Forbidden lines in a newly formed H II region

The initial evolution of a spherically symmetric H II region forming around a star of 36 solar masses is calculated numerically using a program based on a finite-difference scheme extrapolated from those previously proposed for solving hydrodynamic problems involving shock waves. The helium abundance is assumed to be 10 per cent, and no account is taken of magnetic fields, turbulent motions, or interstellar grains. The evolution of the positions of the hydrogen and helium ionization fronts is plotted as a function of time, and the total spectra of the forbidden emission lines of C II, N II, N III, O II, O III, Ne II, and Ne III are computed at each time. The intensities of the 29 strongest lines at five different epochs are compared with a rough estimate of the total strength of H-beta. The temporal evolution of the nebula and the emission lines is described qualitatively.