distribution in the pre-SN model and its chemical composition in the transition region between the hydrogen-rich envelope and metal/helium core. The radioactive $^{56}$Ni mass is reliably measured by the light curve tail after the plateau phase. As the pre-SN model of SN 1999em, we consider non-evolutionary models similar to those of SN 1987A (Utrobin [@utr04]), but for the outer layers assume the standard solar composition with the mass fraction of hydrogen $X=0.735$, helium $Y=0.248$, and the metallicity $Z=0.017$ (Grevesse & Sauval [@gs98]) taking a normal spiral nature of the host galaxy NGC 1637 into account. The best version of such a fitting was obtained with the optimal model that is characterized by the basic parameters: the pre-SN radius of 500 $R_{\sun}$, the ejecta mass of 19 $M_{\sun}$, and the explosion energy of $1.3\times10^{51}$ erg (model D11 in Table \[tab:hydmods\]).

The density profile of the pre-SN model consisting of a central white dwarf like core and an outer envelope with the size of the red supergiant is shown in Fig. \[fig:rhomr\]. In the calculations, the 1.58 $M_{\sun}$ central core is removed from the computational mass grid and assumed to collapse to a neutron star, while the rest of the star is 