The expansion of massive young star clusters-Observation meets theory?

Abstract

Context. Most stars form as part of a star cluster. The most massive clusters in the MilkyWay exist in two groups-loose and compact clusters-with significantly dierent 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 20 Myr 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. Aims. We investigate the eect of gas expulsion by a direct comparison between numerical models and observed clusters and concentrate on clusters with masses 103 M. For these clusters the initial conditions before gas expulsion, the characteristic cluster development, its dependence on cluster mass, and the star formation eciency (SFE) are investigated. Methods. We performed N-body simulations of the cluster expansion process after gas expulsion and compared the results with observations. Results. We find that the expansion processes of the observed loose and compact massive clusters are driven by completely dierent 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 1-3 pc at the onset of expansion. It is demonstrated that compact clusters have a much higher eective SFE of 60-70% and are as a result much less aected 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 dier fundamentally. © ESO 2013.