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Spectrophotometry of SN 1996al carried out throughout 15 years is presented.The early photometry suggests that SN 1996al is a Linear type-II supernova,with an absolute peak of Mv ~ -18.2 mag. Early spectra present broad,asymmetric Balmer emissions, with super-imposed narrow lines with P-Cygniprofile, and He I features with asymmetric, broad emission components. Theanalysis of the line profiles shows that the H and He broad components form inthe same region of the ejecta. By day +142, the Halpha profile dramaticallychanges: the narrow P-Cygni profile disappears, and the Halpha is fitted bythree emission components, that will be detected over the remaining 15 yrs ofthe SN monitoring campaign. Instead, the He I emissions become progressivelynarrower and symmetric. A sudden increase in flux of all He I lines is observedbetween 300 and 600 days. Models show that the supernova luminosity issustained by the interaction of low mass (~1.15 Msun) ejecta, expelled in a lowkinetic energy (~ 1.6 x 10^50 erg) explosion, with highly asymmetriccircumstellar medium. The detection of Halpha emission in pre-explosion archiveimages suggests that the progenitor was most likely a massive star (~25 MsunZAMS) that had lost a large fraction of its hydrogen envelope before explosion,and was hence embedded in a H-rich cocoon. The low-mass ejecta and modestkinetic energy of the explosion are explained with massive fallback of materialinto the compact remnant, a 7-8 Msun black hole.

The spectacular evolution of Supernova 1996al over 15 years: a low energy explosion of a stripped massive star in a highly structured environment