Dependency of dynamical ejections of O stars on the masses of very young
  star clusters
  Massive stars can be efficiently ejected from their birth clusters through
encounters with other massive stars. We study how the dynamical ejection
fraction of O star systems varies with the masses of very young star clusters,
Mecl, by means of direct N -body calculations. We include diverse initial
conditions by varying the half-mass radius, initial mass-segregation, initial
binary fraction and orbital parameters of the massive binaries. The results
show robustly that the ejection fraction of O star systems exhibits a maximum
at a cluster mass of $10^{3.5}$ Msun for all models, even though the number of
the ejected systems increases with cluster mass. We show that lower mass
clusters (Mecl ~ 400 Msun ) are the dominant sources for populating the
Galactic field with O stars by dynamical ejections, considering the mass
function of embedded clusters. About 15 per cent (up to 38 per cent, depending
on 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 a
distribution of embedded clusters, will be dynamically ejected to the field.
Individual clusters may eject 100 per cent of their original O star content. A
large fraction of such O stars have velocities up to only 10 km/s. Synthesising
a young star cluster mass function it follows, given the stellar-dynamical
results presented here, that the observed fractions of field and runaway O
stars, and the binary fractions among them can be well understood theoretically
if all O stars form in embedded clusters.
