Something “kicked” this hypervelocity star racing through the Milky Way at 2.1 million km/h
If you were attacked by a predatory vampire star or threatened to be pulled into a duel between two black holes, you would probably run too!
One of these terrifying scenarios likely caused a low-mass star to streak through the Milky Way at 2.1 million km/h.
The star was designated CWISE J124909+362116.0 (J1249+36) and was first discovered by volunteers from the project Backyard Worlds: Planet 9, who are examining the massive amount of data collected by NASA's Wide-field Infrared Survey Explorer (WISE) mission over nearly a decade and a half. J1249+36 immediately stood out due to its enormous speed – about 2.1 million kilometers per hour, which is almost three times faster than the speed of the Sun in its orbit around the center of the Milky Way. The speed of this “hyperspeed” star is so great that it will most likely leave our galaxy altogether.
To unravel the secrets of this ultra-high-speed star, Adam Burgasser, a professor of astronomy and astrophysics at the University of California at San Diego, turned to the V.M. Observatory. Keck in Maunakea, Hawaii, to study its infrared spectrum.
This study showed that the star belongs to the class of the oldest stars in the Milky Way: L-subdwarfs. These stars are very rare and are notable for having very low mass and relatively low temperature.
The team's spectral data were combined with a new set of atmospheric models created specifically for the study of L-subdwarfs. This made it possible to determine the position and speed of J1249+36 in the Milky Way. “This is where the data got very interesting,” Burgasser said in a statement. “Its speed and trajectory showed it was moving fast enough to potentially escape the Milky Way.”
The question is, what set this subdwarf star on its flight path? Well, that brings us to two suspects.
Is this star escaping from a vampire white dwarf?
In the first scenario used to explain the hypervelocity of J1249+36, Burgasser and his colleagues proposed that this low-mass star was once the stellar companion of a dead star called a white dwarf.
White dwarfs are born when smaller stars such as the Sun run out of hydrogen in their cores. When this happens, nuclear fusion in the star stops. As a result, the flow of energy that supports the star against the internal pressure of its own gravity stops. However, white dwarfs in binary systems can “rise from the grave” by feeding on stellar material from a neighboring “donor” star.
This material accumulates on the white dwarf until the mass of the stellar remnant exceeds Chandrasekhar limit, about 1.4 times the mass of the Sun, above which the star can go supernova. This results in a cosmic explosion called a “type Ia supernova,” which completely destroys the white dwarf.
“In this type of supernova, the white dwarf is completely destroyed, and its companion is freed and flies away at the orbital speed at which it originally moved, plus receiving a small additional push from the supernova explosion,” explains Burgasser. “Our calculations show that this scenario works. However, the white dwarf is no longer there, and the remnants of the explosion, which probably occurred several million years ago, have already dissipated, so we do not have definitive evidence that this is its origin.”
Could twin black holes have something to do with this?
In the second scenario the team considered, this hypervelocity star began its life in a globular cluster—a dense, compact collection of stars bound together by gravity. These spherical clusters can contain tens of thousands to many millions of stars.
Stars are concentrated in the center of globular clusters, where, according to scientists, black holes of various masses also hide. These black holes can come together and form binary systems that can eject any stars that get too close to them from their systems.
“When a star collides with a binary black hole, the complex dynamics of this three-body interaction can eject the star directly from the globular cluster,” says Kyle Kremer, a future assistant professor of astronomy and astrophysics at the University of California, San Diego.
Kremer's simulations showed that, in rare cases, such interactions could push a low-mass subdwarf out of a globular cluster and put it on a trajectory similar to that observed for J1249+36.
The team also traced the trajectory of this hypervelocity star to an extremely dense region of space that may actually be the location of an as-yet-undiscovered globular cluster—or perhaps several.
The team will now study the elemental composition of J1249+36 in an attempt to determine which of these emission scenarios is correct. Composition could be a possible clue to origin because when white dwarfs “go nova,” they pollute the stars they eject. In addition, stars born in globular clusters have a special chemical composition.
Whatever the origin of this star, its discovery gives scientists a unique opportunity to study hypervelocity stars in general. And all this is very cool.
Burgasser presented the team's findings at a press conference on Monday (June 10) during the 244th national meeting of the American Astronomical Society (AAS) in Madison, Wisconsin.
