The Sun moves around the Milky Way galaxy at a speed of about 220 kilometers per second, which is nearly half a million miles per hour. However, when a faint red star moving across the sky noticeably fast was discovered, scientists became interested.

Thanks to the efforts of the citizen science initiative Backyard Worlds: Planet 9 and a team of astronomers from across the country, a rare hyper-velocity L subdwarf star speeding through the Milky Way has been discovered. This star might be on a trajectory that could eject it from the Milky Way. The research, led by Professor of Astronomy and Astrophysics Adam Burgasser from the University of California, San Diego, was presented today at a press conference during the 244th national meeting of the American Astronomical Society (AAS) in Madison, Wisconsin.

The star, affectionately named CWISE J124909+362116.0 (“J1249+36”), was first noticed by over 80,000 volunteers in the Backyard Worlds: Planet 9 project, who review vast amounts of data collected during the last 14 years of NASA's Wide-field Infrared Survey Explorer (WISE) mission. This project leverages the ability of humans to recognize patterns and anomalies in a way that is unmatched by computer technology. Volunteers mark moving objects in the data files, and when enough volunteers mark the same object, astronomers investigate it further.

J1249+36 immediately stood out due to its speed across the sky, initially estimated at around 600 kilometers per second (1.3 million miles per hour). At this speed, the star is fast enough to escape the gravity of the Milky Way, making it a potential “hyper-velocity” star.

To better understand the nature of this object, Burgasser turned to the W.M. Keck Observatory in Maunakea, Hawaii, to measure its infrared spectrum. The data revealed that the object is a rare L subdwarf star - a class of stars with very low mass and temperature. Subdwarfs represent the oldest stars in the Milky Way.

Atmospheric Models
Insights into the composition of J1249+36 were enabled by new sets of atmospheric models created by Roman Gerasimov, a former UC San Diego student, who worked with UC LEADS scientist Efrain Alvarado III to generate models specifically tailored for studying L subdwarfs. “It was exciting to see that our models could 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 the position and speed of J1249+36 in space and predict its trajectory through the Milky Way. “This source became very interesting here because its speed and trajectory showed that it moves fast enough to potentially escape the Milky Way,” Burgasser said.

Explaining the Unusual Trajectory
The researchers focused on two possible scenarios to explain the unusual trajectory of J1249+36. In the first scenario, J1249+36 was originally a low-mass companion to a white dwarf. White dwarfs are the cores of stars that have exhausted their nuclear fuel and shut down. When a stellar companion has a very close orbit with a white dwarf, it can transfer mass, resulting in periodic explosions called novae. If the white dwarf accumulates too much mass, it can collapse and explode as a supernova.

“In this type of supernova, the white dwarf is completely destroyed, so its companion is freed and flies away at the speed it originally orbited, plus a bit of extra push 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 happened a few million years ago, have already dispersed, 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 recognizable by its spherical shape. The centers of these clusters are predicted to contain black holes of various masses. These black holes can also form binary systems, and such systems can be excellent catapults for any stars that get too close.

“When a star encounters a binary black hole, the complex dynamics of this three-body interaction can eject that star from the globular cluster,” explained Kyle Kremer, incoming assistant professor in the Department of Astronomy and Astrophysics at UC San Diego. Kremer conducted a series of simulations and found that in rare cases, these types of interactions can eject a low-mass subdwarf from a globular cluster on a trajectory similar to that observed for J1249+36.

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

To determine if any of these scenarios, or some other mechanism, can explain the trajectory of J1249+36, Burgasser said the team plans to take a closer look at its elemental composition. For example, when a white dwarf explodes, it creates heavy elements that could “pollute” the atmosphere of J1249+36 as it was released. Stars in globular clusters and the Milky Way's satellite galaxies also have specific abundance patterns that could reveal the origin of J1249+36.

“Essentially, we are looking for a chemical fingerprint that would indicate from which system this star comes,” said Gerasimov, whose modeling work allows him to measure elemental abundances in cool stars in several globular clusters, work that he is also presenting at the AAS meeting.

Whether J1249+36's fast travel is 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.

Source: University of California