Very Large Telescope Echelle Spectrophotometry of the Planetary Nebula NGC 5307 and Temperature Variations

Echelle spectrophotometry of the planetary nebula NGC 5307 is presented. The data consist of Very Large Telescope Ultraviolet Visual Echelle Spectrograph observations in the 3100-10360 Å range. Electron temperatures and densities have been determined using different line intensity ratios. We determine the H, He, C, and O abundances based on recombination lines; these abundances are almost independent of the temperature structure of the nebula. We also determine the N, O, Ne, S, Cl, and Ar abundances based on collisionally excited lines; the ratios of these abundances relative to that of H depend strongly on the temperature structure of the nebula. From the O II/[O III] line intensity ratios we find a t2 = 0.056 ± 0.005. The chemical composition of NGC 5307 is compared with those of the Sun and the Orion Nebula. From the study of the relative intensities of the O II recombination lines of multiplet 1 in this and other nebulae, it is found that for electron densities smaller than about 5000 cm-3 collisional redistribution is not complete; this effect has to be taken into account to derive the O abundances for those cases in which not all the lines of the multiplet are observed. From the O II λ4649 versus Ne(Cl III) diagram we find a critical electron density of 1325 cm-3 for collisional redistribution of the O II lines of multiplet 1. Based on this diagram, we also argue that the O II and the [O III] lines originate in the same regions. We also find that the radial velocities and the FWHM of the O II and [O III] lines in NGC 5307 are similar, supporting the previous result. These two results imply that for NGC 5307 and probably for many other gaseous nebulae chemical inhomogeneities are not responsible for the large temperature fluctuations observed.