On the O III/O II Problem in Medium and High Excitation Planetary Nebulae

Extract

Numerical models have been constructed for twelve ionization bounded, medium to high excitation planetary nebulae. In most objects the excitation sensitive line ratio (O III) λ 500.9 nm / (O II) λ 372.7 nm is predicted to be too low as compared to observations. A similar systematic discrepancy is observed for [S III] λ 953.2 nm / (S II) λ 672.0 nm. We investigated the following effects on the ionization structure of the nebulae: 0⁺⁺ + H^O → 0⁺ + H⁺ charge exchange reaction, energy distribution of ionizing radiation and density distribution of gas in the nebular shell. The results show that density distribution is the most important factor determining the 0 III/O II and S III/S II line intensity ratios. While a factor of ten decrease in the charge exchange coefficient is required to explain the systematic discrepancy, a reduction of nebular radius by a few percent - truncated nebula (quasi density bounded model, but nebula still optically thick to Lyman photons) - suffices to produce the correct 0 III/O II ratio. Also, a density gradient of n ∼ r⁻¹ to r⁻² yields much better agreement with observations. Realistic variations in stellar spectrum hardly affects the 0 III/O II line intensity ratio.