Stars like the Sun are bound to our galaxy and orbit its center with moderate velocities. Only a few so-called hypervelocity stars are known to travel with velocities so high that they are unbound, meaning they will not orbit the galaxy, but instead will escape its gravity to wander intergalactic space.
A close encounter with the supermassive black hole at the center of the Milky Way is typically presumed the most plausible mechanism for kicking these stars out of the galaxy.
A team of astronomers led by Stephan Geier from the European Southern Observatory in Garching, Germany, observed the high-velocity star known as US 708 with the Echellette Spectrograph and Imager instrument on the 10-meter Keck II telescope to measure its distance and velocity along our line of sight. By carefully combining position measurements from digital archives with newer positions measured from images taken during the course of the Pan-STARRS1 survey, they were able to derive the tangential component of the star’s velocity across our line of sight.
Putting the measurements together, the team determined the star is moving at about 2.7 million mph (1,200 km/s), much higher than the velocities of previously known stars in the Milky Way Galaxy. More importantly, the trajectory of US 708 means the supermassive black hole at the galactic center could not be the source of US 708’s extreme velocity.
US 708 has another peculiar property in marked contrast to other hypervelocity stars: It is a rapidly rotating, compact helium star likely formed by interaction with a close companion. Thus, US 708 could have originally resided in an ultra-compact binary system, transferring helium to a massive white dwarf companion, ultimately triggering a thermonuclear explosion of a type Ia supernova. In this scenario, the surviving companion, US 708, was violently ejected from the disrupted binary as a result and is now traveling with extreme velocity.
These results provide observational evidence of a link between helium stars and thermonuclear supernovae and is a step toward understanding the progenitor systems of these mysterious explosions.