A multinational team of astronomers led by Dr Stephan Geier from the European Southern Observatory in Garching, Germany, has determined that a hypervelocity star known as US 708 is traveling at about 1,200 km per second.
The hypervelocity star US 708. Image credit: Centre de Données astronomiques de Strasbourg / SIMBAD.
US 708 is an extremely helium-rich white dwarf located at a distance of 61,970 light-years away.
It was first discovered in 1982 by Dr Peter Usher of Pennsylvania State University and his colleagues as a faint blue object in the Milky Way halo, but no follow-up observations have been published.
Also known as SDSS J093320.86+441705.4 and HVS 2, the star was rediscovered by astronomers from the Sloan Digital Sky Survey in 2005.
Dr Geier and his colleagues observed US 708 with the Echellette Spectrograph and Imager instrument on the 10-m Keck II telescope to measure its distance and velocity along our line of sight.
An artist impression of the mass-transfer phase followed by a double-detonation supernova that leads to the ejection of US 708. While this illustration shows the supernova, bottom center, and the ejected star, left, at the same time, in reality the supernova would have been faded away long before the star reached that position. Image credit: ESA / Hubble / NASA / S. Geier.
Putting the measurements together, they determined the star is moving at 1,200 km per second – much higher than the velocities of previously known stars in our Milky Way Galaxy.
More importantly, the trajectory of US 708 means the Milky Way’s central supermassive black hole could not be the source of the star’s extreme velocity.
The star 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, it 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 was violently ejected from the disrupted binary as a result, and is now traveling with extreme velocity.
The results appear in the March 6 edition of the journal Science.
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S. Geier et al. 2015. The fastest unbound star in our Galaxy ejected by a thermonuclear supernova. Science, vol. 347, no. 6226, pp. 1126-1128; doi: 10.1126/science.1259063
