The C-C-C bending modes of PAHs: a new emission plateau from 15 to 20 mu m

We have obtained 2.5-45 mu m spectra of a sample of compact H II regions, Y SOs and evolved stars in order to study the origin and evolution of interst ellar Polycyclic Aromatic Hydrocarbon molecules (PAHs). Besides the well-kn own, strong PAH bands at 3.3, 6.2, 7.7, 8.6, and 11.2 mu m, these spectra r eveal for the first time, a ubiquitous emission plateau from 15 to 20 mu m While the overall shape of this plateau is very similar in all sources, the detailed profiles vary from source to source. In particular, some sources show a distinct emission feature at 16.4 mu m. Moreover, the integrated int ensity of this plateau varies relative to the PAH emission features by a fa ctor 10 in our sample. We attribute this 15-20 mu m plateau to a blend of many emission features d ue to the interstellar or circumstellar PAH family present in these sources . Laboratory studies and quantum chemical calculations show that PAH molecu les invariably possess emission features in this wavelength region, arising from C-CC bending modes which cause in- and out-of-plane distortion of the carbon skeleton. These modes are very sensitive to the molecular structure of the specific PAHs present and hence different molecules emit at differe nt wavelengths. Analysis of the available data on the IR characteristics of PAHs show that a collection of PAHs will give rise to a broad plateau in t his region. We have analyzed the size distribution of PAHs giving rise to the IR emissi on spectra of the sources in our samples. While much of the 15-20 mu m plat eau is thought to arise in relatively large PAHs and PAH clusters, we attri bute the 16.4 mu m feature to the small end of the interstellar PAH size di stribution. We conclude that the observed increased strength of the 15-20 m u m plateau relative to the shorter wavelength IR emission features in regi ons of massive star formation is caused by a preponderance of larger PAHs a nd PAH clusters in those sources. Possibly this reflects the importance of coagulation in the dense molecular cloud environment from which these stars are formed.