A deep survey of heavy element lines in planetary nebulae - I. Observations and forbidden-line densities, temperatures and abundances

We present deep optical spectrophotometry of 12 Galactic planetary nebulae (PNe) and three Magellanic Cloud PNe. Nine of the Galactic PNe were observed by scanning the slit of the spectrograph across the nebula, yielding relative line intensities for the entire nebula that are suitable for comparison with integrated nebular fluxes measured in other wavelength regions. In this paper we use the fluxes of collisionally excited lines (CELs) from the nebulae to derive electron densities and temperatures, and ionic abundances. We find that the nebular electron densities derived from optical CEL ratios are systematically higher than those derived from the ratios of the infrared (IR) fine-structure (FS) lines of [O III]. The latter have lower critical densities than the typical nebular electron densities derived from optical CELs, indicating the presence of significant density variations within the nebulae, with the IR CELs being biased towards lower density regions. We find that for several nebulae the electron temperatures obtained from [O II] and [N II] optical CELs are significantly affected by recombination excitation of one or more of the CELs. When allowance is made for recombination excitation, much better agreement is obtained with the electron temperatures obtained from optical [O III] lines. We also compare electron temperatures obtained from the ratio of optical nebular to auroral [O III] lines with temperatures obtained from the ratio of [O III] optical lines to [O III] IR FS lines. We find that when the latter are derived using electron densities based on the [O III] 52 mum/88 mum line ratio, they yield values that are significantly higher than the optical [O III] electron temperatures. In contrast to this, [O III] optical/IR temperatures derived using the higher electron densities obtained from optical [Cl III] lambda5517/lambda5537 ratios show much closer agreement with optical [O III] electron temperatures, implying that the observed [O III] optical/IR ratios are significantly weighted by densities in excess of the critical densities of both [O III] FS lines. Consistent with this, ionic abundances derived from [O III] and [N III] FS lines using electron densities from optical CELs show much better agreement with abundances derived for the same ions from optical and ultraviolet CELs than do abundances derived from the FS lines using the lower electron densities obtained from the observed [O III] 52 mum 88 mum ratios. The behaviour of these electron temperatures, obtained making use of the temperature-insensitive [O III] IR FS lines, provides no support for significant temperature fluctuations within the nebulae being responsible for derived Balmer jump electron temperatures that are lower than temperatures obtained from the much more temperature sensitive [O III] optical lines.