The structure and evolution of rich star clusters in the Large Magellanic Cloud

Surface brightness profiles and color-magnitude diagrams are presented for 18 rich star clusters in the Large Magellanic Cloud (LMC), with ages ~10⁷^-10⁹^ yr. The profiles of the older clusters are well represented by models with a King-like core. The profiles of many of the younger clusters show departures from such models in the form of bumps, sharp "shoulders, " and central dips. These features persist in profiles derived from images from which the bright stars have been subtracted; they therefore appear to reflect real substructure within the clusters. There is an upper limit to the radii of the cluster cores, and this upper limit increases with age from ~1 pc for the youngest clusters, to ~6 pc for the oldest ones. This trend probably reflects expansion of the cores driven by mass loss from evolving stars. Recent models of cluster evolution predict that the cores should expand at a rate that depends on the slope of the initial mass function (IMF). In the context of these models, the data favor an IMF for most of the clusters with a slope slightly flatter than the Salpeter value (for the range of stellar masses 0. 4-14Mₛun_), but with significant cluster-to-cluster variations. If the clusters have undergone violent relaxation, then the small cores of the youngest ones may imply formation from relatively "cool" initial conditions, while the sharp shoulders favor warmer conditions. An alternative interpretation of the shoulders is that they are signatures of merging subcondensations. It seems likely that the high ellipticity observed in many of the young LMC clusters is due, at least in part, to the presence of these subcondensations. Such substructure will be erased as the clusters evolve, and this might account for the general rounder appearance of the older clusters. Finally, since the younger clusters are not relaxed through two-body encounters, their current structure should provide a good guide for selecting initial conditions for theoretical models of globular cluster evolution.