With an estimated distance of 140 parsecs (457 light-years), the L-subdwarf star CWISE J124909+362116.0 (J1249+36 for short) has a total speed of at least 600 km/s, exceeding the local Galactic escape velocity. Remarkably, this star was likely ejected from a globular cluster located in the outer Galactic plane in the past 10-30 million years.
A simulation of a hypothetical J1249+36-white dwarf binary pair ends with the white dwarf exploding into a supernova. Image credit: Adam Makarenko / W.M. Keck Observatory.
J1249+36 was first discovered by citizen scientists as part of the Backyard Worlds: Planet 9 program.
The star immediately stood out because of the speed at which it is moving across the sky, initially estimated at about 600 km/s.
At this speed, the star is fast enough to escape the gravity of the Milky Way, making it a potential hypervelocity star.
To better understand the nature of J1249+36, University of California, San Diego’s Professor Adam Burgasser and his colleagues turned to the W.M. Keck Observatory to measure its infrared spectrum.
These data revealed that the object was a rare L subdwarf — a class of stars with very low mass and temperature.
The spectral data, along with imaging data from several ground-based telescopes, allowed the team to accurately measure J1249+36’s position and velocity in space, and thereby predict its orbit through the Milky Way.
“This is where the source became very interesting, as its speed and trajectory showed that it was moving fast enough to potentially escape the Milky Way,” Professor Burgasser said.
The researchers focused on two possible scenarios to explain J1249+36’s unusual trajectory.
In the first scenario, J1249+36 was originally the low-mass companion of a white dwarf.
When a stellar companion is in a very close orbit with a white dwarf, it can transfer mass, resulting in periodic outbursts called novae. If the white dwarf collects too much mass, it can collapse and explode as a supernova.
“In this kind of supernova, the white dwarf is completely destroyed, so its companion is released and flies off at whatever orbital speed it was originally moving, plus a little bit of a kick from the supernova explosion as well,” Professor Burgasser said.
“Our calculations show this scenario works. However, the white dwarf isn’t there anymore and the remnants of the explosion, which likely happened several million years ago, have already dissipated, so we don’t have definitive proof that this is its origin.”
In the second scenario, J1249+36 was originally a member of a globular cluster, a tightly bound cluster of stars, immediately recognizable by its distinct spherical shape.
The centers of these clusters are predicted to contain black holes of a wide range of masses.
These black holes can also form binaries, and such systems turn out to be great catapults for any stars that happen to wander too close to them.
“When a star encounters a black hole binary, the complex dynamics of this three-body interaction can toss that star right out of the globular cluster,” said Dr. Kyle Kremer, an astronomer at the University of California, San Diego.
The scientists ran a series of simulations and found that on rare occasions these kinds of interactions can kick a low-mass subdwarf out of a globular cluster and on a trajectory similar to that observed for J1249+36.
“It demonstrates a proof of concept, but we don’t actually know what globular cluster this star is from,” Dr. Kremer said.
“Tracing J1249+36 back in time puts it in a very crowded part of the sky that may hide undiscovered clusters.”
To determine whether either of these scenarios, or some other mechanism, can explain J1249+36’s trajectory, the team hopes to look more closely at its elemental composition.
For example, when a white dwarf explodes, it creates heavy elements that could have polluted the atmosphere of J1249+36 as it was escaping.
The stars in globular clusters and satellite galaxies of the Milky Way also have distinct abundance patterns that may reveal the origin of J1249+36.
“We’re essentially looking for a chemical fingerprint that would pinpoint what system this star is from,” said Roman Gerasimov, also from the University of California, San Diego.
“Whether J1249+36’s speedy journey was because of a supernova, a chance encounter with a black hole binary, or some other scenario, its discovery provides a new opportunity for astronomers to learn more about the history and dynamics of the Milky Way.”
The astronomers announced their discovery this week at the 244th Meeting of the American Astronomical Society in Madison, Wisconsin.
_____
Adam Burgasser et al. 2024. A Hypervelocity L Subdwarf Passing Through the Solar Neighborhood. AAS 224, abstract # 3
