Leveraging HST with MUSE: II. Na-abundance variations in intermediate
  age star clusters
  Ancient ($>$10 Gyr) globular clusters (GCs) show chemical abundance
variations in the form of patterns among certain elements, e.g. N correlates
with Na and anti-correlates with O. Recently, N abundance spreads have also
been observed in massive star clusters that are significantly younger than old
GCs, down to an age of $\sim$2 Gyr. However, so far N has been the only element
found to vary in such young objects. We report here the presence of Na
abundance variations in the intermediate age massive star clusters NGC 416
($\sim$6.5 Gyr old) and Lindsay 1 ($\sim$7.5 Gyr old) in the Small Magellanic
Cloud, by combining HST and ESO-VLT MUSE observations. Using HST photometry we
were able to construct ''chromosome maps'' and separate sub-populations with
different N content, in the red giant branch of each cluster. MUSE spectra of
individual stars belonging to each population were combined, resulting in high
signal-to-noise spectra representative of each population, which were compared
to search for mean differences in Na. We find a mean abundance variation of
$\Delta$[Na/Fe]$=0.18\pm0.04$ dex for NGC 416 and $\Delta$[Na/Fe]$=0.24\pm0.05$
dex for Lindsay 1. In both clusters we find that the population that is
enhanced in N is also enhanced in Na, which is the same pattern to the one
observed in ancient GCs. Furthermore, we detect a bimodal distribution of
core-helium burning Red Clump (RC) giants in the UV colour magnitude diagram of
NGC 416. A comparison of the stacked MUSE spectra of the two RCs shows the same
mean Na abundance difference between the two populations. The results reported
in this work are a crucial hint that star clusters of a large age range share
the same origin: they are the same types of objects, but only separated in age.
