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

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.