Carbon onions as possible carriers of the 2175 angstrom interstellar absorption bump

We have studied the ultraviolet (UV) optical response of various hollow and filled spherical multishell fullerenes (commonly called carbon onions) in the wavelength region of the carbon pi-plasmon excitation. Our theoretical approach, which follows a recently proposed growth model, allowed us to investigate the modification of the electronic properties of these clusters brought about by a material (for example, diamond) filling the inner cavity and by a coating layer (for example, water). We found that coated onion particles have optical properties consistent with the UV interstellar dust absorption data, including the constant position of the absorption maximum associated with a large variation of the resonance width.