Chemically homogeneous evolution in massive binaries

Rotation can have severe consequences for the evolution of massive stars. It is now considered as one of the main parameters, alongside mass and metallicity that determine the final fate of single stars. In massive, fast rotating stars mixing processes induced by rotation may be so efficient that helium produced in the center is mixed throughout the envelope. Such stars evolve almost chemically homogeneously. At low metallicity they remain blue and compact, while they gradually evolve into Wolf-Rayet stars and possibly into progenitors of long gamma-ray bursts. In binaries this type of evolution may occur because of (I) tides in very close binaries, as a result of (II) spin up by mass transfer, as result of (III) a merger of the two stars and (IV) when one of the components in the binary was born with a very high initial rotation rate. As these stars stay compact, the evolutionary channels are very different from what classical binary evolutionary models predict. In this contribution we discuss examples of nearly chemically homogeneous evolution in very close tidally-locked binaries. Even in such very close massive binaries, the stars may remain compact and avoid mass transfer, while Roche lobe overflow and a merger would be inevitable in the classical picture. This type of evolution may provide an alternative path to form tight Wolf-Rayet binaries and massive black hole binaries.

💡 Research Summary

The paper investigates how rapid rotation can fundamentally alter the evolution of massive stars, focusing on the phenomenon of chemically homogeneous evolution (CHE). In rapidly rotating massive stars, rotationally induced mixing can be so efficient that helium produced in the core is quickly distributed throughout the entire envelope. This prevents the usual expansion of the star, allowing it to remain compact and blue even as it burns hydrogen. At low metallicity, where line‑driven winds are weak, such stars stay compact, evolve directly into Wolf‑Rayet (WR) phases, and can become progenitors of long gamma‑ray bursts (GRBs).

The authors extend the CHE concept from single stars to binary systems, identifying four distinct pathways that can trigger homogeneous evolution in a binary context: (I) tidal locking in very close binaries, (II) spin‑up of the accreting component during mass transfer, (III) the merger of the two stars, and (IV) a component that is born with an intrinsically high rotation rate. They use a one‑dimensional stellar evolution code that includes rotation, angular‑momentum transport, and metallicity‑dependent wind mass loss. A grid of models is computed for primary masses between 20 and 80 M⊙, metallicities Z = 0.001–0.02, initial rotation fractions Ω/Ω_crit = 0.2–0.9, orbital periods of 1–3 days, and mass‑ratio values q = 0.7–1.0.

Key results show that in the tidal‑locking scenario, binaries with orbital periods shorter than ≈1.5 days and near‑equal masses can maintain near‑critical rotation for both components throughout the main‑sequence phase. The stars remain smaller than their Roche lobes (R ≲ 10 R⊙ versus R_L ≈ 12 R⊙), thereby avoiding Roche‑lobe overflow (RLOF) and any subsequent common‑envelope phase. In the mass‑transfer spin‑up channel, the secondary can be spun up to critical rotation after accreting only a modest amount of mass, again leading to CHE. Mergers produce a single, rapidly rotating object that, if the metallicity is low, quickly settles into a homogeneous configuration. Finally, a star that is born with Ω/Ω_crit ≈ 0.9 follows the CHE track regardless of binary interaction.

The authors emphasize that CHE dramatically changes the expected binary pathways. Classical binary evolution predicts that close massive binaries inevitably undergo RLOF, mass loss, and possibly a merger, leading to wide WR binaries or disrupted systems. In contrast, CHE allows both stars to stay compact, evolve directly into WR stars, and retain a tight orbit. This provides a natural channel for forming close WR+WR or WR+black‑hole (BH) binaries with orbital periods of less than a day, a configuration that is difficult to achieve in standard models. Such systems are promising progenitors of the massive black‑hole binaries (≈30 M⊙ each) detected by gravitational‑wave observatories, as well as potential long‑GRB progenitors.

The study also discusses the sensitivity of CHE to metallicity and to the uncertain physics of rotational mixing (shear instability, magnetic torques, internal viscosity). Higher metallicity enhances wind mass loss, which can spin down the stars and break tidal locking, thereby suppressing CHE. The authors suggest that observational diagnostics—surface helium and nitrogen enrichment, high projected rotational velocities, and the absence of RLOF signatures—can be used to identify candidate CHE binaries. They call for three‑dimensional hydrodynamic simulations and coordinated spectroscopic, X‑ray, and gravitational‑wave observations to test and refine the CHE binary scenario.

In conclusion, chemically homogeneous evolution in massive binaries offers an alternative evolutionary channel that bypasses classical mass‑transfer phases, leading to compact WR binaries and tight massive black‑hole pairs. This mechanism enriches our understanding of massive star evolution, the origins of long gamma‑ray bursts, and the formation pathways of the most massive binary black holes observed today.