The Nickel Mass Distribution of Stripped-Envelope Supernovae: Implications for Additional Power Sources

Abstract

We perform a systematic study of the ${}^{56}$Ni mass ($M_{\mathrm{Ni}}$) of 27 stripped envelope supernovae (SESNe) by modeling their light-curve tails, highlighting that use of ``Arnett's rule'' overestimates $M_{\mathrm{Ni}}$ for SESN by a factor of $\sim$2. Recently, \citet{Khatami2019} presented a new model relating the peak time ($t_{\mathrm{p}}$) and luminosity ($L_{\mathrm{p}}$) of a radioactive-powered SN to its $M_{\mathrm{Ni}}$ that addresses several limitations of Arnett-like models, but depends on a dimensionless parameter, ${\hat{I}}^2$. Using observed $t_{\mathrm{p}}$, $L_{\mathrm{p}}$, and tail-measured $M_{\mathrm{Ni}}$ values for 27 SESN, we observationally calibrate ${\hat{I}}^2$ for the first time. Despite scatter, we demonstrate that the model of \citet{Khatami2019} with empirically-calibrated ${\hat{I}}^2$ values provides significantly improved measurements of $M_{\mathrm{Ni}}$ when only photospheric data is available. However, these observationally-constrained ${\hat{I}}^2$ values are systematically lower than those inferred from numerical simulations, primarily because the observed sample has significantly higher (0.2-0.4 dex) $L_{\mathrm{p}}$ for a given $M_{\mathrm{Ni}}$. While effects due to composition, mixing, and asymmetry can increase $L_{\mathrm{p}}$ current models cannot explain the systematically low ${\hat{I}}^2$ values. However, the discrepancy can be alleviated if $\sim$7--50\% of $L_{\mathrm{p}}$ for the observed sample originates from sources other than ${}^{56}$Ni. Either shock cooling or magnetar spin-down could provide the requisite luminosity. Finally, we find that even with our improved measurements, the $M_{\mathrm{Ni}}$ values of SESN are still a factor of $\sim$3 larger than those of hydrogen-rich Type II SN, indicating that these supernovae are inherently different in terms of their progenitor initial mass distributions or explosion mechanisms.

27 pages, 12 figures, accepted for publication in ApJ