Abstract

More than 20\% of all massive stars in the Milky-way Galaxy have an unusual high velocity of $>30$\,km/s. The origin of this population of runaway OB stars has been puzzling astronomers for more than half a century. In one of the favorite explanations a star is launched from a binary system when its companion explodes in a core-collapse supernova. This mechanism fails to explain the high proportion of runaway stars that ware ejected from clusters, because the ejection occurred well before any star experienced a supernova explosion. We demonstrate that the observed runaways are explained by stellar ejections mediated by strong three-body interactions with a binary star. Such a binary forms during the core collapse of a young ($\aplt 1$\,Myr) and rather low mass ($ 5000-10000$\,\Msun) star cluster. This model reproduces the key characteristics of OB runaways in the Milky-way Galaxy and it explains the $\apgt 100$\,\Msun\, runaway stars around young star clusters, e.g. R136 and Westerlund 2. The high proportion and the distribution in mass of OB runaway stars in the Milky-way Galaxy can be explained if the vast majority of massive stars are born in dense and relatively low mass clusters.