G. Garavini, Stockholms universitet
G. Folatelli, Stockholms universitet
A. Goobar, Stockholms universitet
S. Nobili, Stockholms universitet
G. Aldering, Lawrence Berkeley National Laboratory
A. Amadon, CEA Saclay
R. Amanullah, Stockholms universitet
P. Astier, Laboratoire de Physique Nucléaire et de Hautes Energies
C. Balland, Laboratoire de Physique Nucléaire et de Hautes Energies
G. Blanc, Lawrence Berkeley National Laboratory
M. S. Burns, Colorado College
A. Conley, Lawrence Berkeley National Laboratory
T. Dahlén, AlbaNova Universitetscentrum
S. E. Deustua, Lawrence Berkeley National Laboratory
R. Ellis, California Institute of Technology
S. Fabbro, Universidade de Lisboa
X. Fan, The University of Arizona
B. Frye, Lawrence Berkeley National Laboratory
E. L. Gates, University of California, Santa Cruz
R. Gibbons, Lawrence Berkeley National Laboratory
G. Goldhaber, Lawrence Berkeley National Laboratory
B. Goldman, New Mexico State University
D. E. Groom, Lawrence Berkeley National Laboratory
J. Haissinski, Laboratoire de l'Accélérateur Linéaire
D. Hardin, Laboratoire de Physique Nucléaire et de Hautes Energies
I. M. Hook, University of Oxford
D. A. Howell, Lawrence Berkeley National Laboratory
D. Kasen, Lawrence Berkeley National Laboratory
S. Kent, Fermi National Accelerator Laboratory
A. G. Kim, Lawrence Berkeley National Laboratory
R. A. Knop, Vanderbilt University
B. C. Lee, Lawrence Berkeley National Laboratory
C. Lidman, European Southern Observatory Santiago

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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 λ6355, 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 λ6580 or Hα. At the same epoch C II λ4648.8 at photospheric velocity is probably responsible for the absorption feature at 4500 Å. 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.

Publication Source (Journal or Book title)

Astronomical Journal

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