Context. Growing evidence is revealing 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 is to disentangle observed stars that have evolved in isolation from those that have experienced binary interaction.
Aims. 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 sini) below ∼150 km/s
Methods. We mostly focus on surface nitrogen and helium abundances, but also consider other dynamical signatures that may help to distinguish products of binary interaction from effectively single stars. We extend previous quantitative spectroscopic analyses performed within the framework of the IACOB project and obtain N abundance estimates. We investigate correlations between these abundances and other stellar parameters, such as v sini, effective temperature, surface gravity, and He abundance. As a reference, we use state-of-the-art predictions from single-star evolution models computed using different physical prescriptions.
Results. We find good agreement between our N abundance estimates and previous determinations based on different analysis methodologies and stellar atmosphere codes. We identify clear differences in the N abundance distributions corresponding to three He abundance regimes, defined as He-low (YHe = N(He)/N(H) ≤ 0.08), He-normal (0.08 < YHe ≤ 0.12), and He-rich (YHe > 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 ϵN = log(N/H) + 12 ∼ 8.4 dex. For these stars, we find overall agreement with single-star evolutionary models that include efficient internal mixing and assume moderate-to-low initial rotation (vini /vcrit ≲ 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 both N abundance subgroups among He-rich stars are most plausibly explained as binary products. Furthermore, despite N abundance in He-normal stars with LC IV and V are reproduced by single stellar 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, also incorporating abundances of additional elements, is essential to further constrain the evolutionary channels of the most massive stars during the main sequence.