September 7, 2011
White Dwarfs May Need To Slow Down Before Going Supernova
Older stars may have to slow down their rotations before exploding as supernovae, according to a new study published earlier this month by scientists at the Harvard-Smithsonian Center for Astrophysics (CfA).
In a September 6 press release, the researchers compare the phenomenon to "Speed," a motion picture whose plot revolves around a bus that will explode if it travels too slowly.
According to their findings, which have been published in The Astrophysical Journal Letters, the same thing happens to certain stars, which "might be held up by their rapid spins, and when they slow down, they explode as supernovae."
Furthermore, they claim that there could be thousands of these so-called "time bombs" scattered throughout the galaxy.
"We haven't found one of these 'time bomb' stars yet in the Milky Way, but this research suggests that we've been looking for the wrong signs," astrophysicist Rosanne Di Stefano, one of three authors credited on the study, said in a statement. "Our work points to a new way of searching for supernova precursors."
The specific type of supernovae involved are called Type Ia supernovae, and they occur when the small, dense stars known as white dwarfs destabilize.
"A white dwarf is a stellar remnant that has ceased nuclear fusion," CfA representatives said in their press release. "It typically can weigh up to 1.4 times as much as our Sun--a figure called the Chandrasekhar mass after the astronomer who first calculated it."
"Any heavier, and gravity overwhelms the forces supporting the white dwarf, compacting it and igniting runaway nuclear fusion that blows the star apart," they added, noting that there are two ways that this could happen: "It can accrete gas from a donor star, or two white dwarfs can collide."
Either would result in a Type Ia supernova, but they state that the majority of astronomers believe the first of the two explanations is more likely. However, they say, there should be certain signs if that were the case, such as the detection of hydrogen and helium near the explosion. Those signs are not visible for most Type Ia supernovae, they report.
"Di Stefano and her colleagues suggest that white dwarf spin might solve this puzzle," the researchers say. "A spin-up/spin-down process would introduce a long delay between the time of accretion and the explosion. As a white dwarf gains mass, it also gains angular momentum, which speeds up its spin."
"If the white dwarf rotates fast enough, its spin can help support it, allowing it to cross the 1.4-solar-mass barrier and become a super-Chandrasekhar-mass star," they add. "Once accretion stops, the white dwarf will gradually slow down. Eventually, the spin isn't enough to counteract gravity, leading to a Type Ia supernova."
That spin-down process could mean that as much as a billion years could pass between the time the star stops accreting gas and the time it explodes.
Joining Di Stefano as credited authors on the paper are Rasmus Voss of Radboud University Nijmegen in the Netherlands and J.S.W. Claeys of Universiteit Utrecht, also in The Netherlands. Their research appears in the September 1 issue of The Astrophysical Journal Letters.
Image Caption: New research shows that some old stars known as white dwarfs might be held up by their rapid spins, and when they slow down, they explode as Type Ia supernovae. Thousands of these "time bombs" could be scattered throughout our Galaxy. In this artist's conception, a supernova explosion is about to obliterate an orbiting Saturn-like planet. Credit: David A. Aguilar (CfA)
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