Improved calculations for the C III 1907,1909 and Si III 1883,1892 electron density sensitive emission-line ratios, and a comparison with IUE observations

Atomic data are used in conjunction with the statistical equilibrium code of Dufton (1977) to calculate relative C III and Si III level populations, and hence emission-line strengths for a range of electron temperatures and densities. It is assumed that photoexcitation and deexcitaton rates are negligible in comparison with the corresponding collisional rates, that ionization to and recombination from other ionic levels are slow compared with bound-bound rates, and that all transitions are optically thin. The observed values of R1 and R2 for several planetary nebulae and a symbiotic star, measured from high-resolution spectra obtained with the IUE satellite, lead to electron densities that are compatible, and are also in good agreement with those deduced from line ratios in other species.