Dust formation in the oxygen-rich AGB star IK Tauri

Aims. We model the synthesis of molecules and dust in the inner wind of the oxygen-rich Mira-type star IK Tau by considering the effects of periodic shocks induced by the stellar pulsation on the gas and by following the non-equilibrium chemistry in the shocked gas layers between 1 R⋆ and 10 R⋆. We consider a very complete set of molecules and dust clusters, and combine the nucleation phase of dust formation with the condensation of these clusters into dust grains. We also test the impact of increasing the local gas density. Our derived molecular abundances and dust properties are compared to the most recent observational data. Methods. A semi-analytical formalism based on parameterised fluid equations is used to describe the gas density, velocity, and temperature in the inner wind. The chemistry is described by using a chemical kinetic network of reactions and the condensation mechanism is described by a Brownian formalism. A set of stiff, ordinary, coupled differential equations is solved, and molecular abundances, dust cluster abundances, grain size distributions and dust masses are derived. Results. The shocks drive an active non-equilibrium chemistry in the dust formation zone of IK Tau where the collision destruction of CO in the post-shock gas triggers the formation of C-bearing species such as HCN and CS. Most of the modelled molecular abundances agree well with the latest values derived from Herschel data, except for SO2 and NH3, whose formation may not occur in the inner wind. Clusters of alumina, Al2O3, are produced within 2 R⋆ and lead to a population of alumina grains close to the stellar surface. Clusters of silicates (Mg2SiO4) form at larger radii (r> 3R⋆), where their nucleation is triggered by the formation of HSiO and H2SiO. They efficiently condense and reach their final grain size distribution between ~6 R⋆ and 8 R⋆ with a major population of medium size grains peaking at ~200 Å. This two dust-shell configuration agrees with recent interferometric observations. The derived dust-to-gas mass ratio for IK Tau is in the range 1–6 × 10-3 and agrees with values derived from observations of O-rich Mira-type stars. Conclusions. Our results confirm the importance of periodic shocks in chemically shaping the inner wind of AGB stars and providing gas conditions conducive to the efficient synthesis of molecules and dust by non-equilibrium processes. They indicate that the wind acceleration will possibly develop in the radius range 4–8 R⋆ in IK Tau.