Excitation of emission lines by fluorescence and recombination in IC418
  We compare calculated intensities of lines of CII, NI, NII, OI and OII with a
published deep spectroscopic survey of planetary nebula 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 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 amounts of
excitation by fluorescence. Recombination excites all the lines from 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.
