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Astronomers connect Wolf-Rayet star to supernova

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On May 21, a team of astronomers led by Avishay Gal-Yam of the Weizmann Institute of Science in Israel announced that they have observationally linked a supernova to a Wolf-Rayet star.

A Wolf-Rayet star is a star with more than 20 solar masses that has evolved out of the main sequence. They are characterized by strong stellar winds and a relative lack of hydrogen content compared to other stars. This gives such stars a distinct emission spectrum with bright lines of helium emission coupled with either nitrogen emission (WN sequence) or carbon/oxygen emission (WC sequence).

The observed type IIb supernova, named SN 2013cu, was observed by the Palomar 48-inch telescope in California.

“This supernova was discovered by the Palomar 48-inch telescope in California. The on-duty Palomar Transient Factory team member in Israel promptly sounded an alert about this supernova discovery, enabling another PTF team member to get a spectrum with the Keck Telescope before the Sun rose in Hawaii,” said Carnegie's Mansi Kasliwal, a member of the PTF team. “The global rapid response protocol ensures the Sun never rises for the PTF team!”

The likely progenitor was found about 15 hours after the explosion, using an observational method called flash spectroscopy. Flash spectroscopy, also known as transient-absorption spectroscopy, is an extension of absorption spectroscopy in which the absorbence of wavelengths is measured as a function of time after excitation by a flash of light, as in a supernova explosion. This technique allows astronomers to look at stars out to 100 megaparsecs, about five times further away than they could previously observe.

“When the supernova exploded, it 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 supernovablast 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 Carnegie Institution for Science wrote in a statement. “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.”

The finding is significant because it is the first direct evidence that Wolf-Rayet stars go supernova. While it was theorized that Wolf-Rayet stars explode as type IIb, Ib, or Ic supernovae, it had yet to be confirmed until now. As a result, a rival theory that such stars might have dim or unobservable deaths, much as white dwarfs do, was gaining traction.

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