Dynamical Impact of Ni Heating in the Pair-Instability Supernovae.

We examine the radioactivity heating of Ni decay in the pair-instability supernovae with two-dimensional simulations. Pair-instability supernovae form from the death of very massive stars of 140-260 Msun. Their explosions are triggered by the contract of the core due to the electron-positron pair production instability, which ignites the explosive burning of oxygen and silicon and eventually blows up the entire star without leaving any compact remnants. Depending on the mass of the progenitor star, about 0.1-30 Msun of radioactive isotope, Ni can be synthesized during the explosion. If the amount of Ni exceeds 5 Msun, the decay energy of Ni may power a luminous transit by providing $\sim 10^{51}$ erg of radiation energy. However, such a large energy injection likely not only produces radiation but also changes the dynamics of the ejecta. We investigate the effect of Ni radioactive heating by performing high-resolution two-dimensional simulations and find the Ni heating creates a shell in the inner boundary of a silicon burning shell about 200 days after the explosion. However, it does not dredge up the Ni to further mix with the outer layers of oxygen or beyond. In addition, this shell distorts the inner ejecta without breaking down its spherical symmetry. Therefore, Ni heating to the dynamics of ejecta is not strong enough to alter the change of PISNe spectra through mixing. Nevertheless, the PISNe light curve becomes dimmer because part of the radioactive energy is used to push out the ejecta instead of turning into radiation.