Something has knocked out a star that is hurtling through the galaxy

A lone hypervelocity star is crossing the Milky Way, and there is absolutely no way it will fly out of this galaxy. There are two possible explanations for its origin.

The low-mass star known as CWISE J124909+362116.0 (J1249+36) is hurtling through the Milky Way at a staggering 2.1 million kilometres per hour. First identified by amateur astronomers involved in the Backyard Worlds: Planet 9 project, who are analysing data from NASA’s Wide-field Infrared Survey Explorer (WISE), J1249+36 has a speed nearly 1,500 times the speed of sound on Earth, which means that this strange object may leave this galaxy.

Adam Burgasser, professor of astronomy and astrophysics at the University of California, San Diego, is trying to answer questions about the star’s origin, reports Live Science. In research at the W.M. Keck Observatory, Burgasser and his team found that J1249+36 is a sub-L dwarf, a class of ancient, low-mass stars with a relatively low temperature.

The main question, of course, is the brutal speed of the star – for which there are two possible explanations: the first is that a “vampire” white dwarf could be the cause. In this case, J1249+36 was once the companion of a white dwarf, the remnant of a Sun-like star that has exhausted its nuclear fuel.

In binary systems, however, white dwarfs can suck material from their companion stars (hence the “vampire” epithet), which can sometimes lead to a Type Ia supernova explosion if the white dwarf’s mass reaches a critical limit.

Such an explosion could accelerate J1249+36 to its current velocity. However, as there is no evidence of a white dwarf or of the remnants of the necessary explosion, this theory is, unfortunately, only speculation for the time being. As Burgasser told the paper:

“In a supernova of this type, the white dwarf is completely destroyed, so the companion star escapes and flies on at its original orbital velocity and with a small boost from the supernova explosion. Our calculations suggest that this scenario is workable. However, the white dwarf is no longer there, and the remnants of the explosion, which probably happened millions of years ago, have dissipated, so we have no definitive evidence that this is the origin of the star.”

Adam Burgasser, professor of astronomy and astrophysics at the University of California

So, let’s see the second scenario! It suggests that the star interacted with black holes in a globular cluster. Globular clusters are dense collections of stars bound together by gravity. Stars in the center of the clusters can encounter black hole binaries, and the complex gravitational interactions that occur can then push the stars out at high speeds.

Simulations by Kyle Kremer, an adjunct professor at the University of San Diego, support this possibility and show that such interactions can occasionally push low-mass subclusters such as J1249+36 into similar orbits. As he said:

“When a star encounters a black hole binary, the complex dynamics of this three-body problem can push the star out of the globular cluster.”

Kremer’s team traced the trajectory of this hypervelocity star to an extremely crowded region of space, which may indeed be the site of a currently undiscovered globular cluster – or even more.

The team is now investigating the elemental composition of J1249+36 to determine which of the two ejection scenarios is the correct one. The composition could be a clue to the origin of the star since when white dwarfs become supernovae, they “contaminate” the ejected stars. Stars born in globular clusters also have a distinctive chemical composition.

Whatever the origin of this star, however, this discovery offers scientists a unique opportunity to study hypervelocity stars in more detail.

(cover image: simulated image showing the moment when a low-mass star is about to be ejected by the supernova explosion of a white dwarf, source: Adam Makarenko / W.M. Keck Observatory)