The role of the charge state of PAHs in ultraviolet extinction

Aims. We explore the relation between the charge state of polycyclic aromatic hydrocarbons (PAHs) and the extinction curve morphology. Methods. We fit extinction curves with a dust model including core-mantle spherical particles of mixed chemical composition (silicate core, sp(2) and sp(3) carbonaceous layers), and an additional molecular component. We use exact methods to calculate the extinction due to classical particles and accurate computed absorption spectra of PAHs in different charge states, for the contribution due to the molecular component, along five different lines of sight. Results. A combination of classical dust particles and mixtures of real PAHs satisfactorily matches the observed interstellar extinction curves. Variations of the spectral properties of PAHs in different charge states produce changes consistent with the varying relative strengths of the bump and non-linear far-UV rise.