The CH out-of-plane bending modes of PAH molecules in astrophysical environments

We present 10-15 μm spectra of a sample of H ii regions, YSOs and evolved stars that show strong unidentified infrared emission features, obtained with the ISO/SWS spectrograph on-board ISO. These spectra reveal a plethora of emission features with bands at 11.0, 11.2, 12.0, 12.7, 13.5 and 14.2 μm. These features are observed to vary considerably in relative strength to each-other from source to source. In particular, the 10-15 μm spectra of the evolved stars are dominated by the 11.2 μm band while for H ii regions the 12.7 is typically as strong as the 11.2 μm band. Analysing the ISO data we find a good correlation between the 11.2 μm band and the 3.3 μm band, and between the 12.7 μm and the 6.2 μm band. There is also a correlation between the ratio of the UIR bands to the total dust emission and the 12.7 over 11.2 μm ratio. Bands in the 10-15 μm spectral region are due to CH out-of-plane (OOP) bending modes of polycyclic aromatic hydrocarbons (PAHs). We summarise existing laboratory data and theoretical quantum chemical calculations of these modes for neutral and cationic PAHs. Due to mode coupling, the exact peak position of these bands depends on the number of adjacent CH groups and hence the observed interstellar 10-15 μm spectra can be used to determine the molecular structure of the interstellar PAHs emitting in the different regions. We conclude that evolved stars predominantly inject compact 100-200 C-atom PAHs into the ISM where they are subsequently processed, resulting in more open and uneven PAH structures.