Strongly Bipolar Inner Ejecta of the Normal Type IIP Supernova ASASSN-16at

Bose, SubhashSubhashBoseDong, SuboSuboDongELIAS DE LA ROSA, NANCY DEL CARMENNANCY DEL CARMENELIAS DE LA ROSAShappee, B. J.B. J.ShappeeBersier, DavidDavidBersierBENETTI, StefanoStefanoBENETTIStritzinger, M. D.M. D.StritzingerGrupe, D.D.GrupeKochanek, C. S.C. S.KochanekPrieto, J. L.J. L.PrietoChen, PingPingChenKuncarayakti, H.H.KuncarayaktiMattila, SeppoSeppoMattilaMorales-Garoffolo, AntoniaAntoniaMorales-GaroffoloMorrell, NidiaNidiaMorrellOnori, F.F.OnoriReynolds, Thomas M.Thomas M.ReynoldsSiviero, A.A.SivieroSomero, AuniAuniSomeroStanek, K. Z.K. Z.StanekTerreran, GiacomoGiacomoTerreranThompson, Todd A.Todd A.ThompsonTOMASELLA, LinaLinaTOMASELLAAshall, C.C.AshallGall, ChristaChristaGallGromadzki, M.M.GromadzkiHoloien, T. W. -S.T. W. -S.Holoien2020-12-302020-12-3020192041-8205http://hdl.handle.net/20.500.12386/29335We report distinctly double-peaked Hα and Hβ emission lines in the late-time, nebular-phase spectra (≳200 days) of the otherwise normal at early phases (≲100 days) type IIP supernova ASASSN-16at (SN 2016X). Such distinctly double-peaked nebular Balmer lines have never been observed for a type II SN. The nebular-phase Balmer emission is driven by the radioactive 56Co decay, so the observed line profile bifurcation suggests a strong bipolarity in the 56Ni distribution or in the line-forming region of the inner ejecta. The strongly bifurcated blueshifted and redshifted peaks are separated by ∼3 × 103 km s-1 and are roughly symmetrically positioned with respect to the host-galaxy rest frame, implying that the inner ejecta are composed of two almost-detached blobs. The red peak progressively weakens relative to the blue peak, and disappears in the 740 days spectrum. One possible reason for the line-ratio evolution is increasing differential extinction from continuous formation of dust within the envelope, which is also supported by the near-infrared flux excess that develops after ∼100 days.STAMPAenStrongly Bipolar Inner Ejecta of the Normal Type IIP Supernova ASASSN-16atArticle10.3847/2041-8213/ab05582-s2.0-85063487155000459993200002https://iopscience.iop.org/article/10.3847/2041-8213/ab05582019ApJ...873L...3BFIS/05 - ASTRONOMIA E ASTROFISICA