1. Astronomy Department, MS 105-24, California Institute of Technology, Pasadena, California 91125, USA
2. Department of Astronomy, University of California, Berkeley, California 94720-3411, USA
3. †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.)

Correspondence to: Douglas C. Leonard^1,3 Correspondence and requests for materials should be addressed to D.C.L. (Email: leonard@sciences.sdsu.edu).

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 -ray bursts^{2, 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 symmetry^{5, 6}. Large polarizations have been measured for 'stripped-envelope' (that is, type Ic; ref. 7) supernovae, which confirms their non-spherical morphology^{8, 9}; but the explosions of massive stars with intact hydrogen envelopes^{7, 10} (type II-P supernovae) have shown only weak polarizations at the early times observed^{11, 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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