On the Excitation and Formation of Circumstellar Fullerenes

TL;DR

This study analyzes the infrared spectra of three fullerene-rich planetary nebulae to understand fullerene excitation mechanisms and formation processes, comparing observational data with theoretical models and identifying common spectral features.

Contribution

It provides new insights into the excitation mechanisms of circumstellar fullerenes and explores their formation pathways through spectral analysis and model comparisons.

Findings

Fullerene emission is similar across different nebulae despite varying radiation fields.

Spectral features suggest a link between fullerenes and hydrocarbon solids.

Comparison with models indicates possible excitation mechanisms for fullerenes.

Abstract

We compare and analyze the Spitzer mid-infrared spectrum of three fullerene-rich planetary nebulae in the Milky Way and the Magellanic Clouds; Tc1, SMP SMC16, and SMP LMC56. The three planetary nebulae share many spectroscopic similarities. The strongest circumstellar emission bands correspond to the infrared active vibrational modes of the fullerene species C60 and little or no emission is present from Polycyclic Aromatic Hydrocarbons (PAHs). The strength of the fullerene bands in the three planetary nebulae is very similar, while the ratio of the [NeIII]15.5um/[NeII]12.8um fine structure lines, an indicator of the strength of the radiation field, is markedly different. This raises questions about their excitation mechanism and we compare the fullerene emission to fluorescent and thermal models. In addition, the spectra show other interesting and common features, most notably in the 6-9um region, where a broad plateau with substructure dominates the emission. These features have previously been associated with mixtures of aromatic/aliphatic hydrocarbon solids. We hypothesize on the origin of this band, which is likely related to the fullerene formation mechanism, and compare it with modeled Hydrogenated Amorphous Carbon that present emission in this region.