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

Abstract

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 <= T_e <= 30 000 K) and densities (10⁴ <= N_e <= 10⁷ cm^-3) applicable to gaseous nebulae. The ratio-ratio diagrams (R₁, R₂) and (R₁, R₃) should, in principle, allow the simultaneous determination of T_e and N_e from measurements of the O Iii features in a spectrum. Plasma parameters derived for a sample of mid- to 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 T_e and N_e 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.