Hypervelocity, or fast-moving, stars may have originated from the Milky Way’s stellar disk, and not from the middle of the galaxy as astronomers previously believed, according to new research.

“This discovery dramatically changes our view on the origin of fast-moving stars,” says Monica Valluri, a research professor in the astronomy department at the University of Michigan.

“The fact that the trajectory of this massive fast-moving star originates in the disk rather that at the galactic center indicates that the very extreme environments needed to eject fast-moving stars can arise in places other than around supermassive black holes.”

Producing a fast-moving star requires lots of energy, usually in extreme environments, Valluri says.

The Milky Way contains tens of billions of stars, most of which are distributed in a pizza-like structure called the stellar disk. In 2005, astronomers first discovered fast-moving stars that move more than twice as fast as most other stars—more than 1 million miles per hour, or 500 kilometers per second (310 miles/second), compared to the rest of the galaxy where stars average a bit more than 200 km/sec (124 miles/second).

Less than 30 of these extremely fast-moving stars (generally called “hypervelocity stars”) have been discovered so far.

Flung from the ‘gravitational slingshot’

When binary stars—a pair of stars that orbit around each other while moving through a galaxy—pass too close to a black hole, it captures one of the binary stars, and the other one is flung out in a “gravitational slingshot.” In order to produce the kinds of velocities astronomers measure for hypervelocity stars, the black hole has to be very massive.

Because there’s evidence that there is a supermassive hole at the center of the Milky Way, many astronomers believe that this supermassive black hole ejected the majority of hypervelocity stars.

Valluri and postdoctoral researcher Kohei Hattori were interested in tracing the trajectory of LAMOST-HVS1, a massive fast-moving star that’s closer to the sun than any other hypervelocity stars, to pinpoint where in the Milky Way it was ejected. They used one of the Magellan telescopes in Chile to determine the distance and velocity of the star.

Hattori then joined a group of international scientists in New York last year to participate in a hackathon to download, share, and analyze data from the European Space Agency space mission Gaia, a space astrometry mission to make the largest, most precise three-dimensional map of the Milky Way.

Using the current location and current velocity of the star from Gaia and Magellan, the astronomers were able to trace back its path, or orbit. To their surprise, it appears the star was ejected from the stellar disk, and not from the center of the Milky Way.

“We thought this star came from the Galactic center. But if you look at its trajectory, it is clear that is not related to the Galactic center,” Hattori says. “We have to consider other possibilities for the origin of the star.”

Evidence of a black hole?

The authors theorize that the ejection of this massive star from the stellar disk may be the result of the star experiencing a close encounter with multiple massive stars or an intermediate mass black hole in a star cluster.

Although massive runaway stars that have been ejected from star clusters with speeds of 40-100 km/s (25-62 miles/second) have been known for a long time, none have been observed with the extreme ejection velocity as needed to explain LAMOST-HVS1. Theoretical models for runaway stars that include multiple-massive stars also very rarely produce such extreme velocities, suggesting a more exotic possibility—an intermediate mass black hole.

The computed path of the star originates at a location in the Norma spiral arm that is not associated with previously known massive star clusters. However if this hypothetical star cluster exists, it may be hidden behind the dust in the stellar disk. If scientists can find it, it would provide the first opportunity to directly discover an intermediate mass black hole in the stellar disk of the Milky Way.

Also, the fact that a massive cluster in the stellar disk may have ejected this star hints at the possibility that such star clusters may also have ejected many other fast-moving stars, the researchers say.

Both the Milky Way and Large Magellanic Cloud (a separate small galaxy, orbiting the Milky Way) have some massive star clusters that might be important ejectors of fast-moving stars, contrary to the widely accepted view that interactions with the central black holes in one of these galaxies ejected them.

This would also lead to new insights into the interactions of stars and the possible formation of intermediate mass black holes in star clusters, the researchers say.

The findings appear in the Astrophysical Journal. Additional coauthors are from University of Michigan; the Leibniz Institute for Astrophysics in Potsdam, Germany; the Joint Institute for Nuclear Astrophysics-Center for the Evolution of the Elements; and the University of Chicago.

Source: University of Michigan

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