Stripped-envelope core-collapse supernova ⁵⁶Ni masses: Persistently larger values than supernovae type II

The mass of synthesised radioactive material is an important power source for all supernova (SN) types. Anderson 2019 recently compiled literature values and obtained $^{56}$Ni distributions for different core-collapse supernovae (CC-SNe), showing that the $^{56}$Ni distribution of stripped envelope CC-SNe (SE-SNe: types IIb, Ib, and Ic) is highly incompatible with that of hydrogen rich type II SNe (SNe-II). This motivates questions on differences in progenitors, explosion mechanisms, and $^{56}$Ni estimation methods. Here, we re-estimate the nucleosynthetic yields of $^{56}$Ni for a well-observed and well-defined sample of SE-SNe in a uniform manner. This allows us to investigate whether the observed SN-II--SE-SN $^{56}$Ni separation is due to real differences between these SN types, or because of systematic errors in the estimation methods. We compiled a sample of well observed SE-SNe and measured $^{56}$Ni masses through three different methods proposed in the literature. Arnett's rule -as previously shown - gives $^{56}$Ni masses for SE-SNe that are considerably higher than SNe-II. While for the distributions calculated using both the Khatami&Kasen prescription and Tail $^{56}$Ni masses are offset to lower values than `Arnett values', their $^{56}$Ni distributions are still statistically higher than that of SNe II. Our results are strongly driven by a lack of SE-SN with low $^{56}$Ni masses (that are in addition strictly lower limits). The lowest SE-SN $^{56}$Ni mass in our sample is of 0.015M$_\odot$, below which are more than 25$\%$ of SNe II. We conclude that there exists real, intrinsic differences in the mass of synthesised radioactive material between SNe II and SE-SNe . Any proposed current or future CCSN progenitor scenario and explosion mechanism must be able to explain why and how such differences arise, or outline a yet to be fully explored bias in current SN samples.