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Astron. Astrophys. 349, 898-906 (1999)


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Tip-AGB stellar evolution in the presence of a pulsating, dust-induced "superwind"

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

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

Received 29 January 1998 / Accepted 3 August 1999

Abstract

We present selected "superwind" mass-loss histories and the related tip-AGB stellar evolution models, which have been computed according to the characteristics of a dust-induced, carbon-rich wind, and which include several recent improvements as compared to Schröder et al. (1998). We discuss the (initial) stellar mass-range of 1 to 2.5 [FORMULA], with a nearly solar composition (X=0.28, Y=0.70, Z=0.02). In each time-step, mass-loss rates are used, which are consistent with the actual stellar parameters, and which are based on our pulsating, dust-induced wind models for carbon-rich stars (Fleischer et al. 1992), including a detailed and consistent treatment of dust formation, radiative transfer and radiative wind acceleration.

The resulting "superwind" mass-loss rates reach 2 to [FORMULA]yr-1. For this reason, they become an influential factor of tip-AGB stellar evolution - but also vice versa, since our mass-loss rates vary strongly with effective temperature ([FORMULA] (roughly), see Arndt et al. 1997), reflecting the temperature sensitivity of the dust formation process on a macroscopic scale.

With all tip-AGB models of an initial stellar mass [FORMULA] we find superwinds with a total mass outflow of 0.26 to [FORMULA] during their final [FORMULA] yrs, just as required for PN-formation. Furthermore, a thermal pulse leads to a very short (100 to 200 yrs) interruption of the "superwind" of these models.

A critical (Eddington-like) luminosity [FORMULA] is required for the radiation driven wind models, which our evolution models fail to reach for [FORMULA]. With slightly larger stellar masses, [FORMULA] is near [FORMULA] and thermal pulses can trigger very short "superwind" bursts, as already pointed out by Schröder et al. (1998). We find good agreement between our improved models and the mass-loss characteristics of the thin CO shells found by Olofsson et al. (1990, 1993, 1996, 1998) around some carbon-rich Mira stars.

Key words: stars: carbon – stars: circumstellar matter – stars: evolution – stars: interiors – stars: late-type – stars: mass-loss

Send offprint requests to: K.-P. Schröder (schroder@ast.cam.ac.uk)

© European Southern Observatory (ESO) 1999

Online publication: September 13, 1999

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