If you were attacked by a hungry vampire star or threatened to fall into two warring black holes, you’d probably run away too!
One of these terrifying scenarios is likely responsible for sending a low-mass star hurtling through the Milky Way at a staggering million miles per hour (1.6 million kilometers per hour). That’s about 1,500 times faster than the speed of sound.
The star has the designation CWISE J124909+362116.0 (J1249+36) and was first detected by citizen science volunteers as part of the Backyard Worlds: Planet 9 project, who are examining the vast amount of data collected by NASA’s Wide-field Infrared Survey Explorer (WISE). mission over the course of nearly a decade and a half. J1249+36 immediately stood out for its immense speed, namely 1.3 million mph (2.1 million km/h), almost three times the speed of the Sun in its orbit around the heart of the Milky Way. In fact, the speed of this “hypervelocity” star is so great that it is likely to escape our galaxy entirely.
Related: ‘Vampire stars’ explode after eating too much – AI could help reveal why
Astronomy and astrophysics professor Adam Burgasser turned to the WM Keck Observatory in Maunakea, Hawaii, to observe its infrared spectrum to unravel the mystery of this hypervelocity star.
This investigation revealed that the star belongs to a class of the oldest stars in the Milky Way: L subdwarfs. These stars are very rare and remarkable because of their very low mass and relatively low temperatures.
The team’s spectral data was combined with a new set of atmospheric models built specifically for the study of L subdwarfs. This revealed the position and velocity of J1249+36 in the Milky Way. “This is where the resource became very interesting,” Burgasser said in a statement. “Its speed and trajectory showed it was moving fast enough to escape the Milky Way.
The question is, what launched this subdwarf star on its rapid escape trajectory? Well, that brings us to our two suspects.
Is this star running away from a white dwarf vampire?
In the first scenario used to explain the hypervelocity nature of J1249+36, Burgasser and colleagues hypothesized that the low-mass star was once the stellar companion of a type of “dead” star called a white dwarf.
White dwarfs are born when smaller stars like the Sun run out of hydrogen in their cores. When this happens, the star’s nuclear fusion stops. This cuts off the outward flow of energy that supports the star against the inward pressure of its own gravity. While this ends the lives of lone, isolated stars like the sun, white dwarfs in binary systems can return from the grave by cannibalistically feeding on stellar material stripped from a nearby “donor” star.
This material accumulates on the white dwarf until the mass of this stellar remnant exceeds the Chandrasekhar limit, which is approximately 1.4 times the mass of the Sun, above which a star can go supernova. This results in a type of cosmic explosion called a “type Ia supernova” that completely wipes out 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 at, plus a small kick from the supernova explosion,” Burgasser explained. “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 have no definitive proof that this is its origin.”
Could two black holes have something to do with it?
The second scenario considered by the team shows that this hypervelocity star begins life in a globular cluster, a dense and compact conglomeration of stars bound together by gravity. These globular clusters can contain anywhere from tens of thousands to many millions of stars.
The stars are clustered toward the center of globular clusters, where scientists believe black holes of varying masses also lurk. These black holes can merge to form binary stars that are adept at ejecting any stars that venture too close from their home systems.
“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,” said Kyle Kremer, incoming assistant professor in UC San Diego’s Department of Astronomy and Astrophysics.
Simulations generated by Kremer revealed that, in rare cases, these kinds of interactions can kick a low-mass subdwarf out of a globular cluster and place it on trajectories similar to that observed for J1249+36.
The team also traced the trajectory of this hypervelocity star back to an extremely crowded region of space, which could indeed be the location of a currently undiscovered globular cluster—or maybe. more than one.
The team will now look at the elemental composition of J1249+36 in an attempt to determine which of these ejection scenarios is the correct one. The composition could be a possible indicator of origin, because when white dwarfs “go nova” they pollute the stars they kick off. In addition, stars born in globular clusters have different chemical compositions.
Whatever the origin of this star, its discovery offers scientists a unique opportunity to study hypervelocity stars as a whole. And it’s all very cool.
Burgasser presented the team’s results at a press conference Monday (June 10) during the 244th National Meeting of the American Astronomical Society (AAS) in Madison, Wisconsin.