Very small grain emission in NGC 7023

We observed the reflection nebula NGC7023 at various positions using wide and narrow band photometry centred on the 7.7 and 11.3 μm PAH emission features and wide band photometry at 20 and 25 μm. No significant variations have been detected over the nebula in the 7.3 and 20 μm continuum relative to the total emission in the 6-15 μm range (hereafter Γ). This result supports the hypothesis that most of the continuum emission in both bands come from very small grains. In contrast, the 7.7 and 11.3 μm PAH emission features show clear variations. We find that in weaker radiation fields of the nebula the 11.3 and 7.7 μm features are not the dominant emission components in the 6-15 μm range. The 11.5 μm band increases whereas the 11.3 μm emission feature decreases relative to Γ in the outer parts of the nebula. The effect can be explained by increased hydrogenation of PAH molecules.