Convective H–He interactions in massive population III stellar evolution models

Abstract

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, M⊙, 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 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 $140\, \mathrm{M}_{\odot }$ simulation becomes pair unstable. We also discuss present modelling limitations and the need for 3D hydrodynamic models to fully understand these stellar evolutionary phases.