Tracking a low-mass star as it speeds through the Milky Way

The Sun may appear to be standing still while the planets in its orbit move, but the Sun is actually orbiting the Milky Way Galaxy at an impressive speed of about 220 kilometers per second – nearly half a million miles per hour. As fast as it may seem, scientists noticed this when a faint red star was discovered crossing the sky at a remarkably fast pace.

Thanks to the efforts of a citizen science project called Backyard Worlds: Planet 9 and a team of astronomers from around the country, a rare hypervelocity sub-dwarf L star has been found hurtling through the Milky Way. Even more remarkable, this star may be on a trajectory that will cause it to leave the Milky Way entirely. The research, led by UC San Diego professor of astronomy and astrophysics Adam Burgasser, was presented at a press conference during the 244th National Meeting of the American Astronomical Society (AAS) in Madison, Wisconsin.

The star, charmingly named CWISE J124909+362116.0 (“J1249+36”), was first spotted by more than 80,000 citizen science volunteers participating in the Backyard Worlds: Planet 9 project, who are combing through the vast amount of data collected in the past. 14 years of the NASA Wide-field Infrared Survey Explorer (WISE) mission. This project benefits from the perceptive abilities of humans, who are evolutionarily programmed to look for patterns and detect anomalies in a way that computer technology cannot match. Volunteers tag moving objects in datasets, and when enough volunteers tag the same object, astronomers investigate.

J1249+36 immediately stood out for the speed at which it is moving across the sky, originally estimated to be about 600 kilometers per second (1.3 million miles per hour). At this speed, the star is fast enough to escape the Milky Way’s gravity, making it a potential “hypervelocity” star.

To better understand the nature of this object, Burgasser turned to the WM Keck Observatory at Mauna Kea in Hawaii 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. Subdwarfs represent the oldest stars in the Milky Way.

Insights into the composition of J1249+36 were made possible by a new suite of atmospheric models created by UC San Diego graduate student Roman Gerasimov, who collaborated with UC LEADS expert Efrain Alvarado III to create models specifically tuned for studying L subdwarfs.

“It was exciting to see that our models were able to accurately match the observed spectrum,” said Alvarado, who is presenting his modeling work at the AAS meeting.

The spectral data, along with imaging data from several ground-based telescopes, allowed the team to precisely measure J1249+36’s position and velocity in space, thereby predicting its orbit around the Milky Way.

“That’s where the source became very interesting because its velocity and trajectory showed it was moving fast enough to escape the Milky Way,” Burgasser said.

What made this star?

The researchers looked at two possible scenarios to explain J1249+36’s unusual trajectory. In the first scenario, J1249+36 was originally a low-mass white dwarf companion. White dwarfs are the remnant cores of stars that have exhausted their nuclear fuel and died out. When a companion star is in very close orbit with a white dwarf, it can transfer matter, resulting in periodic explosions called novae. If a white dwarf accumulates 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 ejected and flies away at the orbital speed it was originally traveling at, plus a little kick from the supernova explosion,” Burgasser said. “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.”

In the second scenario, J1249+36 was originally a member of a globular cluster, a tightly bound cluster immediately recognizable by its distinct spherical shape. The centers of these clusters are thought to contain black holes of a wide range of masses. These black holes can also form binaries, and such systems prove to be great catapults for any stars that happen to wander too close to them.

“When a star meets a black hole binary, the complex dynamics of this three-body interaction can eject that star right out of the globular cluster,” explained Kyle Kremer, an incoming assistant professor in UC San Diego’s Department of Astronomy and Astrophysics. Kremer ran a series of simulations and found that, in rare cases, these kinds of interactions can kick a low-mass subdwarf out of a globular cluster and onto a trajectory similar to that observed for J1249+36.

“It shows a proof of concept,” Kremer said, “but we don’t really know what globular cluster this star came from.” Tracking J1249+36 back in time places it in a very crowded part of the sky that may hide undiscovered star clusters.

To determine whether either of these scenarios or some other mechanism can explain J1249+36’s trajectory, Burgasser said the team hopes to take a closer look at its elemental composition. For example, when a white dwarf explodes, it creates heavy elements that could have “polluted” J1249+36’s atmosphere as it escaped. Stars in globular clusters and satellite galaxies of the Milky Way also have distinct abundance patterns that may reveal the origin of J1249+36.

“Essentially, we’re looking for a chemical fingerprint that would determine exactly what system this star came from,” said Gerasimov, whose modeling work allowed him to measure the element abundances of cool stars in several globular clusters, work he is also presenting at the AAS meeting.

Whether J1249+36’s fast journey was caused by a supernova, a chance encounter with a binary black hole, or some other scenario, its discovery provides astronomers with a new opportunity to learn more about the history and dynamics of the Milky Way.