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"Long
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September 06, 2011
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Thousands of “time bomb” stars might dot our galaxy
Sept. 6, 2011
Courtesy of the Harvard-Smithsonian Center for Astrophysics
and World
Science staff
In the Hollywood blockbuster “Speed,” a bomb on a bus is rigged to go off if the bus slows down below 50 miles per hour. The premise—slow down and you explode—makes for a great action movie plot. In this artist's conception, a supernova explosion is about to obliterate an orbiting Saturn-like planet.
(Credit: David A. Aguilar (CfA))
It also happens to have a cosmic equivalent, some astronomers say.
New research indicates some old stars might be held together only by their rapid spins—so when they slow down, they explode. Thousands of these “time bombs” could be scattered throughout our galaxy, astronomers claim.
“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. Our work points to a new way of searching”
for them, said astrophysicist Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
Di Stefano and colleagues studied a type of stellar explosion called a Type
1a supernova. It occurs when an old, compact star known as a white dwarf becomes unstable. A white dwarf is a stellar remnant that has stopped nuclear fusion, the process that powers active stars. A white dwarf can weigh up to 1.4 times as much as our Sun; any heavier, and it would start to shrink rapidly under its own gravity, triggering runaway nuclear fusion that next blows it up.
There are theoretically two ways a white dwarf can exceed the weight limit and explode, or go
“supernova.” It can accumulate gas from a donor star, or two white dwarfs can collide. Most astronomers favor the first scenario as the more typical explanation. But we’d expect to see certain signs if that is correct, and we don’t for most cases, Di Stefano and colleagues say.
For example, we should detect small amounts of hydrogen and helium gas near the explosion, but we don’t. That gas would come from matter that wasn’t accumulated by the white dwarf, or from the disruption of the companion star in the explosion. Astronomers also have looked for the donor star after the supernova
faded from sight, without success.
Di Stefano and colleagues suggest white dwarf spin might solve this puzzle. A process of spinning up, then spinning down, could create a long delay between the gas accumulation and the explosion. As a white dwarf gets heavier, it also spins faster. If it rotates fast enough, its spin can help support its structure, allowing it to cross the 1.4-solar-mass barrier. Then it would be called a “super-Chandrasekhar-mass star” (the 1.4 limit is called the Chandrasekhar mass after the astronomer who first calculated it.)
Once gas accumulation stops, the white dwarf should gradually slow down. Eventually, the spin isn’t enough to counteract gravity, leading to
the supernova. “Our work is new because we show that spin-up and spin-down of the white dwarf have important consequences,” explained Di Stefano.
The whole process could produce a delay of up to a billion years between the end of gas accumulation, or accretion, and the explosion, Di Stefano said. This would allow the companion star to age and evolve into a second white dwarf, and surrounding
gas to dissipate.
In our galaxy, scientists estimate there are three Type 1a supernovae every thousand years. If a typical super-Chandrasekhar-mass white dwarf takes millions of years to spin down and explode, then calculations suggest that there should be dozens of pre-explosion systems within a few thousand light-years of Earth. Those
“supernova precursors” will be hard to find, but upcoming surveys should be able to spot them, Di Stefano said. “We’re looking forward to hunting them out,” added study co-author Rasmus Voss of Radboud University Nijmegen, The Netherlands.
This research appears in a paper in the Sept. 1 issue of the journal
Astrophysical Journal Letters.
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In the Hollywood blockbuster “Speed,” a bomb on a bus is rigged to go off if the bus slows down below 50 miles per hour. The premise—slow down and you explode—makes for a great action movie plot. It also happens to have a cosmic equivalent, some astronomers say. New research indicates some old stars might be held together only by their rapid spins—so when they slow down, they explode. Thousands of these “time bombs” could be scattered throughout our galaxy, astronomers claim. “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. Our work points to a new way of searching for” them, said astrophysicist Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Di Stefano and colleagues studied a type of stellar explosion called a Type Ia supernova. It occurs when an old, compact star known as a white dwarf becomes unstable. A white dwarf is a stellar remnant that has stopped nuclear fusion, the process that powers active stars. A white dwarf can weigh up to 1.4 times as much as our Sun; any heavier, and it would start to shrink rapidly under its own gravity, triggering runaway nuclear fusion that next blows it up. There are theoretically two ways a white dwarf can exceed the weight limit and explode, or go “supernova.” It can accumulate gas from a donor star, or two white dwarfs can collide. Most astronomers favor the first scenario as the more typical explanation. But we’d expect to see certain signs if that is correct, and we don’t for most cases, Di Stefano and colleagues say. For example, we should detect small amounts of hydrogen and helium gas near the explosion, but we don’t. That gas would come from matter that wasn’t accumulated by the white dwarf, or from the disruption of the companion star in the explosion. Astronomers also have looked for the donor star after the supernova faded from sight, without success. Di Stefano and colleagues suggest white dwarf spin might solve this puzzle. A process of spinning up, then spinning down, could create a long delay between the gas accumulation and the explosion. As a white dwarf gets heavier, it also spins faster. If it rotates fast enough, its spin can help support its structure, allowing it to cross the 1.4-solar-mass barrier. Then it would be called a “super-Chandrasekhar-mass star” (the 1.4 limit is called the Chandrasekhar mass after the astronomer who first calculated it.) Once gas accumulation stops, the white dwarf should gradually slow down. Eventually, the spin isn’t enough to counteract gravity, leading to a Type Ia supernova. “Our work is new because we show that spin-up and spin-down of the white dwarf have important consequences. Astronomers therefore must take angular momentum of accreting white dwarfs seriously, even though it’s very difficult science,” explained Di Stefano. Angular momentum is a form of momentum associated with the spin of an object. The spin-down process could produce a delay of up to a billion years between the end of gas accumulation, or accretion, and the explosion, Di Stefano said. This would allow the companion star to age and evolve into a second white dwarf, and any surrounding material to dissipate. In our Galaxy, scientists estimate there are three Type Ia supernovae every thousand years. If a typical super-Chandrasekhar-mass white dwarf takes millions of years to spin down and explode, then calculations suggest that there should be dozens of pre-explosion systems within a few thousand light-years of Earth. Those “supernova precursors” will be hard to find, but upcoming surveys should be able to spot them, Di Stefano said. “We’re looking forward to hunting them out,” added study co-author Rasmus Voss of Radboud University Nijmegen, The Netherlands. This research appears in a paper in the Sept. 1 issue of the journal Astrophysical Journal Letters.