The formation process(es) of fullerenes in space is still uncertain and several mechanisms have been proposed in the literature. In particular, the most accepted idea to explain the simultaneous presence of fullerenes and PAH-like emission in the H-rich circumstellar envelopes of PNe is that these molecular species may be formed from the photochemical processing of a carbonaceous compound with a mixture of aromatic and aliphatic structures, which should be a major constituent of their circumstellar envelopes. Here we present seeing-limited narrow-band mid-IR GTC/CanariCam images of the fullerene-containing PN IC 418. The narrow-band images cover the 9−13, 11.3, and 17.4 μm emission features (and their adjacent continua) in this extended PN. We study the relative sub-arcsecond spatial distribution of the nebula in these filters with the intention of getting some clues about the formation process of fullerenes in H-rich circumstellar environments.

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.