Dust model extinction for fixed dust mass as a function of E_g (top), a₋ (middle) and α (bottom). The solid (dotted) vertical grey lines mark the model (observed) UV bump peak positions. The symbols indicate the observed extinction for R_V = 3.1 (grey open squares) and 5.1 (black filled squares) (Savage & Mathis 1979; Mathis 1990). The orange line shows the intensity of the adopted ISRF plotted as J_λ/λ and normalised to unity.