Effects of shocks in stellar atmosphere models on the emission line spectrum of surrounding H II regions
Emission line studies from H II regions in our own and other galaxies require tools for the inversion of line ratios into desired physical properties. These tools generally come in the form of diagnostic ratios/diagrams that are based on grids of photoionization models. An important input to the photoionization models is the stellar atmosphere spectrum of the ionizing sources. Among a number of potentially problematic biases introduced by a great deal of unknown variables in both the stellar atmosphere and nebular models, the current omission of shocks in the calculation of the former set of models could also threaten the accuracy of the physical interpretation of emission line ratios from H II regions. Current stellar atmosphere models that are crucial inputs to the grid of photoionization models used to generate nebular emission line diagnostic diagrams might produce significant biases due to the omission of shocks. We therefore investigate whether a new generation of photoionization model grids, taking shocks into account, is required to compensate for the biases. We make use of the WM-BASIC stellar atmosphere code, which can account for the extra energetic emission in the stellar spectral energy distribution produced by shocks, in conjunction with the photoionization code MOCASSIN to determine whether shocks produce significant biases in the determination of the physical parameters of the interstellar medium and/or ionizing stellar parameters. We conclude that these effects are only important for stellar sources with effective temperatures lower than 30 kK and in this case they yield artificially high stellar temperatures, electron temperatures and nebular ionization parameters. The magnitude of the effect is also obviously dependent on the strength of the shock and is likely to be unimportant for the majority of stellar sources. Nevertheless, we find our 20 and 30 kK shock models to strongly enhance the He II λ4686 nebular emission line (next to many other lines). This result is however not strong enough to explain previously observed He II λ4686 line emission in the spectra of H II galaxies.