2013A&A...559A..38P

Most stars form as part of a star cluster. The most massive clusters in the Milky Way exist in two groups - loose and compact clusters - with significantly different sizes at the end of the star formation process. After their formation, both types of clusters expand up to a factor 10-20 within the first 20Myr of their development. Gas expulsion at the end of the star formation process is usually regarded as the only possible process that can lead to such an expansion during this early period of development. We investigate the effect of gas expulsion by a direct comparison between numerical models and observed clusters and concentrate on clusters with masses >10³M_☉. For these clusters the initial conditions before gas expulsion, the characteristic cluster development, its dependence on cluster mass, and the star formation efficiency (SFE) are investigated. We performed N-body simulations of the cluster expansion process after gas expulsion and compared the results with observations. We find that the expansion processes of the observed loose and compact massive clusters are driven by completely different physical processes. As expected, the expansion of loose massive clusters is largely driven by the gas loss due to the low SFE of ∼30%. One new revelation is that all the observed massive clusters of this group seem to have a very similar size of 1-3pc at the onset of expansion. It is demonstrated that compact clusters have a much higher effective SFE of 60-70% and are as a result much less affected by gas expulsion. Their expansion is mainly driven by stellar ejections caused by interactions between the cluster members. The reason ejections are so efficient in driving cluster expansion is that they occur dominantly from the cluster centre and over an extended period of time. During the first 10 Myr the internal dynamics of loose and compact clusters thus differ fundamentally.