Interstellar C<sub>60</sub>

Abstract

Fullerenes are cage-like molecules of pure carbon, such as C60, C70, C76, and C84. C60, also known as buckminster- lrene, is the most stable fullerene and has a soccer-ball like structure. The presence of fullerenes in space has been suggested and observationally explored since their first synthesis in the laboratory in 1985 by Harry Kroto and his colleagues which earned them the 1996 Nobel prize in chemistry. C60 (as well as C70) has recently been detected in reflection nebulae, post-asymptotic giant branch (AGB) stars, pre-planetary nebulae, planetary nebulae, Herbig Ae/Be stars, and young stellar objects through their characteristic infrared emission bands. The formation of C60 in interstellar and circumstellar environments is not firmly established. Experimental studies have shown that C60 can be made by gas-phase condensation (e.g. through vaporization of graphite) in a hydrogen-poor environment. In view of the simultaneous detection of C60 and polycyclic aromatic hydrocarbon (PAH) molecules in hydrogen-rich interstellar and circumstellar regions, it has also been suggested that C60 could be generated by the decomposition of hydrogenated amorphous carbon, or the destruction of PAHs, both induced by shocks and/or ultraviolet photoprocessing. The phase (gas or solid) and excitation mechanism of C60 in interstellar and circumstellar conditions are also hotly debated in the literature. One model suggests that C60 is attached to dust and emits in solid-phase at the equilibrium temperature of the dust. Another model suggests that C60 is stochastically excited by ultraviolet photons and emits in the gas-phase. We prefer the latter model as in interstellar and circumstellar conditions the energy content of a C60 molecule is often smaller than the energy of a single starlight photon and C60 is expected to undergo stochastical heating.