Most stars in the Milky Way move in quiet and orderly orbits around the galactic center, but that is not the case for all of them. Every now and then a maverick is caught breaking ranks, zooming along at speeds that will eventually launch him into intergalactic space.
These ‘hypervelocity’ stars are extremely rare, but we’ve just seen a particularly special one. A star called CWISE J124909+362116.0 (J1249+36 for short) not only exceeds the galactic escape velocity by about 600 kilometers (373 miles) per second, it is a very rare type of small, old main sequence star called an L subdwarf. which also happens to be one of the oldest in the Milky Way.
J1249+36 was first spotted by citizen scientists scouring telescope data for signs of the mysterious Planet Nine. J1249+36 is one of just a handful of hyperspeed stars identified in the Milky Way — and while far from the fastest we’ve ever seen, it represents some of the fastest we’ve ever seen. a challenge for astronomers; namely: how did it become so breathtakingly fast?
The discovery was announced at the 244th meeting of the American Astronomical Society, where the paper was recently submitted The astrophysical diary letters.
There are a number of possible explanations for the star’s speed. The researchers examined three.
The first is the expulsion from a binary system that includes a white dwarf star – the leftover core left behind when a Sun-like star runs out of hydrogen, ejects most of its outer material, dies and enters its afterlife. Ultra-dense white dwarfs shine brightly with waste heat rather than nuclear fusion, and can be a bit unstable if they have a binary companion.
If the two stars are in close orbit, the white dwarf can steal material from the companion star. The problem is that the white dwarf has a maximum mass limit. If it gains just a little bit of mass, it can exist through repeated eruptions known as novae. However, if it gains too much mass, it will explode in a Type Ia supernova that completely destroys the white dwarf.
“In this type of supernova, the white dwarf is completely destroyed, so its companion is released and flies away at the orbital speed at which it was originally moving, plus a small jolt from the supernova explosion,” says astrophysicist Adam Burgasser. from the University of California, San Diego.
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“Our calculations show that this scenario works. However, the white dwarf is no longer there and the remnants of the explosion, which probably took place several million years ago, have already disappeared, so we have no definitive proof that this is its origin.” .”
The second possibility is a multi-body interaction that becomes unstable and bypasses one of the objects in the Milky Way. There are environments in the Milky Way that make these interactions more likely, namely globular clusters – dense blobs that can contain millions of stars. Globular clusters, which are thought to contain swarms of black holes at their centers, have a higher-than-usual concentration of black hole binary pairs.
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“When a star encounters a binary star with a black hole, the complex dynamics of this three-body interaction can throw that star straight out of the globular cluster,” says Caltech astrophysicist Kyle Kremer, who will soon join UC San Diego.
This is also plausible; but tracing the star’s trajectory backward has not yet allowed researchers to identify a specific globular cluster as a starting point.
The third option is that J1249+36 does not come from the Milky Way at all, but from one of the many satellite dwarf galaxies orbiting it. A 2017 study into the origins of high-velocity stars found an extragalactic origin plausible. And the researchers’ calculations showed that this is also feasible for J1249+36.
All three options remain on the table. The best way to find out is to examine the star’s chemical composition in more detail. If J1249+36 was the companion of a white dwarf, the supernova could have left behind trace elements that polluted the atmosphere of the L subdwarf. Conversely, globular clusters contain stars that all have similar compositional properties, so it might be possible to link the star to a home population in this way.
And if neither comes to fruition, we may have to look at the Milky Way’s satellites to determine if this small, faint star is an alien from outside the Milky Way, just winking happily as it passes through.
The discovery was presented at the 224th meeting of the American Astronomical Society.