The final 10^5 years of stellar AGB evolution in the presence of a pulsating, dust-induced "superwind"

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

Contributed talk presented at the IAU Symposium 191: AGB Stars, August 27th - September 1st 1998, Montpellier, France

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Abstract:

We have computed mass-loss histories and tip-AGB stellar evolution models in the presence of a dust-induced, carbon-rich "superwind", in the initial mass-range of 1.1 to about 2.5 solar masses and for nearly solar composition (X=0.28, Y=0.70, Z=0.02). Consistent, actual mass-loss rates are used in each time-step, based on pulsating and "dust-driven" stellar wind models for carbon-rich stars (Fleischer et al. 1992) which include a detailed treatment of dust-formation, radiative transfer and wind acceleration. Our tip-AGB mass-loss rates reach about 4*10-5Msun/yr and become an influencial factor of stellar evolution.

Heavy outflows of 0.3 to 0.6 Msun within only 2 to 3*104yrs, exactly as required for PN-formation, occur with tip-AGB models of an initial stellar mass Mi>~ 1.3Msun. The mass-loss of our "superwind" varies strongly witheffective temperature (dot{M} prop Teff-8, see Arndt et al. 1997), reflecting the temperature-sensitive micro-physics and chemistry of dust-formation and radiative transfer on a macroscopic scale. Furthermore, a thermal pulse leads to a very short (100 to 200 yrs) interruption of the "superwind" of these models.

For Mi<~ 1.1Msun, our evolution models fail to reach the (Eddingtonlike) critical luminosity Lc required by the radiatively driven wind models, while for the (initial) mass-range in-between, with the tip-AGB luminosity LtAGB near Lc, thermal pulses drive bursts of "superwind", which could explain the outer shells found with some PN"s. In particular, a burst with a duration of only 800 yrs and a mass-loss of about 0.03 Msun, occurs right after the last AGB thermal pulse of a model with Mi sim 1.1Msun. There is excellent agreement with the thin CO shells found by Olofsson et al. (e.g., 1990, 1998) around some Mira stars.