Parameters of PNe: constant density versus density distribution

We derive the stellar and circumstellar parameters of the Galactic compact planetary nebula Hen 2-90 with two models: the classical constant density nebula model where the emission (especially of the forbiden lines) is formed in a sphere or shell of constant density, and a model where the emission is formed in a shell with a $r^{-2}$ density distribution. Depending on the density $N_{\rm e,0}$ in the second model, the resulting range of valid values can deviate from the classical model results. This seems to be especially true for elemental abundances as is shown exemplary for S$^{+}$. We find that the abundance of S$^{+}$ is larger for the models with density distribution than for a constant density. A careful analysis of data with an appropriate input model is advisable when determining stellar and circumstellar parameters of planetary nebulae not only in our Galaxy.