“Stellar paternity tests” link orphaned stars to their origins in the Milky Way

In the chaotic environment of open star clusters, strong gravitational interactions between bodies can throw individual stars well outside the cluster, even outside our galaxy, the Milky Way. Now, for the first time, researchers have mapped several of these stars, which exist outside the galaxy’s disk, to their original clusters using new data from the European Space Agency’s Gaia mission.

Researchers from Lehigh University today presented their results, “Stellar Paternity Tests: Matching High-Latitude B Stars to the Open Clusters of Their Birth,” at a news conference at the 243rd Meeting of the American Astronomical Society (AAS) in New Orleans.

“By tracing them back in time to see where they originated, we are able to associate 15 of them with the star clusters where they were born,” said Professor Virginia (Ginny) McSwain. associate professor of physics at Lehigh University. “If we can say with high certainty where some of these stars came from, we will know more about the history of star clusters in the Milky Way.”

Most stars outside the Milky Way’s thin disk, which includes the spiral arms with a thicker diameter at the center, are more than eight billion years old and formed early in the history of the Milky Way. galaxy. Given their great age, it is not surprising that they traveled far from their birthplace.

Since almost all of our galaxy’s star formation occurs in the thin disk, hot B-type stars are rarely found outside of this region. Yet a small number of these young stars, 10 to 100 million years old, lie at high altitudes above and below the disk, likely ejected from their birth clusters within the last few million years. .

“Hot stars don’t often venture out of the disk, so when they do, they are visibly displaced,” said Brandon Schweers, a Lehigh University undergraduate who has conducted key research on the project. “The ‘parent’ clusters likely ejected most of these B-type stars when close three- or four-body gravitational interactions threw a member of the cluster, sending them flying away from the plane of the Milky Way.”

One star studied was thrown at a particularly high speed, so it could have been ejected during a supernova in a nearby binary star system, said Schweers, an astrophysics student. Stars can even be projected and then returned and projected again using a slingshot.

Although these “orphan” stars had been known for two decades, none had been mapped to their place of origin because quality data was not available to trace them back to their beginnings. However, using data from the Gaia mission, researchers were able to decipher the movements of stars with greater precision than was previously available.

Using trajectories to go back in time

The Gaia mission, launched in 2013, aims to study more than a billion stars in the Milky Way and create a precise three-dimensional map of the galaxy. The data includes unprecedented position measurements for stars and radial velocity measurements for the 150 million brightest objects.

Based on Gaia data released in 2022, Lehigh researchers traced the kinematic trajectories of 95 high-latitude B stars and about 1,400 known galactic open clusters to identify times in the past when they might have crossed paths and where an ejection could have occurred.

“Using their 3D positions and 3D velocities in space, we were able to calculate the trajectories of every high-latitude cluster and star over the past 30 million years,” McSwain said. They used the open source Python galpy package for galactic dynamics analysis to model the gravitational field of the galaxy at each point.

Once they identified potential matches, they compared the color and brightness of each ejected star to the Hertzsprung-Russell (HR) diagram, a color magnitude diagram, for each open cluster. An open cluster typically has thousands of stars of the same age and composition, located at the same distance.

“The shape of the HR diagram depends mainly on the age of the cluster, so we can tell whether the ejected star is of a similar age to its potential siblings,” McSwain said. Applying the HR test further narrowed the list of potential matches.

Finally, they analyzed the base densities of each cluster that constituted a possible match. Higher density clusters have more strong gravitational interactions between their members, giving them the greatest potential for ejecting stars.

Paternity tests prove positive

Combining these tools, the researchers confirmed positive paternity matches for 15 orphan stars. It was this galactic genealogical tracing that gave Schweers the idea for the title of the presentation.

“When I reached the stage of comparing the color and brightness of potential matches and discarding those that showed poor correlation in the HR diagrams, I felt like I was comparing the “DNA” of orphan stars and their potential siblings.’” Schweers said, reminding him of “The Maury Povich Show.”

“I think everyone has heard the saying ‘You’re not the father’ that comes from that show. For a lot of these clusters, I was basically telling them that they’re not the parents of these orphan stars, so I came up with the name ‘Stellar Paternity Tests,'” he said.

Based on their trajectory calculations, the researchers estimate that the ejections took place around 5 to 30 million years ago, “throwing abandoned stars across the Milky Way at speeds of 30 to 220 kilometers/second (67,000 to 490,000 mph) to their current location. ,” they wrote. “Our results provide a measure of the ejection age of each orphan star, providing new insight into the relative importance of dynamic versus supernova ejection in young open clusters.”

Although they were able to identify a number of distant stars, some could not be very plausibly linked to the Milky Way’s disk, which could provide evidence for other unusual scenarios, they added. . These could include rare star formation in molecular clouds outside the disk, or even relics of past dwarf galaxies that merged with the Milky Way in the past.

Christopher J. Aviles Bramer, an undergraduate student in astrophysics, graduating in 2022, contributed to the research project.