Hunting UV-dim stars in Magellanic Cloud star clusters with HST

Nearly all young and intermediate-age star clusters in the Large (LMC) and Small Magellanic Clouds typically exhibit complex color-magnitude diagrams (CMDs). In addition to the extended main-sequence turn-offs (eMSTOs), commonly observed in clusters younger than 2 Gyr, the star clusters with ages smaller than 800 Myr display split main sequences (MSs). The interpretation of these observations is still widely debated. Initially explained by prolonged/multiple star formation, eMSTO clusters were considered youthful versions of ancient globular clusters (GCs) with multiple populations. Alternatively, different stellar rotation rates could mimic age differences at the turn-off. Recently, a new phenomenon has been reported in the ~1.5 Gyr cluster NGC 1783, where a fraction of the MS turn-off stars appears abnormally dim in the UV. The colors and magnitudes of these ‘UV-dim’ stars can be reproduced by simulated stars obscured by a ring of dust at the periphery of the excretion disc. The presence of dust may challenge the previous conclusion concerning the role of age and/or rotation in star clusters. Indeed, the eMSTO can be populated by dusty stars and the observed UV-dim stars include the tail of the distribution, where the dust ring is seen along the line of sight. If dust and rotation alone explain the eMSTOs, the origin of the eMSTOs and the formation of multiple populations in GCs are different phenomena. In this master thesis, I derived high-precision multi-band photometry from the Hubble Space Telescope to perform a detailed study of the 20-Myr old LMC cluster NGC 1818. I found that UV-dim stars are also present in this cluster thus demonstrating that they are not a peculiarity of NGC1783. This discovery provides a fundamental contribution to constrain the role of dust, age, and rotation in shaping the eMSTO, and to understand the star-formation history in star clusters.