THE EFFECTS OF ROTATION ON s -PROCESS NUCLEOSYNTHESIS IN ASYMPTOTIC GIANT BRANCH STARS

In this paper, we analyze the effects induced by rotation on low-mass asymptotic giant branch stars. We compute two sets of models, M = 2.0 M sub([middot in circle]) at [Fe/H] = 0 and M = 1.5 M sub([middot in circle]) at [Fe/H] = -1.7, by adopting main-sequence rotation velocities in the range 0-120 km s super(-1). At high metallicity, we find that the Goldreich-Schubert-Fricke instability, active at the interface between the convective envelope and the rapid rotating core, contaminates the super(13)C-pocket (the major neutron source) with super(14)N (the major neutron poison), thus reducing the neutron flux available for the synthesis of heavy elements. As a consequence, the yields of heavy-s elements (Ba, La, Nd, Sm) and, to a lesser extent, those of light-s elements (Sr, Y, Zr) decrease with increasing rotation velocities up to 60 km s super(-1). However, for larger initial rotation velocities, the production of light-s and, to a lesser extent, that of heavy-s, begins again to increase, due to mixing induced by meridional circulations. At low metallicity, the effects of meridional circulations are important even at rather low rotation velocity. The combined effect of the Goldreich-Schubert-Fricke instability and meridional circulations determines an increase of light-s and, to a lesser extent, heavy-s elements, while lead is strongly reduced. For both metallicities, the rotation-induced instabilities active during the interpulse phase reduce the neutron-to-seed ratio, so that the spectroscopic indexes [hs/ls] and [Pb/hs] decrease by increasing the initial rotation velocity. Our analysis suggests that rotation could explain the spread in the s-process indexes, as observed in s-process enriched stars at different metallicities.