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 initialmasses of 13-40M_⊙ and initial rotation velocities of 0- 450 km s^-1 that we investigate, we find amixed helium-oxygen (He-O) shell with a significant total He-O mass and with a helium to oxygen mass ratio in the range of 0.5-2 only for a small fraction of the models. While the shell formation due to mixing is instigated by rotation, the precollapse rotation rate is not very high. The fraction ofmodels with a shell of He-O composition required for an energetic collapse-induced thermonuclear explosion is small, as is the fraction of modelswith high specific angular momentum,which can aid the thermonuclear explosion by retarding the collapse. 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 He-O 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.