Spectroscopy of polycyclic aromatic hydrocarbons and very small grains in photodissociation regions

We have coupled a singular value decomposition method with a Monte Carlo search algorithm to analyse the mid-infrared ISOCAM spectral maps of photodissociation regions (PDRs) in NGC 7023 and ρ Oph-SR3.Three different spectra and their associated spatial distribution were extracted from this analysis. It is shown that they can be associated with polycyclic aromatic hydrocarbons (PAHs) in their cationic and neutral forms and a third population of carbonaceous very small grains (VSGs). The method allows for the first time (i) to separate the contribution of neutral PAHs to the interstellar emission spectrum from that of cationic PAHs; (ii) to show that the 7.8 μm component of the “7.7 μm” broad feature is carried by VSGs, whereas the 7.6 μm component is due to PAHs; and (iii) to give evidence that free-flying PAHs are produced in PDRs by photoevaporation of VSGs. It is proposed that these carbonaceous VSGs are indeed PAH clusters. We derived a minimal size of 400 carbon atoms per cluster and estimated a UV absorption power of the order of 10-24 W C-1 for their dissociation