The Structure and Properties of H II Regions

The equations of ionization and thermal equilibrium are solved for gaseous nebulae, allowing for an arbitrary density distribution. It is shown, by solving the transfer equation for the ionizing radiation produced by recombinations, that use of the "on-the-spot" approximation (Zanstra) can cause consider- able errors in the temperature structure (up to ~~2OOO0 K near the nebular center) and predicted emission properties. In some cases, certain of the far-infrared forbidden lines of the cooling ions undergo considerable self- absorption. We consider the implications of optical thickness for two of these lines, [0 m] XX88 and 52 ,~, by solving the problem of line transfer. We find that under certain conditions electron temperatures may be raised by up to 5000_10000 K. The inclusion of density fluctuations is found to reduce drastically the ionized region and to raise somewhat the electron temperatures, all other parameters of the model being the same Changes in the gas structure due to variations of the model parameters are investigated For the models calculated, the emission properties of the important lines are presented