A non-spherical core in the explosion of supernova SN 2004dj

• Published in: Nature Vol. 440; no. 7083; pp. 505 - 507
• Main Authors: Leonard, Douglas C., Filippenko, Alexei V., Ganeshalingam, Mohan, Serduke, Franklin J. D., Li, Weidong, Swift, Brandon J., Gal-Yam, Avishay, Foley, Ryan J., Fox, Derek B., Park, Sung, Hoffman, Jennifer L., Wong, Diane S.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 23.03.2006; Nature Publishing; Nature Publishing Group
• Subjects: Astronomy; Earth, ocean, space; Emissions; Exact sciences and technology; Explosions; Gamma rays; Humanities and Social Sciences; Hydrogen; Late stages of stellar evolution (including black holes); letter; multidisciplinary; Polarization; Science; Science (multidisciplinary); Stars; Stars & galaxies; Stellar characteristics and properties; Stellar structure, interiors, evolution, nucleosynthesis, ages; Supernovae; Thinning; Velocity; Collapse; Spherical symmetry; Polarization; Asymmetry; Supernovae explosion; Stellar explosion; Pulsars; Emission line; Type II supernova; Massive stars; Spectropolarimetry
• ISSN: 0028-0836; 1476-4687; 1476-4687; 1476-4679
• DOI: 10.1038/nature04558

How supernovae shape up Supernovae have been subjected to extensive study over the years, particularly since the discovery of their involvement with γ-ray bursts. But there are still some basic questions about them that remain unanswered, for instance, what shape are they? Explosion geometry has been a difficult subject to tackle because supernovae in other galaxies are so distant that they remain point-like in our night sky. Now observations of SN 2004dj, the closest normal Type II-P supernova ever observed, show that the innermost regions of the expanding ejecta are severely distorted, the result of an explosion mechanism that is strongly nonspherical. This property may be inherent to the core-collapse process in all types of supernovae. Multi-epoch spectropolarimetry of a supernova reveals the abrupt appearance of significant polarization when the inner core is first exposed in the thinning ejecta — roughly 90 days after explosion. 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 velocities 1 , associations of supernovae with long-soft γ-ray bursts 2 , 3 , and asymmetries in late-time emission-line profiles 4 . 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.