Abstract
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 ρ=13.′3↔41,300au , an association that has been missed in previous wide binary searches. We estimate a projected velocity difference Δv 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.
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