A mixed helium-oxygen shell in some core-collapse supernova progenitors

TL;DR

This study models rotating massive stars to identify the formation of mixed helium-oxygen shells prior to core collapse, revealing their limited occurrence and potential implications for supernova mechanisms and nucleosynthesis.

Contribution

It demonstrates that mixed helium-oxygen shells form in a small subset of rotating massive star models, challenging the prevalence of collapse-induced thermonuclear explosions in supernovae.

Findings

Mixed helium-oxygen shells form in a small fraction of models.

Shell formation is due to rotational mixing at moderate rotation rates.

Implications for supernova diversity and nucleosynthesis processes.

Abstract

We evolve models of rotating massive stars up to the stage of iron core collapse using the MESA code and find a shell with a mixed composition of primarily helium and oxygen in some cases. In the parameter space of initial masses of 13-40 Mo and initial rotation velocities of 0-450 km/s that we investigate, we find a mixed helium-oxygen shell with a significant mass only for a small fraction of the models. While the shell is formed due to rotational mixing, the pre-collapse rotation rate is not very high, as required for an energetic collapse-induced thermonuclear explosion. Our results suggest that the collapse-induced thermonuclear explosion mechanism that was revisited recently can account for at most a small fraction of core-collapse supernovae. The presence of such a mixed helium-oxygen shell still might have some implications for core-collapse supernovae, such as some nucleosynthesis processes when jets are present, or might result in peculiar sub-luminous core-collapse supernovae.