Are stripped envelope supernovae really deficient in $^{56}$Ni?

Recent works have indicated that the $^{56}$Ni masses estimated for Stripped Envelope SNe (SESNe) are systematically higher than those estimated for SNe II. Although this may suggest a distinct progenitor structure between these types of SNe, the possibility remains that this may be caused by observational bias. One important possible bias is that SESNe with low $^{56}$Ni mass are dim, and therefore they are more likely to escape detection. By investigating the distributions of the $^{56}$Ni mass and distance for the samples collected from the literature, we find that the current literature SESN sample indeed suffers from a significant observational bias, i.e., objects with low $^{56}$Ni mass - if they exist - will be missed, especially at larger distances. Note, however, that those distant objects in our sample are mostly SNe Ic-BL. We also conducted mock observations assuming that the $^{56}$Ni mass distribution for SESNe is intrinsically the same with that for SNe II. We find that the $^{56}$Ni mass distribution of the detected SESNe samples moves toward higher mass than the assumed intrinsic distribution, because of the difficulty in detecting the low-$^{56}$Ni mass SESNe. These results could explain the general trend of the higher $^{56}$Ni mass distribution (than SNe II) of SESNe found thus far in the literature. However, further finding clear examples of low-$^{56}$Ni mass SESNe ($leq 0.01M_{odot}$) is required to add weight to this hypothesis. Also, the objects with high $^{56}$Ni mass ($gtrsim 0.2 M_{odot}$) are not explained by our model, which may require an additional explanation.