The impact of updated Zr neutron-capture cross sections and new asymptotic giant branch models on our understanding of the S process and the origin of stardust

We present model predictions for the Zr isotopic ratios produced by slow neutron captures in C-rich asymptotic giant branch (AGB) stars of masses 1.25-4 M_⊙ and metallicities Z = 0.01-0.03, and compare them to data from single meteoritic stardust silicon carbide (SiC) and high-density graphite grains that condensed in the outflows of these stars. We compare predictions produced using the Zr neutron-capture cross sections from Bao et al. and from n-TOF experiments at CERN, and present a new evaluation for the neutron-capture cross section of the unstable isotope ⁹⁵Zr, the branching point leading to the production of ⁹⁶Zr. The new cross sections generally present an improved match with the observational data, except for the ⁹²Zr/⁹⁴Zr ratios, which are on average still substantially higher than predicted. The ⁹⁶Zr/⁹⁴Zr ratios can be explained using our range of initial stellar masses, with the most ⁹⁶Zr-depleted grains originating from AGB stars of masses 1.8-3 M_⊙ and the others from either lower or higher masses. The ^{90, 91}Zr/⁹⁴Zr variations measured in the grains are well reproduced by the range of stellar metallicities considered here, which is the same needed to cover the Si composition of the grains produced by the chemical evolution of the Galaxy. The ⁹²Zr/⁹⁴Zr versus ²⁹Si/²⁸Si positive correlation observed in the available data suggests that stellar metallicity rather than rotation plays the major role in covering the ^{90, 91, 92}Zr/⁹⁴Zr spread.