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.

中文翻译：

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在 MUSE 中利用 HST：II。中年星团的Na丰度变化

远古（$>$10 Gyr）球状星团（GC）在某些元素之间以模式形式显示化学丰度变化，例如 N 与 Na 相关，与 O 反相关。 最近，在大质量星团中也观察到 N 丰度分布比旧的 GC 年轻得多，低至 $\sim$2 Gyr。然而，到目前为止，N 是在这些年轻物体中发现的唯一变化的元素。我们在此通过结合 HST 和 ESO-VLT 报告了小麦哲伦星云中中年龄大质量星团 NGC 416（$\sim$6.5 Gyr 旧）和 Lindsay 1（$\sim$7.5 Gyr 旧）中 Na 丰度变化的存在缪斯观察。使用 HST 光度法，我们能够在每个集群的红巨星分支中构建“染色体图”并分离具有不同 N 含量的亚群。将属于每个族群的单个恒星的 MUSE 光谱组合起来，产生代表每个族群的高信噪比光谱，将其进行比较以寻找 Na 的平均差异。我们发现 NGC 416 的 $\Delta$[Na/Fe]$=0.18\pm0.04$ dex 和 Lindsay 1 的 $\Delta$[Na/Fe]$=0.24\pm0.05$ dex 的平均丰度变化. 在两个集群中，我们发现 N 增加的种群也增加了 Na，这与在古代 GC 中观察到的模式相同。此外，我们在 NGC 416 的紫外色级图中检测到核-氦燃烧红团 (RC) 巨星的双峰分布。 两个 RC 叠加 MUSE 光谱的比较显示两个群体之间的平均 Na 丰度差异相同.