Formation of Interstellar C₆₀ from Silicon Carbide Circumstellar Grains

We have conducted laboratory experiments with analog crystalline silicon carbide (SiC) grains using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). The 3C polytype of SiC was used - the type commonly produced in the envelopes of asymptotic giant branch (AGB) stars. We rapidly heated small (∼50 nm) synthetic SiC crystals under vacuum to ∼1300 K and bombarded them with 150 keV Xe ions. TEM imaging and EELS spectroscopic mapping show that such heating and bombardment leaches silicon from the SiC surface, creating layered graphitic sheets. Surface defects in the crystals were found to distort the six-membered rings characteristic of graphite, creating hemispherical structures with diameters matching that of C₆₀. Such nonplanar features require the formation of five-membered rings. We also identified a circumstellar grain, preserved inside the Murchison meteorite, that contains the remnant of an SiC core almost fully encased by graphite, contradicting long-standing thermodynamic predictions of material condensation. Our combined laboratory data suggest that C₆₀ can undergo facile formation from shock heating and ion bombardment of circumstellar SiC grains. Such heating/bombardment could occur in the protoplanetary nebula phase, accounting for the observation of C₆₀ in these objects, in planetary nebulae (PNs) and other interstellar sources receiving PN ejecta. The synthesis of C₆₀ in astronomical sources poses challenges, as the assembly of 60 pure carbon atoms in an H-rich environment is difficult. The formation of C₆₀ from the surface decomposition of SiC grains is a viable mechanism that could readily occur in the heterogeneous, hydrogen-dominated gas of evolved circumstellar shells.