VLT Echelle Spectrophotometry of the Planetary Nebula NGC 5307 and Temperature Variations

摘要

Echelle spectrophotometry of the planetary nebula NGC 5307 is presented. Thedata consists of VLT UVES observations in the 3100 to 10360 A range. Electrontemperatures and densities have been determined using different line intensityratios. We determine the H, He, C, and O abundances based on recombinationlines, these abundances are almost independent of the temperature structure ofthe nebula. We also determine the N, O, Ne, S, Cl, and Ar abundances based oncollisionally excited lines, the ratios of these abundances relative to that ofH depend strongly on the temperature structure of the nebula. From theOII/[OIII] ratio we find a t^2 = 0.056+-0.005. The chemical composition of NGC5307 is compared with those of the Sun and the Orion nebula. From the study ofthe relative intensities of the OII recombination lines of multiplet 1 in thisand other nebulae it isfound that for electron densities smaller than about5000 cm-3 collisional redistribution is not complete, this effect has to betaken into account to derive the O abundances for those cases in which not allthe lines of the multiplet are observed. From the 4649 OII versus N_e$(ClIII)diagram we find a critical electron density of 1325 cm-3 for collisionalredistribution of the OII lines of multiplet 1. Also based on this diagram weargue that the OII and the [OIII] lines originate in the same regions. We alsofind that the radial velocities and the FWHM of the OII and [OIII] lines in NGC5307 are similar supporting the previous result. These two results imply thatfor NGC 5307 and probably for many other gaseous nebulae chemicalinhomogeneities are not responsible for the large temperature fluctuationsobserved.