Scientists at UC Berkeley have found a new breed of interstellar explosion — just as UCSB astrophysicists had predicted.
Dovi Poznanski, a postdoctoral fellow at UC Berkeley, discovered the possible occurrence of a new type of supernova — a bright, violent blast that results from the death of certain stars — in telescope data from 2002. Scientists at UCSB’s Kavli Institute for Theoretical Physics had predicted such an explosion from theoretical calculations a few years prior.
The event itself was dubbed an “.Ia” supernova by Chris Stubbs, a professor at Harvard University, as a play on the more common Ia category of supernovae, due to the .Ia being a tenth as bright and as lasting as Ia supernovae.
The initial theory was proposed by Lars Bildsten, a professor at KITP, and his fellow researchers. According to Ken Shen, a UCSB graduate student who collaborated on the paper, supernovae are categorized based on what pathway the dying star takes as it produces the observable explosion. This new type of supernovae did not fit into existing categorizations because it is believed to follow a process that is powered by a completely different material component.
“There are two main types of supernovae. One involves the collapse of a very large star, and the other — the type that we are concerned with — involves nuclear fusion, which produces the energy that we see [as light],” Shen said. “Typically, these supernovae, classified as Ia, produce light from the fusion of carbon nuclei. The ‘.Ia’ supernova, [the one we theorized] is the result of a unique process that leads to helium acting as the fuel for this nuclear fusion rather than the carbon that would be typical of an Ia explosion.”
Bildsten said the .Ia supernovae were not the result of a single collapsing star, but rather the collapse of a binary star system, where a pair of stars orbit each other. The gravity of one star is great enough to pull the helium off of the other star, which eventually causes the larger star to reach critical mass and explode.
“[It’s caused] by a special kind of binary. One star, a white dwarf, is nearly pure helium, has about the radius of the earth, and is orbiting so closely to the other more massive star that helium is pulled off onto the surface of this larger star,” Bildsten said. “This accumulated ‘ocean’ of helium is where the explosion occurs, ejecting all the accumulated matter.”
In terms of what observers see, the .Ia supernova is distinguished from other types of supernova in duration and intensity, being both shorter by a matter of months and only a tenth as bright as an Ia supernova. Bildsten said this occurs since the amount of matter involved in a .Ia supernova is far smaller than that of an Ia.
“It’s a much smaller amount of matter [that is involved in the phenomena,] the layer that explodes is about 10 percent of the mass of the sun,” Bildsten said. “In a typical Ia supernova, the exploding layer is about 150 percent the mass of the sun.”
Shen said he was not terribly surprised that the theorized supernova was found in data from older, unrelated projects.
“From the numbers we’ve calculated, we expect to see about one of these events per year in new surveys,” Shen said. “It’s not tremendously surprising that one of these events was hiding in old observational data.”
However, Shen said this should not create the impression that such events are common. High-powered telescopes can see the supernovae frequently because of the size of the observable universe, but such explosions occur far less often than normal supernovae.
“The Milky Way would get around one regular supernova a century,” Shen said. “But only one of this new type every 30 centuries.”
The paper containing the theory was published in 2007, and Bildsten said that for two years, there was no known instance of the type of supernova the theory described.
“When we showed our predictions to observers and asked them if they had seen anything like this, their answer was ‘no,'” Bildsten said.
Shen said he expects that many more examples of the event will be found in the near future.
“We only have one event, and we are reluctant to try to describe any greater implications it might have,” Shen said. “However, because of the number of supernova surveys, we are going to find many more in the next few years.”