Precessional dynamics of black hole triples: binary mergers with near-zero effective spin

The binary black hole mergers detected by Advanced LIGO/Virgo have shown no evidence of large black hole spins. However, because LIGO/Virgo best measures the effective combination of the two spins along the orbital angular momentum ($\chi_{\rm eff}$), it is difficult to distinguish between binaries with slowly-spinning black holes and binaries with spins lying in the orbital plane. Here, we study the spin dynamics for binaries with a distant black hole companion. For spins initially aligned with the orbital angular momentum of the binary, we find that $\chi_{\rm eff}$ "freezes" near zero as the orbit decays through the emission of gravitational waves. Through a population study, we show that this process predominantly leads to merging black hole binaries with near-zero $\chi_{\rm eff}$. We conclude that if the detected black hole binaries were formed in triples, then this would explain their low $\chi_{\rm eff}$ without the need to invoke near-zero spins or initially large spin-orbit angles.