ejected by the SN explosion, which is modelled by a piston action near the edge of the central core. The pre-SN model has a heterogeneous chemical composition containing a 5.6 $M_{\sun}$ helium core and a 11.9 $M_{\sun}$ outer shell of the solar chemical composition (Fig. \[fig:chcom\]). Note that the helium cores up to about 8 $M_{\sun}$ are consistent with the observations (Sect. \[sec:grg-chcom\]). There is no sharp boundary between the hydrogen-rich and helium-rich layers in the ejecta of the optimal model. Hydrogen-rich material is mixed into the central region, and helium-rich material, in turn, is mixed outwards. It is evident that such a distribution of hydrogen and helium implies a strong mixing at the helium/hydrogen composition interface. The fact that the radioactive $^{56}$Ni is confined to the innermost ejected layers suggests its weak mixing during the SN explosion.

In Fig. \[fig:lmbol\] we show the very good match between the bolometric light curve calculated for the optimal hydrodynamic model and the one observed for SN 1999em (Elmhamdi et al. [@edc03]). Note that hereafter $t_{obs}$ is the time in the observer’s frame of reference. The model agrees well with the observed tail of the bolometric light curve for the total $^{56}$Ni mass 