C{sub 60} IN REFLECTION NEBULAE

Abstract

The fullerene C{sub 60} has four infrared-active vibrational transitions at 7.0, 8.5, 17.4, and 18.9 {mu}m. We have previously observed emission features at 17.4 and 18.9 {mu}m 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{sub 60} by detecting a third emission feature at 7.04 {+-} 0.05 {mu}m in NGC 7023. We also report the detection of these three C{sub 60} features in the reflection nebula NGC 2023. Our spectroscopic mapping of NGC 7023 shows that the 18.9 {mu}m C{sub 60} feature peaks on the central star and that the 16.4 {mu}m 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{sub 60}. We find that 0.1%-0.6% of interstellar carbon is in C{sub 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{sub 60} in the ISM.