Journal
ASTROPHYSICAL JOURNAL
Volume 805, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/805/2/92
Keywords
galaxies: star clusters: general; methods: numerical; open clusters and associations: general; stars: kinematics and dynamics; stars: massive
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Massive stars can be efficiently ejected from their birth star 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, M-ecl, 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) M-circle dot for all models, even though the number of ejected systems increases with cluster mass. We show that lower mass clusters (M-ecl approximate to 400 M-circle dot) are the dominant sources for populating the Galactic field with O stars by dynamical ejections, considering the mass function of embedded clusters. About 15% (up to approximate to 38%, depending on the cluster models) of O stars of which a significant fraction are binaries, and which would have formed in a approximate to 10 Myr epoch of star formation in a distribution of embedded clusters, will be dynamically ejected to the field. Individual clusters may eject 100% of their original O star content. A large fraction of such O stars have velocities up to only 10 km s(-1). 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.
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