Age as a major factor in the onset of multiple populations in stellar clusters

Abstract

It is now well established that globular clusters (GCs) exhibit star-to-star\nlight-element abundance variations (known as multiple stellar populations,\nMPs). Such chemical anomalies have been found in (nearly) all the ancient GCs\n(more than 10 Gyr old) of our Galaxy and its close companions, but so far no\nmodel for the origin of MPs is able to reproduce all the relevant observations.\nTo gain new insights into this phenomenon, we have undertaken a photometric\nHubble Space Telescope survey to study clusters with masses comparable to that\nof old GCs, where MPs have been identified, but with significantly younger\nages. Nine clusters in the Magellanic Clouds with ages between $\\sim$ 1.5-11\nGyr have been targeted in this survey. We confirm the presence of multiple\npopulations in all clusters older than 6 Gyr and we add NGC 1978 to the group\nof clusters for which MPs have been identified. With an age of $\\sim$ 2 Gyr,\nNGC 1978 is the youngest cluster known to host chemical abundance spreads found\nto date. We do not detect evident star-to-star variations for slightly younger\nmassive clusters ($\\sim$ 1.7 Gyr), thus pointing towards an unexpected age\ndependence for the onset of multiple populations. This discovery suggests that\nthe formation of MPs is not restricted to the early Universe and that GCs and\nyoung massive clusters share common formation and evolutionary processes.\n