Published online by Cambridge University Press: 30 March 2011
Aromatic carbon, in some form, has been an essential ingredient by and large in allmodels of the extinction curve, since the original proposal to attribute the bump at 217.5nm to “astronomical graphite”. This aromatic carbon is most naturally identified, in up todate models, with a population of Polycyclic Aromatic Hydrocarbons (PAHs), free and/orclustered. In all models, this PAH population accounts for the far–UV nonlinear rise inthe extinction curve, contributes to the bump and possibly part of the large set ofunidentified, discrete absorption features in the visible (the Diffuse InterstellarBands). We review the current state of our understanding of the contribution of PAHs tointerstellar extinction, and what constraints can be imposed on the PAH population byfitting extinction models to observations.