Polycyclic Aromatic Hydrocarbons and the Extinction Curve

Aromatic carbon, in some form, has been an essential in- gredient by and large in all models of the extinction curve, since the original proposal to attribute the bump at 217.5 nm to "astronomical graphite". This aromatic carbon is most naturally identified, in up to date models, with a population of Polycyclic Aromatic Hydrocarbons (PAHs), free and/or clustered. In all models, this PAH population ac- counts for the far-UV nonlinear rise in the extinction curve, contributes to the bump and possibly part of the large set of unidentified, discrete absorption features in the visible (the Diffuse Interstellar Bands). We review the current state of our understanding of the contribution of PAHs to interstellar extinction, and what constraints can be imposed on the PAH population by fitting extinction models to observations. 1 Interstellar extinction before PAHs Interstellar extintion has a long, winding story. It dates as far back in time as 1774, with sir William Herschel noticing a region in the Scorpio constellation remarkably devoid of stars, which he called a "hole in the sky". The fact that such regions were a common occurrence in the Milky Way was clear in the early years of 1900, with the systematic observations of Barnard, as was stated in a textbook of the time (Clerke 1903), and it began to be hypothesised that they were not actually holes in the fabric of the sky, but instead might be "obscured" by some intervening material. For some decades, this hypothesis was the topic of some heated debates, some of which are on record in the proceedings of academic meetings of the time (Shapley & Curtis 1921). It was only about in 1930 that firm evidence of interstellar extinction became available (Trumpler 1930).