Neutral and ionized PAHs: contribution to the interstellar extinction

Abstract

A detailed study of the UV-to-near-IR absorption spectra of neutral and ionized polycyclic aromatic hydrocarbons (PAHs) is reported. The experimental results obtained using Matrix isolation spectroscopy combined with the general spectroscopic properties of PAHs derived from theoretical modeling are compared with astronomical observations. From this study, it is concluded that: (i) small neutral PAHs (less than 25 carbon atoms) absorb only in the UV and cannot contribute to the known diffuse interstellar bands, although they may contribute to the extinction curve. A search for their spectral signatures should provide a test to the PAH model and should allow us to set significant limits for the abundances of specific PAHs; (ii) small ionized PAHs (less than 25 carbon atoms) absorb in the visible and near-IR close to the position of well-known diffuse interstellar bands. Based on these promising results it is argued that PAH ions constitute good candidates to explain the diffuse interstellar bands. An assessment of the PAH/diffuse interstellar band proposal is made and predictions for new diffuse interstellar bands to be searched for are presented.