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

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

Astronomy & Astrophysics, 349, 898-906 (1999)

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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 Msun, 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 3*10-5 Msunyr-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 (dot{M} propto Teff-8 (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 Mi ga 1.3 Msun we find superwinds with a total mass outflow of 0.26 to ga 0.55 Msun during their final 3*104yrs, 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 Lc is required for the radiation driven wind models, which our evolution models fail to reach for Mi la 1.1 Msun. With slightly larger stellar masses, LtAGB is near Lc 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