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Mixed aliphatic and aromatic composition of evaporating very small grains in NGC 7023 revealed by the 3.4/3.3 $μ$m ratio

In photon-dominated regions (PDRs), UV photons from nearby stars lead to the evaporation of very small grains (VSGs) and the production of gas-phase polycyclic aromatic hydrocarbons (PAHs). Our goal is to achieve better insight into the composition and evolution of evaporating very small grains (eVSGs) and PAHs through analyzing the infrared (IR) aliphatic and aromatic emission bands. We combined spectro-imagery in the near- and mid-IR to study the spatial evolution of the emission bands in the prototypical PDR NGC 7023. We used near-IR spectra obtained with AKARI to trace the evolution of the 3.3$μ$m and 3.4$μ$m bands, which are associated with aromatic and aliphatic C-H bonds on PAHs. The spectral fitting involves an additional broad feature centred at 3.45$μ$m. Mid-IR observations obtained with Spitzer are used to discriminate the signatures of eVSGs, neutral and cationic PAHs. We correlated the spatial evolution of all these bands with the intensity of the UV field to explore the processing of their carriers. The intensity of the 3.45$μ$m plateau shows an excellent correlation with that of the 3.3$μ$m aromatic band (correlation coefficient R = 0.95), indicating that the plateau is dominated by the emission from aromatic bonds. The ratio of the 3.4$μ$m and 3.3$μ$m band intensity ($I_{3.4}/I_{3.3}$) decreases by a factor of 4 at the PDR interface from the more UV-shielded to the more exposed layers. The transition region between the aliphatic and aromatic material is found to correspond spatially with the transition zone between neutral PAHs and eVSGs. We conclude that the photo-processing of eVSGs leads to the production of PAHs with attached aliphatic sidegroups that are revealed by the 3.4$μ$m emission band. Our analysis provides evidence for the presence of very small grains of mixed aromatic and aliphatic composition in PDRs.