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Massive stars can be efficiently ejected from their birth clusters throughencounters with other massive stars. We study how the dynamical ejectionfraction of O star systems varies with the masses of very young star clusters,Mecl, by means of direct N -body calculations. We include diverse initialconditions by varying the half-mass radius, initial mass-segregation, initialbinary fraction and orbital parameters of the massive binaries. The resultsshow robustly that the ejection fraction of O star systems exhibits a maximumat a cluster mass of $10^{3.5}$ Msun for all models, even though the number ofthe ejected systems increases with cluster mass. We show that lower massclusters (Mecl ~ 400 Msun ) are the dominant sources for populating theGalactic field with O stars by dynamical ejections, considering the massfunction of embedded clusters. About 15 per cent (up to 38 per cent, dependingon the cluster models) of O stars of which a significant fraction are binaries,and which would have formed in a 10 Myr epoch of star formation in adistribution of embedded clusters, will be dynamically ejected to the field.Individual clusters may eject 100 per cent of their original O star content. Alarge fraction of such O stars have velocities up to only 10 km/s. Synthesisinga young star cluster mass function it follows, given the stellar-dynamicalresults presented here, that the observed fractions of field and runaway Ostars, and the binary fractions among them can be well understood theoreticallyif all O stars form in embedded clusters.

Dependency of dynamical ejections of O stars on the masses of very young star clusters