Rubin Data Preview 1: Extending the View of Unresolved Binary Stars in 47 Tucanae

Abstract: Until now, the study of unresolved main sequence binary stars in globular clusters has been possible almost exclusively in their central regions with deep Hubble Space Telescope (HST) observations. We present the first detection of unresolved main-sequence binary stars in the outer field of 47 Tucanae using Rubin Observatory's Data Preview 1 (DP1). Our analysis exploits deep $i$ vs. $g-i$ colour-magnitude diagrams beyond the cluster's half-light radius, reaching almost to the tidal radius. The high-quality photometry allowed to identify unresolved binaries with mass ratios $q$ larger than 0.7. The derived binary fraction of $f_\mathrm{bin}(q>0.7) = 0.016 \pm 0.005$ stands in contrast to the significantly lower values in the cluster innermost regions, as measured from HST photometry. This result provides new empirical input for testing physical processes that drive the formation and evolution of binary stars in globular clusters. It also demonstrates Rubin's unique wide-field and high-precision photometric capabilities to address a broader range of outstanding questions in star cluster research. Future full data releases will enable to significantly expand the study of dense stellar systems across the Milky Way.