Flipping spins in mass transferring binaries and origin of spin-orbit misalignment in binary black holes

TL;DR

This paper models how mass transfer in close binaries can cause the donor star's spin to flip onto the orbital plane, significantly affecting the spin-orbit alignment in binary black hole systems.

Contribution

It derives new orbit-averaged equations for spin evolution during mass transfer and demonstrates the potential for large spin flips, impacting black hole spin predictions.

Findings

Mass transfer can cause donor spin flips of over 30% of initial mass.

Spin flips dominate over tidal effects in certain stellar mass ranges.

Result supports minimal contribution of stellar spin to black hole effective spin in binaries.

Abstract

Close stellar binaries are prone to undergo a phase of stable mass transfer in which a star loses mass to its companion. Assuming that the donor star loses mass along the instantaneous interstellar axis, we derive the orbit-averaged equations of motion describing the evolution of the donor rotational angular momentum vector (spin) which accompanies the transfer of mass. We consider: (i) a model in which the mass transfer rate is constant within each orbit and (ii) a phase-dependent rate in which all mass per orbit is lost at periapsis. In both cases, we find that the ejection of $\gtrsim 30$ per cent of the donor's initial mass causes its spin to nearly flip onto the orbital plane of the binary, independently of the initial spin-orbit alignment. Moreover, we show that the spin flip due to mass transfer can easily dominate over tidal synchronisation in any giant stars and main-sequence stars with masses $\sim1.5$ to $5\,\rm M_\odot$. Finally, the general equations of motion, including tides, are used to evolve a realistic population of massive binary stars leading to the formation of binary black holes. Assuming that the stellar core and envelope are fully coupled, the resulting tilt of the first-born black hole reduces its spin projection onto the orbit normal by a factor $\sim\mathcal{O}(0.1)$. This result supports previous studies in favour of an insignificant contribution to the effective spin projection, $\chi_{\rm eff}$, in binary black holes formed from the evolution of field binaries.