Applying Bayesian statistics to identify binaries within the open cluster NGC 6819

We use a Bayesian statistics software suite called BASE-9 to photometrically identify binary stars within the open cluster NGC 6819. We compile photometry from the Gaia, 2MASS, and Pan-STARRS surveys. Using Gaia kinematics and distances, we identify likely cluster members. We then feed these probable cluster members into BASE-9 to recover the cluster's age, distance, reddening, and metallicity as well as star-by-star masses and mass ratios for probable binaries. We identify binaries as those objects that have a posterior distribution of secondary mass whose median value is ≥ 3σ away from zero. We compare NGC 6819's age, distance, reddening, metallicity and binary fraction derived using BASE-9 to previous literature values, and find good agreement. Importantly, our binary study extends ~5 magnitudes fainter than previous radial-velocity binary surveys, enabling investigations of binary properties as a function of primary-star mass.