Temperature and density in gaseous nebulae.

The equations of statistical equilibrium for the five level atom have been solved exactly for several ions of the series2p',3p'(q = 2,3,4) and resultsarepresentedfortheions:N n,O rn, Ne rn,Ne v,Narv,S m,Cl II, Cl Iv, Ar rn, Ar v, K Iv and Ca v. The computations have been applied to the fairly regniar, highly stratified planetary nebula NGC 7662 to investigate whether the derived temperalure and density conditions in the regions where the different radiations originate can be made consistent with theoretical models of the nebula. Almost all the forbidden emission line intensity ratios sensitive to the electron temperature and/or density so far measured in NGC 7662 have been used. An agreement with the detailed ionization stratification models developed for NGC 7662 or with the general typical model of a planetary nebula is not clearly achieved. Thdications are found that probably some large inaccuracy still exists in the atomic data for specific ions. However, for a better understanding of the actual accuracy of each theoretical ratio of forbidden emission line intensities, improved observations with the largest spatial resolution as well as a closer knowledge of their accuracy for each ratio would be quite useful. On the other hand, realistic theoretical models of planetary nebulae probably must take into account the specific structures and inhomogeneities of the nebular gases. Key word : statistical equilibrium equations - transition probability - collision strength - planetary nebulae - ionization structure