Structural Properties of Massive Young Clusters

We have retrieved multicolor WFPC2/Hubble Space Telescope data from the STScI archive for 27 nearby massive (>~3×10⁴ M_solar) young (<20 Myr) star clusters (MYCs). The data represents the most complete sample of clearly resolved MYCs to date. We have analyzed their structural properties and have found that they can be classified as either super-star clusters (SSCs) or as scaled OB associations (SOBAs). SSCs have a compact core possibly surrounded by a halo while SOBAs have no core. A morphological sequence can be established from SSCs with weak halos to SSCs with strong halos to SOBAs, and we propose that this is linked to the original mass distribution of the parent giant molecular clouds. Our results indicate that a significant fraction of the stars in MYCs dissipate on timescales of 10 Gyr or less due to the extended character of some of the clusters. Also, SSCs with ages younger than 7 Myr have smaller cores on average than those with ages older than 7 Myr, confirming predictions of numerical simulations with mass loss.