Origin of quasi-periodic shells in dust forming AGB winds

We have combined time dependent hydrodynamics with a two-fluid model for dust driven AGB winds. Our calculations include self-consistent gas chemistry, grain formation and growth, and a new implementation of the viscous momentum transfer between grains and gas. This allows us to perform calculations in which no assumptions about the completeness of momentum coupling are made. We derive new expressions to treat time dependent and non-equilibrium drift in a hydro code. Using a stationary state calculation for IRC +10216 as an initial model, the time dependent integration leads to a quasi-periodic mass loss in the case where dust drift is an taken into account. The time scale of the variation is of the order of a few hundred years, which corresponds to the time scale needed to explain the shell structure of the envelope of IRC +10216 and other AGB and post-AGB stars, which has been a puzzle since its discovery. No such periodicity is observed in comparison models without drift between dust and gas.