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

Astronomy & Astrophysics, 335, L9-L12 (1998)

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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 1.2Msun stellar mass at the foot-AGB and only about 0.2Msun 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*104yrs, masses of several 0.01Msun, 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 propto Teff-8 --- 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*104yrs on the tip-AGB --- if the stellar luminosity is close to the critical (Eddington-like) luminosity log Lc (around 3.5 to 3.7, depending on Teff), while the star has only la 0.2Msun left to lose towards the exposure of its hot core.