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

CONTEXT

A chemical scenario was proposed for photon-dominated regions (PDRs) according to which UV photons from nearby stars lead to the evaporation of very small grains (VSGs) and the production of gas-phase polycyclic aromatic hydrocarbons (PAHs).

AIMS

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.

METHODS

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 the IRC instrument onboard AKARI to trace the evolution of the 3.3 m and 3.4 m bands, which are associated with aromatic and aliphatic C-H bonds on PAHs. The spectral fitting involved an additional broad feature centered at 3.45 m that is often referred to as the plateau. Mid-IR observations obtained with the IRS instrument onboard the Space Telescope were used to distinguish the signatures of eVSGs and neutral and cationic PAHs. We correlated the spatial evolution of all these bands with the intensity of the UV field given in units of the Habing field to explore how their carriers are processed.

RESULTS

The intensity of the 3.45 m plateau shows an excellent correlation with that of the 3.3 m aromatic band (correlation coefficient R = 0.95) and a relatively poor correlation with the aliphatic 3.4 m band (R=0.77). This indicates that the 3.45 m feature is dominated by the emission from aromatic bonds. We show that the ratio of the 3.4 m and 3.3 m band intensity (/) decreases by a factor of 4 at the PDR interface from the more UV-shielded layers ( ~ 150, / = 0.13) to the more exposed layers ( > 1 × 10, / = 0.03). The intensity of the 3.3 m band relative to the total neutral PAH intensity shows an overall increase with , associated with an increase of both the hardness of the UV field and the H abundance. In contrast, the intensity of the 3.4 m band relative to the total neutral PAH intensity decreases with , showing that their carriers are actively destroyed by UV irradiation and are not efficiently regenerated. The transition region between the aliphatic and aromatic material is found to correspond spatially with the transition zone between neutral PAHs and eVSGs.

CONCLUSIONS

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 m emission band. Our analysis provides evidence for the presence of very small grains of mixed aromatic and aliphatic composition in PDRs.