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 〈 ψ 〉 = 6.9 − 4.1 + 5.4 degrees, while for e > 0.7, it rises to 〈 ψ 〉 = 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.