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The fraction of massive stars in young stellar clusters is of importance asthey are the dominant sources of both mechanical and radiative feedback,strongly influencing the thermal and dynamical state of their birthenvironments. It turns out that a significant fraction of massive stars escapefrom their parent cluster via dynamical interactions of single stars and/ormultiple stellar systems. M 17 is the nearest giant H II region hosting a veryyoung and massive cluster: NGC 6618. Our aim is to identify stars brighter thanG < 21 mag that belong to NGC 6618, including the (massive) stars that may haveescaped since its formation, and to determine the cluster distance and age. Weidentified 42 members of NGC 6618 of which eight have a spectral type of O,with a mean distance of 1675 pc and a transversal velocity dispersion of about3 km/s , and a radial velocity dispersion of 6 km/s. Another ten O stars areassociated with NGC 6618, but they cannot be classified as members due to poorastrometry or high extinction. We have also identified six O star runaways. Therelative transverse velocity of these runaways ranges from 10 to 70 km/s andtheir kinematic age ranges from about 100 to 750 kyr. Given the alreadyestablished young age of NGC 6618 (< 1 Myr), this implies that massive starsare being ejected from the cluster already directly after or during the clusterformation process. When constructing the initial mass function, one has to takeinto account the massive stars that have already escaped from the cluster, thatis, about 30% of the O stars of the original population of NGC 6618. Thetrajectories of the O runaways can be traced back to the central 0.25 pc regionof NGC 6618. The good agreement between the evolutionary and kinematic age ofthe runaways implies that the latter provides an independent way to estimate (alower limit to) the age of the cluster.

The early evolution of young massive clusters. II. The kinematic history of NGC 6618 / M 17