We investigate the effect of mixing of radioactive nickel ( 56 Ni) on the early-time color evolution of Type Ib and Ic supernovae (SNe Ib/Ic) using multi-group radiation hydrodynamics simulations. We consider both helium-rich and helium-poor progenitors. Mixing of 56 Ni is parameterized using a Gaussian distribution function. We find that the early-time color evolution with a weak 56 Ni mixing is characterized by three different phases: initial rapid reddening, blueward evolution due to the delayed effect of 56 Ni heating, and redward evolution thereafter until the transition to the nebular phase. With a strong 56 Ni mixing, the second phase disappears. 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 56 Ni 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 56 Ni 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 56 Ni mixing in SNe Ib/Ic.

We present the results of wide integral-field near-infrared (1.0-1.8 µm) spectroscopic observations of the southeastern shell of the young core-collapse supernova remnant (SNR) G11.2−0.3. We first construct [Fe II] 1.644 µm line images of three bright clumps from the obtained spectral image cubes and compare them with those of other transitions such as [Fe II] 1.257, [Fe II] 1.534 and He I 1.083 µm line images. This allows us to estimate the electron density (∼ 4,700-9,400 cm −3 ) and extinction (A V ∼ 16-20 mag) of the shell, including detailed two-dimensional distribution of the properties in the brightest clump, as well as the discovery of a faint high-velocity (∼ -440 km s −1 ) component in the clump. Our SNR shock model calculations estimate the preshock number density of ∼ 250-500 cm −3 and shock speed of ∼ 80-250 km s −1 in the [Fe II]-emitting region of the SNR. The comparison between the observed and modelled radial profiles of the line intensities and their ratios reveals that the shell is composed of multiple thin filaments which have been likely formed in episodic mass loss processes of a progenitor star. The discovery of the faint high-velocity component supports the interpretation that the southeastern shell of G11.2−0.3 is mainly composed of circumstellar material with contamination by supernova ejecta and also that its ejected material was expelled primarily in the southeast-northwest direction. Subject headings: ISM: individual objects (G11.2−0.3) -ISM: supernova remnants -infrared: ISM -shock waves