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

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

Jul 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998a%26a...335l...9s&link_type=abstract

Astronomy and Astrophysics, v.335, p.L9-L12 (1998)

Astronomy and Astrophysics

Astrophysics

25

Stars: Carbon, Stars: Circumstellar Matter, Stars: Evolution, Stars: Interiors, Stars: Late-Type, Stars: Mass Loss

Scientific paper

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 Msun stellar mass at the foot-AGB and only about 0.2 Msun 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 x 10(4) yrs, masses of several 0.01 Msun, 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 ~ T_eff(-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 x 10(4) yrs 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 T_eff), while the star has only la 0.2 Msun left to lose towards the exposure of its hot core.

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