d such a test in Galactic  regions; they find that @pauldrach01 atmospheres fit the observed \[\]/\[\] versus \[\]/\[\] relation, whereas stellar models that assume LTE do not. In another test, @pauldrach01 argued that their models successfully reproduce the observed far–UV spectra of hot stars, as opposed to other models. Finally, @snc argue that @pauldrach01 atmospheres should be more realistic than those of @sk because the latter neglect line broadening, and thus underestimate line blanketing. As a result, CoStar atmospheres have significantly higher ionizing fluxes, especially at energies exceeding the He$^+$ edge.

Given these problems, and that the CoStar atmospheres predict much higher \[\]/\[\] line ratios than are observed, we will use the @pauldrach01 atmospheres in this paper. Still, that the Pauldrach spectra are better does not mean they are correct; the CoStar–Pauldrach discrepancy should serve as some warning of the current uncertainties regarding hot star spectra—a critical input to the models.

We also consider Wolf–Rayet (WR) stars in simple nebulae. We use WN and WC model spectra compiled by @snc, which were generated using the code of @hillmill. For a given , these model WR stars yield much lower \[\]/\[\] ratios compared to Pauldrach O stars: at ${\hbox{T$_{eff}$}}=50,000$ K, the difference is a factor of $30$ for WN, and a factor of $10^4$ for WC stars. This is because of the very strong line blanketing found in WR stars. Since Wolf–Rayet stars can reach much hotter temperatures than main sequence stars, a ${\hbox{T$_{eff}$}}\la 140,000$ K WC star can reach $[{\hbox{{\rm Ne}\kern 0.1em{\sc iii}}}]/[{\hbox{{\rm Ne}\kern 0.1em{\sc ii}}}]\sim10$ (comparable to the ratio produced by a ${\hbox{T$_{eff}$}}=50,000$ K Pauldrach O star). Similarly, a $120,000$ K WN star can reach \[\]/\[\]$=100$. Thus, given these stellar atmospheres, only a WN star can give rise to a neon ratio between $10$ and $100$.

As a result, there are mid–infrared line ratio regimes that only Wolf–Rayet s