On the origin of thin detached gas shells around AGB stars - Insights fromtime-dependent hydrodynamical simulations

We have applied two different computer codes to study the time-dependent hy drodynamics of circumstellar gas/dust shells of AGE stars in their final st ages of evolution. A two-component radiation hydrodynamics code is designed to model a stellar wind driven by radiation pressure on dust grains. Combi ned with detailed stellar evolution calculations, this approach allows us t o simulate the dynamical response of the AGE wind envelope and the emergent spectral energy distribution to temporal changes of the stellar luminosity and mass loss rate. A completely independent one-component, Godunov-type h ydrodynamics code, which is particularly well suited to resolve shock front s, is used to check the results obtained with the numerically more diffusiv e two-component code. First, we verify that a presumed short episode of high mass loss translates into a correspondingly narrow, high-density shell moving through the circu mstellar envelope, provided that the mass loss rate, and hence the outflow velocity, is essentially constant during the mass ejection. In principle, t his scenario remains a viable explanation for the existence of the very thi n molecular shells recently detected around some carbon-rich AGE stars. Second, we discovered that an alternative mechanism producing very thin she lls of greatly enhanced gas density can operate in the dusty outflows from AGE stars: the interaction of a faster inner wind running into a slower out er wind, sweeping up matter at the interface between both type of winds. Ba sed on different numerical simulations and on a simple analytical model, we show that this mechanism easily leads to the formation of very thin shells without the need to invoke large variations of the mass loss rate on very short time scales. Finally, we demonstrate that a typical helium-shell flash induces both a ma ss loss 'eruption' and a two-wind interaction due to the increased outflow velocity during the high mass loss phase, leading to the formation of a thi n compressed gas shell. Very likely, this mechanism is responsible for the origin of the CO shells found around some semiregular, optically visible ca rbon stars, the most prominent example being TT Cygni.