The tight subgiant branch of the intermediate-age star cluster NGC 411
  implies a single-aged stellar population
  The presence of extended main-sequence turn-off (eMSTO) regions in
intermediate-age star clusters in the Large and Small Magellanic Clouds is
often interpreted as resulting from extended star-formation histories (SFHs),
lasting $\geq$ 300 Myr. This strongly conflicts with the traditional view of
the dominant star-formation mode in stellar clusters, which are thought of as
single-aged stellar populations. Here we present a test of this interpretation
by exploring the morphology of the subgiant branch (SGB) of NGC 411, which
hosts possibly the most extended eMSTO among all known intermediate-age star
clusters. We show that the width of the NGC 411 SGB favours the single-aged
stellar population interpretation and rules out an extended SFH. In addition,
when considering the red clump (RC) morphology and adopting the unproven
premise that the widths of all features in the colour--magnitude diagram are
determined by an underlying range in ages, we find that the SFH implied is
still very close to that resulting from a single-aged stellar population, with
a minor fraction of stars scattering to younger ages compared with the bulk of
the population. The SFHs derived from the SGB and RC are both inconsistent with
the SFH derived from the eMSTO region. NGC 411 has a very low escape velocity
and it has unlikely undergone significant mass loss at an early stage, thus
indicating that it may lack the capacity to capture most of its initial,
expelled gas from stellar evolutionary processes, a condition often required
for extended SFHs to take root.
