The importance of 56Ni in shaping the light curves of type II supernovae

What intrinsic properties shape the light curves of Type II supernovae (SNe)? To address this question we derive observational measures that are robust (i.e., insensitive to detailed radiative transfer) and constrain the contribution from $^{56}$Ni, as well as a combination of the envelope mass, progenitor radius, and explosion energy. By applying our methods to a sample of type II SNe from the literature we find that $^{56}$Ni contribution is often significant. In our sample its contribution to the time weighted integrated luminosity during the photospheric phase ranges between 8% and 72% with a typical value of 30%. We find that the $^{56}$Ni relative contribution is anti-correlated with the luminosity decline rate. When added to other clues, this in turn suggests that the flat plateaus often observed in type II SNe are not a generic feature of the cooling envelope emission, and that without $^{56}$Ni many of the SNe that are classified as II-P would have shown a decline rate that is steeper by up to 1 mag/100 d. Nevertheless, we find that the cooling envelope emission, and not $^{56}$Ni contribution, is the main driver behind the observed range of decline rates. Furthermore, contrary to previous suggestions, our findings indicate that fast decline rates are not driven by lower envelope masses. We therefore suggest that the difference in observed decline rates is mainly a result of different density profiles of the progenitors.