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.