  Context. The compositional properties of hydrogenated amorphous carbons are
known to evolve in response to the local conditions. Aims. To present a model
for low-temperature, amorphous hydrocarbon solids, based on the microphysical
properties of random and defected networks of carbon and hydrogen atoms, that
can be used to study and predict the evolution of their properties in the
interstellar medium. Methods. We adopt an adaptable and prescriptive approach
to model these materials, which is based on a random covalent network (RCN)
model, extended here to a full compositional derivation (the eRCN model), and a
defective graphite (DG) model for the hydrogen poorer materials where the eRCN
model is no longer valid. Results. We provide simple expressions that enable
the determination of the structural, infrared and spectral properties of
amorphous hydrocarbon grains as a function of the hydrogen atomic fraction, XH.
Structural annealing, resulting from hydrogen atom loss, results in a
transition from H-rich, aliphatic-rich to H-poor, aromatic-rich materials.
Conclusions. The model predicts changes in the optical properties of
hydrogenated amorphous carbon dust in response to the likely UV photon-driven
and/or thermal annealing processes resulting, principally, from the radiation
field in the environment. We show how this dust component will evolve,
compositionally and structurally in the interstellar medium in response to the
local conditions.
