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Astron. Astrophys. 335, L9-L12 (1998)


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Letter to the Editor

A theoretical model for episodic mass-loss producing detached shells around bright carbon stars

K.-P. Schröder, J.M. Winters, T.U. Arndt and E. Sedlmayr

Technische Universität Berlin, Institut für Astronomie und Astrophysik, Sekr. PN 8-1, Hardenbergstrasse 36, D-10623 Berlin, Germany

Received 15 April 1998 / Accepted 13 May 1998

Abstract

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  [FORMULA] stellar mass at the foot-AGB and only about 0.2  [FORMULA] 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 [FORMULA] yrs, masses of several 0.01 [FORMULA], 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 [FORMULA] - 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 [FORMULA] yrs on the tip-AGB - if the stellar luminosity is close to the critical (Eddington-like) luminosity log [FORMULA] (around 3.5 to 3.7, depending on [FORMULA]), while the star has only [FORMULA] 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

Send offprint requests to: K.-P. Schröder (e-mail: schroder@weizen.physik.tu-berlin.de)

© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998

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