Don't Slow Down White Dwarf, You Might Explode


Astrophysicists believe there are two ways to trigger a Type 1a supernova. The first is when a white dwarf “steals” gas from a binary partner, grows to a certain mass limit and then explodes. The second is when two white dwarfs collide (as Larry O’Hanlon calls them, “loving dwarfs”), the mass limit is immediately exceeded and they explode.

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The debate as to which mechanism is most common has raged for decades, and only recently, it was thought there were more “loving dwarfs” than “thieving dwarfs” accreting matter from binary, sun-like partner stars.

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(In a nutshell, astronomers haven’t detected hydrogen and helium in the Type 1a supernova signatures, suggesting there was no accretion disk. Also, the lack of an X-ray “fuse” hints that no material is being accreted before detonation. Therefore the colliding scenario must be right, regardless of how “unlikely” it may seem.)

This finding didn’t sit well as many astrophysicists believed there should be more “thieving dwarfs” causing Type 1a supernovae.

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However, as announced by Harvard-Smithsonian Center for Astrophysics (CfA) researchers today, the thieving scenario might be right after all. And it’s all to do with how fast white dwarfs spin.

The research is published in the Sept. 1 issue of The Astrophysical Journal Letters.

The logic works like this: In the “thieving white dwarf scenario,” as a spinning white dwarf steals matter from a neighboring sun-like star, it will gain angular momentum — basically, its spin will speed up as it gains mass.

Usually, if the mass of a white dwarf star exceeds the Chandrasekhar mass limit — approximately 1.4 solar masses — a Type 1a supernova will occur. However, the CfA researchers have found that as the white dwarf is fed, its increased angular momentum can keep the white dwarf stable even if its mass exceeds the Chandrasekhar limit.

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As noted in the CfA press release, the white dwarf becomes a beefed-up, fast-spinning, super-Chandrasekhar mass white dwarf.

But there’s a catch. Once the supply of material accreting onto the thieving white dwarf is cut off, its spin will gradually slow down, meaning its angular momentum will decrease. At a certain spin speed, the star will destabilize and explode as a (delayed) Type 1a supernova. Although the white dwarf borrowed more time by spinning fast, once its supply is diminished, the death clock starts ticking.

This is good news for white dwarf fast-spinners, the time between accretion cut-off and supernova could be up to a billion years. And here’s the kicker: it may explain why there is no accretion on the white dwarf before it goes supernova; the star that it stole the matter has itself turned into a white dwarf star — the accreting matter is long gone.

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“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 supernova precursors,” said CfA astrophysicist Rosanne Di Stefano, lead scientist of this research.

I suppose the movie title “The Quick and the Dead” could be very apt for white dwarf stars. Or, perhaps: “The Quick Die Later.”

Image credit: David A. Aguilar (CfA)

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