A Wolf–Rayet-like progenitor of SN 2013cu from spectral observations of a stellar wind
The explosive fate of massive Wolf–Rayet stars^1 (WRSs) is a key open question in stellar physics. An appealing option is that hydrogen-deficient WRSs are the progenitors of some hydrogen-poor supernova explosions of types IIb, Ib and Ic (ref. 2). A blue object, having luminosity and colours consistent with those of some WRSs, has recently been identified in pre-explosion images at the location of a supernova of type Ib (ref. 3), but has not yet been conclusively determined to have been the progenitor. Similar work has so far only resulted in non-detections^4. Comparison of early photometric observations of type Ic supernovae with theoretical models suggests that the progenitor stars had radii of less than 1012 centimetres, as expected for some WRSs^5. The signature of WRSs, their emission line spectra, cannot be probed by such studies. Here we report the detection of strong emission lines in a spectrum of type IIb supernova 2013cu (iPTF13ast) obtained approximately 15.5 hours after explosion (by 'flash spectroscopy', which captures the effects of the supernova explosion shock breakout flash on material surrounding the progenitor star). We identify Wolf–Rayet-like wind signatures, suggesting a progenitor of the WN(h) subclass (those WRSs with winds dominated by helium and nitrogen, with traces of hydrogen). The extent of this dense wind may indicate increased mass loss from the progenitor shortly before its explosion, consistent with recent theoretical predictions^6.
Additional Information© 2014 Macmillan Publishers Limited. All Rights Reserved. Received 23 October 2013 Accepted 25 March 2014 Published online 21 May 2014. This research was supported by the I-CORE programme 'The Quantum Universe' of the Planning and Budgeting Committee and The Israel Science Foundation. A.G.-Y. acknowledges support by grants from the ISF, BSF, GIF, Minerva and FP7/ERC, and a Kimmel Investigator award. M.M.K. acknowledges support from Hubble and Carnegie-Princeton fellowships. E.O.O. acknowledges the Arye Dissentshik Career Development Chair and a grant from the Israeli MOST. J.C.W. is supported in part by the NSF. J.M.S. is supported by an NSF Postdoctoral Fellowship. A.V.F. acknowledges financial support from the TABASGO Foundation, the Richard and Rhoda Goldman Fund, the Christopher R. Redlich Fund and the NSF. The National Energy Research Scientific Computing Center, supported by the Office of Science of the US Department of Energy, provided staff, computational resources and data storage for this project. The Oskar Klein Centre is funded by the Swedish Research Council. We thank K. Clubb, O. Fox, P. Kelly, S. Tang and B. Sesar for their help with observations, and J. Groh, P. Crowther, M. Bersten, C. Fransson and E. Nakar for advice. Some of the data presented here were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA. The observatory was made possible by the generous financial support of the W. M. Keck Foundation.
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