ASASSN-15no: The Supernova that plays hide-and-seek

We report the results of our follow-up campaign of the peculiar supernova ASASSN-15no, based on optical data covering ~300 d of its evolution. Initially the spectra show a pure blackbodycontinuum. After few days, the HeI λλ 5876 transition appears with a P-Cygni profile andan expansion velocity of about 8700 km s-1. Fifty days after maximum, the spectrum showssigns typically seen in interacting supernovae. A broad (FWHM~ 8000 km s-1) Ha becomesmore prominent with time until ~150 d after maximum and quickly declines later on. At thesephases Hastarts to show an intermediate component, which together with the blue pseudocontinuumare clues that the ejecta begin to interact with the circumstellar medium (CSM).The spectra at the latest phases look very similar to the nebular spectra of stripped-envelopeSNe. The early part (the first 40 d after maximum) of the bolometric curve, which peaks at aluminosity intermediate between normal and superluminous supernovae, is well reproducedby a model in which the energy budget is essentially coming from ejecta recombination and56Ni decay. From the model, we infer a mass of the ejecta Mej = 2.6Mo˙ an initial radiusof the photosphere R0 = 2.1 × 1014 cm; and an explosion energy Eexpl = 0.8 × 1051 erg. Apossible scenario involves a massive and extended H-poor shell lost by the progenitor star afew years before explosion. The shell is hit, heated, and accelerated by the supernova ejecta. The accelerated shell+ejecta rapidly dilutes, unveiling the unperturbed supernova spectrumbelow. The outer ejecta start to interact with a H-poor external CSM lost by the progenitorsystem about 9-90 yr before the explosion.