The survival of 205Pb in intermediate-mass AGB stars

The now extinct 205 Pb is a pure s-process radionu- clide (t1=2 =1 : 51 0 7 y) of possible substantial cosmochemical interest. As a necessary complement to the detailed theoreti- cal study of the nuclear physics and astrophysics aspects of the 205 Pb - 205 Tl pair carried out by Yokoi et al. (1985), and to the recent calculation of the 205 Pb production in Wolf-Rayet stars by Arnould et al. (1997), this paper addresses for the rst time in some detail the question of the survival of this radionuclide in thermally pulsing AGB stars. This problem is made difcult by the high sensitivity to temperature and density of the rates of the weak interaction processes that are able to produce or destroy 205 Pb. In view of this sensitivity, a recourse to detailed stellar models is mandatory. With the help of some simplify- ing assumptions concerning in particular the third dredge-up characteristics, some of which (like its depth) being considered as free parameters, predictions are made for the 205 Pb contam- ination of the stellar surface at the end of a pulse-interpulse cycle following a series of a dozen of pulses in three different intermediate-mass stars (M =3 M , Z= 0.02; M =6 M , Z = 0.02; M =3 M , Z= 0.001). It is concluded that the chances for a signicant 205 Pb surface enrichment are likely to increase with M for a given Z, or to increase with decreasing Z for a given M. More specically, following the considered pulses at least, the enrichment appears to be rather unlikely in the 3M 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 (3M,Z = 0.02) star could possibly experience some 205 Pb en- richment following later pulses than the ones considered in this paper.