THE SURVIVAL OF PB-205 IN INTERMEDIATE-MASS AGB STARS

The now extinct Pb-205 is a pure s-process radionuclide (t(1/2) = 1.5 10(7) y) of possible substantial cosmochemical interest. As a necessar y complement to the detailed theoretical study of the nuclear physics and astrophysics aspects of the Pb-205-Tl-205 pair carried out by Yoko i et al. (1985), and to the recent calculation of the Pb-205 productio n in Wolf-Rayet stars by Arnould et al. (1997), this paper addresses f or the first time in some detail the question of the survival of this radionuclide in thermally pulsing AGE stars. This problem is made diff icult by the high sensitivity to temperature and density of the rates of the weak interaction processes that are able to produce or destroy Pb-205. In view of this sensitivity, a recourse to detailed stellar mo dels is mandatory. With the help of some simplifying assumptions conce rning in particular the third dredge-up characteristics, some of which (like its depth) being considered as free parameters, predictions are made for the Pb-205 contamination of the stellar surface at the end o f a pulse-interpulse cycle following a series of a dozen of pulses in three different intermediate-mass stars (M = 3 M-circle dot,Z = 0.02; M = 6 M-circle dot,Z = 0.021 M = 3 M-circle dot,Z = 0.001). It is conc luded that the chances for a significant Pb-205 surface enrichment are likely to increase with M for a given Z, or to increase with decreasi ng Z for a given M. More specifically, following the considered pulses at least, the enrichment appears to be rather unlikely in the 3 M-cir cle dot star with Z = 0.02, while it seems to be much more probable in the other two considered stars. It is also speculated that the (3 M-c ircle dot,Z = 0.02) star could possibly experience some Pb-205 en rich ment following later pulses than the ones considered in this paper.