Dust temperature and time-dependent effects in the chemistry of
  photodissociation regions
  When studying the chemistry of PDRs, time dependence becomes important as
visual extinction increases, since certain chemical timescales are comparable
to the cloud lifetime. Dust temperature is also a key factor, since it
significantly influences gas temperature and mobility on dust grains,
determining the chemistry occurring on grain surfaces. We present a study of
the dust temperature impact and time effects on the chemistry of different
PDRs, using an updated version of the Meijerink PDR code and combining it with
the time-dependent code Nahoon. We find the largest temperature effects in the
inner regions of high $G$$_{\mathrm{0}}$ PDRs, where high dust temperatures
favour the formation of simple oxygen-bearing molecules (especially that of
O$_2$), while the formation of complex organic molecules is much more efficient
at low dust temperatures. We also find that time-dependent effects strongly
depend on the PDR type, since long timescales promote the destruction of
oxygen-bearing molecules in the inner parts of low $G$$_{\mathrm{0}}$ PDRs,
while favouring their formation and that of carbon-bearing molecules in high
$G$$_{\mathrm{0}}$ PDRs. From the chemical evolution, we also conclude that, in
dense PDRs, CO$_2$ is a late-forming ice compared to water ice, and confirm a
layered ice structure on dust grains, with H$_2$O in lower layers than CO$_2$.
Regarding steady state, the PDR edge reaches chemical equilibrium at early
times ($\lesssim$10$^5$ yr). This time is even shorter ($<$10$^4$ yr) for high
$G$$_{\mathrm{0}}$ PDRs. By contrast, inner regions reach equilibrium much
later, especially low $G$$_{\mathrm{0}}$ PDRs, where steady state is reached at
$\sim$10$^6$-10$^7$ yr.
