A new approach to the puzzle of the ultraviolet interstellar extinction bump

We present a model that is able to shed light on the long-standing problem of the attribution of the UV interstellar extinction band at 4.6 mu m. The model relies on a basic physical description of the electronic structure of carbon materials and is supported by laboratory simulations of UV processing of interstellar grains. The UV bump is attributed to a population of nano-sized, UV-processed hydrogenated amorphous carbon grains: the bump carrier carbons (BCCs). Specifically, we model the feature with a linear combination of absorption from different BCC populations present in interstellar regions sampled along a line of sight. The observed bump width variations are the result of different contributions of BCC grains along different lines of sight. The absorption from less processed particles prevails for wider bumps (denser regions), while more processed grains dominate in the case of sharper features (diffuse medium).