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Spectroscopic observations and analysis of the peculiar SN 1999aa

  • Author(s): Garavini, G.
  • Folatelli, G.
  • Goobar, A.
  • Nobili, S.
  • Aldering, G.
  • Amadon, A.
  • Amanullah, R.
  • Astier, P.
  • Balland, C.
  • Blanc, G.
  • Burns, M.S.
  • Conley, A.
  • Dahlen, T.
  • Deustua, S.E.
  • Ellis, R.
  • Fabbro, S.
  • Fan, X.
  • Frye, B.
  • Gates, E.L.
  • Gibbons, R.
  • Goldhaber, G.
  • Goldman, B.
  • Groom, D.E.
  • Haissinski, J.
  • Hardin, D.
  • Hook, I.M.
  • Howell, D.A.
  • Kasen, D.
  • Kent, S.
  • Kim, A.G.
  • Knop, R.A.
  • Lee, B.C.
  • Lidman, C.
  • Mendez, J.
  • Miller, G.J.
  • Moniez, M.
  • Mourao, A.
  • Newberg, H.
  • Nugent, P.E.
  • Pain, R.
  • Perdereau, O.
  • Perlmutter, S.
  • Prasad, V.
  • Quimby, R.
  • Raux, J.
  • Regnault, N.
  • Rich, J.
  • Richards, G.T.
  • Ruiz-Lapuente, P.
  • Sainton, G.
  • Schaefer, B.E.
  • Schahmaneche, K.
  • Smith, E.
  • Spadafora, A.L.
  • Stanishev, V.
  • Walton, N.A.
  • Wang, L.
  • Wood-Vasey, W.M.
  • et al.
Abstract

We present an extensive new time series of spectroscopic data of the peculiar SN 1999aa in NGC 2595. Our data set includes 25 optical spectra between -11 and +58 days with respect to B-band maximum light, providing an unusually complete time history. The early spectra resemble those of an SN 1991T-like object but with a relatively strong Ca H and K absorption feature. The first clear sign of Si II lambda6355, characteristic of Type Ia supernovae, is found at day -7, and its velocity remains constant up to at least the first month after B-band maximum light. The transition to normal-looking spectra is found to occur earlier than in SN 1991T, suggesting SN 1999aa as a possible link between SN 1991T-like and Branch-normal supernovae. Comparing the observations with synthetic spectra, doubly ionized Fe, Si, and Ni are identified at early epochs. These are characteristic of SN 1991T-like objects. Furthermore, in the day -11 spectrum, evidence is found for an absorption feature that could be identified as high velocity C II lambda6580 or H alpha. At the same epoch C III lambda4648.8 at photospheric velocity is probably responsible for the absorption feature at 4500 8. High-velocity Ca is found around maximum light together with Si II and Fe II confined in a narrow velocity window. Implied constraints on supernovae progenitor systems and explosion hydrodynamic models are briefly discussed.

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