Hypervelocity stars are solitary stars moving fast enough to escape the gravitational grasp of the Milky Way Galaxy. Discovered in 2014, LAMOST-HVS1 is a massive hypervelocity star that’s closer to the Sun than any other of its kind. The star is 8.3 times the mass of the Sun, 4 times hotter and about 3,400 times brighter (if viewed from the same distance). Using one of the Magellan Telescopes in Chile as well as the data from ESA’s star-mapping Gaia satellite, a team of astronomers recreated the trajectory of LAMOST-HVS1. The trajectory shows the star was ejected from the inner stellar disk of the Milky Way 33 million years ago, not the Galactic center as previously believed.
This is an artist’s impression of the hypervelocity star LAMOST-HVS1 speeding away from the visible part of the Milky Way Galaxy. Image credit: Ben Bromley / University of Utah.
Our Milky Way Galaxy contains 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 mph (500 km per second). Less than 30 of these hypervelocity stars have been discovered so far.
“When binary stars 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,” explained University of Michigan astronomer Kohei Hattori and colleagues.
“In order to produce the kinds of velocities we 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 the majority of hypervelocity stars were ejected by this supermassive black hole.”
The researchers were interested in tracing the trajectory of LAMOST-HVS1, which is 42,400 light-years away from Earth and 62,000 light-years from the Milky Way’s center, to pinpoint where in the Galaxy it was ejected.
Using the current location and current velocity of LAMOST-HVS1 derived from one of the Magellan telescopes and Gaia, they were able to trace back its path.
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. We have to consider other possibilities for the origin of the star,” Dr. Hattori said.
Hattori et al recreated the trajectory of LAMOST-HVS1. Image credit: Kohei Hattori / University of Michigan.
The scientists 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.
“Theoretical models for runaway stars that include multiple-massive stars very rarely produce such extreme velocities, suggesting a more exotic possibility — an intermediate mass black hole,” they said.
“The computed path of LAMOST-HVS1 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 it is found, 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 this star may be ejected from a massive cluster in the stellar disk hints at the possibility that many other fast-moving stars may also have been ejected from such star clusters.”
The study was published in the Astrophysical Journal.
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Kohei Hattori et al. 2019. Origin of a Massive Hyper-runaway Subgiant Star LAMOST-HVS1: Implication from Gaia and Follow-up Spectroscopy. ApJ 873, 116; doi: 10.3847/1538-4357/ab05c8
