YOUNG RUNAWAY STARS

Most stars have spatial velocities within a few km/s of the average velocity of their surroundings and can be considered to be gravitationally trapped. There are, however,  a small number of stars that move at velocities of tens or even hundreds of km/s with respect to their environment and that will eventually escape from there. The archetypal runaway stars are mu Col and AE Aur that escaped in opposite directions from the vicinity of the Orion Nebula Cluster about 2.5 million years ago at velocities of order 100 km/s. Can we find “young” (i. e. recently ejected) runaway stars in regions of star formation? We will present the cases of an O6 star that was ejected about 6,000 years ago from the Welch ring of ultracompact HII regions  in W51N and review the case of the BN/KL region in Orion from where six runaways ejected only 500 years ago have been detected. Finally, we will discuss the mechanisms that have been proposed to produce runaway stars.