Leveraging HST with MUSE: II. Na-abundance variations in intermediate age star clusters

Martocchia, S, Kamann, S, Saracino, S, Usher, CG, Bastian, N, Rejkuba, M, Latour, M, Lardo, C, Cabrera-Ziri, I, Dreizler, S, Kacharov, N, Kozhurina-Platais, V, Larsen, S, Mancino, S, Platais, I and Salaris, M Leveraging HST with MUSE: II. Na-abundance variations in intermediate age star clusters. Monthly Notices of the Royal Astronomical Society. ISSN 0035-8711 (Accepted)

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2009.10023v1.pdf - Accepted Version

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.