Investigating Mass Segregation of the Binary Stars in the Open Cluster NGC 6819

Claire Zwicker, Aaron M. Geller, Anna C. Childs, Erin Motherway, Ted von Hippel

Published 2023-08-29Version 1

We search for mass segregation in the intermediate-aged open cluster NGC 6819 within a carefully identified sample of likely cluster members. Using photometry from the Gaia, 2MASS, and Pan-STARRS surveys as inputs for a Bayesian statistics software suite, BASE-9, we identify a rich population of (photometric) binaries and derive posterior distributions for the cluster age, distance, metallicity and reddening as well as star-by-star photometric membership probabilities, masses and mass ratios (for binaries). Within our entire sample, we find 2781 likely cluster members and 831 binaries. We select a main-sequence 'primary sample' with 14.85 < G < 19.5 containing 1515 likely cluster members and 256 binaries with mass ratios q > 0.5, to investigate for mass segregation. Within this primary sample, we find the binary radial distribution is significantly shifted toward the cluster center as compared to the single stars, resulting in a binary fraction that increases significantly toward the cluster core. Furthermore, we find that within the binary sample, more massive binaries have more centrally concentrated radial distributions than less massive binaries. The same is true for the single stars. As the cluster has persisted through several half-mass relaxation times, and the expected mass-segregation timescale for stars in our primary sample is also significantly shorter than the cluster age, we interpret these results as strong evidence for mass segregation in the cluster. Importantly, this is the first study to investigate mass segregation of the binaries in the open cluster NGC 6819.