Hidden Companions to Intermediate-mass Stars. XVI. Unveiling a 41,300 au Companion to the Very Eccentric Binary Zeta Boötis

Zeta Boötis is a nearby 2.2 M ⊙ + 2.2 M ⊙ 125 yr period visual binary distinguished by its very high eccentricity e ≈ 0.98. Here we report that HIP 71759 is a very wide 1.65 M ⊙ companion to ζ Boo at a projected separation <?CDATA ρ =13\,. 3↔ 41,300\,au?> ρ = 13 .′ 3 ↔ 41,300 au , an association that has been missed in previous wide binary searches. We estimate a projected velocity difference Δ v 2D = 0.86 ± 0.20 km s −1 compared to the Newtonian 2D escape velocity v esc,2D = 0.5 km s −1 . We propose that the triple system is gravitationally bound and that it was once much more compact, with the outer companion exciting high amplitude Kozai–Lidov eccentricity oscillations in ζ Boo as it also migrated outwards by removing energy from the inner binary. The Galactic tidal field eventually shut off the triple interaction, freezing the e ≈ 0.98 eccentricity of ζ Boo.