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Very massive 140-260 Msun stars can die as highly-energetic pair-instabilitysupernovae (PI SNe) with energies of up to 100 times those of core-collapse SNethat can completely destroy the star, leaving no compact remnant behind. Theseexplosions can synthesize $0.1-30$ Msun of radioactive Ni56, which can causethem to rebrighten at later times when photons due to Ni56 decay diffuse out ofthe ejecta. However, heat from the decay of such large masses of Ni56 couldalso drive important dynamical effects deep in the ejecta that are capable ofmixing elements and affecting the observational signatures of these events. Wehave now investigated the dynamical effect of Ni56 heating on PI SN ejecta withhigh-resolution two-dimensional hydrodynamic simulations performed with theCASTRO code. We find that expansion of the hot Ni56 bubble forms a shell at thebase of the silicon layer of the ejecta about 200 days after the explosion butthat no hydrodynamical instabilities develop that would mix Ni56 with theSi/O-rich ejecta. However, while the dynamical effects of Ni56 heating may beweak they could affect the observational signatures of some PI SNe by divertingdecay energy into internal expansion of the ejecta at the expense ofrebrightening at later times.

Gas Dynamics of the Nickel-56 Decay Heating in Pair-Instability Supernovae