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Ultracool stars take ‘wild rides’ as they orbit around the Milky Way

Washington, June 10 (ANI): Astronomers have found that stars of a recently discovered type, dubbed ‘ultracool subdwarfs’, take some pretty wild rides as they orbit around the Milky Way, following paths that are very different from those of typical stars.

The finding, by Adam Burgasser and John Bochanski of MIT, clarifies the origins of these peculiar, faint stars, and may provide new details on the types of stars the Milky Way has acquired from other galaxies.

Ultracool subdwarfs were first recognized as a unique class of stars in 2003, and are distinguished by their low temperatures (“ultracool”) and low concentrations of elements other than hydrogen and helium (“subdwarf”).

They sit at the bottom end of the size range for stars, and some are so small that they are closer to the planet-like objects called brown dwarfs.

Only a few dozen ultracool subdwarfs are known today, as they are both very faint – up to 10,000 times fainter than the Sun – and extremely rare.

Burgasser, associate professor of physics at MIT and lead author of the study, was intrigued by the fast motions of ultracool subdwarfs, which zip past the Sun at astonishing speeds.

“Most nearby stars travel more or less in tandem with the Sun tracing circular orbits around the center of the Milky Way once every 250 million years,” he explained.The ultracool subdwarfs, on the other hand, appear to pass us by at very high speeds, up to 500 km/s, or over a million miles per hour.

Burgasser’s team of astronomers assembled measurements of the positions, distances and motions of roughly two dozen of these rare stars.

Robyn Sanderson, co-author and MIT graduate student, then used these measurements to calculate the orbits of the subdwarfs using a numerical code developed to study galaxy collisions.

Despite doing similar calculations for other types of low-mass stars, “these orbits were like nothing I’d ever seen before,” said Sanderson.

Sanderson’s orbit calculations confirm that all of the ultracool subdwarfs are part of the Milky Way’s halo, a widely dispersed population of stars that likely formed in the Milky Way’s distant past.

However, one of the subdwarfs, a star in the constellation Virgo, has an orbit indicating that it might have a very different lineage, possibly extragalactic.

“Our calculations show that this subdwarf travels up to 200,000 light years away from the center of the Galaxy, almost 10 times farther than the Sun,” said Bochanski, a postdoctoral researcher in Burgasser’s group at MIT.

This is farther than many of the Milky Way’s nearest galactic neighbors, suggesting that this particular subdwarf may have originated somewhere else. (ANI)

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