Spin-Orbit Alignment of Early-type Astrometric Binaries and the Origin of Slow Rotators

The spin-orbit alignment of binary stars traces their formation and accretion history. Previous studies of spin-orbit alignment have been limited to small samples, slowly rotating solar-type stars, and/or wide visual binaries that not surprisingly manifest random spin-orbit orientations. We analyze 917 Gaia astrometric binaries across periods P = 100-3000 days (a = 0.5-5 au) that have B8-F1 IV/V primaries (M ₁ = 1.5-3 M _⊙) and measured projected rotational velocities v sin i. The primary stars in face-on orbits exhibit substantially smaller v sin i compared to those in edge-on orbits at the 6σ level, demonstrating significant spin-orbit alignment. The primaries in our astrometric binaries are rotating more slowly than their single-star or wide-binary counterparts and therefore comprise the slow-rotator population in the observed bimodal rotational velocity distribution of early-type stars. We discuss formation models of close binaries where some of the disk angular momentum is transferred to the orbit and/or secondary spin, quenching angular momentum flow to the primary spin. The primaries in astrometric binaries with small mass ratios q = M ₂/M ₁ < 0.3 possess even smaller v sin i, consistent with model predictions. Meanwhile, astrometric binaries with large eccentricities e > 0.4 do not display spin-orbit alignment or spin reduction. Using a Monte Carlo technique, we measure a spin-orbit alignment fraction of F _align = 75% ± 5% and an average spin reduction factor of 〈S _align〉 = 0.43 ± 0.04. We conclude that 75% of close A-type binaries likely experienced circumbinary disk accretion and probably formed via disk fragmentation and inward disk migration. The remaining 25%, mostly those with e > 0.4, likely formed via core fragmentation and orbital decay via dynamical friction.