EMISSION-LINES OF [NE-IV] IN THE OPTICAL AND ULTRAVIOLET-SPECTRA OF GASEOUS NEBULAE

Recent R-matrix calculations of electron impact excitation rates in Ne rv are used to calculate emission line ratio-ratio diagrams involving both the ultraviolet (1602, 2422 and 2424 Angstrom and optical (4714, 4716, 4724 and 4726 ii) [Ne IV] transitions, for a range of electron temperatures (T-e=10 000-30 000K) 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 m easurements of the [Ne IV] lines in a spectrum. Plasma parameters dedu ced for a sample of high-excitation planetary nebulae, using a combina tion of observational data obtained with the IUE satellite and the Ham ilton Echelle Spectrograph (HES) on the 3-m Shane Telescope at the Lic k Observatory, are found to show generally excellent internal consiste ncy. In addition, they are in good agreement with the values of T-e an d N-e estimated from other high-excitation line ratios in the HES spec tra, and by previous authors using infrared and ultraviolet transition s in [O IV] and [Ne V]. These results provide observational support fo r the accuracy of the theoretical [Ne IV] ratios, and hence the atomic data adopted in their derivation. An inspection of IUE and GHRS/HST s pectra of the symbiotic stars Z And and RR Tel reveals asymmetries in the line profile of the [Ne IV] 2s(2)2p(34)S-2s(2)2p(32)P(1/2,3/2) dou blet at 1602 Angstrom, hence allowing the measurement of the wavelengt h separation of the S-4-P-2(1/2) and S-4-P-2(3/2) components. The sepa ration is found to be 0.21 +/- 0.02 Angstrom in good agreement with th e theoretical estimate of 0.16 +/- 0.03 Angstrom; this is the first ti me (to our knowledge) that this quantity has been experimentally deter mined.