2020MNRAS.497.1619M

Mixing of ⁵⁶Ni, whose nuclear decay energy is a major luminosity source in stripped-envelope supernovae, is known to affect the observational properties of stripped-envelope supernovae such as light-curve and colour evolution. Here we systematically investigate the effect of ⁵⁶Ni mixing on the photospheric velocity evolution in stripped-envelope supernovae. We show that ⁵⁶Ni mixing significantly affects the early photospheric velocity evolution. The photospheric velocity, which is often used to constrain the ejecta mass and explosion energy, significantly varies by just changing the degree of ⁵⁶Ni mixing. In addition, the models with a small degree of ⁵⁶Ni mixing show a flattening in the early photospheric velocity evolution, while the fully mixed models show a monotonic decrease. The velocity flattening appears in both helium and carbon+oxygen progenitor explosions with a variety of ejecta mass, explosion energy, and ⁵⁶Ni mass. Some stripped-envelope supernovae with early photospheric velocity information do show such a flattening. We find that Type Ib SN 2007Y, which has early photospheric velocity information, has a signature of a moderate degree of ⁵⁶Ni mixing in the photospheric velocity evolution and about half of the ejecta is mixed in it. The immediate spectroscopic follow-up observations of stripped-envelope supernovae shortly after the explosion providing the early photospheric evolution give an important clue to constrain ⁵⁶Ni mixing in the ejecta.