Most of the stars in the Milky Way follow a calm and orderly orbital path around the galactic center, but this is not the case for all. Every once in a while an individual is caught breaking ranks, blasting through at a speed that will eventually see them ejected cleanly into intergalactic space.
These “hypervelocity” stars are extremely rare, but we’ve just spotted a particularly special example. 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, ancient main-sequence star called an L subdwarf, which it also happens to be one of the oldest in the Milky Way.
First spotted by citizen scientists scouring telescope data for signs of the mysterious Planet Nine, J1249+36 is one of the few hypervelocity stars identified in the Milky Way – and while it’s far from the fastest we’ve seen, it represents something of a challenge to astronomers; namely, how did it get so breathtakingly fast?
The discovery was announced at the 244th meeting of the American Astronomical Society, with a paper recently submitted to The Astrophysical Journal Letters.
There are several possible explanations for the star’s speed. The researchers examined three of them.
The first is an ejection from a binary system that includes a white dwarf star—the remnant core left behind when a Sun-like star runs out of hydrogen, ejects most of its outer material, dies, and enters its afterlife. Superdense white dwarfs glow hot with residual heat, not fusion, and can be somewhat 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 an upper mass limit. If it gains only a little mass, it can maintain its existence through repeated eruptions known as novae. However, if it gains too much mass, it will explode in a Type Ia supernova, which completely obliterates the white dwarf.
“In this kind of supernova, the white dwarf is completely destroyed, so its companion breaks free and takes off at whatever orbital speed it was traveling at originally, plus a little kick from the supernova explosion,” says astrophysicist Adam Burgaser of 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 remains of the explosion, which probably happened several million years ago, have already dissipated, so we have no definitive proof that this is its origin.”
The second possibility is a multi-body interaction that becomes unstable and rams one of the objects into the entire galaxy. There are environments in the Milky Way that make these interactions more likely, namely globular clusters—dense orbs that can contain millions of stars. Thought to contain swarms of black holes at their centers, globular clusters have a higher than usual concentration of black hole binaries.
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“When a star collides with a binary black hole, the complex dynamics of this three-body interaction can throw that star right 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 backwards has not yet allowed researchers to identify a specific globular cluster as its starting point.
The third possibility is that J1249+36 is not from the Milky Way at all, but is one of the many dwarf satellite galaxies orbiting it. A 2017 study investigating the origins of hypervelocity stars found their extragalactic origin to be plausible. And the researchers’ calculations showed that this is also possible for J1249+36.
All three options remain on the table. The best way to understand this will be to look at the chemical composition of the star in more detail. If J1249+36 was a white dwarf companion, the supernova may have left a trail of elements that contaminated the L subdwarf’s atmosphere. Conversely, globular clusters contain stars that have similar compositional properties, so it may be possible for a star to associate with a native population in this way.
And if neither of those works, we may have to look to the Milky Way’s companions to determine if this small, faint star is unknown outside the galaxy, just winking healthily as it passes through.
The discovery was presented at the 224th meeting of the American Astronomical Society.
