We predict intensities of lines of CII, NI, NII, OI and OII and compare them
with a deep spectroscopic survey of IC 418 to test the effect of excitation of
nebular emission lines by continuum fluorescence of starlight. Our calculations
use a nebular model and a synthetic spectrum of its central star to take into
account excitation of the lines by continuum fluorescence and recombination.
The NII spectrum is mostly produced by fluorescence due to the low excitation
conditions of the nebula, but many CII and OII lines have more excitation by
fluorescence than recombination. In the neutral envelope, the NI permitted
lines are excited by fluorescence, and almost all the OI lines are excited by
recombination. Electron excitation produces the forbidden optical lines of OI,
but continuum fluorescence excites most of the NI forbidden line intensities.
Lines excited by fluorescence of light below the Lyman limit thus suggest a new
diagnostic to explore the photodissociation region of a nebula.Comment: 2 pages, 4 figures, to appear in proceedings of the IAU Symposium
283: "Planetary Nebulae: An Eye to the Future", Eds.: A. Manchado, L.
Stanghellini and D. Schoenberne