591 high velocity star candidates have been discovered in the Milky Way halo

A representation of the Milky Way with the 591 high velocity star candidate in the halo (Image courtesy KONG Xiao of NAOC)
A representation of the Milky Way with the 591 high velocity star candidate in the halo (Image courtesy KONG Xiao of NAOC)

An article published in “The Astrophysical Journal Supplement Series” reports the discovery of 591 high velocity star candidates in the Milky Way’s halo. A team of researchers used data from the Data Release 7 of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and the Data Release 2 of the Gaia space probe to find these stars whose velocity is very high compared to the average of the Milky Way stars. The data indicates that 43 of these stars may have enough velocity to escape the galaxy’s gravity.

Stellar kinematics studies the movement of stars independently of its origin. Stars with a very high velocity compared to the average have been discovered since 2005, and before this research, over 550 were known. Over time, different subclasses have been created for high velocity stars: hypervelocity stars, runaway stars, hyper-runaway stars, and fast halo stars. Their identification is not only a scientific curiosity but offers information on what is happening in the galaxy that hosts them.

This new research identified a number of high velocity star candidate that, if all confirmed, would double the total known number. To achieve such a result, the researchers examined the spectral data of over ten million stars. This huge amount of data was found in two large databases: that of LAMOST, the largest Chinese optical telescope, and that of Gaia, a special ESA space telescope that has exactly the purpose of mapping the sky.

This study concerned the kinematics of stars to trace their movements and therefore their velocity but also their chemical composition to try to understand their origin. The result was the discovery of 591 high velocity star candidate in the galactic halo, a region surrounding the Milky Way. At least 43 of them show such a velocity that they have a greater than 50% chance of ending up ejected from the Milky Way. The chemical composition of the candidates shows low metallicity, which means the elements heavier than hydrogen and helium, in most of them. The low metallicity suggests that the bulk of the halo formed with the absorption of dwarf galaxies by the Milky Way.

The combination of data from two large astronomical databases doubled the number of known high velocity stars. In this case, probably the information obtained concerns mostly the galactic halo with clues to its formation, but in other cases, the results could concern other areas of the Milky Way. The great astronomical surveys are used precisely to offer information to astronomers for various types of research, and their combination can help to better understand different aspects of the evolution of the Milky Way and its parts.

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