  Recent Herschel and ALMA observations of Photodissociation Regions (PDRs)
have revealed the presence of a high thermal pressure (P ~ 10^7-10^8 K cm-3)
thin compressed layer at the PDR surface where warm molecular tracer emission
(e.g. CH+, SH+, high-J CO, H2,...) originate. These high pressures (unbalanced
by the surrounding environment) and a correlation between pressure and incident
FUV field (G0) seem to indicate a dynamical origin with the radiation field
playing an important role in driving the dynamics. We investigate whether
photoevaporation of the illuminated edge of a molecular cloud could explain
these high pressures and pressure-UV field correlation. We developed a 1D
hydrodynamical PDR code coupling hydrodynamics, EUV and FUV radiative transfer
and time-dependent thermo-chemical evolution. We applied it to a 1D
plane-parallel photoevaporation scenario where a UV-illuminated molecular cloud
can freely evaporate in a surrounding low-pressure medium. We find that
photoevaporation can produce high thermal pressures and the observed P-G0
correlation, almost independently from the initial gas density. In addition, we
find that constant-pressure PDR models are a better approximation to the
structure of photoevaporating PDRs than constant-density PDR models, although
moderate pressure gradients are present. Strong density gradients from the
molecular to the neutral atomic region are found, which naturally explain the
large density contrasts (1-2 orders of magnitude) derived from observations of
different tracers. The photoevaporating PDR is preceded by a low velocity shock
(a few km/s) propagating into the molecular cloud. Photoevaporating PDR models
offer a promising explanation to the recent observational evidence of dynamical
effects in PDRs.
