A THEORETICAL-MODEL FOR EPISODIC MASS-LOSS PRODUCING DETACHED SHELLS AROUND BRIGHT CARBON STARS

We present stellar evolution models of the final AGE phase, in which t he star undergoes heavy and optically thick mass-loss (''superwind'' p hase). Our computations are based on consistent, pulsating wind models for carbon-rich stars and include a detailed treatment of dust format ion, radiative transfer and wind acceleration (Fleischer et al. 1992). For a specific mass range, around 1.2 M-. stellar mass at the foot-AG E and only about 0.2 M-. wide, we find particularly pronounced episodi c mass-loss which is consistent with all properties of the detached CO shells found by Olofsson et al. (1990, 1993, 1996) around bright carb on stars: kinematic ages of 1 to 2 x 10(4)yrs, masses of several 0.01 M-., and a mass-loss duration of less than several thousand years. The physics, micro-physics, and chemistry of our dust-induced superwind i s essential for understanding such details of the final stellar mass-l oss history. Unlike other superwind models, our mass-loss rate depends very sensitively on the stellar temperature - about proportional to T -eff(-8) - and our models require a minimum luminosity to be surmounte d. Together, that yields a much pronounced mass-loss variation with th e late thermal pulses. In particular, our models suggest the formation of CO shells in the final 2 to 6 x 10(4)yrs on the tip-AGE - if the s tellar luminosity is close to the critical (Eddington-like) luminosity log L-c (around 3.5 to 3.7, depending on T-eff), while the star has o nly less than or similar to 0.2M(.) left to lose towards the exposure of its hot core.