Massive Stars: Their Birth Sites and Distribution

The stellar IMF has been found to be an invariant Salpeter power-law (α = 2.35) above about 1M⊙, but at the same time a massive star typically has more than one companion. This constrains the possible formation scenarios of massive stars, but also implies that the true, binary-star corrected stellar IMF could be significantly steeper than Salpeter, α > 2.7. A significant fraction of all OB stars are found relatively far from potential birth sites which is most probably a result of dynamical ejections from cores of binary-rich star clusters. Such cores form rapidly due to dynamical mass segregation, or they are primordial. Probably all OB stars thus form in stellar clusters together with low-mass stars, and they have a rather devastating effect on the embedded cluster by rapidly driving out the remaining gas leaving expanding OB associations and bound star clusters. The distributed population of OB stars has a measured IMF with α ≈ 4, which however, does not necessarily constitute a different physical mode for isolated star formation. A steep field-star IMF is obtained naturally because stars form in clusters which are distributed according to a power-law cluster mass function.