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Abstract

An important and perhaps critical clue to the mechanism driving the explosion of massive stars as supernovae is provided by the accumulating evidence for asymmetry in the explosion. Indirect evidence comes from high pulsar velocities1, associations of supernovae with long-soft γ-ray bursts2,3, and asymmetries in late-time emission-line profiles4. Spectropolarimetry provides a direct probe of young supernova geometry, with higher polarization generally indicating a greater departure from spherical symmetry5,6. Large polarizations have been measured for ‘stripped-envelope’ (that is, type Ic; ref. 7) supernovae, which confirms their non-spherical morphology8,9; but the explosions of massive stars with intact hydrogen envelopes7,10 (type II-P supernovae) have shown only weak polarizations at the early times observed11,12. Here we report multi-epoch spectropolarimetry of a classic type II-P supernova that reveals the abrupt appearance of significant polarization when the inner core is first exposed in the thinning ejecta (90 days after explosion). We infer a departure from spherical symmetry of at least 30 per cent for the inner ejecta. Combined with earlier results, this suggests that a strongly non-spherical explosion may be a generic feature of core-collapse supernovae of all types, where the asphericity in type II-P supernovae is cloaked at early times by the massive, opaque, hydrogen envelope.

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Acknowledgements

We thank D. Kasen for discussions. Support for this research was provided by the National Science Foundation, NASA, and the Sylvia and Jim Katzman Foundation. A.V.F. is grateful for a Miller Research Professorship at UC Berkeley, during which part of this work was completed.

Author information

Author notes

    • Douglas C. Leonard
    •  & Derek B. Fox

    †Present addresses: Department of Astronomy, San Diego State University, San Diego, California 92182, USA (D.C.L.); Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, Pennsylvania 16802, USA (D.B.F.)

Affiliations

  1. Astronomy Department, MS 105-24, California Institute of Technology, Pasadena, California 91125, USA

    • Douglas C. Leonard
    • , Avishay Gal-Yam
    •  & Derek B. Fox
  2. Department of Astronomy, University of California, Berkeley, California 94720-3411, USA

    • Alexei V. Filippenko
    • , Mohan Ganeshalingam
    • , Franklin J. D. Serduke
    • , Weidong Li
    • , Brandon J. Swift
    • , Ryan J. Foley
    • , Sung Park
    • , Jennifer L. Hoffman
    •  & Diane S. Wong

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Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

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Correspondence to Douglas C. Leonard.

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https://doi.org/10.1038/nature04558

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