Abstract
Detailed ionization structure models of gaseous nebulae, for different electron densities and ionizing radiation fields are . The observed [S II]γ[N II] line intensity ratios in the Orion Nebula are greater than those predicted by our models. Possible explanations for this discrepancy are analyzed. It is found that the ionization structure of the heavy elements critically depends on the electron density. From the computations it follows that in order to infer by observation the amount of neutral helium in a given H II region, it is necessary that not only the degree of ionization of heavy elements should be obtained, but also that its electron density should be known.
