The BANANA Project. VII. High Eccentricity Predicts Spin–Orbit Misalignment in Binaries

The degree of spin–orbit alignment in a population of binary stars can be determined from measurements of their orbital inclinations and rotational broadening of their spectral lines. Alignment in a face-on binary guarantees low rotational broadening, while alignment in an edge-on binary maximizes the rotational broadening. In contrast, if spin–orbit angles ( ψ ) are random, rotational broadening should not depend on orbital inclination. Using this technique, we investigated a sample of 2727 astrometric binaries from Gaia DR3 with F-type primaries and orbital periods between 50 and 1000 days (separations 0.3–2.7 au). We found that ψ is strongly associated with e , the orbital eccentricity. When e < 0.15, the mean spin–orbit angle is $\langle \psi \rangle ={6.9}_{-4.1}^{+5.4}$ degrees, while for e > 0.7, it rises to $\langle \psi \rangle ={46}_{-24}^{+26}$ degrees. These results suggest that some binaries are affected by processes during their formation or evolution that excite both orbital eccentricity and inclination.