  We use the Wide Field Camera 3 (WFC3) onboard the Hubble Space Telescope
(HST) to obtain deep, high resolution images of two intermediate-age star
clusters in the Large Magellanic Cloud of relatively low mass ($\approx$ $10^4$
$M_{\odot}$) and significantly different core radii, namely NGC2209 and
NGC2249. For comparison purposes, we also re-analyzed archival HST images of
NGC1795 and IC2146, two other relatively low mass star clusters. From the
comparison of the observed color-magnitude diagrams with Monte Carlo
simulations, we find that the main sequence turnoff (MSTO) regions in NGC2209
and NGC2249 are significantly wider than that derived from simulations of
simple stellar populations, while those in NGC1795 and IC2146 are not. We
determine the evolution of the clusters' masses and escape velocities from an
age of 10 Myr to the present age. We find that the differences among these
clusters can be explained by dynamical evolution arguments if the currently
extended clusters (NGC2209 and IC2146) experienced stronger levels of initial
mass segregation than the currently compact ones (NGC2249 and NGC1795). Under
this assumption, we find that NGC2209 and NGC2249 have estimated escape
velocities $V_{\rm esc}$ $\geq$ 15 km s$^{-1}$ at an age of 10 Myr, large
enough to retain material ejected by slow winds of first-generation stars,
while the two clusters that do not feature extended MSTOs have $V_{\rm esc}$
$\leq$ 12 km s$^{-1}$ at that age. These results suggest that the extended MSTO
phenomenon can be better explained by a range of stellar ages rather than a
range of stellar rotation velocities or interacting binaries.

Solar and Stellar Astrophysics;Astrophysics of Galaxies