1995A&A...299..715P

We analyse the Hα emission line in the late time spectra of the type II-Ib SN 1993J at epochs 0.5-1 year after the explosion. The observed Hα profile is fitted assuming that ionized hydrogen is distributed in a shell with H⁺ density maximum at 8900-9300km/s, inner edge at about 7500km/s and outer edge at about 11400km/s. These velocities remained constant for over 6 months. The mass of H⁺ in the shell required for the Hα emission to be reproduced by recombination is 0.05-0.2M_☉. Energy deposition by the γ-rays generated by the radioactive decay of 0.075M_☉ of ⁵⁶Ni is insufficient to explain the observed Hα flux after an epoch about 150d. The interaction of the ejecta with the circumstellar (CS) wind is probably the dominant energy source for the Hα luminosity at t≥0.5yr. However, the standard model of the ejecta-wind interaction requires a mass loss rate {dot}(M)≃210^–5(u/10km/s)M_☉/yr, which exceeds by approximately a factor 10 the upper limit derived from the Hα width and from the lack of profile evolution. Wind clumpiness may significantly increase the efficiency of the conversion of the luminosity produced by the ejecta-wind interaction into Hα radiation. The required average wind density would thus decrease and so would the estimated mass loss rate.