Top-down formation of fullerenes in the interstellar medium

¹ Université de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France
e-mail: olivier.berne@gmail.com
² CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
³ Department of Physics, PO Box 64, University of Helsinki, 00014 Helsinki, Finland
⁴ Institut Utinam, CNRS UMR 6213, OSU THETA, Université de Franche-Comté, 41bis avenue de l’Observatoire, 25000 Besançon, France

Received: 14 November 2014
Accepted: 12 March 2015

Abstract

Fullerenes have recently been detected in various circumstellar and interstellar environments, raising the question of their formation pathway. It has been proposed that they can form at the low densities found in the interstellar medium by the photo-chemical processing of large polycyclic aromatic hydrocarbons (PAHs). Following our previous work on the evolution of PAHs in the NGC 7023 reflection nebula, we evaluate, using photochemical modelling, the possibility that the PAH C₆₆H₂₀ (i.e. circumovalene) can lead to the formation of the C₆₀ fullerene upon irradiation by ultraviolet photons. The chemical pathway involves full dehydrogenation of C₆₆H₂₀, folding into a floppy closed cage and shrinking of the cage by loss of C₂ units until it reaches the symmetric C₆₀ molecule. At 10′′ from the illuminating star and with realistic molecular parameters, the model predicts that 100% of C₆₆H₂₀ is converted into C₆₀ in ~10⁵ yr, a timescale comparable to the age of the nebula. Shrinking appears to be the kinetically limiting step of the whole process. Hence, PAHs larger than C₆₆H₂₀ are unlikely to contribute significantly to the formation of C₆₀, while PAHs containing between 60 and 66 C atoms should contribute to the formation of C₆₀ with shorter timescales, and PAHs containing fewer than 60 C atoms will be destroyed. Assuming a classical size distribution for the PAH precursors, our model predicts that absolute abundances of C₆₀ are up to several 10^-4 of the elemental carbon, that is, less than a percent of the typical interstellar PAH abundance, which is consistent with observational studies. According to our model, once formed, C₆₀ can survive much longer (> 10⁷ yr for radiation fields below G₀ = 10⁴) than other fullerenes because of the remarkable stability of the C₆₀ molecule at high internal energies. Hence, a natural consequence is that C₆₀ is more abundant than other fullerenes in highly irradiated environments.