High velocity (HiVel) stars move sufficiently fast so that they could escape from the Galaxy. Since the first hypervelocity star was discovered by Brown (2005), more than 30 hypervelocity stars have been found. Most of these are more than 2-4 solar mass late B-type stars in the Galactic halo. HiVel stars are intriguing because they not only flag the presence of extreme dynamical and astrophysics processes, but also can be used as dynamical traces of properties of the Galaxy. In particular, the origin of HiVel stars can provide useful information about the environments from which they are produced.

Recently, Dr. Cuihua Du from College of Astronomy and Space Sciences, University of Chinese Academy of Sciences, Dr. Heidi Jo Newberg from Rensselaer Polytechnic Institute (RPI), USA, and other collaborators found 24 late type HiVel stars from Gaia and LAMOST data. Of these, 6 stars belong to hypervelocity stars. This work was recently published in Astrophysical Journal Letters.

In order to further explore the origin of these HiVel stars, they traced the orbits of each HiVel star to derive probability parameters which are used to classify these HiVel stars. Of these, 5 stars are from the tidal debris of disrupted dwarf galaxy and 19 stars are runaway-star candidates which are thought to have formed in the disk and “kicked out” into the halo. But majority of runaway stars in the literature are high-mass O and B type stars. Our studies show it is possible for low-mass late type stars with high ejection velocities. Their chemical and kinematic characteristic provide important information for the origin of HiVel stars and the formation of the Milky Way Galaxy.

Figure: The orbit of the some represented HiVel stars in Galactocentric Coordinates. The red dot represents the Sun and the black dot represents the Galactic Center. The thin lines show 100 random orbits from the uncertainties.