adopt here a distance of 11.7 Mpc (Leonard et al. [@lknt03]), a recession velocity to SN 1999em of 800 kms$^{-1}$ (Leonard et al. [@lfg02]), an explosion date of JD 2451476.77, and a total extinction $A_V=0.31$ (Baron et al. [@bbh00]; Hamuy et al. [@hpm01]; Elmhamdi et al. [@edc03]).

Supernova modelling and input physics {#sec:snmodel}
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Keeping in mind the importance of a hydrodynamic study and of atmosphere modelling, including the time-dependent kinetics and energy balance for the interpretation of the SN 1987A phenomenon, we use this approach to investigate SN 1999em.

A hydrodynamic model is computed in terms of radiation hydrodynamics in the one-group approximation taking into account non-LTE effects on the average opacities and the thermal emissivity, effects of nonthermal ionization, and a contribution of lines to opacity as in the case of SN 1987A (Utrobin [@utr04]). Note that the bolometric luminosity of an SN is calculated by including the retardation and limb-darkening effects.

The atmosphere model includes the time-dependent ionization and excitation kinetics of hydrogen and other elements, the time-dependent kinetics of molecular hydrogen, and the time-dependent energy balance (Utrobin & Chugai [@uc05]). The density distribution, chemical composition, radius of the photosphere, and effective temperature are provided by the 