Stability of interstellar fullerenes under high-dose γ-irradiation

It is generally assumed that the large spherical carbon clusters can be generated within circumstellar envelopes of mass-losing carbon-rich stars. The detection of fullerenes in carbonaceous chondrites, in the geological strata of the Cretaceous-Tertiary and the Permian-Triassic boundary layers, associated with bolide impacts, can be considered as an indirect evidence of their formation in circumstellar envelopes. On the other hand, a question arises on the fate of fullerenes in harsh radiation environments: namely, in close proximity to the stars where they have been produced (the gas phase), and in the solid phase after their incorporation into the interstellar grains. The existence of the large spherical carbon clusters must finally depend on competition between the rates of their formation and radiation decomposition. In the present work we estimated the capability of C-60 fullerene to withstand prolonged gamma-irradiation, under the doses exceeding 6 MGy (1 Gy = 1 J kg(-1)), in the presence and absence of liquid water. The results of high-performance liquid chromatographic and infrared spectroscopic measurements of the irradiated products suggest general pathways of C-60 radiolysis in the aqueous phase to be polymerization (most likely cross-linking), breaking the carbon backbone, C-H group formation, and incorporation of oxygen atoms as carbonyl and OH groups. In the absence of water, attachment of other radicals generated in the environment is possible, along with the polymerization. As a whole, crystalline C-60 fullerene exhibited an extremely high stability. A similar behavior can be expected in different space environments, where the large carbon clusters are formed, incorporated into interstellar dust particles and subsequently into comets, and travel through the Universe. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.