When does the onset of multiple stellar populations in star clusters occur? Detection of enriched stellar populations in NGC 2121

Star-to-star light-element abundance variations, know as multiple stellar populations (MPs), are common in almost all Galactic globular clusters. Recently, MPs have also been detected in a number of massive clusters with ages in excess of 2 Gyr in the Large Magellanic Cloud (LMC), thus indicating that age is likely a control parameter for the presence of MPs. However, to conclusively confirm this notion, additional studies of star clusters in the LMC’s age gap' of 3--6 Gyr are required. Here, we use {\sl Hubble Space Telescope} observations to study the 3 Gyr-old cluster NGC 2121. Compared with so-called simple’ stellar population models, the cluster’s red-giant branch exhibits an apparent spread in a specific color index that is sensitive to intrinsic chemical spreads. The observed spread can be explained by an intrinsic spread in nitrogen abundance of $\sim$0.5–1.0 dex}. NGC 2121 has a comparable mass to its younger counterparts without MPs, indicating that cluster mass might not be the dominant parameter controlling the presence of MPs. The transition phase between the occurrence of clusters with or without MPs seems to occur at ages of 2–3 Gyr, indicating that cluster age may play a dominant role in the establishment of MPs.