Helium against nitrogen abundances for the studied sample. Circles and crosses indicate Likely Single and binary stars, respectively, while identified runaway stars are highlighted by red symbols. Typical uncertainties in He and N abundances are marked with a gray error bar in the upper left corner. HD 226868 (optical counterpart of Cyg X-1) is surrounded by a green circle. Blue and gray shadowed strips delimit the locations of two published solar-abundance single-star-evolution tracks. Baseline abundances for these models are marked with a bluish-gray dot. The blue open contours surround the star sample with N and He abundance patterns not covered by the single-star evolutionary models considered.
There is increasing evidence that single-star evolutionary models are unable to reproduce all of the observational properties of massive stars. Binary interaction has emerged as a key factor in the evolution of a significant fraction of massive stars. In this study, we investigate the helium (Y(He)) and nitrogen surface abundances in a comprehensive sample of 180 Galactic O-type stars with projected rotational velocities of ≤150 km/s. We found a subsample (~20% of the total, and ~80% of the stars with Y(He) ≥ 0.12) with a Y(He) and nitrogen abundance combined pattern that is unexplainable by single-star evolution. We argue that the stars with anomalous surface abundance patterns are binary interaction products.
