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An asymmetric explosion as the origin of spectral evolution diversity in type Ia supernovae

Abstract

Type Ia supernovae form an observationally uniform class of stellar explosions, in that more luminous objects have smaller decline-rates1. This one-parameter behaviour allows type Ia supernovae to be calibrated as cosmological ‘standard candles’, and led to the discovery of an accelerating Universe2,3. Recent investigations, however, have revealed that the true nature of type Ia supernovae is more complicated. Theoretically, it has been suggested4,5,6,7,8 that the initial thermonuclear sparks are ignited at an offset from the centre of the white-dwarf progenitor, possibly as a result of convection before the explosion4. Observationally, the diversity seen in the spectral evolution of type Ia supernovae beyond the luminosity–decline-rate relation is an unresolved issue9,10. Here we report that the spectral diversity is a consequence of random directions from which an asymmetric explosion is viewed. Our findings suggest that the spectral evolution diversity is no longer a concern when using type Ia supernovae as cosmological standard candles. Furthermore, this indicates that ignition at an offset from the centre is a generic feature of type Ia supernovae.

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Figure 1: Comparison between HVG type Ia SN 2002bo and LVG type Ia SN 1998bu.
Figure 2: Relations between the features in early and late phases.
Figure 3: A schematic picture of the structure of type Ia supernovae ejecta.
Figure 4: Expectations from a hydrodynamic explosion model.

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Acknowledgements

We thank W. Hillebrandt for discussions. This study is partly based on observations obtained at the Gemini Observatory, Chile (GS-2009B-Q-8, GS-2008B-Q-32/40/46), the Magellan Telescopes, Chile, and by ESO Telescopes at the La Silla or Paranal Observatories under programme 080.A-0516. This research made use of the SUSPECT archive, at the Department of Physics and Astronomy, University of Oklahoma. This work was supported by World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan. K.M. was supported by a JSPS Grant-in-Aid for young scientists; S.B. acknowledges partial support from ASI contracts ‘COFIS’; M.S. was supported by the National Science Foundation; F.K.R. was supported through the Emmy Noether Program of the German Research Foundation and by the Cluster of Excellence ‘Origin and Structure of the Universe’; G.F. and M.H. acknowledge support from Iniciativa Cientifica Milenio and CONICYT programmes FONDECYT/FONDAP/BASAL; J.S. is a Royal Swedish Academy of Sciences Research Fellow supported by the Knut and Alice Wallenberg Foundation; and S.T. acknowledges support from the Transregional Collaborative Research Centre under the programme ‘The dark Universe’. The Dark Cosmology Centre is funded by the Danish National Research Foundation.

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Authors

Contributions

S.B. and K.M. found the relation between the velocity gradient and the nebular velocity, and initiated and organized the project. K.M. wrote the manuscript with the assistance of M.S., J.S., G.F. and S.T. S.B. is responsible for the late-phase spectrum of SN 1997bp. M.S., G.F. and M.H are responsible for acquisition and reduction of SN 2007on, SN 2007sr and SN 2009ab. F.K.R. and K.M. are responsible for the explosion simulation. All the authors contributed to discussions.

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Correspondence to K. Maeda.

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This file contains Supplementary Notes 1-3, Supplementary Table 1, Supplementary Figures 1-3 with legends and References. (PDF 365 kb)

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Maeda, K., Benetti, S., Stritzinger, M. et al. An asymmetric explosion as the origin of spectral evolution diversity in type Ia supernovae. Nature 466, 82–85 (2010). https://doi.org/10.1038/nature09122

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