The 15-20 μm Spitzer Spectra of Interstellar Emission Features in NGC 7023

We present 15-20 μm long-slit spectra of NGC 7023 from the Infrared Spectrograph (IRS) on Spitzer. We observe recently discovered interstellar emission features at 15.9, 16.4, 17.0, 17.4, 17.8, and 18.9 μm, throughout the reflection nebula. The 16.4 μm emission feature peaks near the photodissociation front northwest of the star, as the aromatic emission features (AEFs) at 3.3, 6.2, and 11.3 μm do. The 16.4 μm emission feature is thus likely related to the AEFs and radiates by nonequilibrium emission. The new 18.9 μm emission feature, by contrast, decreases monotonically with stellar distance. We consider candidate species for the 18.9 μm feature, including polycyclic aromatic hydrocarbons, fullerenes, and diamonds. We describe future laboratory and observational research needed to identify the 18.9 μm feature carrier