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