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Astron. Astrophys. 357, 1013-1019 (2000)

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6. Summary

We have observed an emission plateau which extends from 15 to 20 µm in some young stellar objects, compact H II regions, and evolved objects (planetary nebulae) which show the PAH bands. The relative strength of this plateau, relative to the shorter wavelength PAH bands varies by about a factor 10 between the different sources. Although the global profile of the plateau is very similar between the different sources, significant variations in the substructure are apparent. The two YSO's, IRAS 03260 and CD 42, and the PN, NGC 7027, exhibit a clear 16.4 µm feature on top of this plateau, which contains about 10% of the total flux in the plateau. Taking this all together, we interpret this plateau as a collection of blended emission features carried by the same family of PAH molecules and PAH clusters responsible for the PAH bands and the 6-9 µm plateau underlying the 6.2 and 7.7 µm feature.

Using laboratory experimental spectra taken from the Hudgins dataset, and quantum chemical calculations preformed by Bauschlicher on the IR modes expected in this wavelength region for neutral and cationic PAHs, we show that bands in this wavelength region arise from C-C-C bending modes which cause in- and out-of-plane distortions of the carbon skeleton. These C-C-C bending modes are very sensitive to the individual molecular structure of the PAH and, hence, different molecules emit at (slightly) different positions. A family of PAHs will therefore give rise to a plateau in this region (see Fig. 5.). Spectral substructure present in this plateau, such as the 16.4 µm band, seems to be a characteristic of the presence of asymmetry in the molecular structure, such as a pentagon, which tends to produce IR spectral activity around this wavelength. In that case, this band is likely carried by relatively small PAHs ([FORMULA] 50 C-atoms). Further laboratory and/or quantum chemical calculations may well provide important clues to the identity of the emitting species.

Following theoretical studies of the IR emission of a size distribution of PAHs, we attribute the overall 15-20 µm plateau emission to relatively large PAHs or PAH clusters containing typically [FORMULA] 2000 carbon-atoms (but recognizable spectral structure is likely carried by smaller species; see above). The intensity ratio of the 15-20 µm plateau to the UIR features is then a measure for the relative abundances of big PAH clusters and small PAH molecules. We can therefore attribute the observed variations in this ratio, to variations in the size distribution of PAHs and PAH clusters. Analysis of our data shows that the size distribution shifts to larger sizes in H II regions such as S 106 compared to the Herbig AeBe stars in our sample or to the PN, NGC 7027. Possibly, this reflects the effects of coagulation which may have been more important in the dense environment which lead to massive star formation than in regions where intermediate mass stars form. Whether the dominance of small sizes in the PNe, NGC 7027, reflects the irrelevance of coagulation in this environment or the effects of shattering in the (weak) shocks associated with its outflows is presently not clear.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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