Dust-gas chemistry in AGB outflows: Chemical modelling of dust-gas
  chemistry within AGB outflows I. Effect on the gas-phase chemistry
  Chemical modelling of AGB outflows is typically focused on either
non-thermodynamic equilibrium chemistry in the inner region or photon-driven
chemistry in the outer region. We include, for the first time, a comprehensive
dust-gas chemistry in our AGB outflow chemical kinetics model, including both
dust-gas interactions and grain-surface chemistry. The dust is assumed to have
formed in the inner region, and follows an interstellar-like dust-size
distribution. Using radiative transfer modelling, we obtain dust temperature
profiles for different dust types in an O-rich and a C-rich outflow. We
calculate a grid of models, sampling different outflow densities, drift
velocities between the dust and gas, and dust types. Dust-gas chemistry can
significantly affect the gas-phase composition, depleting parent and daughter
species and increasing the abundance of certain daughter species via
grain-surface formation followed by desorption/sputtering. Its influence
depends on four factors: outflow density, dust temperature, initial
composition, and drift velocity. The largest effects are for higher density
outflows with cold dust and O-rich parent species, as these species generally
have a larger binding energy. At drift velocities larger than $\sim 10$ km
s$^{-1}$, ice mantles undergo sputtering; however, they are not fully
destroyed. Models with dust-gas chemistry can better reproduce the observed
depletion of species in O-rich outflows. When including colder dust in the
C-rich outflows and adjusting the binding energy of CS, the depletion in C-rich
outflows is also better reproduced. To best interpret high-resolution molecular
line observations from AGB outflows, dust-gas interactions are needed in
chemical kinetics models.
