The upper main sequence in h and hi Persei

Binary interactions play a crucial role in the evolution and eventual outcomes of massive stars. Rich young open clusters provide large co-eval populations that can be used to constrain the effects of binary evolution. The Double Cluster, h and ̧hi Persei, with a turn-off mass well above the threshold for core-collapse supernovae, is an excellent test bed for these processes. We used the observed stellar populations of h and ̧hi Persei, together with the results of a similar study of NGC 663, to constrain the evolutionary pathways of massive stars and, in particular, the role of binary interaction. We present optical spectroscopy of around 80 stars, covering the entire upper main sequence of NGC 869 and most of the brightest B-type stars in NGC 884, and used it to obtain spectral types and derive stellar parameters. We combined this dataset with Gaia astrometry to evaluate their evolutionary status and possible evolutionary paths. We find that 40% of the stars along the photometric sequence of NGC 869 have parallaxes consistent with the cluster mean but proper motions inconsistent at the 2σ level, suggesting some sort of dynamical perturbation in this field. We derived atmospheric parameters for almost 60 stars in the upper main sequence (B2,V and earlier) and found broad agreement between two independent fully automated procedures. We identified the cluster turn-offs between B1,V and B1.5,--,2,IV and differentiated this feature from the vicinity of the terminal main sequence (TAMS), where we found a population of giants between B1 and B1.5 with a high fraction of Be stars. Three luminous supergiants (HD 14134, HD 14143, HD 13854) are clearly detached from the rest of the population in all our diagrams. The bulk of the population in all of the massive clusters studied in the Perseus arm is consistent with single-star evolutionary models at a given age, but some B-type supergiants are much more luminous than any other cluster star. Their typical cluster kinematics and stellar parameters suggest a merger origin. Other lower-luminosity B-type supergiants seem consistent with the rest of the population. Kinematic analysis has indicated that most Be stars have not been dynamically perturbed by a supernova explosion. Those below the turn-off are likely to have been formed via case A mass transfer. The more massive Be stars above the turn-off do not display any other characteristic indicating a distinct evolutionary population.