Author affiliations
1 Laboratoire de Physique Nucléaire et de Haute Energies, CNRS-IN2P3, Université Paris VI and VII, Paris, France
2 Department of Physics, Stockholm University, Albanova University Center, S-106 91 Stockholm, Sweden
3 E. O. Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720
4 Visiting Astronomer, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under cooperative agreement with the National Science Foundation
5 DAPNIA-SPP, CEA Saclay, 91191 Gif-sur-Yvette, France
6 Université Paris Sud and APC, 11 Place Marcelin Berthelot, F-75231 Paris Cedex 05, France
7 Osservatorio Astronomico di Padova, INAF, vicolo dell'Osservatorio 5, 35122 Padua, Italy
8 Department of Physics, University of California, Berkeley, CA 94720-7300
9 American Astronomical Society, 2000 Florida Avenue NW, Suite 400, Washington, DC 20009
10 California Institute of Technology, MS 105-24, 1201 East California Boulevard, Pasadena, CA 91125
11 CENTRA and Departamento de Física, Instituto Superior Técnico, Universidade Técnica de Lisboa, Avenida Rovisco Pais, 1049-001 Lisbon, Portugal
12 Steward Observatory, University of Arizona, 933 North Cherry Avenue, Room N204, Tucson, AZ 85721
13 Lick Observatory, P.O. Box 85, Mount Hamilton, CA 95140
14 Department of Physics and Astronomy, Vanderbilt University, P.O. Box 1807, Station B, Nashville, TN 37240
15 Department of Astronomy, New Mexico State University, MSC 4500, P.O. Box 30001, Las Cruces, NM 88011
16 Laboratoire de l'Accélerateur Linéaire, IN2P3-CNRS, Université Paris Sud, B.P. 34, 91898 Orsay Cedex, France
17 Department of Physics, University of Oxford, Nuclear and Astrophysics Laboratory, Keble Road, Oxford OX1 3RH, UK
18 Department of Astronomy and Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
19 Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510
20 European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
21 Isaac Newton Group, Apartado de Correos 321, 38780 Santa Cruz de La Palma, Canary Islands, Spain
22 Department of Astronomy, University of Barcelona, Martí i Franques 1, E-08028 Barcelona, Spain
23 Department of Astronomy, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182-1221
24 Department of Astronomy, University of Illinois, 1002 West Green Street, Urbana, IL 61801
25 Laboratoire de l'univers et de ses Theories, UMR 8102, CNRS, Observatoire de Paris, Section de Meudon, F-92195 Meudon Cedex, France
26 Department of Physics, Rensselaer Polytechnic Institute, SC1C25, Troy, NY 12180
27 Princeton University Observatory, Peyton Hall, Princeton University, Princeton, NJ 08544
28 Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803
29 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
, and Δm15 further underlines the unique characteristics of SN 1999ac. We find convincing evidence of C II λ6580 in the day -15 spectrum with ejection velocity v > 16,000 km s-1, but this signature disappears by day -9. This rapid evolution at early times highlights the importance of extremely early-time spectroscopy.