1996A&A...305L..21O

Available optical and UV spectral observations of the planetary nebula NGC6302 are analyzed. Temperature diagnostic ratios of high excitation species ([ArV], [NeIV], [NeV]) yield low temperatures typical of photoionized nebulae. The presence of a significant amount of hot, collisionally ionized gas is therefore excluded, and other arguments agree with this conclusion. The fact that observed [NeV] FIR lines are relatively faint cannot be therefore attributed to an unusually large NeV temperature, as in Rowlands et al. (1994: RHH94). Rather, it indicates that the ground state collision strengths are overestimated by a factor of ∼3. A major consequence is that available photoionization models of AGN may largely overestimate the strength of [NeV] FIR lines, and this should be considered when interpreting ISO data. More generally, much care should be taken in modeling ground state lines of high-Z species in 'cool' photoionized gas because their collision strengths are often dominated by strong resonances close to the threshold and whose positions may be quite uncertain. This letter is also intended to urge theoretical/experimental work aimed at determining the position of autoionizing levels within {DELTA}S≲3.10⁴cm^–1 from the ground state of astrophysically abundant ions.