The detection of buckminsterfullerene (C₆₀) in interstellar and circumstellar environments has changed the paradigm of chemical complexity in the hostile conditions of space. Despite the ubiquitous presence of C₆₀ in the interstellar medium (ISM), its chemical formation pathway remains uncertain. The creation of large, pure carbon-bearing molecules in the hydrogen-rich interstellar environment has been the subject of speculation, with several proposed mechanisms involving polycyclic aromatic hydrocarbons (PAHs) or hydrogenated amorphous carbon (HACs). Recently, we conducted laboratory transmission electron microscopy (TEM) experiments that suggest another route to C₆₀ formation: the shock-heating of 3C-polytype silicon carbide grains, commonly produced circumstellar shells and found in meteorites. Our imaging measurements showed the formation of graphite layers and then carbon nanobuds, with diameters matching that of C₆₀, on the surfaces of analog SiC grains, when heated to 1000 K and subjected to ion bombardment. Further experiments, done with the Hitachi HF5000 TEM at the University of Arizona, have shown that that carbon nanostructures form with simply rapid heating to 1000 K. Under these conditions, mimicking shocks in circumstellar shells, silicon atoms readily evaporated from the SiC surface, forming graphitic layers on the grain surface. These layers were mobile as the temperature increased, with the folding and curling of the graphite sheets, producing possible nanotubes. These experimental results have been verified with molecular dynamic (MD) simulations of rapid heating of layered graphite. The computations showed widespread distortion and disintegration of the graphite layers, creating carbon fragments with both 5 and 6 -membered rings that rearrange with atom migration over a series of steps to C₆₀. These experiments and simulations lend strong support to the hypothesis that C₆₀ and other fullerenes and nanostructures are synthesized by the shock-heating of common SiC grains in stellar envelopes at the end of the asymptotic giant branch (AGB) phase, where shocks are common as the mass loss increases significantly. It also suggests that fullerenes and other pure carbon structures may be present in primitive meteorites, and represent an important carrier of this element within the solar system.

Jacob J. Bernal

Graduate Research Assistant, University of Arizona

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