The long-standing problem of 176Lu/176Hf branching: A new approach with full stellar evolutionary models

We present an analysis of the s-process branching at A=176, which is determined by the two s-only isotopes 176Lu and 176Hf. This work is motivated by the discrepancy between the current theoretical expectations and solar-system abundances of the two isotopes involved in the branching. Asymptotic Giant Branch models have been calculated with the FRANEC evolutionary code, coupling a full nuclear reaction network with the equation describing the physical evolution of the stellar structure. With respect to previous studies, we can better follow the complex interplay between mixing and burning, which both strongly influence this branching. We show that a substantial reduction of the mixing velocity within the convective zone generated by a He-shell flash (or thermal pulse) may alleviate the discrepancy between the theoretical expectations and the observed solar abundances. Nonetheless, we reckon that a definitive solution of this problem has to be searched in the nuclear coupling scheme between the thermally populated levels in 176Lu.