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

摘要

We present 15 - 20 micron long-slit spectra, from the Infrared Spectrograph(IRS) on Spitzer, of NGC 7023. We observe recently-discovered interstellaremission features, at 15.9, 16.4, 17.0, 17.4, 17.8, and 18.9 microns,throughout the reflection nebula. The 16.4 micron emission feature peaks nearthe photodissociation front northwest of the star, as do the aromatic emissionfeatures (AEFs) at 3.3, 6.2 and 11.3 microns. The 16.4 micron emission featureis thus likely related to the AEFs and radiates by non-equilibrium emission.The new 18.9 micron emission feature, by contrast, decreases monotonically withstellar distance. We consider candidate species for the 18.9 micron feature,including polycyclic aromatic hydrocarbons, fullerenes, and diamonds. Wedescribe future laboratory and observational research needed to identify the18.9 micron feature carrier.