C$_{60}$ in Photodissociation Regions
  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.
