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

Abstract

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 (T_e=10 000-30 000 K) and electron densities (N_e=10^2-10^6.5 cm^-3) appropriate to gaseous nebulae. These diagrams should, in principle, allow the simultaneous determination of T_e and N_e 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 T_e and N_e 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] 2s^22p^3 ^4S-2s^22p^3 ^2P_1/2, 3/2 doublet at 1602 A, hence allowing the measurement of the wavelength separation of the ^4S-^2P_1/2 and ^4S-^2P_3/2 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.