The Origin of OB Runaway Stars

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Science  17 Nov 2011:
DOI: 10.1126/science.1211927


About 20% of all massive stars in the Milky Way have unusually high velocities, the origin of which has puzzled astronomers for half a century. We argue that these velocities originate from strong gravitational interactions between single stars and binaries in the centers of star clusters. The ejecting binary forms naturally during the collapse of a young (~ < 1 Myr) star cluster. This model replicates the key characteristics of OB runaways in our galaxy, and it explains the ~> 100 M runaway stars around young star clusters (e.g., R136 and Westerlund 2). The high proportion and the distributions in mass and velocity of runaways in the Milky Way is reproduced if the majority of massive stars are born in dense and relatively low-mass (5000 to 10,000 M) clusters.