The role of radioactive nickel in shaping the plateau phase of Type II supernovae

In the present study, we systematically explore the effect of the radioactive ⁵⁶Ni and its mixing properties in the ejecta on the plateau of Type IIP supernovae (SNe IIP). We evaluate the importance of ⁵⁶Ni in shaping light curves of SNe IIP by simulating light curves for two red supergiant models using different amounts of ⁵⁶Ni and with different types of mixing: uniform distribution of ⁵⁶Ni out to different fractions of the envelope and 'boxcar' distribution of ⁵⁶Ni. We find, similarly to previous studies, that ⁵⁶Ni extends duration of the plateau. We find a formula to estimate the extension based on the observed bolometric light curves and show that for most SNe IIP ⁵⁶Ni extends the plateau by about 20 per cent. Another effect of ⁵⁶Ni consists in reduction of the plateau decline rate, i.e. ⁵⁶Ni presented in the ejecta flattens the plateau. Our simulations suggest that for typical SNe IIP it can reduce the decline rate by about 1 mag per 100 d. We find that for the contribution of ⁵⁶Ni seen in most supernovae (SNe) our simulated bolometric light curves resemble observed ones for various types of ⁵⁶Ni mixing. We thereby cannot determine the level of ⁵⁶Ni mixing in these SNe based on the light curve alone. However, for SN 2009ib we find that only a model where ⁵⁶Ni is mixed significantly throughout most of the hydrogen envelope is consistent with the observed light curve.