Using data from the Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST) and ESA’s star-mapping satellite Gaia, astronomers have discovered 591 new high-velocity stars in the halo of our Milky Way Galaxy.

The positions and orbits of the newly-discovered high-velocity stars. Image credit: Xiao Kong, China’s National Astronomical Observatories.

The positions and orbits of the newly-discovered high-velocity stars. Image credit: Xiao Kong, China’s National Astronomical Observatories.

High-velocity stars are members of Milky Way’s halo, moving very fast in highly elliptical orbits around the center of the galaxy.

There are four subclasses of these stars: hypervelocity stars (fastest stars in the Galaxy), runaway stars, hyper-runaway stars, and fast halo stars.

These stars mark the presence of extreme dynamical and astrophysical processes, especially when a star approaches or even exceeds the escape velocity of the Milky Way at its position.

They provide probes for a wide range of Galactic science, on scales from a few light-years near the central supermassive black hole to the distant halo.

They also provide insights to the dynamical mechanisms that produce their extreme velocities, and for example, their distributions in space and velocity can reveal the existence of a binary massive black hole.

“Though rare in the Milky Way, high-velocity stars, with unique kinematics, can provide deep insight into a wide range of Galactic science,” said co-author Professor Youjun Lu, an astronomer at China’s National Astronomical Observatories and the University of Chinese Academy of Sciences.

“The 591 high-velocity stars discovered this time doubled the total number previously discovered, bringing the current total number exceeding 1,000,” added lead author Dr. Yinbi Li, an astronomer at China’s National Astronomical Observatories.

The astronomers analyzed data from the LAMOST Data Release 7 and the second Gaia data release.

They identified a total of 591 high-velocity stars, including at least 43 stars that are unbound to the Milky Way with escape probabilities larger than 50%.

The majority of them are giant stars; about 14% are metal-rich halo stars.

“The two massive databases provide us unprecedented opportunity to find more high-velocity stars, and we did it,” said co-author Ali Luo, also from China’s National Astronomical Observatories and the University of Chinese Academy of Sciences.

In order to further investigate their origin, the team tracked the orbits of 591 high-velocity stars back in time.

They found that about 15% of stars are from the Galactic center, 55% are from the Galactic disk, and 30% of stars have extragalactic origins.

“Their low metallicities indicate that the bulk of the stellar halo formed as a consequence of the accretion and tidal disruption of dwarf galaxies,” said senior author Professor Gang Zhao, also from China’s National Astronomical Observatories and the University of Chinese Academy of Sciences.

“The discovery of these high-velocity stars tells us that the combination of multiple large surveys in the future will help us to discover more high-velocity stars and other rare stars, which will be used to study the unsolved mystery about our Galaxy.”

The discovery is reported in a paper in the Astrophysical Journal Supplement Series.

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Yin-Bi Li et al. 2021. 591 High-velocity Stars in the Galactic Halo Selected from LAMOST DR7 and Gaia DR2. ApJS 252, 3; doi: 10.3847/1538-4365/abc16e