2003ApJ...599..312A

We present a new 0".9 resolution 3.29 µm narrowband image of the reflection nebula NGC 7023. We find that the 3.29 µm infrared emission feature (IEF) in NGC 7023 is brightest in narrow filaments northwest of the illuminating star HD 200775. These filaments have been previously seen in images of extended red emission, K', near-infrared H₂emission lines, the 6.2 and 11.3 µm IEFs, and HCO⁺. We also detect 3.29 µm IEF emission faintly but distinctly between the filaments and the star. The 3.29 µm IEF image is in marked contrast to narrowband continuum images at 2.09, 2.14, and 2.18 µm, which show an extended emission peak midway between the filaments and the star, and much fainter emission near the filaments. The [2.18 µm]-[3.29 µm] color shows a wide variation, ranging from colors of 3.4-3.6 mag at the 2 µm continuum emission peak to a color of 5.5 mag in the 3.29 µm IEF emission filaments. A color of 5.5 mag is 1 mag higher than any published value in reflection nebulae. We observe [2.18 µm]-[3.29 µm] to increase smoothly with increasing projected distance from the illuminating star in NGC 7023, up until the brightest 3.29 µm IEF filament, suggesting that the main difference between the spatial distributions of the 2 µm continuum and the 3.29 µm IEF emission is related to the incident stellar flux. The 3.29 µm IEF is widely attributed to an aromatic CH stretch, while the 2 µm continuum emission may be due to transient heating of tiny grains, fluorescence of aromatic molecules, or photoluminescence of larger grains. Our results suggest that the 3.29 µm IEF carriers are likely to be distinct from but related to the 2 µm continuum emitters. Our findings also imply that in NGC 7023 the 2 µm continuum emitters are mainly associated with H I, while the 3.29 µm IEF carriers are primarily found in warm H₂, but that both the 2 µm continuum emitters and the 3.29 µm IEF carriers can survive in H I or H₂.