An HST/WFPC2 Survey of Bright Young Clusters in M31 III. Structural
  Parameters
  Surface brightness profiles for 23 M31 star clusters were measured using
images from the Wide Field Planetary Camera 2 on the Hubble Space Telescope,
and fit to two types of models to determine the clusters' structural
properties. The clusters are primarily young (~10^8 yr) and massive (~10^4.5
solar masses), with median half-light radius 7 pc and dissolution times of a
few Gyr. The properties of the M31 clusters are comparable to those of clusters
of similar age in the Magellanic Clouds. Simulated star clusters are used to
derive a conversion from statistical measures of cluster size to half-light
radius so that the extragalactic clusters can be compared to young massive
clusters in the Milky Way. All three sets of star clusters fall approximately
on the same age-size relation. The young M31 clusters are expected to dissolve
within a few Gyr and will not survive to become old, globular clusters.
However, they do appear to follow the same fundamental plane relations as old
clusters; if confirmed with velocity dispersion measurements, this would be a
strong indication that the star cluster fundamental plane reflects universal
cluster formation conditions.
