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Letter to the Editor
A theoretical model for episodic mass-loss producing detached shells around bright carbon stars
T.U. Arndt and
Received 15 April 1998 / Accepted 13 May 1998
We present stellar evolution models of the final AGB phase, in which the star undergoes heavy and optically thick mass-loss ("superwind" phase). Our computations are based on consistent, pulsating wind models for carbon-rich stars and include a detailed treatment of dust formation, radiative transfer and wind acceleration (Fleischer et al. 1992). For a specific mass range, around 1.2 stellar mass at the foot-AGB and only about 0.2 wide, we find particularly pronounced episodic mass-loss which is consistent with all properties of the detached CO shells found by Olofsson et al. (1990, 1993, 1996) around bright carbon stars: kinematic ages of 1 to 2 yrs, masses of several 0.01 , and a mass-loss duration of less than several thousand years.
The physics, micro-physics, and chemistry of our dust-induced superwind is essential for understanding such details of the final stellar mass-loss history. Unlike other superwind models, our mass-loss rate depends very sensitively on the stellar temperature - about - and our models require a minimum luminosity to be surmounted. Together, that yields a much pronounced mass-loss variation with the late thermal pulses. In particular, our models suggest the formation of CO shells in the final 2 to 6 yrs on the tip-AGB - if the stellar luminosity is close to the critical (Eddington-like) luminosity log (around 3.5 to 3.7, depending on ), while the star has only left to lose towards the exposure of its hot core.
Key words: stars: carbon stars: circumstellar matter stars: evolution stars: interiors stars: late-type stars: mass loss
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© European Southern Observatory (ESO) 1998
Online publication: June 12, 1998