Spectral observations of a supernova just hours after the explosion have revealed the composition of the destroyed progenitor star, indicating that it was a massive Wolf-Rayet star. The work, published in Nature, offers insights into the fate of such stars, which has been a long-standing question in the field of
stellar physics. Wolf-Rayet stars are evolved, massive stars that are losing mass rapidly by means of a very strong stellar wind, the nature of which gives these stars a unique spectral signature. Avishay Gal-Yam (Weizmann Institute of Science) and colleagues of the intermediate Palomar Transient Factory (iPTF) collaboration have detected Wolf-Rayet-like wind signatures in early spectra from a type IIb supernova (named SN 2013cu) obtained around 15 hours after the explosion. This discovery leads the authors to infer that the supernova progenitor was a Wolf-Rayet star. They note that the observations provide clues about the final events that precede the explosion and propose that the extent of this dense wind may indicate increased mass loss from the progenitor shortly before its explosion, consistent with recent theoretical predictions. Wolf-Rayet stars have been suspected to be progenitors of supernovae and this work provides a direct confirmation.
 
Jesper Sollerman and Francesco Taddia at the Department of Astronomy at Stockholm University have contributed to the early observations of SN 2013cu with the acquisition of an optical spectrum at 3 days after explosion. This spectrum, obtained at the Nordic Optical Telescope, was crucial to verify the quick disappearance of the early Wolf-Rayet wind features, previously detected by the Keck telescope.
 
The early discovery of SN 2013cu was possible thanks to the iPTF collaboration, led by Prof. Shri Kulkarni of the California Institute of Technology. iPTF and its precursor PTF have discovered more than 2000 supernovae during their four and a half years of observations by using a robotic observing system mounted on the 48-inch Samuel Oschin Telescope on Palomar Mountain near San Diego,  California. iPTF is a scientific collaboration among the California Institute of Technology, Los Alamos National Laboratory, the University of Wisconsin, Milwaukee, the Oskar Klein Center in Stockholm, the Weizmann Institute of Science, the TANGO Program of the University System of Taiwan, and the Kavli Institute for the Physics and Mathematics of the Universe.
 
Contact
Francesco Taddia (Stockholm University, Astronomy department, Stockholm, Sweden)
Tel: +08-55378511; E-mail: francesco.taddia@astro.su.se