NASA/ESA/G. Bacon (STScI). We've learned a lot about a hypervelocity star escaping the galaxy (note that this is an artist's rendition of a very different HVS, giant blue star HE 0437-5439)

A star named US 708 has been spotted leaving our galaxy at 4.3 million kilometers an hour (2.7 million mph), the fastest exit speed ever observed. US 708's haste has been attributed to a supernova going off nearby, shedding light on how one such type of supernova occurs.

Stars moving fast enough to escape a galaxy's gravity have been observed often enough to be named hypervelocity stars (HVSs). Last year, an entire cluster was found to have been expelled by the supergiant galaxy M87. However, US 708 is distinctive both for the speed at which it is moving and for being the only HVS known that is a compact helium star.

Now, these new measurements published in Science cast doubt on the previously favored theory for the origins of HVSs.

“According to the widely accepted theory for the acceleration of hypervelocity stars, a close binary is disrupted by the supermassive black hole (SMBH) in the center of our Galaxy, and one component is ejected as a HVS,” the authors note. They agree this theory fits well with many HVSs, most of which are young main sequence stars.

However, US 708 doesn't fit so well. It is believed stars require very close companion stars to evolve into the hot subdwarf form of US 708. Although it is theoretically possible for such a close binary to have an encounter with more massive objects that send one of the stars off at great speed while leaving the other behind, it is very improbable.

On the other hand, the alternative hypothesis to explain HVSs—where the acceleration is a product of the shockwave from a nearby supernova—fits very well with a lost binary companion. If the supernova in question was Type Ia, which is thought to occur in close binary systems where gasses are stripped from a donor star onto a white dwarf, the pieces come together very well.

By measuring US 708's motion and tracing its path backwards, Dr. Stephan Geier of the European Southern Observatory and co-authors calculated that it probably crossed the galactic disk around 14 million years ago and almost certainly did not come from the center of the galaxy. They propose that it was once paired with a carbon-oxygen white dwarf slightly more massive than the sun.

Credit: Geier et al. Simulations of where US 708 may have crossed the galactic plane. Black dot is the galactic center, star is the sun, triangle is the US 708's current position.

The two stars are thought to have had an orbital period of around 10 minutes, which is astonishing to non-astronomers, but consistent with the Helium star/white dwarf binary SDSS J065133+284423's period of 12 minutes. Such a close association with a larger star would cause a hot subdwarf to spin very fast. Geier's team measured US 708's spin at an impressive 115km/s, much faster than similar stars that never had companions, but a quarter of the speed their modeling predicted. The difference may be explained through the transfer of angular momentum to the former companion star before the explosion.

The authors note that black hole slingshots remain the more likely explanation for most HVSs, but propose US 708 could tell us a lot about Ia supernovae.