The s -Process in Massive Stars at Low Metallicity: The Effect of Primary 14 N from Fast Rotating Stars

Abstract

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 14N. Part of the nitrogen produced in the H-burning shell diffuses by rotational mixing into the He core where it is converted to 22Ne providing additional neutrons for the s-process. We present nucleosynthesis calculations for a 25 M -->☉ star at [Fe/H] = –3, –4, where about 0.8% in mass is made of primary 22Ne 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 22Ne and by nuclear uncertainties, first of all by the 22Ne(α, n)25Mg reaction. Finally, assuming that 22Ne is primary in the considered metallicity range, the s-process efficiency decreases with metallicity due to the effect of the major neutron poisons 25Mg and 22Ne. 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.