The IACOB project

Context. In recent years, a growing amount of evidence has revealed the crucial role of binarity in massive star evolution. This additional complexity compounds the uncertainties that still affect single-star evolution models and demands a refinement of the available observational constraints. A first crucial step toward this goal involves disentangling observed stars that have evolved in isolation from those that have experienced binary interaction. Aims. We aim to investigate the possible evolutionary origins of a sample of 117 Galactic O-type stars with luminosity classes V to III and projected rotational velocities ( v sin i ) below ∼150 km s −1 . Methods. We mostly focused on surface nitrogen and helium abundances but also considered other dynamical signatures that may help distinguish products of binary interaction from effectively single stars. We have therefore extended previous quantitative spectroscopic analyses performed within the framework of the IACOB project, and we obtained N abundance estimates. We investigated the correlations between these abundances and other stellar parameters, such as v sin i , effective temperature, surface gravity, and He abundance. As a reference, we used state-of-the-art predictions from single-star evolution models computed using different physical prescriptions. Results. We found good agreement between our N abundance estimates and previous determinations based on different analysis methodologies and stellar atmosphere codes. We identified clear differences in the N abundance distributions corresponding to three He abundance regimes, defined as He-low ( Y He = N (He)/ N (H)≤0.08), He-normal (0.08 < Y He ≤ 0.12), and He-rich ( Y He > 0.12). We argue that the abundance estimates for the He-low group, as well as for some additional stars with abnormally low N abundances, are likely spurious determinations. For the He-normal group, the N abundance distribution peaks slightly above the expected birth value and extends up to ϵ N = log(N/H)+12 ∼ 8.4 dex. For these stars, we found an overall agreement with single-star evolutionary models that include efficient internal mixing and assume moderate-to-low initial rotation ( v ini / v crit ≲ 0.2). In contrast, the He-rich group exhibits a bimodal N abundance distribution, with one peak at ∼8.1 dex corresponding to mildly enriched stars and a second more enriched peak around ∼8.5 dex. None of these stars are consistent with predictions from state-of-the-art single-star evolutionary models. Conclusions. We argue that the two N abundance subgroups among the He-rich stars are most plausibly explained as binary products. Furthermore, despite the N abundance in He-normal stars with luminosity classes (LC) IV and V being reproduced by single-star evolutionary models with efficient mixing models, the same models predict a higher N abundance than observed for stars in this group with LC III. This indicates that rotational mixing alone is unable to explain the observed distribution of N abundances among stars with normal He abundances. A future comprehensive study of surface abundances in O-type supergiants (LC I and II) and fast rotators that also incorporates the abundances of additional elements is essential to further constraining the evolutionary channels of the most massive stars during the main sequence.