Milone, A. P.A. P.MiloneBEDIN, LuigiLuigiBEDINPiotto, G.G.PiottoMarino, A. F.A. F.MarinoCASSISI, SantiSantiCASSISIBellini, A.A.BelliniJerjen, H.H.JerjenPIETRINFERNI, AdrianoAdrianoPIETRINFERNIAparicio, A.A.AparicioRich, R. M.R. M.Rich2020-04-022020-04-0220150035-8711http://hdl.handle.net/20.500.12386/23793Recent studies have shown that the extended main-sequence turn-off (eMSTO) is a common feature of intermediate-age star clusters in the Magellanic Clouds (MCs). The most simple explanation is that these stellar systems harbour multiple generations of stars with an age difference of a few hundred million years. However, while an eMSTO has been detected in a large number of clusters with ages between ̃1-2 Gyr, several studies of young clusters in both MCs and in nearby galaxies do not find any evidence for a prolonged star formation history, i. e. for multiple stellar generations. These results have suggested alternative interpretation of the eMSTOs observed in intermediate-age star clusters. The eMSTO could be due to stellar rotation mimicking an age spread or to interacting binaries. In these scenarios, intermediate-age MC clusters would be simple stellar populations, in close analogy with younger clusters. Here, we provide the first evidence for an eMSTO in a young stellar cluster. We exploit multiband Hubble Space Telescope photometry to study the ̃300-Myr old star cluster NGC 1856 in the Large Magellanic Cloud and detected a broadened MSTO that is consistent with a prolonged star formation which had a duration of about 150 Myr. Below the turn-off, the main sequence (MS) of NGC 1856 is split into a red and blue component, hosting 33 ± 5 and 67 ± 5 per cent of the total number of MS stars, respectively. We discuss these findings in the context of multiple-stellar-generation, stellar-rotation, and interacting-binary hypotheses.STAMPAenArticle10.1093/mnras/stv8292-s2.0-84938096444000360824000027https://academic.oup.com/mnras/article/450/4/3750/9928532015MNRAS.450.3750MFIS/05 - ASTRONOMIA E ASTROFISICA