2016MNRAS.456.3296B

Spectrophotometry of SN 1996al carried out throughout 15 yr 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 superimposed narrow lines with P-Cygni profile, and Hei features with asymmetric broad emission components. The analysis of the line profiles shows that the H and He broad components form in the same region of the ejecta. By day +142, the Hα profile dramatically changes: the narrow P-Cygni profile disappears, and the Hα is fitted by three emission components that will be detected over the remaining 15 yr of the supernova (SN) monitoring campaign. Instead, the Hei emissions become progressively narrower and symmetric. A sudden increase in flux of all Hei lines is observed between 300 and 600 d. Models show that the SN luminosity is sustained by the interaction of low-mass ( ∼ 1.15 M_☉) ejecta, expelled in a low kinetic energy ( ∼ 1.6 x 10⁵⁰ erg) explosion, with highly asymmetric circumstellar medium. The detection of Hα emission in pre-explosion archive images suggests that the progenitor was most likely a massive star ( ∼ 25 M_☉ ZAMS) 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 modest kinetic energy of the explosion are explained with massive fallback of material into the compact remnant, a 7-8-M_☉ black hole.