Circumstellar nebulae of WR and OB stars with strong winds: Simultaneous excitation by stellar radiation and shocks

We use a two-phase model for the structure of the circumstellar nebulae of hot stars to analyze the radiative cooling of a dense, compact cloud behind the shock produced by the compression of the cloud by hot gas from the stellar wind, taking into account ionization and heating by radiation from the central star. We can distinguish three stages of the evolution of the cloud during its compression. In the first stage, relevant for the entire cloud before compression and the gas ahead of the shock front, the state of the gas is determined purely by ionization by the stellar radiation. The next stage is characterized by the simultaneous action of two gas excitation mechanisms—photoionization by the stellar radiation and shock heating. In this stage, the gas intensively radiates thermal energy received at the shock front. After radiative cooling, in the final stage, ionization and heating of the gas are again determined mainly by the star. To compute the spectrum of the cloud radiation, we solved for the propagation of a plane-parallel, homogeneous flux through the shock front in the radiation field of the hot star. The computations show that a combination of two excitation mechanisms considerably enriches the theoretical spectrum. The relative intensities of emission lines of a single cloud may resemble either those for an HII region or of a supernova remnant.