A new paper from Sambaran Banerjee, Aleksandra Olejak, and Krzysztof Belczyński asks whether the mild preference for aligned spins seen in LIGO/Virgo’s catalogue of binary black hole mergers can help us distinguish between formation in isolated field binaries versus dynamical assembly in young massive star clusters. The spin alignment of merging black holes had previously been proposed as a way to favour the isolated binary channel, since field binaries might retain a memory of their shared orbital history in their spins, while cluster-assembled pairs would not.
The paper shows that this distinction is less clean than assumed. Young massive clusters also produce a spin alignment asymmetry, because many of the merging pairs within them were originally born as binary stars and retain partial alignment despite subsequent dynamical perturbations. Both channels, combined with realistic natal kick distributions, reproduce the observed spin alignment distribution. Disentangling the two will require more precise measurements from future gravitational wave observations rather than spin alignment alone. StarTrack population synthesis, with Universe@Home providing part of the computational support, underpins the isolated binary component of the analysis.
Link to paper: https://arxiv.org/abs/2302.10851