Emission lines of [O II] in the optical and ultraviolet spectra of planetary nebulae

Abstract

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 (Te = 7500-20 000 K) and electron densities (Ne = 101.5–105 cm−3) appropriate to planetary nebulae. These diagrams should, in principle, allow the simultaneous determination of Te and Ne 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 Te and Ne 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.