The 15-20 μm PAH emission features: probes of individual PAHs?

Context. Spectral features between about 15-20 mu m are commonly associated with polycyclic aromatic hydrocarbons (PAHs). With the NASA Spitzer Space Telescope these features are reported routinely, and as such, warrant a deeper molecular explanation. Aims. We aim to determine the characteristics of the group of carriers of the plateau and the distinct sub-features at 15.8, 16.4, 17.4, 17.8 and 18.9 mu m and to draw astronomical implications from these spectra. Methods. We analyse and interpret the spectra of 15 different sources using the NASA Ames PAH IR spectroscopic database. Results. The bands within the 15-20 mu m region show large variations. Except for the 16.4 mu m band, there is also no connection, both in band strength and feature classification, with the mid-IR PAH bands. Of the PAH spectra considered, only those from species containing pendent rings show one common characteristic: a band near the astronomical 16.4 mu m position. However, coupling with the carbon skeleton's core influences its precise position in the spectrum. Compact PAHs in the size range 50-130 carbon atoms, consistently show a strong band near the astronomical 17.4 mu m band position. Conclusions. The 15-20 mu m region is the transition zone from PAH nearest neighbour modes to full-skeleton modes. We conclude that a few individual PAHs dominate the astronomical PAH family when clear features are prominent. In the few cases of a broad plateau, the PAH family would be far richer. Although PAHs containing pendent rings showed promise explaining the astronomical 16.4 mu m band, coupling with the skeleton core and the inherent strong quartet mode expected around 13.5 mu m, make it a less viable candidate. The number of large PAHs in the database becomes a limitation in studying the emission between 15-20 mu m and longward. Computation of larger PAH spectra should therefore be stimulated, especially for understanding the forthcoming far-IR data expected from Herschel, SOFIA and ALMA.