The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point

TLDR: In this paper, a power-law distribution of small a-C grains and log-normal distributions of large a-SilFe and aC(:H) grains are used to explore dust evolution in the interstellar medium, and it is shown that mantle accretion in molecular clouds and UV photo-processing in photodominated regions are likely the major drivers of dust evolution.

Abstract: Context. The evolution of amorphous hydrocarbon materials, a-C(:H), principally resulting from ultraviolet (UV) photon absorption-induced processing, are likely at the heart of the variations in the observed properties of dust in the interstellar medium.Aims. The consequences of the size-dependent and compositional variations in a-C(:H), from aliphatic-rich a-C:H to aromatic-rich a-C, are studied within the context of the interstellar dust extinction and emission.Methods. Newly-derived optical property data for a-C(:H) materials, combined with that for an amorphous forsterite-type silicate with iron nano-particle inclusions, a-SilFe , are used to explore dust evolution in the interstellar medium.Results. We present a new dust model that consists of a power-law distribution of small a-C grains and log-normal distributions of large a-SilFe and a-C(:H) grains. The model, which is firmly anchored by laboratory-data, is shown to quite naturally explain the variations in the infrared (IR) to far-ultraviolet (FUV) extinction, the 217 nm UV bump, the IR absorption and emission bands and the IR-mm dust emission.Conclusions. The major strengths of the new model are its inherent simplicity and built-in capacity to follow dust evolution in interstellar media. We show that mantle accretion in molecular clouds and UV photo-processing in photo-dominated regions are likely the major drivers of dust evolution.