The state of the lone star: Tracking a low-mass star as it travels through the Milky Way

It may look like the sun is stationary while the planets in its orbit are moving, but the sun is actually spinning around 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, when a faint red star was discovered crossing the sky at a remarkably fast rate, 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 hypervelocity sub-L star has been found racing through the Milky Way. Surprisingly, this star may be on a trajectory that causes it to leave the Milky Way altogether. The research, led by University of California Astronomy and Astrophysics Professor 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 some of the more than 80,000 citizen science volunteers who participated in the Backyard Worlds: Planet 9 project, which analyzes a variety of big data collected in the past. 14 years since NASA’s Wide-field Infrared Survey Explorer (WISE) mission. This project takes advantage of the acumen of humans, who are evolutionarily programmed to look for patterns and spot anomalies in a way unmatched by computer technology. Volunteers tag moving objects in data files, and when enough volunteers tag the same object, astronomers investigate.

J1249+36 immediately caught the eye because of the speed at which it moves across the sky, first 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 on Mauna Kea, 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.

The look at J1249+36’s composition was made possible by a new set of atmospheric models created by UC San Diego graduate student Roman Gerasimov, who worked with UC LEADS researcher Efrain Alvarado III to generate models specifically tuned to studied subdwarfs L.

“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, and thereby predict its orbit through the Milky Way.

“This is where the source became very interesting, as its velocity and trajectory indicated that it was moving fast enough to potentially escape the Milky Way,” Burgasser said.

What struck this star?

The researchers focused on two possible scenarios to explain the unusual trajectory of J1249+36. In the first scenario, J1249+36 was originally the low-mass companion of a white dwarf. White dwarfs are the remnant cores of stars that have depleted their nuclear fuel and died. When a companion star is in very close orbit to a white dwarf, it can transfer mass, resulting in periodic explosions called novae. If the white dwarf gathers too much mass, it may collapse and explode as a supernova.

“In this type of supernova, the white dwarf is completely destroyed, so its companion is ejected and flies off at whatever orbital speed it was moving at first, plus a little shock 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 likely happened several million years ago, have already dispersed, so we have no conclusive evidence that this is his origin.”

In the second scenario, J1249+36 was originally a member of a globular cluster, a tightly bound group of stars immediately recognizable by its distinctive 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 prove to be excellent catapults for any stars that happen to wander too close to them.

“When a star collides with a binary black hole, the complex dynamics of this three-body interaction can throw that star right out of the globular cluster,” explained Kyle Kremer, an 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 eject a low-mass subdwarf from a globular cluster and on a trajectory similar to that observed for J1249+36.

“It demonstrates a proof of concept,” Kremer said, “but we don’t actually know which globular cluster this star is from.” Tracing J1249+36 back in time places it in a very crowded part of the sky that could hide undetected clusters.

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 may 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.

“We’re basically looking for a chemical fingerprint that will determine which system this star is from,” said Gerasimov, whose modeling work has enabled him to measure the elemental abundances of cool stars in several clusters. globular, work he is also presenting at the AAS Meeting.

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