Emission lines of O III in the optical and ultraviolet spectra of planetary nebulae

Recent R-matrix calculations of electron impact excitation rates in O iii are used to calculate electron temperature and density-dependent emission line ratios R1 = I(4363 Å)/I(4960 Å + 5007 Å), R2 = I(1661 Å + 1667 Å)/I(4960 Å + 5007 Å) and R3 = I(2322 Å)/I(1661 Å + 1667 Å), for a range of electron temperatures (7500 ≤ Te ≤ 30 000 K) and densities (104 ≤ Ne ≤ 107 cm−3) applicable to gaseous nebulae. The ratio-ratio diagrams (R1, R2) and (R1, R3) should, in principle, allow the simultaneous determination of Te and Ne from measurements of the O iii features in a spectrum. Plasma parameters derived for a sample of midto high-excitation planetary nebulae from (R1, R2) and (R1, R3) measurements, produced using a combination of ultraviolet spectra obtained with the International Ultraviolet Explorer (IUE) and optical data from a number of observing runs, are found to show excellent internal consistency. They also show, in general, good agreement with the values of Te and Ne estimated from other line ratios in the nebulae, therefore providing observational support for the accuracy of the theoretical ratios and hence the atomic data adopted in their derivation.