On the presence of large aromatic molecules in the Interstellar Medium inferred from IR data

We propose a consistent explanation of the so-called ''unidentified'' IR emission bands of the Interstellar Medium observed at 3.3 - 6.2 - 7.7 - 8.6 - 11.3 μm. Following Sellgren (1984), we consider the transient heating of Very Small Grains (∼ 50 atoms) to a peak temperature of ∼ 1000 K by the absorption of a single UV photon and estimate the subsequent IR emission. We estimate the emission spectrum of a typical polycyclic aromatic hydrocarbon, coronene: C24H12, and find a suggestive spectroscopic agreement of the main spectral features with the astronomical ones. This explanation of the observed emission bands can account for their excitation mechanism which was not hitherto explained. New IR absorption spectra of large PAH molecules are presented with the cross section of the different vibrational modes. The corresponding emissions are calculated and it is shown that the agreement found for coronene is not casual: a family of PAH molecules can explain the astronomical observations. We can even specify that compact PAHs give a better agreement than non-compact ones. Different implications of this identification are reviewed such as the abundance of these molecules. We also mention several open problems