Supernova 1996L: evidence of a strong wind episode before the explosion

Observations of the Type II supernova (SN) 1996L reveal the presence of a slowly expanding (v~ 700 km s^-1) shell at ~ 10^16 cm from the exploding star. Narrow emission features are visible in the early spectra superposed on the normal SN spectrum. Within about two months these features develop narrow symmetric P Cygni profiles. About 100 d after the explosion the light curve suddenly flattens, the spectral lines broaden and the Hα flux becomes larger than that expected from a purely radioactive model. These events are interpreted as signatures of the onset of the interaction between the fast moving ejecta and a slowly moving outer shell of matter ejected before the SN explosion. At about 300 d the narrow lines disappear and the flux drops until the SN fades away, suggesting that the interaction phase is over and that the shell has been swept away. Simple calculations show that the superwind episode started 9 yr before the SN explosion and lasted 6 yr, with an average MsolarM=10^-3 M_solar yr^-1. Even at very late epochs (up to day 335) the typical forbidden lines of [O i], Ca ii], [Fe ii] remain undetected or very weak. Spectra after day 270 show relatively strong emission lines of He i. These lines are narrower than other emission lines coming from the SN ejecta, but broader than those from the circumstellar material (CSM). These high-excitation lines are probably the result of non-thermal excitation and ionization caused by the deposition of the gamma-rays emitted in the decay of radioactive material mixed in the He layer.