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This work was conducted as part of the SPECPDR program, dedicated to thestudy of very small particles and astrochemistry, in Photo-Dissociation Regions(PDRs). We present the analysis of the mid-IR spectro-imagery observations ofCed 201, NCG 7023 East and North-West and rho-Ophiuchi West filament. Using thedata from all four modules of the InfraRed Spectrograph onboard the SpitzerSpace Telescope, we produced a spectral cube ranging from 5 to 35 um for eachone of the observed PDRs. The resulting cubes were analysed using Blind SignalSeparation methods (NMF and FastICA). For Ced 201, rho-Ophiuchi West filamentand NGC 7023 East, we find that two signals can be extracted from the originaldata cubes, which are 5 to 35 um spectra. The main features of the firstspectrum are a strong continuum emission at long wavelengths, and a broad 7.8um band. On the contrary, the second spectrum exhibits the classical AromaticInfrared Bands (AIBs) and no continuum. The reconstructed spatial distributionmaps 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 thePDR. The study of the spectral energy distribution of Ced 201 up to 100 umsuggests that, in cool PDRs, the 5-25 um continuum is carried by Very SmallGrains (VSGs). The AIB spectra in the observed objects can be interpreted asthe contribution of neutral and positively-charged Polycyclic AromaticHydrocarbons (PAHs). We extracted the 5 to 25 um emission spectrum of VSGs incool PDRs, these grains being most likely carbonaceous. We show that thevariations of the mid-IR (5-35 um) spectra of PDRs can be explained by thephoto-chemical processing of these VSGs and PAHs, VSGs being the progenitors offree PAH.Comment: 14 pages, 11 figures, to appear in A&

Analysis of the emission of very small dust particles from Spitzer spectro-imagery data using blind signal separation methods