THE s-PROCESS IN MASSIVE STARS AT LOW METALLICITY: THE EFFECT OF PRIMARY N-14 FROM FAST ROTATING STARS

The goal of this Letter is to analyze the impact of a primary neutron source on the s-process nucleosynthesis in massive stars at halo metallicity. Recent stellar models including rotation at very low metallicity predict a strong production of primary N-14. Part of the nitrogen produced in the H-burning shell diffuses by rotational mixing into the He core where it is converted to Ne-22 providing additional neutrons for the s-process. We present nucleosynthesis calculations for a 25 M-circle dot star at [Fe/H] = -3, -4, where about 0.8% in mass is made of primary, Ne-22 in the convective He-burning core. The usual weak s-process shape is changed by the additional neutron source with a peak between Sr and Ba, where the s-process yields increase by orders of magnitude with respect to the yields obtained without rotation. Iron seeds are fully consumed and the maximum production of Sr, Y, and Zr is reached. On the other hand, the s-process efficiency beyond Sr and the ratio Sr/Ba are strongly affected by the amount of Ne-22 and by nuclear uncertainties, first of all by the Ne-22(alpha, n)Mg-25 reaction. Finally, assuming that Ne-22 is primary in the considered metallicity range, the s-process efficiency decreases with metallicity due to the effect of the major neutron poisons Mg-25 and Ne-22. This work represents a first step toward the study of primary neutron source effect in fast rotating massive stars, and its implications are discussed in the light of spectroscopic observations of heavy elements at halo metallicity.