C_(60) in Reflection Nebulae

Abstract

The fullerene C_(60) has four infrared-active vibrational transitions at 7.0, 8.5, 17.4, and 18.9 μm. We have previously observed emission features at 17.4 and 18.9 μm in the reflection nebula NGC 7023 and demonstrated spatial correlations suggestive of a common origin. We now confirm our earlier identification of these features with C_(60) by detecting a third emission feature at 7.04 ± 0.05 μm in NGC 7023. We also report the detection of these three C_(60) features in the reflection nebula NGC 2023. Our spectroscopic mapping of NGC 7023 shows that the 18.9 μm C_(60) feature peaks on the central star and that the 16.4 μm emission feature due to polycyclic aromatic hydrocarbons peaks between the star and a nearby photodissociation front. The observed features in NGC 7023 are consistent with emission from UV-excited gas-phase C_(60). We find that 0.1%-0.6% of interstellar carbon is in C_(60); this abundance is consistent with those from previous upper limits and possible fullerene detections in the interstellar medium (ISM). This is the first firm detection of neutral C_(60) in the ISM.

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