The abundances are solar for all the elements. We show also the shock models mentioned in the previous section. From the diagrams in Figure 2 we obtain the following results. H ratios are strongly dependent on the ionization parameter U .
H depend only weakly on the density and U .
H , we computed models with metallicities half-solar and three and five times solar. These models, together with the solar abundance model and the data points, are plotted in Figures 3a-d . H is equivalent to the effect of a higher density. It can be seen that the data on this diagram are compatible with a higher metallicity in the nuclear gas. H ratios presented by the data.