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J. H. An, K. Sellgren (OSU)
We present ground-based images of the 3.3 micron aromatic emission feature, the 2.09 and 2.14 micron narrow-band continua, and the 2.12 micron 1--0 S(1) emission line of H2 in the reflection nebula NGC 7023. We find, as have previous authors, that the H2 emission is concentrated in narrow filaments northwest of the illuminating star, HD 200775. We observe that the 3.3 micron emission is also very bright in the filaments, but that there is also a fainter, more extended region of 3.3 micron emission between the filaments and the star. The spatial coincidence of strong 3.3 micron emission and strong H2 emission in the NGC 7023 filaments is in marked contrast to the Orion Bar, where the 3.3 micron emission peaks 0.03 pc closer to the star than does the H2 emission. The 2.1 micron narrow-band continuum is not strong in the filaments, but rather is brightest in the region between the filaments and the star. The 2.1 micron continuum in visual reflection nebulae has been attributed to non-equilibrium heating of tiny grains by single stellar photons, electronic or molecular fluorescence of polycyclic aromatic hydrocarbon (PAH) molecules, or luminescence of aromatic grains. The 3.3 micron aromatic emission feature has been attributed to PAH molecular emission or thermal emission (equilibrium or non-equilibrium) from aromatic grains. There is a clear spatial separation between the 2.1 micron continuum, which is weak in the filaments and strong in the region between the star and the filaments, and the 3.3 micron aromatic emission feature, which is strong in the filaments and weak between the star and the filaments. This leads us to suggest that the 2.1 micron continuum and the 3.3 micron emission feature arise either from different interstellar materials or arise in the same interstellar material but by very different excitation mechanisms.
The author(s) of this abstract have provided an email address for comments about the abstract: sellgren@astronomy.ohio-state.edu