title: Top-down formation of fullerenes in the interstellar medium.
abstract: Fullerenes have been recently 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 modeling, the possibility that the PAH C66H20(i.e. circumovalene) can lead to the formation of C60upon 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 C2units until it reaches the symmetric C60molecule. At 10" from the illuminating star and with realistic molecular parameters, the model predicts that 100% of C66H20is converted into C60in ~ 105years, 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 C66H20are unlikely to contribute significantly to the formation of C60, while PAHs containing between 60 and 66 C atoms should contribute to the formation of C60with shorter timescales, and PAHs containing less than 60 C atoms will be destroyed. Assuming a classical size distribution for the PAH precursors, our model predicts absolute abundances of C60are up to several 10-4of the elemental carbon, i.e. less than a percent of the typical interstellar PAH abundance, which is consistent with observational studies. According to our model, once formed, C60can survive much longer (> 107years for radiation fields belowG0= 104) than other fullerenes because of the remarkable stability of the C60molecule at high internal energies. Hence, a natural consequence is that C60is more abundant than other fullerenes in highly irradiated environments.
authors.affiliation: Université de Toulouse; UPS-OMP; IRAP; Toulouse, France ; CNRS; IRAP; 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France.
authors.forename: O
authors.initials: O
authors.lastname: Berné
authors.affiliation: Department of Physics, PO Box 64, University of Helsinki, 00014, Helsinki, Finland ; Institut Utinam, CNRS UMR 6213, OSU THETA, Université de Franche-Comté, 41bis avenue de l'Observatoire, 25000 Besançon, France.
authors.forename: J
authors.initials: J
authors.lastname: Montillaud
authors.affiliation: Université de Toulouse; UPS-OMP; IRAP; Toulouse, France ; CNRS; IRAP; 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France.
authors.forename: C
authors.initials: C
authors.lastname: Joblin
keywords: ISM: molecules; Methods: numerical; astrochemistry; molecular processes