Institute of Astronomy

The origin of OB runaways and the Astronomical Multipurpose Software Environment

SpeakerTalk DateTalk Series
Simon Portegies Zwart (Leiden)15 March 2012Institute of Astronomy Colloquia

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.

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