Fullerenes and proto-fullerenes in interstellar carbon dust

TL;DR

This paper investigates the spectral features of hydrogenated amorphous carbon dust, proposing that certain IR emission lines are from proto-fullerenes rather than C60 molecules, and models their evolution in space.

Contribution

It introduces a model linking HAC dust evolution to fullerene formation and reinterprets IR features as proto-fullerenes, not C60, in astronomical spectra.

Findings

Spectral features at 7.05, 8.5, 17.4, 18.9 μm are linked to proto-fullerenes.

The 16.4 μm feature indicates pentagonal carbon rings for fullerene formation.

Many objects' IR features are from proto-fullerenes, not C60.

Abstract

Laboratory spectra of hydrogenated amorphous carbon (HAC) particles prepared under a variety of conditions show spectral features at 7.05, 8.5, 17.4 and 18.9 {\mu}m (1418, 1176, 575 & 529 cm-1) that have been associated with emission from C60 molecules. These lines occur in the spectra even though C60 molecules as such are not present in our samples. It appears that these four spectral lines in HAC can instead be associated with precursor molecules or "proto-fullerenes" that subsequently react to yield C60. We develop a model tracing the evolution and de-hydrogenation of HAC dust and show that the observation of an emission feature at 16.4 {\mu}m (610 cm-1) in astronomical spectra signals the presence of the pentagonal carbon rings required for the formation of fullerenes. We suggest that the set of four IR emission lines previously identified with C60 in many objects that also show the 16.4 {\mu}m feature and other polycyclic aromatic hydrocarbon bands arise from proto-fullerenes rather than C60. Tc1 is an example of a source in which de-hydrogenation has proceeded to the point where only fullerenes are present.