Following the Tides: Stellar Streams in Open Clusters with Gaia DR3

In this study, Ira Sharma and collaborators explore tidal tails, long streams of stars that stretch out from open star clusters due to the gravitational pull of the Milky Way. These structures are important because they reveal how clusters lose stars over time, as well as how they move through the Galaxy. Past attempts to detect these features were limited by faint signals and contamination from background stars. With the unprecedented precision of the Gaia mission, however, astronomers can now identify these elusive stellar streams more clearly.

Clusters and Methods

The team focused on five open clusters, BH 164, Alessi 2, NGC 2281, NGC 2354, and M67, spanning a wide range of ages from 60 million years to 4 billion years. Using Gaia DR3 data, they developed a method that combines machine learning, clustering algorithms, and statistical checks to identify tidal tails. They filtered the data carefully, selecting stars with reliable measurements of motion and distance, and then applied an algorithm called DBSCAN to group stars belonging to each cluster. From there, they used tools such as color-magnitude diagrams (to compare the brightness and color of stars) and principal component analysis (to reveal the preferred direction of elongation) to separate tail stars from unrelated stars in the field.

Analysis of the Tails

Once the tails were identified, the researchers investigated their properties. They corrected for observational distortions and modeled the clusters’ orbits within the Milky Way to determine the direction of leading and trailing tails. The stellar populations in the tails were compared to those in the cluster cores by looking at luminosity functions, which show how many stars are found at each brightness. Interestingly, the tails generally lacked massive stars and instead had a higher fraction of binary stars. This suggested that interactions inside the clusters may have preferentially ejected binary systems into the tails. For two of the clusters, M67 and NGC 2281, the authors were also able to study rotational motion and discovered significant rotation for the first time.

Results

The results confirmed tidal tails in all five clusters. The youngest cluster, BH 164, had the shortest tails, while the oldest, M67, had the longest, stretching more than 100 parsecs. In every case, the tails extended well beyond the theoretical “Jacobi radius,” the distance at which the Galaxy’s gravity begins to dominate over the cluster’s. For M67 and NGC 2281, the rotation measurements fit well with standard Newtonian dynamics, and the mass of each cluster was estimated using Plummer model fits. Overall, the tails contained between 100 and 200 stars each, with lengths spanning 40–100 parsecs.

Conclusion

In conclusion, Sharma and colleagues showed that optimized statistical methods can detect tidal tails even at distances of up to a kiloparsec, much farther than earlier techniques allowed. These methods not only confirm the presence of tidal streams in nearby open clusters but also open the door to uncovering similar structures in more distant systems. Such findings refine our understanding of how clusters evolve under the influence of the Milky Way’s gravity and highlight the ongoing role of Gaia in revolutionizing stellar dynamics studies.

Source: Sharma