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

Abstract

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 56 Ni, which can cause them to rebrighten at later times when photons due to 56 Ni decay diffuse out of the ejecta. However, heat from the decay of such large masses of 56 Ni 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 56 Ni heating on PI SN ejecta with high-resolution two-dimensional hydrodynamic simulations performed with the CASTRO code. We find that expansion of the hot 56 Ni bubble forms a shell at the base of the silicon layer of the ejecta ∼200 days after the explosion but that no hydrodynamical instabilities develop that would mix 56 Ni with the 28 Si/ 16 O-rich ejecta. However, while the dynamical effects of 56 Ni 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.