Context. This work was conducted as part of the SPECPDR program, dedicated to the study of very small particles and astrochemistry, in Photo-Dissociation Regions (PDRs). Aims. We present the analysis of the mid-IR spectro-imagery observations of Ced 201, NCG 7023 East and North-West and ρ Ophiuchi West filament. Methods. Using the data from all four modules of the InfraRed Spectrograph onboard the Spitzer Space Telescope, we produced a spectral cube ranging from 5 to 35 µm, for each one of the observed PDRs. The resulting cubes were analysed using Blind Signal Separation methods (NMF and FastICA). Results. For Ced 201, ρ Ophiuchi West filament and NGC 7023 East, we find that two signals can be extracted from the original data cubes, which are 5 to 35 µm spectra. The main features of the first spectrum are a strong continuum emission at long wavelengths, and a broad 7.8 µm band. On the contrary, the second spectrum exhibits the classical Aromatic Infrared Bands (AIBs) and no continuum. The reconstructed spatial distribution maps show that the latter spectrum is mainly present at the cloud surface, close to the star whereas the first one is located slightly deeper inside the PDR. The study of the spectral energy distribution of Ced 201 up to 100 µm suggests that, in cool PDRs, the 5-25 µm continuum is carried by Very Small Grains (VSGs). The AIB spectra in the observed objects can be interpreted as the contribution of neutral and positively-charged Polycyclic Aromatic Hydrocarbons (PAHs). Conclusions. We extracted the 5 to 25 µm emission spectrum of VSGs in cool PDRs, these grains being most likely carbonaceous. We show that the variations of the mid-IR (5-35 µm) spectra of PDRs can be explained by the photo-chemical processing of these VSGs and PAHs, VSGs being the progenitors of free PAHs.

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