Chemical modelling of dust-gas chemistry within AGB outflows III.
  Photoprocessing of the ice and return to the ISM
  To explain the properties of dust in the interstellar medium (ISM), the
presence of a refractory organic mantle is necessary. The outflows of AGB stars
are among the main contributors of stellar dust to the ISM. We present the
first study of the refractory organic contribution of AGB stars to the ISM.
Based on laboratory experiments, we included a new reaction in our extended
chemical kinetics model: the photoprocessing of volatile complex ices into
inert refractory organic material. The refractory organic feedback of AGB
outflows to the ISM is estimated using observationally motivated parent species
and grids of models of C-rich and O-rich outflows. Refractory organic material
is mainly inherited from the gas phase through accretion onto the dust and
subsequent photoprocessing. Grain-surface chemistry, initiated by
photodissociation of ices, produces only a minor part and takes place in a
sub-monolayer regime in almost all outflows. The formation of refractory
organic material increases with outflow density and depends on the initial
gas-phase composition. While O-rich dust is negligibly covered by refractory
organics, C-rich dust has an average coverage of $3-9\%$, but can be as high as
$8-22\%$. Although C-rich dust does not enter the ISM bare, its average
coverage is too low to influence its evolution in the ISM or significantly
contribute to the coverage of interstellar dust. This study opens up questions
on the coverage of other dust-producing environments. It highlights the need
for an improved understanding of dust formation and for models specific to
density structures within the outflow.
