Nonthermal ionization and excitation in Type IIb supernova 1993J

A non-LTE study of Type IIb supernova 1993J in the galaxy M 81 accounting for nonthermal ionization and line blocking effects is carried out. Hydrodynamical models and theoretical spectra clearly show that nonthermal ionization and excitation dominate after the second maximum, at day similar to 30, and play a decisive role in reproducing both a smooth tail of the light curve and an emergence of helium lines in the spectrum similar to those observed. Based on our model of supernova 1993J, we predict that the light curves of Type Ib supernovae should be subject to nonthermal ionization and excitation at earlier times than even that of supernova 1993J. To fit the bolometric and visual light curves of supernova 1993J, an outer layer of similar to 1 M. has to be helium-rich hydrogen shell with a hydrogen mass fraction of similar to 0.1. In this shell there is no nearly pure helium mantle as contrasted to most of the evolutionary models at the time of explosion. The fact that such a distribution of hydrogen results in a characteristic maximum of hydrogen number density at velocity of similar to 8600 km s(-1) in the expelled envelope is well consistent with late time observations of H alpha emission at epochs of 0.5 - 1 year after the explosion. An emergence of helium lines between day 24 and day 30 illustrated by the evolution of calculated profile of the He I line lambda 6678 Angstrom completely fits the spectral observations of supernova 1993J. The bolometric and visual light curves and the spectral evolution of helium lines are consistent with a mass of the ejected envelope of similar to 2.4 M. including a hydrogen mass of similar to 0.12 M., an explosion energy of similar to 1.6 . 10(51) ergs, and a mass of radioactive Ni-56 of similar to 0.078 M.. It is found that the bulk of the radioactive material should be confined to layers of the ejected envelope expanding with velocities less than similar to 3800 km s(-1). In our model, the outburst of supernova 1993J is interpreted as the explosion of a similar to 4 M. red supergiant undergoing core collapse and leaving a neutron star in a binary system. The progenitor is supposed to have a helium core mass of similar to 3 M. corresponding to a similar to 13 M. main-sequence star. Supernova 1993J adds evidence to the scenario that Type Ib supernovae originate from moderately massive stars on the main sequence which have lost their hydrogen envelopes in interacting binary systems. It is shown that there are strong arguments in favor of a fundamental similarity between the explosions of Type IIb, Ib, and II-P supernovae.