Lone star status: tracking a low-mass star as it sweeps through the Milky Way

It may seem as if the Sun is standing still while the planets move in its orbit, but in reality the Sun is orbiting the Milky Way Galaxy at an impressive speed of about 140 miles per second – almost half a million miles per hour. As fast as that may seem, when a faint red star was discovered crossing the sky at a noticeably fast pace, scientists took notice.

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 high-speed L subdwarf star has been found speeding through the Milky Way. Even more remarkable, this star may be on a trajectory that would see it leave the Milky Way entirely. The research, led by Adam Burgasser, professor of astronomy and astrophysics at the University of California, San Diego, 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 some of the more than 80,000 citizen science volunteers participating in the Backyard Worlds: Planet 9 project, who were sifting through vast amounts of data stored in the past had been collected. 14 years by NASA’s Wide-field Infrared Survey Explorer (WISE) mission. This project harnesses the keen ability of humans, who are evolutionarily programmed to look for patterns and detect anomalies in a way unmatched by computer technology. Volunteers tag moving objects in databases and when enough volunteers tag the same object, astronomers investigate.

J1249+36 was immediately noticeable because of the speed at which it moves through the air, initially estimated at 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 in Mauna Kea, Hawaii to measure its infrared spectrum. These data showed 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.

The insight into the composition of J1249+36 was made possible by a new series of atmosphere models created by UC San Diego alumnus Roman Gerasimov, who collaborated with UC LEADS scientist Efrain Alvarado III to generate models specifically tailored to studying L -subdwarves.

“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, together with image data from several ground-based telescopes, allowed the team to accurately measure the position and speed of J1249+36 in space and thus predict its trajectory through the Milky Way.

“This is where the source became very interesting, because its speed and trajectory showed that it was moving fast enough to possibly escape the Milky Way,” Burgasser said.

What gave this star a kick?

Researchers focused on two possible scenarios to explain J1249+36’s unusual trajectory. In the first scenario, J1249+36 was originally the light companion of a white dwarf. White dwarfs are the leftover cores of stars that have used up their nuclear fuel and become extinct. 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 gathers too much mass, it could collapse and explode as a supernova.

“In this type of supernova, the white dwarf is completely destroyed, so its companion is released and flies off at the orbital speed it was originally moving at, plus a bit of a 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 took place several million years ago, have already disappeared, so we have no definitive proof that this is the cause. origin.”

In the second scenario, J1249+36 was originally part of a globular cluster, a tightly bound star cluster, immediately recognizable by its distinctive spherical shape. The centers of these clusters are predicted to contain black holes with a wide range of masses. These black holes can also form binary stars, and such systems prove to be great catapults for stars that happen to get too close.

“When a star encounters a binary with a black hole, the complex dynamics of this three-body interaction can throw that star straight out of the globular cluster,” explains Kyle Kremer, a new assistant professor in the Department of Astronomy and Astrophysics at UC San Diego. Kremer ran a series of simulations and found that in rare cases, these types of interactions can kick a low-mass subdwarf out of a globular cluster and follow a trajectory similar to that observed for J1249+36.

“It shows a proof of concept,” Kremer said, “but we don’t actually know which globular cluster this star comes from.” If J1249+36 is traced back in time, it will be in a very busy part of the sky, where undiscovered star clusters may be hidden.

To determine whether one 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. The stars in globular clusters and satellite galaxies of the Milky Way also have distinct patterns in their abundance that could reveal the origins of J1249+36.

“We are essentially looking for a chemical fingerprint that can identify which system this star comes from,” says Gerasimov, whose modeling work has allowed him to measure the element abundances of cool stars in several globular clusters. AAS meeting.

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