Self-consistent dynamic models of steady ionization fronts. I. weak-D and weak-R fronts

We present a method for including steady state gas flows in the plasma physics code Cloudy, which was previously restricted to modeling static configurations. The numerical algorithms are described in detail, together with an example application to plane-parallel ionization-bounded H II regions. As well as providing the foundation for future applications to more complex flows, we find the following specific results regarding the effect of advection on ionization fronts in H II regions: (1) Significant direct effects of advection on the global emission properties occur only when the ionization parameter is lower than is typical for H II regions. For higher ionization parameters, advective effects are indirect and largely confined to the immediate vicinity of the ionization front. (2) The overheating of partially ionized gas in the front is not large, even for supersonic (R-type) fronts. For subsonic (D-type) fronts we do not find the temperature spike that has been previously claimed. (3) The most significant morphological signature of advective fronts is an electron density spike that occurs at the ionization front whenever the relative velocity between the ionized gas and the front exceeds about one-half the ionized isothermal sound speed. Observational evidence for such a spike is found in [N II] λ6584 images of the Orion bar. (4) Plane-parallel, weak-D fronts are found to show at best a shallow correlation between mean velocity and ionization potential for optical emission lines even when the flow velocity closely approaches the ionized sound speed. Steep gradients in velocity versus ionization, such as those observed in the Orion Nebula, seem to require transonic flows, which are only possible in a diverging geometry when radiation forces are included.