Fullerenes in circumstellar and interstellar environments
  We recently identified several emission bands in the Spitzer-IRS spectrum of
the unusual planetary nebula Tc 1 with the infrared active vibrational modes of
the neutral fullerene species C60 and C70. Since then, the fullerene bands have
been detected in a variety of sources representing circumstellar and
interstellar environments. Abundance estimates suggest that C60 represents
~0.1%-1.5% of the available carbon in those sources. The observed relative band
intensities in various sources are not fully compatible with single-photon
heating and fluorescent cooling, and are better reproduced by a thermal
distribution at least in some sources. The observational data suggests that
fullerenes form in the circumstellar environments of evolved stars, and survive
in the interstellar medium. Precisely how they form is still a matter of
debate.
