AGB stars.
A study of the type presented here is important for understanding how these assumptions affect the infrared spectrum of the post-AGB stars. The results of this study may be helpful for understanding the observations made in e.g. the IRAS color-color diagram. This in turn may lead to an improved understanding of mass loss processes, hydrodynamic interactions and evolutionary rates of the central star. Eventually it may lead to better evolutionary calculations. However, we are still far from this goal and our study should only be viewed as a small step towards achieving this goal.