An artist’s concept of what the system looked like shortly before it boomed.
NASA, ESA and P. Ruiz Lapuente (University of Barcelona); Cut and colored by S. Geier
Most of the stars in our galaxy orbit around the galactic core in an orderly fashion. But in recent years, researchers have discovered a class of stars that move remarkably fast, in many cases fast enough to reach an escape velocity – they will eventually leave the Milky Way for intergalactic space.
What can explain a star moving at nearly 1,000 kilometers per second? One model concerns our galaxy’s supermassive black hole. When a binary star system comes close to it, orbital interactions can keep one member of the binary star orbiting the black hole, while the other is flung out of the system at high speeds. Several of the hypervelocity stars can trace their path back to the galactic core, lending support to this model.
But apparently not all. Researchers revisited a hypervelocity star called US 708 and found that it is both the fastest-moving star we’ve seen in our galaxy and that it doesn’t originate from the galactic core. Instead, the star’s properties suggest it was blasted to its current speed by what’s called a double-blast supernova.
US 708 was first recognized as a hypervelocity star in 2005. It is part of an unusual class of stars called hot subdwarfs. These are the helium-burning former cores of red giants, left over after all the hydrogen has been removed. Due to the process of stripping, they have a low mass (about half the mass of the Sun) and spin to high rotational speeds.
Where does all the hydrogen end up? Usually it’s stripped by a compact companion that’s close to Earth — half of the hot subdwarfs we’ve observed have a companion orbiting Earth with a period of less than 30 days, some with a companion to orbit the Earth in less than a day. This companion extracts matter from the red giant, leaving behind its helium-burning core.
If the companion is a neutron star or a black hole, it’s not so bad. However, if the companion is a white dwarf (a carbon-oxygen remnant of a Sun-like star), it can become explosive. If enough material accumulates on the surface of the white dwarf, it can cause a thermonuclear explosion, which we call a Type Ia supernova. In this case, the explosion was likely a so-called “double blast” — first the gas on the white dwarf’s surface explodes, creating a shock wave that in turn triggers a thermonuclear explosion of its carbon/oxygen core.
The new observations of US 708 show it rotating rapidly, which is equivalent to an object that has spun by stripping off its outer layers. But it’s not at the full speed we’d expect, suggesting the explosion also changed its rotation.
But the new measurements also show that at 1200 kilometers per second it is the fastest moving star in our galaxy. Combined with archival images, the astronomers were also able to trace its path back in time, showing that it has passed through the galactic disk and is currently making its way into the halo, all without ever approaching the central black part of the Milky Way. to have come. hole.
Suggesting that his speed was generated by his companion’s explosion. To reach these speeds, it must have been very close to a very large white dwarf. The team behind the new observations suggests it orbited a white dwarf about the mass of the sun with an orbital period of less than 10 minutes — extremely close.
Further observations should now look for the presence of elements forged in the supernova on the surface of US 708 to confirm this model (we need ultraviolet spectroscopy for this). It would also be interesting to model the parental system. With a short orbital period and the inflated size of a red giant, these may have been fainter stars and closer to orbit in a distorted, divided envelope.
Science2014. DOI: 10.1126/science.1259063 (About DOIs).