Tidal Spin-up of Black Hole Progenitor Stars

Gravitational wave observations indicate the existence of merging black holes (BHs) with high spin ($a\gtrsim0.3$), whose formation pathways are still an open question. A possible way to form those binaries is through the tidal spin-up of a Wolf-Rayet (WR) star by its BH companion. In this work, we investigate this scenario by directly calculating the tidal excitation of oscillation modes in WR star models, determining the tidal spin-up rate, and integrating the coupled spin-orbit evolution for WR-BH binaries. We find that for short-period orbits and massive WR stars, the tidal interaction is mostly contributed by standing gravity modes, in contrast to Zahn's model of travelling waves which is frequently assumed in the literature. The standing modes are less efficiently damped than traveling waves, meaning that prior estimates of tidal spin-up may be overestimated. We show that tidal synchronization is rarely reached in WR-BH binaries, and the resulting BH spins have $a \lesssim 0.4$ for all but the shortest period ($P_{\rm orb} \! \lesssim 0.5 \, {\rm d}$) binaries. Tidal spin-up in lower-mass systems is more efficient, providing an anti-correlation between the mass and spin of the BHs, which could be tested in future gravitational wave data. Nonlinear damping processes are poorly understood but may allow for more efficient tidal spin-up. We also discuss a new class of gravito-thermal modes that appear in our calculations.