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The Mystery of the Runaway Star

Astronomers thought they knew why one otherwise ordinary star is shooting away from the Milky Way with a speed that exceeds the galactic escape velocity. Now they’re not so sure

In 1988, the astrophysicist Jack Hills at Los Alamos National Laboratories wondered what might happen if a binary star system were to wander too close to the supermassive black hole at the centre of our galaxy. He reasoned that it ought to be possible for the black hole to swallow one star while sending the other shooting away, like a galactic catapult.

These runaway stars would be no ordinary objects. Hills calculated that they would have velocities exceeding 1000 km/s relative to the Milky Way’s rest frame. That’s more than the escape velocity of the galaxy.

The only trouble was that no astronomer had ever seen such a runaway star. Hills told them to look harder and sure enough, astronomers began to spot otherwise ordinary stars travelling at hypervelocities. Their discovery was a testament to the predictive powers of gravitational theory and a triumph for Hills.

Now there’s a puzzle. Although astronomers have found several hypervelocity runaways they’ve been able to measure the proper motion of only one–HD 271791, a star about 11 times the mass of the sun and the first known to be escaping our galaxy.

Knowing the proper motion is handy because it allows astronomers to trace the trajectory of the star back to its origin.

The trouble is that when researchers do this for HD 271791, it leads to the edge of the galactic disc, more than 3000 light years from the supermassive black hole at the galactic centre. Whichever way you look at it, that’s bad news for Hills’ theory.

So what else can accelerate stars beyond the galactic escape velocity, asks Vasilii Gvaramadze at Moscow State University. There are several possibilities. One idea is that the star might have been torn from the clutches of the Milky Way by the tidal forces associated with a close encounter with satellite galaxy. Gvaramadze says that doesn’t seem likely because there is no sign of such an encounter that could have occurred within the lifetime of HD 271791.

Another possibility (and the one most popular with other astronomers) is that HD 271791 was once part of a binary system whose partner exploded in a supernova. This sent HD 271791 on its current trajectory. But Gvaramadze dismisses this too because HD 271791’s velocity is just too big to have been kick-started in this way.

That leaves one other idea: that HD 271791’s velocity is the result of a much more complex interaction between three or four stars. Gvaramadze suggests that this might have involved two binary systems or a binary system interacting with a giant star some 300 times the size of the sun. The end result of this would have been a catapult effect that ejected HD 271791 at hypervelocities.

Of course, that doesn’t exclude the possibility that there may have been a supernova involved somewhere. Proponents of the supernova idea are likely to say that Gvaramadze has overcomplicated matters.

How to settle the matter? More clues are likely to come from the study of the composition of HD 271791. A nearby supernova would have taken its toll on this star’s make up.

And determining the proper motions of other runaway stars would help to give us a better understanding of this population of extraordinary stars.

Astronomers may even find that some of them do come from the galactic centre, which would give Hills the experimental evidence he needs to confirm his idea, albeit a little later than expected.

Ref: On the Origin of the Hypervelocity Runaway Star HD271791

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