|Annu. Rev. Astron. Astrophys. 1999. 37:
Copyright © 1999 by . All rights reserved
2.3. Strategies for Abundance Work
There is much that is unknown about QSO line-forming regions. We do not, for example, have a clear picture of the overall geometry or the spatial variations of key parameters. But we do not need this information for abundance work. The emission lines from photoionized clouds are controlled fundamentally by the energy balance and microphysics. The strategy for abundance studies is to identify line ratios that have significant abundance sensitivities and minimal dependence on other unknown or uncertain parameters. For example, we can minimize the sensitivity to large-scale geometric effects by comparing lines that form as much as possible in the same gas. Detailed simulations are often needed to identify useful line ratios and quantify their parameter sensitivities. Simple analytic expressions can be used for some applications, and they can help, in any case, provide physical insight into the emission-line behaviors.
Below we review some of the basic principles of photoionization and emission line formation. See Osterbrock (1989), Mihalas (1978) for further reviews, Davison & Netzer (1979), Kwan & Krolick (1981), Ferland & Shields (1985), Netzer (1990) for applications to QSOs, and Ferland et al. (1998) for more on the numerical simulations and input atomic data.