Determine how massive binaries shrink to very tight (≲ few au) separations

Characterize the physical mechanisms and timescales by which massive binaries shrink from their initial separations to very tight (≲ few au) configurations observed today, identifying the contributions of disc-driven migration, multi-scale fragmentation, and hierarchical dynamical evolution.

Background

Observations show massive binaries at very tight separations, but theory has struggled to explain how systems evolve from initial formation scales to such compact orbits. The authors note bimodal separation distributions and multiple formation channels, suggesting that subsequent migration must play a key role.

This statement in the introduction frames the central challenge of linking initial assembly with later orbital shrinkage to sub-au scales, which their simulation partially addresses via disc interactions and circumbinary phases while leaving the final steps of extreme hardening unresolved.