2011A&A...536A..90P

Massive Galactic clusters (>1000M_☉) exhibit a clear correlation between cluster density, size, and age and can be sorted into two categories, i.e. starburst and leaky clusters. The reason for having two types of massive clusters is still an open question; however, the answer is probably connected to the different formation histories of the two types. In this study we concentrate on leaky clusters and investigate possible formation scenarios and the gas expulsion phase. We use the existing observational data and numerical results of embedded cluster properties. Assuming that a clear correlation between cluster density, size, and age exists, it is shown that the density-radius development over time for embedded clusters can be approximated by ρ≃100*r^–1.3M_☉/pc³. The consequences for the star formation process in leaky clusters are discussed and found to favour an inside-out star formation scenario with an initially low, but later accelerated, star formation rate. It is shown how the leaky clusters form in a unique, sequential manner and how rapid gas expulsion is responsible for the 80-90% mass loss over the next 20Myr.