The forbidden line spectrum in the edges of H II regions

It is shown analytically that the motion of exciting stars can affect the forbidden emission-line spectra of H II regions in an observable manner. A model is used which comprises a small solid angle of a spherical nebula that is symmetric with respect to the direction of stellar motion and consists of a mixture of H, He, C, N, O, and Ne with ionization states up to He II, C III, N III, O III, and Ne III. Consideration is given to the case of a 36-solar-mass O5 star with an effective temperature of 43,000 K moving at a velocity of 10 km/s in a nebula having a density of 10 times the proton mass per cu cm. The results obtained indicate that: (1) the region where forbidden lines characteristic of ionization fronts are produced is broadened considerably at the trailing edge of the nebula; (2) the spectrum in this region is comparable to that of fossil H II regions; and (3) the ionization front at the leading edge is sharper, the temperature there is higher, and the forbidden-line spectrum is similar to that of young ionization fronts.