Asymmetries in core-collapse supernovae from maps of radioactive 44Ti in Cassiopeia A

• Published in: Nature (London) 2014-02, Vol.506 (7488), p.339-342
• Main Authors: Grefenstette, B. W., Harrison, F. A., Boggs, S. E., Reynolds, S. P., Fryer, C. L., Madsen, K. K., Wik, D. R., Zoglauer, A., Ellinger, C. I., Alexander, D. M., An, H., Barret, D., Christensen, F. E., Craig, W. W., Forster, K., Giommi, P., Hailey, C. J., Hornstrup, A., Kaspi, V. M., Kitaguchi, T., Koglin, J. E., Mao, P. H., Miyasaka, H., Mori, K., Perri, M., Pivovaroff, M. J., Puccetti, S., Rana, V., Stern, D., Westergaard, N. J., Zhang, W. W.
• Format: Article
• Language: English
• Subjects: 639/33/34/864; 639/33/34/866; Humanities and Social Sciences; letter; multidisciplinary; Science
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature12997

The observation of non-uniformly distributed titanium emission in the interior of Cassiopeia A, a core-collapse supernova, is an indicator of asymmetries in the stellar explosion and provides strong evidence for the development of low-mode convective instabilities in such supernovae. Cassiopeia A — remnant of an asymmetric explosion Most simulations of stellar core collapse events indicate that the explosions are asymmetric, but the resulting shapes differ in the various models. Brian Grefenstette et al . analysed the distribution of radioactive titanium-44 in Cassiopeia A, a young core-collapse remnant, as a proxy for explosion asymmetry. They report a degree of non-uniform distribution in the unshocked interior of Cas A greater than that expected from a spherical explosion, yet not as pronounced as would follow a highly bipolar explosion. On the basis of these findings, the authors conclude that the type of explosion for the Cas A core-collapse explosion was part-way between the two extremes of asymmetry. Asymmetry is required by most numerical simulations of stellar core-collapse explosions, but the form it takes differs significantly among models. The spatial distribution of radioactive 44 Ti, synthesized in an exploding star near the boundary between material falling back onto the collapsing core and that ejected into the surrounding medium 1 , directly probes the explosion asymmetries. Cassiopeia A is a young 2 , nearby 3 , core-collapse 4 remnant from which 44 Ti emission has previously been detected 5 , 6 , 7 , 8 but not imaged. Asymmetries in the explosion have been indirectly inferred from a high ratio of observed 44 Ti emission to estimated 56 Ni emission 9 , from optical light echoes 10 , and from jet-like features seen in the X-ray 11 and optical 12 ejecta. Here we report spatial maps and spectral properties of the 44 Ti in Cassiopeia A. This may explain the unexpected lack of correlation between the 44 Ti and iron X-ray emission, the latter being visible only in shock-heated material. The observed spatial distribution rules out symmetric explosions even with a high level of convective mixing, as well as highly asymmetric bipolar explosions resulting from a fast-rotating progenitor. Instead, these observations provide strong evidence for the development of low-mode convective instabilities in core-collapse supernovae.