PASADENA, Calif.- An international team of astronomers at the California Institute of Technology, University of Toronto, and Lawrence Berkeley National Laboratory have discovered a supernova more massive than previously believed possible. This has experts rethinking their basic understanding of how stars explode as supernovae, according to a paper to be published in Nature on September 21.
The lead author of the study, University of Toronto postdoctoral researcher Andy Howell, identified a Type Ia supernova, named SNLS-03D3bb, in a distant galaxy 4 billion light years away that originated from a dense evolved star, termed a “white dwarf,” whose mass is far larger than any previous example. Type Ia supernovae are thermonuclear explosions that destroy white dwarfs when they accrete matter from a companion star.
The discovery was made possible through images taken as part of a long-term survey for distant supernovae with the Canada France Hawaii Telescope. Follow-up spectroscopy led by Richard Ellis, Steele Family Professor of Astronomy at Caltech, with the 10-meter Keck Telescope was key to determining the unusually high mass of the new event.
Researchers say the surprisingly high mass of SNLS-03D3bb has opened up a Pandora’s box on the current understanding of Type Ia supernovae and, in particular, how well they might be used for future precision tests of the nature of the mysterious “dark energy” responsible for the acceleration of the cosmic expansion.
Current understanding is that Type Ia supernova explosions occur when the mass of a white dwarf approaches 1.4 solar masses, or the “Chandrasekhar limit.” This important limit was calculated by Nobel laureate Subramanyan Chandrasekhar in 1930, and is founded on well-established physical laws. Decades of astrophysical research have been based upon the theory. Yet somehow the star that exploded as SNLS-03D3bb reached about two solar masses before exploding.
“It should not be possible to break this limit,” says Howell, “but nature has found a way! So now we have to figure out how nature did it.”
In a separate “News & Views” article on the research in the same issue of Nature, University of Oklahoma professor David Branch has dubbed this the “Champagne Supernova,” since extreme explosions that offer new insight into the inner workings of supernovae are an obvious cause for celebration.
The team speculates that there are at least two possible explanations for how this white dwarf got so fat before it went supernova. One is that the original star was rotating so fast that centrifugal force kept gravity from crushing it at the usual limit. Another is that the blast was in fact the result of two white dwarfs merging, and that the body was only briefly more massive than the Chandrasekhar limit before exploding.
Since Type Ia supernovae usually have about the same brightness, they can be used to map distances in the universe. In 1998 they were used to make the surprising discovery that the expansion of the universe is accelerating. Although the authors are confident that the discovery of a supernova that doesn’t follow the rules does not undermine this result, it will make them more cautious about using them to measure distance in the future.
Ellis summarizes: “This is a remarkable discovery that in no way detracts from the beautiful results obtained so far by many teams, which convincingly demonstrate the cosmic acceleration and hence the need for dark energy. However, what it does show is that we have much more to learn about supernovae if we want to use them with the necessary precision in the future. This study is an important step forward in this regard.”
Peter Nugent, a staff scientist with the of the scientific computing group at Lawrence Berkeley National Laboratory, is a co-author of the Nature paper.
