Evidence for energizing of H alpha emission in type II supernovae by ejecta-wind interaction.

A simple one-zone model of Hα luminosity of late time type II supernovae (τ > 100 d), energized by ^56^Co-^56^Fe decay, was applied to the analysis of the observed Hα evolution. Of the six available SNeII with known Hα fluxes at late times, three reveal the extra Hα flux above the radioactive model. This excess is interpreted as an effect of an energy release due to ejecta-wind interaction. Two SNeII, 1979C and 1987F, with the particularly strong Hα excess, show additional spectroscopic signatures, which can be attributed to the dense wind around supernovae. In SN 1987F the wind density parameter M^bar^/u_w_ is of the order of 10^17^ g cm^-1^, thus being high enough to provide a noticeable contribution arising from ejecta-wind interaction to the overall optical luminosity at late times (τ > 150 d). The hard X-ray (kT~50 keV) luminosity of the outer shock wave must have been, in this case, of the order of 10^42^ erg s^-1^ at 150 d. The modelling of the Hα profile in SN 1987F at τ = 150 d provides compelling evidence in favour of the existence of an inner radiative shock wave.