Abstract
We consider the family of photoionization models which are optically thin to the ionizing radiation, have low enough density that collisional de-excitation is unimportant, and have normal (cosmic) chemical abundances. The line ratios from all such models with a given form of the ionizing spectrum can be derived by means of a scaling law from a single numerical calculation and expressed as a of one physical parameter. The value of the parameter is uniquely determined from the observed line ratios. The resulting model is in reasonably good agreement with the typical or composite QS0 spectrum and with line intensities for two recently observed individual QS0s. We therefore suggest that much of the emission may come from optically thin gas, although some of the observational evidence (especially the strength of Mg ii A2800) indicates that thick gas may also be present in some objects. Subject headings: nebulae - quasi-stellar sources or objects
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