Do O-stars form in isolation?

Around 4 per cent of O-stars are observed in apparent isolation, with no associated cluster, and no indication of having been ejected from a nearby cluster. We define an isolated O-star as a star >17.5 M⊙ in a cluster with total mass <100 M⊙ which contains no other massive (>10 M⊙) stars. We show that the fraction of apparently isolated O-stars is reproduced when stars are sampled (randomly) from a standard initial mass function and a standard cluster mass function of the form N(M) ∝M−2. This result is difficult to reconcile with the idea that there is a fundamental relationship between the mass of a cluster and the mass of the most massive star in that cluster. We suggest that such a relationship is a typical result of star formation in clusters, and that ‘isolated O-stars’ are low-mass clusters in which massive stars have been able to form.