Formation of Interstellar C60 from Silicon Carbide Circumstellar Grains

The detection of C₆₀ in astronomical sources has changed our notions concerning the chemical complexity of the interstellar medium (ISM), and the repositories of gas-phase carbon. Furthermore, the C₆₀⁺ cation has been proposed as a carrier of a few diffuse interstellar bands. Despite its ubiquitous presence, the mechanism of C₆₀ formation remains the subject of speculation. Several mechanisms have been proposed, including destruction of polycyclic aromatic hydrocarbons (PAHs). Here we propose a new synthetic route to interstellar C₆₀ from shock heating of SiC grains in the protoplanetary nebulae phase. We have conducted laboratory experiments with analog SiC grains using techniques of transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). The silicon carbide 3C polytype was used - the type commonly produced in the envelopes of AGB stars. We rapidly heated small SiC crystals ( 50 nm) under vacuum to 1300 K thermally and with ion bombardment. TEM imaging and EELS spectroscopic mapping shows that such heating leaches silicon from the silicon carbide surface, creating layered graphitic sheets. Surface defects in the crystal were found to distort the six-membered rings characteristic of graphite, creating hemispherical structures with diameters matching that of C₆₀. Such non-planar features require the formation of 5-membered rings. We have subsequently identified circumstellar grains, preserved inside primitive meteorites, that surprisingly contain a SiC core surrounded by graphite, contradicting long-standing thermodynamic predictions. The proposed mechanism readily explains the presence of C₆₀ in proto-planetary (PPNe) and planetary nebulae (PNe). Subsequent injection of C₆₀, a chemically-robust molecule, into the diffuse ISM from PNe accounts for its presence in other interstellar sources.