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Hypervelocal stars are solitary stars that move fast enough to escape the gravitational hold of the Milky Way galaxy. Discovered in 2014, LAMOST-HVS1 is a huge hypervelocity star closer to the Sun than any other of its kind. The star is 8.3 times the mass of the sun, 4 times warmer and about 3400 times brighter (if viewed from the same distance). Using one of the Magellan telescopes in Chile as well as data from Gaia's ESA star mapping satellite, a team of astronomers has reproduced the trajectory of LAMOST-HVS1. The trajectory shows that the star was ejected from the Milky Way's internal stellar disk 33 million years ago, and not from the Galactic center as previously thought.
It is an artistic impression of the star of hypervitesse LAMOST-HVS1 which is rapidly moving away from the visible part of the Milky Way. Image Credit: Ben Bromley / University of Utah.
Our Milky Way contains billions of stars, most of which are spread in a pizza-shaped structure called a stellar disk.
In 2005, astronomers discovered for the first time fast stars that move more than twice as fast as most other stars – more than 1 million mph (500 km / s). Until now, less than 30 stars of hypervelocity have been discovered.
"When the binary stars pass too close to a black hole, one of the binary stars is captured, and the other is projected in slingshot," explained astronomer Kohei Hattori of the University of Michigan and his colleagues.
"In order to produce the types of speeds we measure for hypervelocity stars, the black hole must be very massive."
"Because it is proven that there is a supermassive hole in the center of the Milky Way, many astronomers believe that the majority of hypervelocity stars have been ejected by this supermassive black hole."
Researchers were interested in tracing the trajectory of LAMOST-HVS1, 42,400 light-years away from Earth and 62,000 light-years away from the center of the Milky Way, to locate its ejection into the galaxy.
Using the current location and speed of LAMOST-HVS1 derived from one of the Magellan and Gaia telescopes, they were able to retrace their path.
To their surprise, it seems that the star was ejected from the star disk and not from the center of the Milky Way.
"We thought this star was coming from the galactic center. But if you look at its trajectory, it's clear that it's not related to the galactic center. We must consider other possibilities for the origin of the star, "said Dr. Hattori.
Hattori et al recreated the trajectory of LAMOST-HVS1. Image credit: Kohei Hattori / University of Michigan.
Scientists assume that the ejection of this massive star from the stellar disk could be the result of a close encounter between several massive stars or a black hole of intermediate mass in a cluster of stars.
"Theoretical models for fugitive stars that include multi-mass stars very rarely produce such extreme velocities, suggesting a more exotic possibility – a black hole of intermediate mass," they said.
"The calculated path of LAMOST-HVS1 originates from a location in Norma's spiral arm that is not associated with the massive star clusters previously known."
"However, if this hypothetical star group exists, it can be hidden behind the dust in the star disk. If found, this will be the first opportunity to directly discover a black hole of intermediate mass in the stellar disk of the Milky Way. "
"In addition, the fact that this star can be ejected from a massive group in the star disk suggests the possibility that many other fast-moving stars might also have been ejected from these star groups."
The study was published in the Astrophysical Journal.
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Kohei Hattori et al. 2019. LAMOST-HVS1: Gaia involvement and tracking spectroscopy. ApJ 873, 116; doi: 10.3847 / 1538-4357 / ab05c8
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