Systematic investigation of the effect of 56Ni mixing in the early photospheric velocity evolution of stripped-envelope supernovae

TL;DR

This study systematically explores how varying degrees of 56Ni mixing influence the early photospheric velocity evolution in stripped-envelope supernovae, revealing that mixing significantly alters velocity profiles and can be inferred from early observations.

Contribution

It provides the first systematic analysis of 56Ni mixing effects on photospheric velocity evolution in stripped-envelope supernovae, highlighting the importance of early velocity measurements.

Findings

56Ni mixing significantly affects early photospheric velocity evolution.

Velocity flattening indicates a small degree of 56Ni mixing.

SN 2007Y shows evidence of moderate 56Ni mixing.

Abstract

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 color 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 a 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.