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

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1. Introduction

Searching for evidence of episodic mass loss of evolved stars on the asymptotic giant branch (AGB), Olofsson et al. (1996) noted the existence of detached narrow shells (width-to-radius [FORMULA]) in molecular line emission of CO around the carbon stars R Scl, U Ant, S Sct and TT Cyg. These objects were chosen from a sample of visually bright C-stars detected in circumstellar CO radio emission by Olofsson et al. (1993). All of these objects are irregular or semiregular variables, and their position in the IRAS two-color diagram (regions VIa and VIb, respectively; cf. van der Veen & Habing 1988) indicates the existence of detached circumstellar dust shells. In the case of U Ant, careful analysis of IRAS images has indeed revealed a detached dust shell surrounding an extended inner shell, which is probably also detached (Izumiura et al. 1997). The apparent size of this inner dust shell roughly coincides with the ring of CO emission reported by Olofsson et al. (1996). Very recently, Olofsson et al. (1998, 2000) have succeeded in spatially resolving the CO shell around TT Cyg through interferometric observations and found an even 10 times smaller width-to-radius value of [FORMULA]. Absolute radii of the CO shells are estimated to lie between about 1 (R Scl) and 3 (TT Cyg) times [FORMULA] cm. Since their discovery, the origin of the circumstellar CO shells has been a matter of debate.

Thin gas shells are not uncommon in astrophysics. Usually, they are formed by two-wind interaction, such as in planetary nebulae and even supernova remnants. However, it is unclear how the wind interaction mechanism can operate in the much slower, dust-driven winds of AGB stars. Although it has been speculated that the occurrence of "thermal pulses" might play a key role, hydrodynamical models describing the response of a dusty AGB wind to the short-term variations of the stellar parameters during a helium-shell flash have not been computed before.

The primary purpose of this work is to investigate two competing scenarios currently being considered as possible explanations for the existence of thin circumstellar CO shells: Mass loss `eruption' versus two-wind interaction (for a recent discussion see Olofsson et al. 2000and references therein). We find that a typical helium-shell flash leads to a cooperation of both mechanisms, combining their advantages and eliminating their problems. This sequence of events is very likely the correct explanation for the origin of the observed thin CO shells.

Before presenting the main results in Sect. 3, we briefly summarize in Sect. 2 the basic features of the two different time-dependent hydrodynamics codes which were employed as the main tools for the present investigation. After discussing some open questions in Sect. 4, we finally summarize our conclusions in Sect. 5.

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000