Excitation of emission lines by fluorescence and recombination in IC 418

TL;DR

This study predicts and compares emission line intensities in IC 418, demonstrating that fluorescence significantly influences nebular spectra and proposing new diagnostics for photodissociation regions.

Contribution

It introduces a detailed model combining fluorescence and recombination to explain emission lines, highlighting fluorescence's role in nebular diagnostics.

Findings

Fluorescence dominates NII emission due to low excitation conditions.

Many CII and OII lines are primarily excited by fluorescence.

NI and OI lines are excited by fluorescence and recombination, respectively.

Abstract

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.