Preprint typeset using LATEX style emulateapj v. 5/2/11 C60 IN PHOTODISSOCIATION REGIONS

Recent studies have confirmed the presence of buckminsterfullerene (C60) in different interstellar and circumstellar environments. However, several aspects regarding C60 in space are not well under-stood yet, such as the formation and excitation processes, and the connection between C60 and other carbonaceous compounds in the interstellar medium, in particular polycyclic aromatic hydrocarbons (PAHs). In this paper we study several photodissociation regions (PDRs) where C60 and PAHs are detected and the local physical conditions are reasonably well constrained, to provide observational insights into these questions. C60 is found to emit in PDRs where the dust is cool (Td = 20 − 40 K) and even in PDRs with cool stars. These results exclude the possibility for C60 to be locked in grains at thermal equilibrium in these environments. We observe that PAH and C60 emission are spatially uncorrelated and that C60 is present in PDRs where the physical conditions (in terms of radiation field and hydrogen density) allow for full dehydrogenation of PAHs, with the exception of Ced 201. We also find trends indicative of an increase in C60 abundance within individual PDRs, but these trends are not universal. These results support models where the dehydrogenation of carbonaceous species is the first step towards C60 formation. However, this is not the only parameter involved and C60 formation is likely affected by shocks and PDR age. Subject headings: astrochemistry – infrared: ISM – ISM: lines and bands – ISM: molecules – photon-dominated region 1