Astronomers have spotted white dwarfs moving faster than any free-moving star seen before – so fast they must have been launched by supernovae.
Two white dwarfs rocketing through space are the fastest free-moving stars that we know about and they could explain how some supernovae form.
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| The white dwarf on the right is stealing matter from a neighbouring star. It will eventually explode as a type Ia supernova, propelling its companion away NASA/JPL-Caltech |
Type Ia supernovae are so reliably bright that astronomers use them as measuring sticks for assessing how far away distant stars and galaxies are. These supernovae tend to occur when a white dwarf siphons off matter from a neighbouring star and starts to swell, until it eventually becomes massive enough to explode.
But simulations have shown that it is possible for these supernova explosions to occur when two white dwarfs spiral towards each other and collide. White dwarfs are so small and dense that they can orbit each other at close range faster than any other star type, so that when one of a pair explodes, the other is launched through space at high speed.
Evidence for supernovae formed this way has proven elusive, but Kareem El-Badry at Harvard University and his colleagues have now used the Gaia star catalogue to identify four apparent white dwarfs that are travelling fast enough to provide evidence. Data from Gaia helps do this because the space observatory precisely measures a star’s position, distance from us and motion, known as astrometry.
The four white dwarfs are moving at more than 1000 kilometres per second. “If you find a white dwarf going that fast, it’s basically guaranteed that it came from some white dwarf binary where one exploded. There’s just no other conceivable channel we can think of to make something go that fast,” says El-Badry.
One of these stars is travelling at 2285 km/s and another at 1694 km/s, which are the highest velocities for free-moving stars ever recorded, although stars orbiting a black hole have been found to go faster. All four stars seem to be made almost entirely of carbon and oxygen, which is another predicted hallmark of stars shot from a supernova, as this process strips away helium and hydrogen from the stellar surface.
El-Badry and his team estimate that supernovae generated in such collisions might make up half of all type Ias, but they will need to locate more fast-moving white dwarfs to be certain, he says.
Finding these supernova remnants is no easy task, says Sarah Casewell at the University of Leicester, UK. “It’s very hard to chase down what might have escaped from a type Ia supernova,” she says. “The fact that Gaia provides us with so much astrometry has really made this happen.”
These stars are also moving fast enough to escape the Milky Way’s gravitational pull. In a few hundred million years, they will probably leave our galaxy entirely, says El-Badry. Depending on how often supernovae kick out fast-moving stars, we might even be able to see one arriving in the Milky Way from another galaxy. “It’s basically the only reasonable prospect we have of observing stars from other galaxies,” he says.
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