Evaporating Very Small Grains as tracers of the UV radiation field in
  Photo-dissociation Regions
  Context. In photo-dissociation regions (PDRs), Polycyclic Aromatic
Hydrocarbons (PAHs) could be produced by evaporation of Very Small Grains
(VSGs) by the impinging UV radiation field from a nearby star. Aims. We
investigate quantitatively the transition zone between evaporating Very Small
Grains (eVSGs) and PAHs in several PDRs. Methods. We study the relative
contribution of PAHs and eVSGs to the mid-IR emission in a wide range of
excitation conditions. We fit the observed mid-IR emission of PDRs by using a
set of template band emission spectra of PAHs, eVSGs and gas lines. The fitting
tool PAHTAT (PAH Toulouse Astronomical Templates) is made available to the
community as an IDL routine. From the results of the fit, we derive the
fraction of carbon f_eVSG locked in eVSGs and compare it to the intensity of
the local UV radiation field. Results. We show a clear decrease of f_eVSG with
increasing intensity of the local UV radiation field, which supports the
scenario of photo-destruction of eVSGs. Conversely, this dependence can be used
to quantify the intensity of the UV radiation field for different PDRs,
including non resolved ones. Conclusions. PAHTAT can be used to trace the
intensity of the local UV radiation field in regions where eVSGs evaporate,
which correspond to relatively dense (nH = [100, 10^5 ] cm-3) and UV irradiated
PDRs (G0 = [100, 5x10^4]) where H2 emits in rotational lines.
