Spatially Resolved O II Recombination Line Observations of the Ring Nebula, NGC 6720

We present new long-slit CCD spectra of O II permitted lines and [O III] forbidden lines in the Ring Nebula NGC 6720. These observations provide spatially resolved information on both O II and [O III] over the 70'' diameter of the main shell. We find significant differences in the spatial distribution of the O II lines and [O III] λ4959. The [O III] emission follows the Hβ emission measure, peaking slightly radially inward from the Hβ peak. The O II emission peaks inside the [O III] emission. This suggests that radiative recombination may not be the primary mechanism for producing the O II lines. O+2 abundances derived from O II lines are 5-10 times larger than those derived from [O III] in the region within 20'' of the central star. Outside of this region, however, the O II-derived and [O III]-derived abundances agree to within 0.2-0.3 dex. The electron temperature derived from [O III] lines rises smoothly from about 10,000 K in the outer shell to about 12,000 K in the center; we see no evidence for a temperature jump that would be associated with a shock. If temperature fluctuations are responsible for the discrepancy in O+2 abundances, the average temperature would have to be approximately 6500 K in the He+2 zone and about 9000 K in the outer shell in order to force the [O III]-derived abundance to equal that derived from O II. This would conflict with ionization models for planetary nebulae, which predict that the temperature is higher in the He+2 region close to the ionizing star. We therefore argue that temperature fluctuations cannot explain the abundance discrepancy. A comparison of the spatial distribution of O II emission with the location of dusty knots shows that the O II recombination lines do not peak where the dense knots are located, creating difficulties for models that explain the recombination line/forbidden line discrepancy by density fluctuations. We examine the possibility that high-temperature dielectronic recombination in a central hot bubble enhances the recombination line strengths in the central part of the nebula. However, comparison of recombination rates with collisional excitation rates shows that the increase in recombination emission due to dielectronic recombination at T ≈ 105 K is not sufficient to overcome the increase in collisonally excited emission. We are unable to find a completely satisfactory model to explain the discrepancy between recombination line and forbidden line abundances.