Which carriers for the unidentified IR emission bands? Observations and laboratory simulations

Abstract

The spectra of samples of pyrolysed polycyclic aromatic hydrocarbons (PAHs) are shown to present some striking similarities to the interstellar IR emission bands observed at 3.3, 6.2, 7.6, 7.8, 8.6, 11.3 µm and to the related features at 3.5 and 12.7 µm. The spectral variations which are observed during the pyrolysis of several PAHs: phenanthrene C₁₄H₁₀, coronene C₂₄H₁₂ and ovalene C₃₂H₁₄, are discussed in relation to the spectral diversity observed in the astronomical spectra. In the laboratory, the spectral changes are found to be related to the polymerization of PAHs, which induces the formation of very large PAHs and leads to various reactions between hydrogen and PAHs: H-transfer, H-loss upon aromatization and possibly H-capture by other species. These processes give characteristic features which seem to have their counterpart in astronomical spectra. This further constrains the identity of the carriers of the emission bands and the physico-chemical conditions which prevail in the regions where they are observed. A tentative identification is given, which attributes the classical emission bands to large (possibly ionised) PAHs, the anomalous 3.51(3.53) µm band to superhydrogenated PAHs, the broad 8.7 µm band observed in HII regions to very large PAH systems, and the 12.7 µm band to (slightly) dehydrogenated PAHs. The 11.0 µm band might be indicative of more severe dehydrogenation. The proposed identifications have yet to be further tested by dedicated observational and laboratory work.