An optimal hydrodynamic model for the normal type IIP supernova 1999em

Context. There is still no consensus about progenitor masses of type IIP supernovae. Aims. We study a normal type IIP SN 1999em in detail and compare it to a peculiar type IIP SN 1987A. Methods. We computed the hydrodynamic and time-dependent atmosphere models interpreting simultaneously both the photometric and spectroscopic observations. Results. The bolometric light curve of SN 1999em and the spectral evolution of its H alpha line are consistent with a presupernova radius of 500 +/- 200 R circle dot, an ejecta mass of 19.0 +/- 1.2 M circle dot, an explosion energy of (1.3 +/- 0.1) x 10(51) erg, and a radioactive Ni-56 mass of 0.036 +/- 0.009 M circle dot. A mutual mixing of hydrogen-rich and helium-rich matter in the inner layers of the ejecta guarantees a good fit of the calculated light curve to that observed. Based on the hydrodynamic models in the vicinity of the optimal model, we derive the approximate relationships between the basic physical and observed parameters. The hydrodynamic and atmosphere models of SN 1999em are inconsistent with the short distance of 7.85 Mpc to the host galaxy. Conclusions. We find that the hydrogen recombination in the atmosphere of a normal type IIP SN 1999em, as well as most likely other type IIP supernovae at the photospheric epoch, is essentially a time-dependent phenomenon. It is also shown that in normal type IIP supernovae the homologous expansion of the ejecta in its atmosphere takes place starting from nearly the third day after the supernova explosion. A comparison of SN 1999em with SN 1987A reveals two very important results for supernova theory. First, the comparability of the helium core masses and the explosion energies implies a unique explosion mechanism for these core collapse supernovae. Second, the optimal model for SN 1999em is characterized by a weaker Ni-56 mixing up to approximate to 660 km s(-1) compared to a moderate Ni-56 mixing up to similar to 3000 km s(-1) in SN 1987A, hydrogen being mixed deeply downward to similar to 650 km s(-1).