NGC 346: Tracing the Evolution of a Super Star Cluster

We discuss how star formation (SF) has been progressing - in space and time - in NGC 346, the site of most intense SF in the Small Magellanic Cloud (SMC), as derived from the analysis of optical broad (˜ V and ˜ I) and narrow band ˜ H_alpha) Hubble Space Telescope (HST) images. Our analysis reveals that NGC 346 experienced different regimes of SF, in a number of compact sub-clusters. In the youngest ones, we find a puzzling and intriguing deficiency of massive stars, either suggestive of an evolution of the initial mass function with time, with the youngest sub-clusters not having had sufficient time to build the more massive stars yet, or that high and low-mass stars may form through different mechanisms. The combination of the broad and narrow-band data allowed us to identify bona fide pre-main sequence (PMS) stars that are actively accreting material from their circumstellar disks. Interestingly, the identified PMS stars show a bimodal age distribution - peaked respectively at ˜ 1 and ˜ 20 Myr - with the two generations of stars that appear to be spatially independent