A Superfast Star from Far, Far Away

The Magellanic Clouds
The Large and Small Magellanic Clouds are about 165,000 and 200,000 light-years distant, respectively. They're dimly visible to the naked eye in the south circumpolar sky. The Large Magellanic Cloud (LMC) in particular contains several sites of furious ongoing star formation.
Akira Fujii
From huge new databases such as the Sloan Digital Sky Survey, astronomers have fished up 10 stars that are moving so fast (in terms of radial velocity: speed toward or away from us, as measured by Doppler shift) that they're destined to escape clean out of our Milky Way galaxy.

What could have set them going so fast? The only mechanism that really works well enough involves a fast-orbiting binary star making a close pass by the supermassive black hole at the Milky Way's center. The tidal effect of the hole's strong gravity could pull the binary apart and send one of the two stars flying off at more than 1,000 km per second, while swallowing the other. (Technical details.) This should happen about once every 100,000 years on average, given the density of stars in the Milky Way's center, and that's enough to account for the number of hypervelocity stars that are being discovered.

But one of those 10 stars poses a problem for this idea. It's so far from the Milky Way's center that, even at its fast clip, it would have taken 100 million years to get there. However, the star is young, judging by its spectral type and the fast rate it's using up its nuclear fuel. It can't even have been born more than about 35 million years ago.

The discoverers proposed in 2005 that this star, named HE 0437-5439, came not from the Milky Way but from our neighboring dwarf galaxy the Large Magellanic Cloud, which the star is actually much closer to. Now a team of astronomers from the Carnegie Institution and Queens University, Belfast, has found more evidence. The team studied the star's spectrum in new detail and found that, like the stars of the Large Magellanic Cloud, it has about half the heavy-element abundance of young stars born in the Milky Way.

Moreover, they confirmed that the star is a fast rotator, a sign that it may once have been part of a close binary. The binary could have been torn asunder and the star flung off by an encounter with a black hole of only 1,000 solar masses.

"This is the first observational clue that a massive black hole exists somewhere in the LMC," says lead author Alcestes Bonanos. "We look forward to finding out where this black hole might be."

Such an "intermediate mass" black hole would be far smaller than the one in the center of the Milky Way, which is now thought to have about 3.5 million solar masses. So it's plausible that it would have given no other sign until now.

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