New technique reveals supernova progenitor

A star in a distant galaxy explodes as a supernova: While observing a galaxy known as UGC 9379 (left; image from the Sloan digital Sky Survey) located about 350 million light-years from us, the team discovered a new source of bright blue light (right; image from the 60″ robotic telescope at Palomar Observatory). This white-hot young supernova marked the explosive death of a massive star in that distant galaxy. A detailed study of the spectrum (the distribution of colors composing the light from the supernova) using a technique called “flash spectroscopy” revealed the signatures of a wind blown by the aging star just prior to its terminal explosion, and allowed to determine what elements were abundant on the surface of the dying star as it was about to explode as a supernova, providing important information about how massive stars evolve just prior to their death and the origin of crucial elements such as carbon, nitrogen, and oxygen.

Avishay Gal-Yam of the Weizmann Institute of Science

Wolf-Rayet stars are very large and very hot. Astronomers have long wondered whether they are the progenitors of certain types of supernovae. New work from the Palomar Transient Factory (PTF) team, including Carnegie’s Mansi Kasliwal, is homing in on the answer. They have identified a Wolf-Rayet star as the likely progenitor of a recently exploded supernova.

Wolf-Rayet 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.

It is thought that Wolf-Rayet stars explode as type IIb, Ib, or Ic supernovae. Yet direct evidence linking these types of supernovae to their progenitor stars has heretofore been missing.

The team, led by Avishay Gal-Yam of the Weizmann Institute of Science in Israel, applied a novel observational method called flash spectroscopy to identify the likely progenitor of a type IIb supernova called SN 2013cu just over 15 hours after it exploded.

“This supernova was discovered by the Palomar 48-inch telescope in California. The on-duty PTF 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,” Kasliwal said. “The global rapid response protocol ensures the Sun never rises for the PTF team!”

When the supernova exploded, 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 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.

Previously when looking for a pre-explosion star using the Hubble Space Telescope, astronomers could only look over a range of about 20 megaparsecs. But using these new tools they can increase that distance by a factor of five, allowing them to identify many more supernova progenitors.

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