2012A&A...542A..98J

The properties of hydrogenated amorphous carbon (a-C:H) dust evolve in response to the local radiation field in the interstellar medium (ISM) and the evolution of these properties is particularly dependent upon the particle size. A model for finite-sized, low-temperature amorphous hydrocarbon particles, based on the microphysical properties of random and defected networks of carbon and hydrogen atoms, with surfaces passivated by hydrogen atoms, has been developed. The eRCN/DG and the optEC_(s) models have been combined, adapted and extended into a new optEC_(s)(a) model that is used to calculate the optical properties of hydrocarbon grain materials down into the sub-nanometre size regime, where the particles contain only a few tens of carbon atoms. The optEC_(s)(a) model predicts a continuity in properties from large to small (sub-nm) carbonaceous grains. Tabulated data of the size-dependent optical constants (from EUV to cm wavelengths) for a-C:H (nano-)particles as a function of the bulk material band gap [E_g(bulk)], or equivalently the hydrogen content, are provided. The effective band gap [E_g(eff.)] is found to be significantly larger than E_g(bulk) for hydrogen-poor a-C(:H) nano-particles and their predicted long-wavelength (λ> 30µm) optical properties differ from those derived for interstellar polycyclic aromatic hydrocarbons (PAHs). The optEC_(s)(a) model is used to investigate the size-dependent structural and spectral evolution of a-C(:H) materials under ISM conditions, including: the IR-FUV extinction, the 217nm bump and the infrared emission bands. The model makes several predictions that can be tested against observations.