weakly to affect the ionization and excitation of hydrogen and other elements in the atmosphere at the plateau phase. Thus, it is possible to conclude that the hydrogen recombination in the atmosphere of SN 1999em during the whole plateau phase is essentially a time-dependent phenomenon.

In turn, the short distance results in the hydrodynamic model with the following parameters: the pre-SN radius of 375 $R_{\sun}$, the ejecta mass of 16 $M_{\sun}$, and the explosion energy of $6.86\times10^{50}$ erg (model D07 in Table \[tab:hydmods\]). In this model the central core of 1.4 $M_{\sun}$ is assumed to collapse to a neutron star. The hydrodynamic model fits the observed bolometric light curve of SN 1999em but for the $^{56}$Ni mass of 0.0162 $M_{\sun}$, the most of $^{56}$Ni being mixed in the velocity range $\le 580$ kms$^{-1}$ (Fig. \[fig:hmd785\]a). The photospheric velocity curve runs well below the observed points (Fig. \[fig:hmd785\]b), and it might be expected that the spectral lines in this model would be narrower than those observed. Indeed, on days 26.24 and 52.14 both the emission and absorption components of the H$\alpha$ line computed with the time-dependent approach with model B are significantly narrower than those observed in SN 1999em (Figs. \[fig:hmd785\]c 