2021ApJ...918...89A

We perform a systematic study of the ⁵⁶Ni mass (M_Ni) of 27 stripped-envelope supernovae (SESNe) by modeling their light-curve tails, highlighting that use of "Arnett's rule" overestimates M_Ni for SESNe by a factor of ∼2. Recently, Khatami & Kasen presented a new model relating the peak time (t_p) and luminosity (L_p) of a radioactively powered supernova to its M_Ni that addresses several limitations of Arnett-like models, but depends on a dimensionless parameter, β. Using observed t_p, L_p, and tail-measured M_Ni values for 27 SESNe, we observationally calibrate β for the first time. Despite scatter, we demonstrate that the model of Khatami & Kasen with empirically calibrated β values provides significantly improved measurements of M_Ni when only photospheric data are available. However, these observationally constrained β values are systematically lower than those inferred from numerical simulations, primarily because the observed sample has significantly higher (0.2-0.4 dex) L_p for a given M_Ni. While effects due to composition, mixing, and asymmetry can increase L_p none can explain the systematically low β values. However, the discrepancy can be alleviated if ∼7%-50% of L_p for the observed sample comes from sources other than radioactive decay. Either shock cooling or magnetar spin-down could provide the requisite luminosity. Finally, we find that even with our improved measurements, the M_Ni values of SESNe are still a factor of ∼3 larger than those of hydrogen-rich Type II SNe, indicating that these supernovae are inherently different in terms of the initial mass distributions of their progenitors or their explosion mechanisms.