A Self-Consistent Bayesian Investigation of Star Cluster Binary Populations

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About 2/3 of the stars in our galaxy reside in close pairs, called “binary stars”. Many of these stars are born within larger families known as “star clusters”. Yet despite how common and important binary stars are, vital questions remain. The investigator will use newly available observations and state-of-the-art software to produce a large database of the empirical properties of binary stars and their host star clusters. The investigator will then use this database to examine theoretical predictions. This database will be made publicly available through a custom website and will enable many follow-up studies by the astronomical community. The investigator will also mentor up to nine undergraduate. Each student will learn skills in research methods, coding, data analysis, and web development. The investigator will help to advance a more diverse, sophisticated, and highly competitive STEM workforce.The investigator and his team will self-consistently characterize the binary star populations in hundreds of open star clusters, across a wide range of cluster properties, using a Bayesian Analysis of Stellar Evolution (BASE-9) tool. For each cluster, the investigator will compile available photometry from Gaia, 2MASS and Pan-STARRS, and use these data within the BASE-9 software to (a) identify photometric binaries and determine their masses and mass ratios, (b) establish the cluster age, distance, reddening and metallicity, and (c) derive star-by-star photometric membership probabilities and masses. The investigator will use this database to examine predicted trends in binary frequency and mass-ratio distribution across different clusters (as functions of, e.g., age, metallicity, density, cluster mass, etc.) as predicted by stellar dynamical and star formation models, and within each individual cluster (e.g., as a function of distance from the cluster center) as predicted by two-body relaxation and mass segregation effects. The investigator will also develop a custom online interactive table and plotting utility that will provide public access to this database of binary and cluster parameters. Most students working with the investigator will be recruited from the Chicago area through an existing network that the investigator has developed as part of the NSF-funded Research Experiences for Undergraduates (REU) program he directs at Northwestern. In addition to the directed research projects, these students will also create their own research websites and help to develop the interactive website that will host the binary star and star cluster database. Over the summer, each student will be fully integrated into Northwestern’s astrophysics REU program, which provides numerous professional development workshops, including coding and high-performance computing, science communication and career planning discussions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.