Surface chemical abundance patterns can identify massive stars that previously accreted material from binary companions, according to research published in Nature Astronomy. The findings allow researchers to reconstruct the evolutionary history of progenitor binary systems from observed surface compositions.
Around 70% of unevolved massive stars form in close binary systems where mass exchange is inevitable. Most products of these interactions appear as single stars because they lose their companions, merge with them, or leave faint, low-mass remnants. Up to 30% of core-hydrogen-burning stars and 70% of core-helium-burning stars are expected to have undergone binary interaction, yet identifying past mass transfer has historically been difficult because envelope mixing in single stars and wind mass loss also expose thermonuclear processed material.
Tracking Abundance Branches
Researchers analysed a grid of massive binary evolution models computed with Modules for Experiments in Stellar Astrophysics (MESA). The simulations tracked mass and angular momentum transfer, differential rotation, tides, and an extended nuclear network following stable carbon, nitrogen, and oxygen isotopes through hydrogen burning.
The models revealed that post-mass-transfer stars populate two distinct branches in nitrogen-to-carbon and nitrogen-to-oxygen abundance diagrams. While mass donors and self-stripping single stars occupy a branch framed by complete CNO-equilibrium dilution, mass gainers uniquely populate an upper branch near the CN-equilibrium dilution line. In mass gainers, accreting helium-enriched material triggers fast thermohaline mixing across the envelope on a thermal timescale of roughly 10,000 years. Subsequent slower thermohaline and rotational mixing draws unprocessed carbon down into the CN-burning layer at the envelope base, lifting CN-processed matter to the surface and elevating the nitrogen-to-carbon ratio.
Application to Known Stars
The team applied an analytic framework based on these signatures to several well-studied OB stars. For the naked-eye star γ Columbae, an apparent single star of about 6.0 solar masses previously suggested to be an envelope-stripped star, the analysis showed it is instead a mass gainer.
The nitrogen-to-oxygen ratio of γ Columbae indicated it accreted roughly 0.8 solar masses of CNO-equilibrium material with an average helium mass fraction of approximately 90%. This accretion required an initial donor star of at least 14 solar masses and an initial binary mass ratio below 0.35, pointing to highly non-conservative mass transfer. The researchers also applied the framework to HD 48279, HD 93840, ζ Ophiuchi, and the supernova SN 1987A.
