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

• Published in: The Astrophysical journal Vol. 975; no. 1; pp. 153 - 169
• Main Authors: Smith, Chase L., Moe, Maxwell, Kratter, Kaitlin M.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.11.2024; IOP Publishing
• Subjects: Accretion disks; Alignment; Angular momentum; Angular velocity; Astrometric binary stars; Binary stars; Early-type stars; Fluid flow; Fragmentation; Mass ratios; Orbit decay; Orbits; Rotation; Spin reduction; Stellar evolution; Velocity distribution
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ad6dd2

Abstract 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 6 σ 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.