Global mass segregation in hydrodynamical simulations of star formation

Recent analyses of mass segregation diagnostics in star-forming regions invite a comparison with the output of hydrodynamic simulations of star formation. In this work we investigate the state of mass segregation of 'stars' (i.e. sink particles in the simulations) in the case of hydrodynamical simulations which omit feedback. We first discuss methods to quantify mass segregation in substructured regions, either based on the minimum spanning tree (Allison's Λ), or through analysis of correlations between stellar mass and local stellar surface number densities. We find that the presence of even a single 'outlier' (i.e. a massive object far from other stars) can cause the Allison Λ method to describe the system as inversely mass segregated, even where in reality the most massive sink particles are overwhelmingly in the centres of the subclusters. We demonstrate that a variant of the Λ method is less susceptible to this tendency but also argue for an alternative representation of the data in the plane of stellar mass versus local surface number density. The hydrodynamical simulations show global mass segregation from very early times which continues throughout the simulation, being only mildly influenced during subcluster merging. We find that up to ≈2-3 per cent of the 'massive' sink particles (m > 2.5 M⊙) are in relative isolation because they have formed there, although other sink particles can form later in their vicinity. Ejections of massive sinks from subclusters do not contribute to the number of isolated massive sink particles, as the gravitational softening in the calculation suppresses this process.