Most of the stars in the Milky Way move in a quiescent, orderly orbit around the galactic center, but this is not true for all. Every now and then, a maverick is caught breaking the line, whizzing along at a speed that eventually catapults him into intergalactic space.
These “hypervelocity” stars are extremely rare, but we’ve just spotted a particularly special example. Named CWISE J124909+362116.0 (J1249+36 for short), the star not only beats the Galactic escape velocity at around 600 kilometers per second, it’s a very rare type of small, ancient main-sequence star called an L subdwarf, which also happens to be one of the oldest in the Milky Way dragee.
Spotted for the first time 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 far from the fastest we’ve ever seen, it represents something a challenge for astronomers; specifically, how did it happen so incredibly fast?
The discovery was announced at the 244th meeting of the American Astronomical Society, with her paper recently presented The Astrophysical Journal Letters.
There are a number of possible explanations for the star’s velocity. The researchers examined three of them.
The first is an expulsion from a binary that includes a white dwarf—the core remnant left behind when a Sun-like star runs out of hydrogen, ejects most of its outer material, dies, and enters its afterlife. Ultradense white dwarfs burn by residual heat rather than fusion and can be a bit unstable if they have a binary companion.
If two stars are in close orbit, the white dwarf can steal material from the companion star. The problem is that a white dwarf has an upper mass limit. If it picks up just a bit of matter, it can maintain its existence through repeated eruptions known as novae. However, if it gains too much mass, it explodes in a Type Ia supernova, completely obliterating the white dwarf.
“In this kind of supernova, the white dwarf is completely destroyed, so its companion is ejected and flies away at the orbital speed it was originally traveling on, plus a little kick 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 remains of the explosion, which probably occurred several million years ago, have already dissipated, so we do not have definitive proof that this is its origin.” .”
The second possibility is a many-body interaction that becomes unstable and yet one of the objects appears across the galaxy. There are environments in the Milky Way that make these interactions more likely, namely globular clusters—dense spheres that can contain millions of stars. Globular clusters that are supposed to contain swarms of black holes at their centers have a higher than normal concentration of binary black hole pairs.
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“When a star collides with a black hole binary, the complex dynamics of this three-body interaction can eject that star right out of the globular cluster,” says Caltech astrophysicist Kyle Kremer, who will soon join UC San Diego.
This is also likely; but retracing the star’s trajectory has not yet allowed researchers to identify a specific globular cluster as its starting point.
A third possibility is that J1249+36 is not from the Milky Way at all, but one of the many satellite dwarf galaxies that orbit it. A 2017 study investigating the origin of hypervelocity stars found an 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 figure this out will be to examine the star’s chemical composition in more detail. If J1249+36 was a white dwarf companion, the supernova may have left behind trace elements that contaminate the atmosphere of the L subdwarf. Conversely, globular clusters contain stars that all have similar compositional properties, so it may be possible to connect the star to its host population in this way.
And if neither of those works, we may have to look to the satellites of the Milky Way to see if this small, dim star is an alien outside the galaxy, just winking in greeting as it passes by.
The discovery was presented at the 224th meeting of the American Astronomical Society.