Mixed aliphatic and aromatic composition of evaporating very small
  grains in NGC 7023 revealed by the 3.4/3.3 $\mu$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$\mu$m and
3.4$\mu$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$\mu$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$\mu$m plateau shows an
excellent correlation with that of the 3.3$\mu$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$\mu$m and 3.3$\mu$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$\mu$m emission band. Our
analysis provides evidence for the presence of very small grains of mixed
aromatic and aliphatic composition in PDRs.
