2021MNRAS.500.2685C

In Pop III stellar models, convection-induced mixing between H- and He-rich burning layers can induce a burst of nuclear energy and thereby substantially alter the subsequent evolution and nucleosynthesis in the first massive stars. We investigate H-He shell and core interactions in 26 stellar evolution simulations with masses 15-140, M_☉, using five sets of mixing assumptions. In 22 cases H-He interactions induce local nuclear energy release in the range ∼10⁹-10^13.5 L_☉. The luminosities on the upper end of this range amount to a substantial fraction of the layer's internal energy over a convective advection time-scale, indicating a dynamic stellar response that would violate 1D stellar evolution modelling assumptions. We distinguish four types of H-He interactions depending on the evolutionary phase and convective stability of the He-rich material. H-burning conditions during H-He interactions give ¹²C/¹³C ratios between ≃ 1.5 to ∼1000 and [C/N] ratios from ≃ -2.3 to ≃ 3 with a correlation that agrees well with observations of CEMP (carbon-enhanced metal-poor) no stars. We also explore Ca production from hot CNO breakout and find the simulations presented here likely cannot explain the observed Ca abundance in the most Ca-poor CEMP-no star. We describe the evolution leading to H-He interactions, which occur during or shortly after core-contraction phases. Three simulations without an H-He interaction are computed to Fe-core infall and a 140 M_☉ simulation becomes pair unstable. We also discuss present modelling limitations and the need for 3D hydrodynamic models to fully understand these stellar evolutionary phases.