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Astron. Astrophys. 357, 180-196 (2000)

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On the origin of thin detached gas shells around AGB stars

Insights from time-dependent hydrodynamical simulations

M. Steffen and D. Schönberner

Astrophysikalisches Institut Potsdam, 14482 Potsdam, Germany (MSteffen, DeSchoenberner@aip.de)

Received 15 September 1999 / Accepted 13 March 2000

Abstract

We have applied two different computer codes to study the time-dependent hydrodynamics of circumstellar gas/dust shells of AGB stars in their final stages of evolution. A two-component radiation hydrodynamics code is designed to model a stellar wind driven by radiation pressure on dust grains. Combined with detailed stellar evolution calculations, this approach allows us to simulate the dynamical response of the AGB 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 hydrodynamics code, which is particularly well suited to resolve shock fronts, is used to check the results obtained with the numerically more diffusive 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 circumstellar envelope, provided that the mass loss rate, and hence the outflow velocity, is essentially constant during the mass ejection. In principle, this scenario remains a viable explanation for the existence of the very thin molecular shells recently detected around some carbon-rich AGB stars.

Second, we discovered that an alternative mechanism producing very thin shells of greatly enhanced gas density can operate in the dusty outflows from AGB stars: the interaction of a faster inner wind running into a slower outer wind, sweeping up matter at the interface between both type of winds. Based 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 mass 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 thin compressed gas shell. Very likely, this mechanism is responsible for the origin of the CO shells found around some semiregular, optically visible carbon stars, the most prominent example being TT Cygni.

Key words: stars: AGB and post-AGB – stars: circumstellar matter – stars: mass-loss – stars: individual: TT Cygni – hydrodynamics – radiative transfer

Send offprint requests to: Matthias Steffen

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000
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