drastically reduce the stored energy and completely determine the evolution of the bolometric luminosity during the adiabatic cooling phase (Figs. \[fig:snphs\] and \[fig:swbrck\]a). Such behavior of the luminosity lasts till the gas and radiation temperatures drop to the critical value in the subphotospheric layers, and a recombination of hydrogen, the most abundant element in the ejected envelope, starts. In these layers hydrogen becomes partially ionized by around day 18. For the adiabatic cooling phase, the color and effective temperatures drop rapidly from $1.35 \times 10^{5}$ K and $9.32 \times 10^{4}$ K, respectively, at day 0.93 to 6650 K and 6560 K at day 18. Their ratio also reduces as the contribution of scattering processes to the opacity decreases.

In an optically thick medium, the strong shock wave propagates almost adiabatically throughout the stellar matter. When the shock wave emerges on the pre-SN surface, the adiabatic regime is broken and transforms into the isothermal regime. This transformation takes place during the adiabatic cooling phase and gives rise to a thin, dense shell at the outer edge of the ejected envelope. The dense shell is arising from day 0.945 to day 1.022, reaching the density contrast of $\sim$ 210 at a velocity 