With velocities of 1,694 and 2,285 kilometers per second, they shatter the radial velocity record for any known star.
Until recently, astronomers knew just a dozen stars with trajectories that would allow them to “escape” the pull of our galaxy, all guided by nearby powerful stellar explosions. But now, a new study led by astronomer Kareem El-Badry of the Harvard/Smithsonian Center for Astrophysics has revealed the existence of six other previously unknown ‘runaway’ stars, two of which (J1235 and J0927, two small white) smashed the current radial velocity record for any known star, at 1,694 and 2,285 kilometers per second, respectively. And if it is true that some stars that orbit near Sagittarius A*, the central black hole of our galaxy, it must be said that those stars are “locked” in circular orbits by the gravity of the black hole and will never be able to follow a trajectory that will lead them to leave the Milky Way. The two newly discovered hypervelocity stars will also help explain how stars formed rare supernovae which gave them the momentum they needed to overcome the gravity of the Milky Way. El-Badry and his team, who have just reported their discovery in an article published on the pre-publication server ‘ arXiv’ , used data from the Gaia survey, the European mission which is preparing a catalog with the location, the distance and motion of more than a billion stars in our galaxy. The supernovae that catapulted these stars belong to a special class known as Type 1a. Noted for their usefulness in determining astronomical distances (because they always explode with the same brightness), type 1a supernovae occur in binary star systems, where a small, dense white dwarf slowly absorbs and digests matter from its companion as it that the two stars approach each other. They orbit in time.
But there comes a time when the white dwarf has absorbed so much material from the other star that it literally suffers from severe “indigestion.” In fact, when it reaches 1.4 solar masses (called ‘Chandrasekhar Mass’ from the name of the Indian-American theoretical physicist who calculated it), the star can no longer resist its own weight and collapses on itself, giving rise to a massive explosion . Even if it’s not an explosion like the others. Indeed, there are still open questions regarding type 1a supernovae. In theory, white dwarf binaries reaching Chandrasekhar mass should be rarer than they are. In other words, they are being observed more than one might expect. Which led astronomers to consider an alternative method of causing such a supernova: a double detonation. In this scenario, a white dwarf steals helium from the shell of its nearby star, and the helium explodes first, causing a shock wave that then triggers a second detonation, this time from the star’s carbon core. Under these circumstances, and provided that the celebrated star has a large enough carbon nucleus, the white dwarf can go supernova without ever reaching the Chandrasekhar limit. In this double detonation scenario, simulations show that the remnants of the companion star are hurled into space, with a speed similar to the one it had while still orbiting its now deceased partner. This process allows the “escape” to reach breakneck speeds through (and eventually out of) the Milky Way. «These runaway white dwarfs – the authors write in their article – are smoking guns of doubly degenerate detonations. If you find a white dwarf going that fast, it’s guaranteed to come from a white dwarf binary system in which it exploded. There’s no other conceivable channel we can think of to make something go so fast.” Of course, single-hit supernovae can also produce runaway stars, albeit at slower speeds. In those cases, it is the remnants of the exploding star itself (and not those of a companion) that reach extreme speeds. Such events are called 1ax-type supernovae, in which the explosion fails to completely destroy the star, leaving behind the fast-moving remnants of the white dwarf’s core. Thanks to work like that of El-Badry and his colleagues, researchers can now use differences in observed speeds to determine the different origins of runaway stars and classify them accordingly. As the population of known escapees grows, it will be possible to determine how often each type of supernova occurs.