Unequal-mass highly spinning binary black hole mergers in the stable mass transfer formation channel

Context. The growing database of gravitational wave (GW) detections with binary black holes (BHs) merging in the distant Universe contains subtle insights into their formation scenarios. Aims. We investigated one of the puzzling properties of detected GW sources, namely, the possible (anti)correlation between the mass ratio q of BH-BH binaries and their effective spin χ_eff. In particular, unequal-mass systems tend to exhibit higher spins than those with nearly equal-mass BH components. Methods. We used rapid binary evolution models to demonstrate that the isolated binary evolution followed by efficient tidal spin-up of stripped helium core produces a similar pattern in χ_eff versus q distributions of BH–BH mergers. Results. In our models, the progenitors of unequal BH-BH systems in the stable mass transfer formation scenario are more likely to efficiently shrink their orbits during the second Roche-lobe overflow than the binaries that evolve into nearly equal-mass component systems. This makes it easier for unequal-mass progenitors to enter the tidal spin-up regime and later merge due to GW emission. Our results are, however, sensitive to some input assumptions, especially the stability of mass transfer and the angular momentum loss during nonconservative mass transfer. We note that mass transfer prescriptions widely adopted in rapid codes favor the formation of BH–BH merger progenitors with unequal masses and moderate separations. We compared our results with detailed stellar model grids and found reasonable agreement after appropriate calibration of the physics models. Conclusions. We anticipate that future detections of unequal-mass BH–BH mergers could provide valuable constraints on the role of the stable mass transfer formation channel. A significant fraction of BH-BH detections with mass ratio q ∈ (0.4 ‑ 0.7) would be consistent with having a mass ratio reversal scenario during the first relatively conservative mass transfer and a non-enhanced angular momentum loss during the second highly nonconservative mass transfer phase.