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 Ni-56 and its mixing properties in the ejecta on the plateau of Type IIP supernovae (SNe IIP). We evaluate the importance of Ni-56 in shaping light curves of SNe IIP by simulating light curves for two red supergiant models using different amounts of Ni-56 and with different types of mixing: uniform distribution of Ni-56 out to different fractions of the envelope and 'boxcar' distribution of Ni-56. We find, similarly to previous studies, that Ni-56 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 Ni-56 extends the plateau by about 20 per cent. Another effect of Ni-56 consists in reduction of the plateau decline rate, i.e. Ni-56 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 Ni-56 seen in most supernovae (SNe) our simulated bolometric light curves resemble observed ones for various types of Ni-56 mixing. We thereby cannot determine the level of Ni-56 mixing in these SNe based on the light curve alone. However, for SN 2009ib we find that only a model where Ni-56 is mixed significantly throughout most of the hydrogen envelope is consistent with the observed light curve. Our light curves are available via link https://wwwmpa.mpa-garching.mpg.de/ccsnarchive/data/Kozyreva2018/