UR: Mass segregation in star clusters: nature or nurture?

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The undergrad research series is where we feature the research that you’re doing. If you’ve missed the previous installments, you can find them under the “Undergraduate Research” category here.

Are you doing an REU this summer? Were you working on an astro research project during this past school year? If you, too, have been working on a project that you want to share, we want to hear from you! Think you’re up to the challenge of describing your research carefully and clearly to a broad audience, in only one paragraph? Then send us a summary of it!

You can share what you’re doing by clicking here and using the form provided to submit a brief (fewer than 200 words) write-up of your work. The target audience is one familiar with astrophysics but not necessarily your specific subfield, so write clearly and try to avoid jargon. Feel free to also include either a visual regarding your research or else a photo of yourself.

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Carleen Markey

Purdue University

This guest post was written by Carleen Markey. Carleen is an undergraduate Physics and Statistics student at Purdue University who spent this past summer working on this research with Dr. Dávid Guszejnov and Dr. Stella Offner at the University of Texas at Austin. The results of this project have been submitted to the Research Notes of the American Astronomical Society under the title “Origin of Mass Segregation in Stellar Clusters in STARFORGE Simulations”.

Observed stellar clusters often contain stars which are segregated by mass, with the massive stars found in the cluster centers. These massive stars were either born there or they start in a random distribution of stars and move to the centers through gravitational interactions. Identifying the mechanism of mass segregation will help to constrain models of how stars form and evolve.

As the formation of stellar clusters takes millions of years, I tested the mass segregation mechanism by analyzing magneto-hydrodynamic star-formation simulations from the STARFORGE simulation suite. Using a density-based spatial clustering algorithm, I found clustering on two different scale sizes, clusters at a 1 parsec scale and subclusters at a 0.25 parsec scale. For each cluster and subcluster, we tracked the mass segregation ratio (lambda) over time, which measures the degree of mass segregation in the cluster. Throughout their lifetimes, both clusters and subclusters showed signs of massive stars both being born in the centers of clumps of newborn stars and moving to the centers of clusters and subclusters as these clumps merged to create these structures.

At the beginning, the massive stars were found on the outer edges of the cluster, instead of toward the center or randomly distributed, so lambda was less than or equal to 1 as in the left figure. This was because they were in the widely separated subclusters within the clusters, with each subcluster containing 1-2 massive stars. Therefore at this point, the distances between these subclusters dominate the mass segregation ratio. The first sign of dynamical effects moving massive stars to the centers were these small star-formation sites merging due to gravity and ejecting stars, causing lambda to decrease at ~0.8-1.1 Myr and ~1.5-1.75 Myr in the subcluster and at ~0.6-0.75 Myr, ~1.1-1.3 Myr, and ~1.5-2.0 Myr in the cluster. However, we then saw that the central migration of the cluster’s massive stars dominated over all other gravitational processes and caused both subclusters and clusters to end up with a lambda greater than one, indicating eventual mass segregation.

Overall, I found that mass segregation happened because of both nature and nurture. Stars were born at mass segregated sites with 1-2 massive stars. Over time, the subclusters and clusters then become more mass segregated through gravitational interactions.

On the left is mass segregation ratio versus cluster age, with dynamical interactions highlighted. Note: lambda of 1 equals log lambda of 0 on this scale. The right hand figure shows a set of clusters and subclusters when the cluster is ~0.6 Myr old, with cluster boundaries outlined. Taken and modified for clarity from the corresponding research note, “Origin of Mass Segregation in Stellar Clusters in STARFORGE Simulations”.

If you are an undergraduate that took part in an REU this summer and would like to share your research on Astrobites, please contact us at [email protected]

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