The IACOB project XIII. Helium enrichment in O-type stars as a tracer of past binary interaction

There is increasing evidence that single-star evolutionary models are inadequate to reproduce all 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 ($Y_{\mathrm{H}e}$) and nitrogen (${\epsilon }_{\mathrm{N}}$) surface abundances in a comprehensive sample of 180 Galactic O-type stars with projected rotational velocities $v\sin (i)\le 150\mathrm{k}m\cdot {\mathrm{s}}^{-1}$. We found a subsample ($\sim 20\mathrm{\%}$ of the total, and $\sim 80\mathrm{\%}$ of the stars with $Y_{\mathrm{H}e}\ge 0.12$) with a $Y_{\mathrm{H}e}$ and ${\epsilon }_{\mathrm{N}}$ combined pattern unexplainable by single-star evolution. We argue that the stars with anomalous surface abundance patterns are binary interaction products.