Convective H-He Interactions in Massive Population III Stellar Evolution Models

Abstract

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 $15 - 140\,\mathrm{M}_{\odot}$, using five sets of mixing assumptions. In 22 cases H-He interactions induce local nuclear energy release in the range $ \sim 10^{9} - 10^{13.5}\,\mathrm{L}_{\odot}$. The luminosities on the upper end of this range amount to a substantial fraction of the layer's internal energy over a convective advection timescale, 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 $^{12}\mathrm{C}/^{13}\mathrm{C}$ ratios between $\approx 1.5$ to $\sim 1000$ and [C/N] ratios from $\approx -2.3 $ to $\approx 3$ with a correlation that agrees well with observations of CEMP-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 a H-He interaction are computed to Fe-core infall and a $140\,\mathrm{M}_{\odot}$ simulation becomes pair-unstable. We also discuss present modelling limitations and the need for 3D hydrodynamic models to fully understand these stellar evolutionary phases.