Type Ib/Ic Supernovae: Effect of Nickel Mixing on the Early-time Color Evolution and Implications for the Progenitors

We investigate the effect of mixing of radioactive nickel (Ni-56) on the early-time color evolution of Type Ib and Ic supernovae (SNe Ib/Ic) using multigroup radiation hydrodynamics simulations. We consider both helium-rich and helium-poor progenitors. Mixing of Ni-56 is parameterized using a Gaussian distribution function. We find that the early-time color evolution with a weak Ni-56 mixing is characterized by three different phases: initial rapid reddening, blueward evolution due to the delayed effect of Ni-56 heating, and redward evolution thereafter until the transition to the nebular phase. With a strong Ni-56 mixing, the color evolution is characterized by a monotonic reddening. We compare our models with the early-time color evolution of several SNe Ib/Ic (SN 1999ex, SN 2008D, SN 2009jf, iPTF13bvn, SN 1994I, SN 2007gr, SN 2013ge, and 2017ein) and find signatures of relatively weak and strong Ni-56 mixing for SNe Ib and SNe Ic, respectively. This suggests that SNe Ib progenitors are distinct from SN Ic progenitors in terms of helium content and that Ni-56 mixing is generally stronger in the carbon-oxygen core and weaker in the helium-rich envelope. We conclude that the early-time color evolution is a powerful probe of Ni-56 mixing in SNe Ib/Ic.