Stellar Separation Shapes Spin–Orbit Alignment in Visual Binaries

Abstract Stellar binaries may form through several formation pathways, including disk or core fragmentation. Their spin–orbit angles are a signature of formation, although individual measurements for visual binaries are limited and broad. A seminal work by A. Hale found that visual binaries with separations ≲30 au tend to be more aligned, which laid the groundwork for binary formation theories. However, A. B. Justesen & S. Albrecht found that underestimated stellar radii lead to inaccurate spin–orbit angles and that Kolmogorov–Smirnov (KS) statistics do not provide meaningful population-level constraints, even with updated radii. Using a hierarchical Bayesian model to reanalyze their dataset, we find evidence with a Bayes factor of 12 for two subpopulations of spin–orbit angles separated by a ∼31–38 au cutoff. Binaries inside (outside) the cutoff are more (less) aligned, consistent with a Fisher distribution with κ = 48 ( κ = 6). We also find possible indications of a secondary cutoff at ∼10–17 au, although more data are required to resolve this prediction. These cutoffs may mark transitions between formation pathways: closer-in binaries tend to form aligned in a shared protostellar disk, while wider binaries tend to form less aligned through turbulent fragmentation.