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

BENETTI, StefanoStefanoBENETTIZAMPIERI, LucaLucaZAMPIERIPASTORELLO, AndreaAndreaPASTORELLOCAPPELLARO, EnricoEnricoCAPPELLAROPumo, M. L.M. L.PumoELIAS DE LA ROSA, NANCY DEL CARMENNANCY DEL CARMENELIAS DE LA ROSAOchner, P.P.OchnerTerreran, G.G.TerreranTOMASELLA, LinaLinaTOMASELLATaubenberger, S.S.TaubenbergerTURATTO, MassimoMassimoTURATTOMorales-Garoffolo, A.A.Morales-GaroffoloHarutyunyan, A.A.HarutyunyanTARTAGLIA, LeonardoLeonardoTARTAGLIA2020-10-162020-10-1620180035-8711http://hdl.handle.net/20.500.12386/27863We 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 blackbody continuum. After few days, the HeI λλ 5876 transition appears with a P-Cygni profile and an expansion velocity of about 8700 km s-1. Fifty days after maximum, the spectrum shows signs typically seen in interacting supernovae. A broad (FWHM ∼ 8000 km s-1) Hα becomes more prominent with time until ∼150 d after maximum and quickly declines later on. At these phases Hαstarts to show an intermediate component, which together with the blue pseudo-continuum are 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-envelope SNe. The early part (the first 40 d after maximum) of the bolometric curve, which peaks at a luminosity intermediate between normal and superluminous supernovae, is well reproduced by a model in which the energy budget is essentially coming from ejecta recombination and 56Ni decay. From the model, we infer a mass of the ejecta Mej = 2.6 M☉; an initial radius of the photosphere R0 = 2.1 × 1014 cm; and an explosion energy Eexpl = 0.8 × 1051 erg. A possible scenario involves a massive and extended H-poor shell lost by the progenitor star a few years before explosion. The shell is hit, heated, and accelerated by the supernova ejecta. The accelerated shell+ejecta rapidly dilutes, unveiling the unperturbed supernova spectrum below. The outer ejecta start to interact with a H-poor external CSM lost by the progenitor system about 9-90 yr before the explosion.STAMPAenASASSN-15no: the Supernova that plays hide-and-seekArticle10.1093/mnras/sty1662-s2.0-85043509170000429276700021https://academic.oup.com/mnras/article/476/1/261/49195692018MNRAS.476..261BFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation::PE9_6 Stars and stellar systems