Polycyclic Aromatic Hydrocarbon Emission in the 15-21 Micron Region

Observations from the Spitzer Space Telescope have drawn attention to spectroscopic structure longward of 15 μm that is associated with objects showing prominent unidentified infrared (UIR) bands in the mid-IR. If polycyclic aromatic hydrocarbons (PAHs) are indeed responsible for the UIR features, longer wavelength emission arising from out-of-plane PAH skeletal vibrations is required. Here we compare some of the Spitzer spectra with spectra from the Infrared Space Observatory and analyze these data in terms of the PAH model utilizing the spectra of neutral PAHs from the Ames PAH IR spectral database. The 14-21 μm emission spectra from the H II region S106, the young stellar object CD -42°11721, the reflection nebula NGC 7023, and the H2 ridge in LkHα 234 are presented. We show that while the emission in this region can be quite variable, the bulk of these variations can be accommodated by variations in PAH population.