1999MNRAS.304...27K

Recent R-matrix calculations of electron impact excitation rates in O II are used to calculate the emission-line ratio–ratio diagrams (R₁, R₂), (R₁, R₃) and (R₁, R₄), where R₁ = I(3729 Å)/I(3726 Å), R₂ = I(7320 Å)/I(3726 + 3729 Å), R₃ = I(7330 Å)/I(3726 + 3729 Å) and R₄ = I(2470 Å)/I(3726 + 3729 Å), for a range of electron temperatures (T_e = 7500–20 000 K) and electron densities (N_e = 10^1.5–10⁵ cm^–3) appropriate to planetary nebulae. These diagrams should, in principle, allow the simultaneous determination of T_e and N_e from measurements of the [O II] emission lines in a spectrum. Plasma parameters deduced for a sample of planetary nebulae, using observational data obtained with the IUE satellite and the Hamilton Echelle spectrograph on the 3-m Shane Telescope at the Lick Observatory, are found to show excellent internal consistency, and to be in generally good agreement with the values of T_e and N_e estimated from other line ratios in the echelle spectra. These results provide observational support for the accuracy of the theoretical ratios, and hence the atomic data adopted in their derivation.