THE EFFECTS OF ROTATION ON<i>s</i>-PROCESS NUCLEOSYNTHESIS IN ASYMPTOTIC GIANT BRANCH STARS

Abstract

In this paper we analyze the effects induced by rotation on low mass\nAsymptotic Giant Branch stars. We compute two sets of models, M=2.0 Msun at\n[Fe/H]=0 and M=1.5 Msun at [Fe/H]=-1.7, respectively, by adopting Main Sequence\nrotation velocities in the range 0 - 120 km/s. At high metallicity, we find\nthat the Goldreich-Schubert-Fricke instability, active at the interface between\nthe convective envelope and the rapid rotating core, contaminates the\n13C-pocket (the major neutron source) with 14N (the major neutron poison), thus\nreducing the neutron flux available for the synthesis of heavy elements. As a\nconsequence, the yields of heavy-s elements (Ba, La, Nd, Sm) and, to a less\nextent, those of light-s elements (Sr, Y, Zr) decrease with increasing rotation\nvelocities up to 60 km/s. However, for larger initial rotation velocities, the\nproduction of light-s and, to a less extent, that of heavy-s begins again to\nincrease, due to mixing induced by meridional circulations. At low metallicity,\nthe effects of meridional circulations are important even at rather low\nrotation velocity. The combined effect of Goldreich-Schubert-Fricke instability\nand meridional circulations determines an increase of light-s and, to a less\nextent, heavy-s elements, while lead is strongly reduced. For both\nmetallicities, the rotation-induced instabilities active during the interpulse\nphase reduce the neutrons-to-seeds ratio, so that the spectroscopic indexes\n[hs/ls] and [Pb/hs] decrease by increasing the initial rotation velocity. Our\nanalysis suggests that rotation could explain the spread in the s-process\nindexes, as observed in s-process enriched stars at different metallicities.\n