Transmission Electron Microscopy of SiC Grains: Insights into the Formation of Interstellar Fullerenes and Carbon Nanotubes

The detection of the fullerenes C₆₀ and C₇₀ in space has transformed our understanding of chemical complexity in the interstellar medium (ISM). It also raises the question of the origin of these large, carbon-bearing species, as well as the possibility of even larger molecules in astrophysical environments. Using transmission electron microscopy (TEM), we have sought to answer such questions with laboratory experiments involving in situ heating of analog silicon carbide (SiC) grains. These heating experiments are designed to simulate shocks occurring in post-AGB stellar envelopes, where fullerenes are observed. Our newest experimental findings reveal that heating the analog SiC grains yields a forest of hemispherical C₆₀-sized nanostructures, which later transform into multi-walled carbon nanotubes (MWCNTs). These MWCNTs are larger than any of the currently-observed interstellar fullerene species, both in overall size and number of carbon atoms. These experimental results suggest that such MWCNTs are likely to form in post-AGB shocks, where the structures, along with the smaller fullerenes, are subsequently injected into the ISM.