Wolf-Rayet Star Linked to Supernova SN 2013cu

A group of astronomers led by Dr Avishay Gal-Yam from Weizmann Institute of Science has identified a mysterious Wolf-Rayet star as the likely progenitor of SN 2013cu, a Type IIb supernova recently discovered in a distant galaxy known as UGC 9379.

This image from the 1.5-m robotic telescope at Palomar Observatory shows SN 2013cu in the galaxy UGC 9379. Image credit: Avishay Gal-Yam et al.

Wolf-Rayet stars are more than 20 times as massive as our Sun and at least 5 times as hot. Because they are relatively rare and often obscured, astronomers don’t know much about how they form, live and explode.

These stars are notable for having strong stellar winds and being deficient in hydrogen when compared with other stars. Taken together, these two factors give Wolf-Rayet stars easily recognizable stellar signatures.

Astronomers have long wondered whether Wolf-Rayet stars are the progenitors of certain types of stellar explosions – type IIb, Ib or Ic supernovae.

Yet, direct evidence linking such supernovae to their progenitor stars has been missing.

Now, Dr Gal-Yam’s team applied a new technique called flash spectroscopy to identify the likely Wolf-Rayet progenitor of the Type IIb supernova SN 2013cu just over few hours after it exploded.

“Newly developed observational capabilities now enable us to study exploding stars in ways we could only dream of before. We are moving towards real-time studies of supernovae,” said Dr Gal-Yam, who is the first author of a paper published in the journal Nature.

When SN 2013cu exploded in the galaxy UGC 9379 (located in the Bootes constellation, about 360 million light years away), its flash ionized its immediate surroundings, giving the astronomers a direct glimpse of the progenitor star’s chemistry.

This opportunity lasts only for a day before the supernova blast wave sweeps the ionization away. So it’s crucial to rapidly respond to a young supernova discovery to get the flash spectrum in the nick of time.

The astronomers triggered ground- and space-based telescopes to observe the event about 5.7 hours and 15 hours after it self-destructed.

The observations found evidence of composition and shape that aligns with that of a Nitrogen-rich Wolf-Rayet star. What’s more, the progenitor star likely experienced an increased loss of mass shortly before the explosion, which is consistent with model predictions for Wolf-Rayet explosions. These techniques shed fresh light on the poorly understood evolution of massive stars.

“This discovery was totally shocking, it opens up a whole new research area for us. With our largest telescopes you might have a chance of getting a spectrum of a Wolf-Rayet star in the nearest galaxies to our Milky Way, perhaps 4 million light years away. SN 2013cu is 360 million light years away – further by almost factor of 100,” said study co-author Prof Peter Nugent of Lawrence Berkeley National Laboratory.

“When a Wolf-Rayet star goes supernova, the explosion typically overtakes the stellar wind and all information about the progenitor star is gone. We got lucky with SN 2013cu – we caught the supernova before it overtook the wind. Shortly after the star exploded, it let out an ultraviolet flash from the shock wave that heated and lit up the wind. The conditions that we observed in this moment were very similar to what was there before the supernova.”