High luminosity, slow ejecta and persistent carbon lines: SN 2009dc challenges thermonuclear explosion scenarios

Creators: Taubenberger, S.; Elias-Rosa, N.

Abstract

Extended optical and near-IR observations reveal that SN 2009dc shares a number of similarities with normal Type Ia supernovae (SNe Ia), but is clearly overluminous, with a (pseudo-bolometric) peak luminosity of log (L) = 43.47 (erg s^(−1)). Its light curves decline slowly over half a year after maximum light [Δm_(15)(B)_true= 0.71], and the early-time near-IR light curves show secondary maxima, although the minima between the first and the second peaks are not very pronounced. The bluer bands exhibit an enhanced fading after ~200 d, which might be caused by dust formation or an unexpectedly early IR catastrophe. The spectra of SN 2009dc are dominated by intermediate-mass elements and unburned material at early times, and by iron-group elements at late phases. Strong C ii lines are present until ~2 weeks past maximum, which is unprecedented in thermonuclear SNe. The ejecta velocities are significantly lower than in normal and even subluminous SNe Ia. No signatures of interaction with a circumstellar medium (CSM) are found in the spectra. Assuming that the light curves are powered by radioactive decay, analytic modelling suggests that SN 2009dc produced ~1.8 M_⊙ of ^(56)Ni assuming the smallest possible rise time of 22 d. Together with a derived total ejecta mass of ~2.8 M_⊙, this confirms that SN 2009dc is a member of the class of possible super-Chandrasekhar-mass SNe Ia similar to SNe 2003fg, 2006gz and 2007if. A study of the hosts of SN 2009dc and other superluminous SNe Ia reveals a tendency of these SNe to explode in low-mass galaxies. A low metallicity of the progenitor may therefore be an important prerequisite for producing superluminous SNe Ia. We discuss a number of possible explosion scenarios, ranging from super-Chandrasekhar-mass white-dwarf progenitors over dynamical white-dwarf mergers and Type I(1/2) SNe to a core-collapse origin of the explosion. None of the models seems capable of explaining all properties of SN 2009dc, so that the true nature of this SN and its peers remains nebulous.

Additional Information

© 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS. Accepted 2010 November 25. Received 2010 November 24; in original form 2010 November 9. Article first published online: 24 Jan. 2011. Based on observations at ESO La Silla, Prog. 083.D-0970 and 184.D-1140 and ESO Paranal, Prog. 083.D-0728. This work is based on observations collected at the 2.2-m Telescope of the Centro Astronómico Hispano Alemán (Calar Alto, Spain), the Italian 3.58-m Telescopio Nazionale Galileo, the 2.56-m Nordic Optical Telescope and the 2.0-m Liverpool Telescope (La Palma, Spain), the 3.58-m New Technology Telescope and 0.60-m Rapid Eye Mount (La Silla, Chile), the 1.82-m Copernico Telescope on Cima Ekar (Asiago, Italy) and the 2×8.2m Large Binocular Telescope (Arizona, US). The Telescopio Nazionale Galileo is operated by the Fundación Galileo Galilei of the Instituto Nazionale di Astrofisica (INAF) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. ESO observations have been performed under programmes 083.D-0728, 083.D-0970 and 184.D-1140. We thank the support astronomers at the Telescopio Nazionale Galileo, the 2.2-m Telescope at Calar Alto, the Nordic Optical Telescope and the Large Binocular Telescope for performing the follow-up observations of SN 2009dc. This research made use of the NASA/IPAC Extragalactic Database (NED), operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration; the Lyon-Meudon Extragalactic Database (LEDA), supplied by the LEDA team at the Centre de Recherche Astronomique de Lyon, Observatoire de Lyon; the Online Supernova Spectrum Archive (SUSPECT), initiated and maintained at the Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma; and the SMOKA archive, operated by the Astronomy Data Center, National Astronomical Observatory of Japan. Some data used in this paper were obtained from the SDSS. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society and the Higher Education Funding Council for England. The SDSS website is http://www.sdss.org/. We also benefited greatly from the information provided by the Bright Supernova web pages (maintained by D. Bishop) as part of the Rochester Academy of Sciences (http://www.RochesterAstronomy.org/snimages). The authors are indebted to the referee, D. Branch, for his constructive comments. Our thanks go to F. K. Röpke, S. A. Sim, I. R. Seitenzahl, A. J. Ruiter, M. Fink, I. Maurer, K. Nomoto and K. Maeda for inspiring discussions, to M. Fink and S. Benítez Herrera for assistance with observations, and to K. Maeda and K. Kawabata for images and spectra of SN 2006gz obtained with the Subaru telescope. ST acknowledges support by the Transregional Collaborative Research Centre TRR 33 'The Dark Universe' of the German Research Foundation (DFG). MC is supported by the Director, Office of Science, Office of High Energy Physics, of the US Department of Energy under Contract No. DE-AC02-05CH11231 and by a grant from the Gordon & Betty Moore Foundation. SB, FB, PAM and MT are partially supported by the PRIN-INAF 2009 with the project 'Supernovae Variety and Nucleosynthesis Yields'. VS acknowledges financial support from Fundação para a Ciência e a Tecnologia under program Ciência 2008. This research has benefited from the European supernova collaboration led by SB.

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