2012MNRAS.426.2318E

We compare calculated intensities of lines of CII, NI, NII, OI and OII with a published deep spectroscopic survey of IC 418. Our calculations use a self-consistent nebular model and a synthetic spectrum of the central star atmosphere to take into account line excitation by continuum fluorescence and electron recombination. We found that the NII spectrum of the s, p and most d states is excited by fluorescence due to the low-excitation conditions of the nebula. Many CII and OII lines have significant amount of excitation by fluorescence. Recombination excites all the lines from the f and g states and most OII lines. In the neutral-ionized boundary, the NI quartet and OI triplet dipole-allowed lines are excited by fluorescence, while the quintet OI lines are excited by recombination. Electron excitation produces the forbidden optical lines of OI, and continuum fluorescence enhances the NI forbidden line intensities. Lines excited by fluorescence of light below the Lyman limit thus suggest a new diagnostic to explore the inner boundary of the photodissociation region of the nebula.