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H.L. Dinerstein (U. Texas, Austin), D. R. Garnett (U. Arizona), C. Pulliam Lafon (Brookhaven Nat. Lab.)
The spectra of ionized nebulae contain weak permitted emission lines of heavy elements such as C, O, N, and Mg, in addition to ``forbidden" (collisionally excited) lines from these species, and recombination lines of H and He. The spectrum of recombining doubly ionized oxygen (O++), the dominant ionization stage in most planetary nebulae, is particularly rich, with over 100 transitions in the optical spectral region. In principle, these lines offer a reliable means for determining O/H abundances, since they do not suffer from the severe dependence on gas temperature that affects the collisionally excited lines. However, application of this method to a few objects, such as the planetary nebula NGC 7009 (e.g. Liu {et al.} 1995, MNRAS, 272, 369) yielded surprisingly high values for O/H. Pulliam, Garnett, & Dinerstein (1997, BAAS, 29, 1233) described new McDonald Observatory measurements of O II lines in about 14 planetary nebulae. Their preliminary analysis yielded high O/H values - greater than solar - for the whole sample. We have reanalyzed these data with special attention to line blending issues and the optimum choice of recombination coefficients, and reach somewhat different conclusions. For several nebulae in our sample the O/H values derived from the recombination lines are in good agreement with the values from the [O III] lines, implying that there cannot be a wide dispersion in the temperature of the emitting region. However, in other nebulae the inferred O/H values are factors of a few higher than the forbidden-line abundances, and above solar. An alternate interpretation is that some additional physical mechanism is enhancing the O II recombination line strengths. Intriguingly, the line strength enhancements are selective to particular transitions, and the patterns of enhancement are consistent from object to object. This may hold the key to uncovering this mechanism, which may affect the observed spectra of nebulae in other ways.