Institution(s): 1. European Southern Observatory
The most massive star clusters forming today go through a phase during which massive, luminous stars coexist with their birth molecular cloud. These clusters collect their mass from a surrounding region much larger than the final cluster size, and therefore their final mass is not set until their parent cloud is completely destroyed. The conditions at these early stages are crucial for determining the initial mass function and cluster structure. I will discuss a comparitive study of two of the most active cluster-forming regions in the Galaxy today, W51 and Sgr B2. In W51, two separate clusters of massive stars have formed within ~5 pc of each other, and both clusters are still accreting from their parent cloud. We observe the kinematics of the most
massive stars in the central clusters and compare the luminosity and dynamically derived masses. In W51, the clusters are presently failing to disrupt their parent clouds, but supernovae will probably eventually remove the gas. In Sgr B2, the massive clusters are driving powerful bubbles, but the gas remains hot and molecular. Some of these bubbles may be driven by supernovae, making Sgr B2 the only region in our Galaxy where supernova feedback fails to halt star formation. The failure of different feedback mechanisms has important implications for the timescale of star formation, setting a lower limit that is in tension with the short single-burst models that fit more evolved young massive clusters.