Tuesday

Stellar clusters have traditionally been thought of as simple stellar populations, with all of the stars within a given cluster having the same chemical composition and age, within some small tolerance. However, all old globular clusters (GCs) in the Galaxy that have been studied in the necessary detail show chemical abundance anomalies (known as multiple populations - MPs) not observed in field stars of the same metallicity. The most common of these are the Na-O and N-C abundance anti-correlations.
The origin of MPs is still under debate, with none of the proposed scenarios able to reproduce the main observational properties of MPs without making ad hoc assumptions.
I will present the results from our ongoing HST Magellanic Cloud Cluster survey, which attempts to shed light on the physical property that controls the onset of MPs. The survey is composed of 12 massive clusters in the Large and Small Magellanic Clouds, spanning a wide range of ages (100 Myr-11 Gyr), but with similar masses. The surprising result we found is that all clusters older than 2 Gyr appear to host MPs, while all clusters younger do not. This finding clearly marks cluster age as one of the major parameter controlling the presence of MPs. This also points towards a fresh new perspective on to the onset of MPs, since this is completely unexpected for all theories for the origin of MPs.