The Structure of Ionization Fronts in H II Regions

Abstract

Spectra have been obtained of bright rim structures in four nearby emission nebulae, NGC 1976, NGC 2264, NGC 6820 and IC 1396. These spectra have been calibrated to give relative line strengths and the ratio of the [O II] λ3727 doublet has been measured. The Balmer decrement is found to be quite constant across the bright rims indicating that collisional excitation is not important and that the internal reddening is fairly constant. From the Balmer decrement the reddening of the objects is determined and the forbidden lines are used to obtain the variation of the electron density, electron temperature and level of ionization across the bright rims. This data was compared with theoretical calculations by Pottasch. It was found that the densities at the ionization fronts agree with the calculations but the variation of density across the rims differs from the theory in one case. The electron temperatures are much cooler than the theoretical values. Consideration of the bright rim in the Orion nebula indicates that there is a hidden source of excitation in an obscured region to the south-east of the nebula. Data on the bright rim in NGC 2264 indicates that there is no connection between it and an infrared source in this region.