The s process in massive stars at low metallicity. Effect of primary N14 from fast rotating stars

Abstract

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