Convective-reactive nucleosynthesis of K, Sc, Cl and p-process isotopes in O-C shell mergers

Abstract

We address the deficiency of odd-Z elements P, Cl, K and Sc in galactic chemical evolution models through an investigation of the nucleosynthesis of interacting convective O- and C shells in massive stars. 3D hydrodynamic simulations of O-shell convection with moderate C-ingestion rates show no dramatic deviation from spherical symmetry. We derive a spherically averaged diffusion coefficient for 1D nucleosynthesis simulations which show that such convective-reactive ingestion events can be a production site for P, Cl, K and Sc. An entrainment rate of $10^{-3}{\mathrm{M}}_{\odot }$/s features overproduction factors $O{P}_{\mathrm{s}}\approx 7$. Full O-C shell mergers in our 1D stellar evolution massive star models have overproduction factors $O{P}_{\mathrm{m}}>1\mathrm{dex}$ but for such cases 3D hydrodynamic simulations suggest deviations from spherical symmetry. p-process species can be produced with overproduction factors of $O{P}_{\mathrm{m}}>1\mathrm{dex}$, e.g. for ${}^{130,132}Ba$. Using the uncertain prediction of the $15{\mathrm{M}}_{\odot }$, $Z=0.02$ massive star model ($O{P}_{\mathrm{m}}\approx 15$) as representative for merger or entrainment convective-reactive events involving O- and C-burning shells, and assume that such events occur in more than 50% of all stars, our chemical evolution models reproduce the observed Galactic trends of the odd-Z elements.

6 pages, 6 figures, accepted for publication in MNRAS