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

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 Å) and optical (4714, 4716, 4724 and 4726 Å) [Ne iv] transitions, for a range of electron temperatures (T_e = 10 000−30 000 K) and electron densities (N_e = 10²−10^6.5 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 [OIV] and [NV]. 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−2s²2p³²P_{1/2, 3/2} doublet at 1602 Å, hence allowing the measurement of the wavelength separation of the ⁴S−²P_½ and ⁴S−²P_3/2 components. The separation is found to be 0.21±0.02 Å, in good agreement with the theoretical estimate of 0.16 ± 0.03 Å; this is the first time (to our knowledge) that this quantity has been experimentally determined.