Discrepancies in the ages of young star clusters; evidence for mergers?
  There is growing evidence that star clusters can no longer be considered
simple stellar populations (SSPs). Intermediate and old age clusters are often
found to have extended main sequence turn-offs (eMSTOs) which are difficult to
explain with single age isochrones, an effect attributed to rotation. In this
paper, we provide the first characterisation of this effect in young (<20Myr)
clusters. We determine ages for 4 young massive clusters (2 LMC, 2 Galactic) by
three different methods: using the brightest single turn-off (TO) star; using
the luminosity function (LF) of the TO; and by using the lowest $L_{\rm bol}$
red supergiant (RSG). The age found using the cluster TO is consistently
younger than the age found using the lowest RSG $L_{\rm bol}$. Under the
assumption that the lowest luminosity RSG age is the `true' age, we argue that
the eMSTOs of these clusters cannot be explained solely by rotation or
unresolved binaries. We speculate that the most luminous stars above the TO are
massive blue straggler stars formed via binary interaction, either as mass
gainers or merger products. Therefore, using the cluster TO method to infer
ages and initial masses of post-main sequence stars such as Wolf-Rayet stars,
luminous blue variables and RSGs, will result in ages inferred being too young
and masses too high.
