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  • Writer's pictureYang Huang

Discovery of two more runaway stars from the LAMOST Spectroscopic Surveys

Updated: Nov 3, 2019

We have discoveried two more HVSs from the LAMOST Spectroscopic Surveys!

A study by our research group led by Prof. Xiaowei Liu (刘晓为) from the South-Western Institute for Astronomy Research (SWIFAR) at Yunnan University reports the discovery of two new unbound hypervelocity stars from the LAMOST spectroscopic surveys. The results were published in The Astrophysical Journal Letters (Huang et al. 2017, ApJL, 847, 9).

Hypervelocity stars (HVSs) are rare objects of extreme fast velocities allowing them to escape the Milky Way galaxy. One of the most commonly proposed scenarios is that they originate near the Galactic center (GC) by dynamical interactions between (binary) stars and the central massive black hole(s) (MBH). Alternative explanations exist, for examples, they could be the tidal debris of an accreted and disrupted dwarf galaxy, or they could be surviving companion stars of Type Ia supernovae.

In addition to exploring the origin of HVSs, scientists also care about these speedy objects since they are powerful tracers to probe the mass distribution of the Milky Way, given the fact that they travel large distances across the Galaxy.

For these reasons, scientists have been searching for HVSs, both systematically and serendipitously, for many years. However, only about 20 HVSs have been identified hitherto, much less than the number of over 2,000 predicted by the theory. Therefore, it is of vital importance to discover more new HVSs.

Our group has carried out a systematic search for HVSs amongst nearly 6.5 million high-quality stellar spectra of 4.4 unique stars collected up to June 2016 by the LAMOST spectroscopic surveys, and have discovered two new HVSs. They are respectively a B2V type star of about 7 Msun with a Galactic rest-frame radial velocity of 502 km/s at a Galactocentric radius of about 21 kpc, and a B7V type star of about 4 Msun with a Galactic rest-frame radial velocity of 408 km/s at a Galactocentric radius of about 30 kpc. The two stars are very bright and the upcoming Gaia data release should provide proper motion measurements accurate enough to give direct constraints on their origins.

We expect more HVSs will be discovered by the on-going LAMOST spectroscopic surveys, providing further constraints on the nature and ejection mechanisms of HVSs.  

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