Stellar separation shapes spin-orbit alignment in visual binaries

TL;DR

This study uses hierarchical Bayesian modeling to analyze spin-orbit angles in visual binaries, revealing potential formation pathway transitions at specific separations.

Contribution

It identifies two distinct subpopulations of binary star spin-orbit alignments separated by a separation cutoff, refining understanding of binary formation mechanisms.

Findings

Binaries within 31-38 AU are more aligned, with a Fisher distribution κ=48.

Binaries outside 31-38 AU are less aligned, with a Fisher distribution κ=6.

Evidence suggests a secondary cutoff at 10-17 AU, indicating possible additional formation transition.

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 (1994) found that visual binaries with separations $\lesssim 30$ AU tend to be more aligned, which laid the groundwork for binary formation theories. However, A. B. Justesen & S. Albrecht (2020) found that underestimated stellar radii lead to inaccurate spin-orbit angles and that 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 $\sim 31-38$ AU cutoff. Binaries inside (outside) the cutoff are more (less) aligned, consistent with a Fisher distribution with $\kappa=48$ ($\kappa=6$). We also find possible indications of a secondary cutoff at $\sim 10-17$ AU, although more data is 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.