The PAH Hypothesis

Abstract

Polycyclic Aromatic Hydrocarbons (PAHs) are undoubtedly present in large amounts in many different astronomical objects as well as in the diffuse interstellar medium. They are detected through their infrared vibrational transitions in the mid-infared but are expected to be observable in other regions of the spectrum such as the UV, the visible and the far-infrared. PAHs are suggested to be the most abundant free organic molecules in the interstellar environment. In the laboratory, investigations have been undertaken to provide the tools for the interpretation of astronomical observations. For example, the recent detection of the overtone of the well-known 3.28 μm band around 1.67 μm has led to the determination of a relatively large minimum average size of about 60 atoms. In the far-infrared, above 15 up to 100 μm, expected transitions from laboratory measurements are scattered all over this region, making the detection of these lines difficult. In the diffuse medium, a complex aromatic network is expected on interstellar grains and its similarity with carbon extracts from primitive meteorites is suggestive of a link between interstellar matter and primitive Solar System bodies, in particular comets.