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

Abstract

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.