Spindown of massive single main sequence stars in the Milky Way

Context. We need to understand the spin evolution of massive stars to compute their internal rotation-induced mixing processes, isolate effects of close binary evolution, and predict the rotation rates of white dwarfs, neutron stars, and black holes. Aims. We discuss the spindown of massive main sequence stars imposed by stellar winds. Methods. We used detailed grids of single-star evolutionary models to predict the distribution of the surface rotational velocities of core hydrogen-burning massive Galactic stars as function of their mass and evolutionary state. We then compared the spin properties of our synthetic populations with appropriately selected subsamples of Galactic main sequence OB-type stars extracted from the IACOB survey. Results. We find that observations and models agree that the surface rotational velocities of massive Galactic stars below ∼40 M ⊙ remain relatively constant during their main sequence evolution. The more massive stars in the IACOB sample appear to spin down less strongly than predicted, while our updated angular momentum loss prescription predicts a faster spindown. Furthermore, the observations show a population of fast rotators, v sin i ≳ 200 km s −1 , persisting for all ages. This is not reproduced by our synthetic single star populations. Conclusions. We conclude that the wind-induced spindown of massive main sequence stars is yet to be fully understood. In this regard, lower stellar wind mass-loss rates than we used here might alleviate the discrepancy between observations and predictions at higher masses, but would likely not solve it. We propose that binary evolution might significantly contribute to the fraction of rapid rotators in massive stars.