The Fate of Carbon in the Interstellar Medium: Experiment, Theory and Observations

Carbon is the fourth most abundant element in the Galaxy. It is created in stars, and enters the
interstellar medium (ISM) through mass loss and supernovae explosions. Astronomical
observations have shown that carbon has a rich chemistry in the ISM, forming molecules as
complex as fullerences. It also bonds to metals and phosphorus to create unusual, “non-
terrestrial” species. Using methods of chemical physics, we have been investigating new
pathways in the interstellar chemistry of carbon. Laboratory solid state experiments and molecular
dynamics (MD) calculations have been conducted, probing the interstellar formation of C 60 .
Using transmission electron microscopy (TEM), we have demonstrated that rapid heating to
~1300 K of analog SiC grains under vacuum creates layered graphitic sheets, which distort into
hemispherical structures with the diameter of C 60 . In the MD simulations, rapid thermal heating
of graphite up to 2000 K, as well as subjecting it to shocks, resulted in the formation of near-
spherical fragments with 5-member and 6-member rings. These results suggest a new formation
interstellar mechanism for C 60 : the shock heating of SiC grains in circumstellar envelopes. In
addition, we have also been conducting gas-phase microwave/millimeter-wave spectroscopy of
small, carbon-bearing molecules bonded to metals or phosphorus. We have currently been
examining MC 2 species and phosphorus-carbon chains- possible fragments from fullerene
formation. New results will be discussed.