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

We present spectrophotometry of 12 Galactic and 3 Magellanic Cloud planetary nebulae (PNe). Nine of the Galactic PNe were observed by scanning the slit across the PN. We use the fluxes of collisionally excited lines (CELs) to derive electron densities (D's) and temperatures (T's), and ionic abundances. We find that the D's derived from optical CEL ratios are systematically higher than those derived from the ratios of the IR fine-structure (FS) lines of [OIII], indicating the presence of significant density variations within the PNe. We also compare T's obtained from the ratio of optical nebular to auroral [OIII] lines with those obtained from the ratio of [OIII] optical to IR FS lines. We find that when the latter are derived using D's based on the [OIII] 52um/88um ratio, they yield values that are significantly higher than the optical [OIII] T's. Contrasting this, [OIII] optical/IR T's derived using the higher D's obtained from [ClIII] 5517A/5537A ratios show much closer agreement with optical [OIII] T's, implying that the observed [OIII] optical/IR ratios are significantly weighted by D's in excess of the critical densities of both [OIII] FS lines. Consistent with this, ionic abundances derived from [OIII] and [NIII] FS lines using D's from optical CELs show much better agreement with abundances derived for the same ions from optical and UV CELs than do abundances derived from the FS lines using the lower D's obtained from the 52um/88um ratios. The behaviour of T's obtained making use of the T-insensitive IR FS lines provides no support for significant T-fluctuations within the PNe that could be responsible for derived Balmer jump T's being lower than those obtained from the much more T-sensitive [OIII] optical lines.