2.2.3. Photoionization Modeling
Photoionization modeling is a necessary component of any abundance analysis. We have seen that the bright line method relies on calibrations from photoionization models as do ionization correction factors in the direct method. Thus, it might be misleading to give modeling a separate category. I do so to point out that with a high signal/noise spectrum, rich in emission lines, it is possible to derive more accurate abundances and to deduce properties of the exciting stars and the nebular geometry. Often, abundance studies deal with surveys of many objects intended to uncover underlying patterns, but in some cases, the accurate abundances of an individual object are critical for a particular problem. In the latter case, a direct abundance analysis can be put on firmer ground by developing a detailed photoionization model of the nebula.
Really wonderful things can be done when spatially resolved observations are combined with detailed photoionization models. I think a great example of this is the work of Baldwin et al. (1991) on the Orion nebula; this is a real treat with regard to getting the most out of one's observations. In this study it is possible to identify geometrical effects (the relative positions of the exciting stars and the bounding molecular clouds), but the most impressive aspect of this work is the detailed investigation of the possible effects of dust within the nebula. Their appendix C is a valuable monograph on the potential effects of dust on HII regions.
Photoionization modeling also allows us to explore different physical processes which may affect abundance derivations. As mentioned above, Shields & Kennicutt (1995) looked at the effects of dust in high metallicity HII regions. They discovered that the dust acts as an additional heating agent, strengthening the collisionally excited optical forbidden lines, which is why one never observes pure Balmer line optical spectra from high metallicity HII regions (even though they are easy to produce in photoionization models without dust). They also confirmed the suggestion by Henry (1993) that the depletion of refractory elements onto grains (specifically Si and Fe) could lead to hotter nebula as the efficiency of particular fine structure lines as coolants was diminished. From the conclusions of Shields & Kennicutt one can take away the following rule of thumb: HII region abundances are generally more secure for low metallicity regions with high temperature exciting stars and generally less secure for high metallicity regions with low temperature exciting stars.