Supernova Progenitor Revealed
May 22, 2014

Exploding Wolf-Rayet Stars Found To Be Responsible For Type IIb Supernovae

redOrbit Staff & Wire Reports - Your Universe Online

For the first time ever, astronomers have directly confirmed that a rare and extremely massive type of star known as a Wolf-Rayet star died in a violent explosion known as a Type IIb supernova, and their observations will help scientists better understand the life and death of the progenitor Wolf-Rayet.

While our sun is 330,000 times more massive than Earth, comprises 99.86 percent of the total mass of the Solar System, and has a surface temperature of approximately 10,000 degrees Celsius, Wolf-Rayet stars are even more impressive. They are 20 times more massive than the Sun and at least five times as hot.

However, Wolf-Rayet stars are relatively rare and are typically obscured, meaning that scientists are limited in their knowledge of how they form and their life-death cycle. However, thanks to the Palomar Transient Factory (iPTF) sky survey and instruments located at the Lawrence Berkeley National Laboratory, that is slowly starting to change.

Using the iPTF and the Berkeley Lab instruments, Prof. Avishay Gal-Yam of the Weizmann Institute of Science in Israel and his colleagues were able to observe supernova SN 2013cu just a few hours after its explosion. They then turned to ground and space-based telescopes to observe the event about 5.7 hours and 15 hours after self-destruction, allowing then to confirm SN 2013cu’s progenitor.

In a paper published online in the journal Nature on Wednesday, the study authors explained that they used a method known as flash spectroscopy, and that they were able to identify the mix of elements released just prior to the explosion so that they could identify the star most likely responsible for the supernova.

The star can be identified by the proportion of elements such as carbon, nitrogen and oxygen that are produced in the nuclear fusion that powers the stars and can be detected in the material that is ejected into space during the supernova process. The opportunity to observe the progenitor star’s chemistry lasted only about one day before the blast wave from the supernova swept away the flash ionization caused by the explosion.

Based on their observations, Prof. Gal-Yam’s team determined that the composition and shape of that progenitor matched that of a nitrogen-rich Wolf-Rayet star, and that the star most likely experienced an increased loss of mass shortly before the actual explosion occurred. This discovery is consistent with simulations of Wolf-Rayet explosions, they explained, and shed new light on the mysteries of the massive star evolutionary process.

“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 Gal-Yam in a statement.

“This is the smoking gun. For the first time, we can directly point to an observation and say that this type of Wolf-Rayet star leads to this kind of Type IIb supernova,” added co-author Peter Nugent, the head of the Berkeley Lab Computational Cosmology Center (C3). “This discovery was totally shocking, it opens up a whole new research area for us.”

Nugent noted that the explosion that occurs when a Wolf-Rayet star goes supernova usually overtakes the stellar wind, resulting in the loss of all the information pertaining to the progenitor star. However, he said that he and his colleagues were fortunate enough to catch SN 2013cu before the supernova overtook the solar wind. The post-explosion ultraviolet flash heated and lit up the wind, allowing the scientists to observe conditions extremely similar to those present just before the supernova occurred.

After making the post-explosion observations, the research team started working backwards. Their analysis of the recorded spectra demonstrated for the first time that SN 2013cu’s progenitor star possessed a nitrogen-rich wind, similar to the Wolf-Rayet stars that have previously been studied in our own galaxy.

“When I identified the first example of a Type IIb supernova in 1987, I dreamed that someday we would have direct evidence of what kind of star exploded. It's refreshing that we can now say that Wolf-Rayet stars are responsible, at least in some cases,” added co-author and University of California, Berkeley astronomy professor Alex Filippenko.