Recent studies have confirmed the presence of buckminsterfullerene ( C _ { 60 } ) in different interstellar and circumstellar environments .
However , several aspects regarding C _ { 60 } in space are not well understood yet , such as the formation and excitation processes , and the connection between C _ { 60 } and other carbonaceous compounds in the interstellar medium , in particular polycyclic aromatic hydrocarbons ( PAHs ) .
In this paper we study several photodissociation regions ( PDRs ) where C _ { 60 } and PAHs are detected and the local physical conditions are reasonably well constrained , to provide observational insights into these questions .
C _ { 60 } is found to emit in PDRs where the dust is cool ( T _ { d } = 20 - 40 K ) and even in PDRs with cool stars .
These results exclude the possibility for C _ { 60 } to be locked in grains at thermal equilibrium in these environments .
We observe that PAH and C _ { 60 } emission are spatially uncorrelated and that C _ { 60 } 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 C _ { 60 } 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 C _ { 60 } formation .
However , this is not the only parameter involved and C _ { 60 } formation is likely affected by shocks and PDR age .