Emission lines of [Ne iv] in the optical and ultraviolet spectra of gaseous nebulae

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Recent R-matrix calculations of electron impact excitation rates in Ne IV are used to calculate emission line ratio–ratio diagrams involving both the ultraviolet (1602, 2422 and 2424 A) and optical (4714, 4716, 4724 and 4726 A) [Ne IV] transitions, for a range of electron temperatures (Te=10 000–30 000 K) and electron densities (Ne=102–106.5 cm−3) appropriate to gaseous nebulae. These diagrams should, in principle, allow the simultaneous determination of Te and Ne from measurements of the [Ne IV] lines in a spectrum. Plasma parameters deduced for a sample of high-excitation planetary nebulae, using a combination of observational data obtained with the IUE satellite and the Hamilton Echelle Spectrograph (HES) on the 3-m Shane Telescope at the Lick Observatory, are found to show generally excellent internal consistency. In addition, they are in good agreement with the values of Te and Ne estimated from other high-excitation line ratios in the HES spectra, and by previous authors using infrared and ultraviolet transitions in [O IV] and [Ne V]. These results provide observational support for the accuracy of the theoretical [Ne IV] ratios, and hence the atomic data adopted in their derivation. An inspection of IUE and GHRS/HST spectra of the symbiotic stars Z And and RR Tel reveals asymmetries in the line profile of the [Ne IV] 2s22p34S–2s22p32P1/2, 3/2 doublet at 1602 A, hence allowing the measurement of the wavelength separation of the 4S–2P1/2 and 4S–2P3/2 components. The separation is found to be 0.21±0.02 A, in good agreement with the theoretical estimate of 0.16±0.03 A; this is the first time (to our knowledge) that this quantity has been experimentally determined.