2024RNAAS...8...55W

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 $\rho =13\buildrel{\,\prime}\over{.} 3\leftrightarrow {\rm{41,300}}\,\mathrm{au}$, 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.