Spatially resolved OII recombination line observations of the Ring Nebula,NGC 6720

We present new long-slit CCD spectra of O II permitted lines and [O III] fo rbidden lines in the Ring Nebula NGC 6720. These observations provide spati ally resolved information on both O II and [O III] over the 70 " diameter o f the main shell. We find significant differences in the spatial distributi on of the O II lines and [O III] lambda 4959. The [O III] emission follows the H beta emission measure, peaking slightly radially inward from the H be ta peak. The O II emission peaks inside the [O III] emission. This suggests that radiative recombination may not be the primary mechanism for producin g the O II lines. O+2 abundances derived from O II lines are 5-10 times lar ger than those derived from [O III] in the region within 20 " of the centra l star. Outside of this region, however, the O II-derived and [O III]-deriv ed 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 t o about 12,000 K in the center; we see no evidence for a temperature jump t hat would be associated with a shock. If temperature fluctuations are respo nsible for the discrepancy in 0" abundances, the average temperature would have to be approximately 6500 K in the He+2 zone and about 9000 K in the ou ter shell in order to force the [O III]-derived abundance to equal that der ived from O II. This would conflict with ionization models for planetary ne bulae, which predict that the temperature is higher in the He+2 region clos e to the ionizing star. We therefore argue that temperature fluctuations ca nnot explain the abundance discrepancy. A comparison of the spatial distrib ution 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, creatin g difficulties for models that explain the recombination line/forbidden lin e discrepancy by density fluctuations. We examine the possibility that high -temperature dielectronic recombination in a central hot bubble enhances th e recombination line strengths in the central part of the nebula. However, comparison of recombination rates with collisional excitation rates shows t hat the increase in recombination emission due to dielectronic recombinatio n at T approximate to 10(5) K is not sufficient to overcome the increase in collisonally excited emission. We are unable to find a completely satisfac tory model to explain the discrepancy between recombination line and forbid den line abundances.