Gas dynamics of the nickel-56 decay heating in pair-instability supernovae

Very massive 140-260 M_☉ stars can die as highly-energetic pair-instability supernovae (PI SNe) with energies of up to 100 times those of core-collapse SNe that can completely destroy the star, leaving no compact remnant behind. These explosions can synthesize 0.1-30 M_☉ of radioactive ⁵⁶Ni, which can cause them to rebrighten at later times when photons due to ⁵⁶Ni decay diffuse out of the ejecta. However, heat from the decay of such large masses of ⁵⁶Ni could also drive important dynamical effects deep in the ejecta that are capable of mixing elements and affecting the observational signatures of these events. We have now investigated the dynamical effect of ⁵⁶Ni heating on PI SN ejecta with high-resolution two-dimensional hydrodynamic simulations performed with the CASTRO code. We find that expansion of the hot ⁵⁶Ni bubble forms a shell at the base of the silicon layer of the ejecta about 200 days after the explosion but that no hydrodynamical instabilities develop that would mix ⁵⁶Ni with the ²⁸Si/¹⁶O-rich ejecta. However, while the dynamical effects of ⁵⁶Ni heating may be weak they could affect the observational signatures of some PI SNe by diverting decay energy into internal expansion of the ejecta at the expense of rebrightening at later times.