Stellar mergers set black-hole natal spins up to a=0.5 in 10% of cluster-formed systems

The study couples direct N-body integration in CMC with detailed MESA post-merger evolution to track angular-momentum deposition into the envelope and core of merger products. Significant mergers (q>0.1) leave rapidly rotating supergiants whose cores retain enough specific angular momentum to avoid the near-zero natal spins predicted by standard wind-driven angular-momentum loss in isolated stars. Roughly half of all black holes in the simulated clusters form through some merger channel, with the q>0.1 subset supplying the high-spin tail.

These results directly confront the isolated-binary formation narrative that dominates current LIGO/Virgo/KAGRA population inferences. Effective-spin distributions inferred from GWTC-3 are broadly consistent with low natal spins; the cluster channel supplies a concrete dynamical pathway that can populate the moderate-spin regime without requiring fine-tuned common-envelope physics. Retention fractions and subsequent dynamical pairing further amplify the contribution of these objects to the detectable merger rate.

The principal limitation is the still-modest number of high-fidelity MESA follow-ups performed on the CMC merger candidates; full population synthesis that self-consistently evolves post-merger structure across the entire cluster grid is required before quantitative rate predictions can be compared with O4/O5 data. Upcoming LVK catalogs that resolve the spin distribution above χ_eff>0.2 will provide the decisive test.