Excitation of emission lines by fluorescence and recombination in IC418

TL;DR

This study compares observed and calculated emission line intensities in planetary nebula IC 418, highlighting the roles of fluorescence and recombination in line excitation, and proposing new diagnostics for nebular boundary analysis.

Contribution

It introduces a self-consistent model incorporating fluorescence and recombination effects to accurately predict emission line intensities in IC 418.

Findings

Fluorescence excites NII, CII, and OII lines under low excitation conditions.

Recombination predominantly excites lines from f and g states.

Fluorescence of light below Lyman limit offers new boundary diagnostics.

Abstract

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.