0 Assigned ID: 6478 ----- Attempting Submission 1 (Thu Apr 10 15:23:42 GMT 2025) ----- 0 ----- Attempting Submission 1 (Sat Jul 26 16:59:45 GMT 2025) ----- Young stellar clusters in Magellanic Clouds (MCs) do not host simple stellar populations, as could be expected. Features, such as the split main sequence (MS) and the extended main sequence turn-off (eMSTO), are easily visible in all clusters younger than 2 Gyrs. Spectroscopic studies suggest that these features arise from different rotation rates among the stars. However, the occurrence of slow rotators poses a serious challenge to our understanding of stellar evolution. Indeed, pre-MS stars are expected to accelerate, so we do not expect to find slow-rotating stars. What causes some stars to slow down while others retain their spin? So far, three main mechanisms have been proposed, involving tidal interaction in binary systems, disc interaction in the pre-MS phase, and stellar mergers. Binary fraction among fast and slow rotators is the key to constrains the mechanism, indeed, each mechanism predicts different binary content. A new method has been developed to analyze a large sample of binaries, effectively distinguishing between slow and fast rotators using the F275W filter. This method has already been applied to three star clusters that are less than 100 million years old. Unfortunately, due to their young age, it is challenging to validate a single mechanism. We propose to observe NGC 1866, a 200-million-year-old star cluster in the Large Magellanic Cloud, using the F275W filter. Given that the mechanism has been active for a longer period, we expect to find more significant differences in binary content. This makes the cluster the smoking gun uncovering the braking mechanism and the origin of the split MS and the eMSTO. This is a proposal aimed at obtaining high accuracy photometry of the MC cluster NGC1866 in the WFC3/UVIS/F275W band. Over the past two decades, our team has significantly advanced the study of multiple stellar populations in star clusters and in the eMSTO and split MS phenomena. We combine expertise in photometry, astrometry, spectroscopy, and theoretical modeling to tackle key questions in stellar astrophysics. Our group includes recognized early-career and senior researchers experienced in HST data analysis and stellar evolution modeling. We have led the main surveys on stellar populations in Magellanic Cloud clusters with HST (e.g. Milone et al. 2009, A&A, 497, 755; Milone et al. 2023, A&A, 672, 161) and various papers on major discoveries in the field (e.g. Milone et al. 2015, MNRAS, 450, 3750; Milone et al. 2018, MNRAS, 477, 2640; Marino et al. 2018, AJ, 156, 116; Cordoni et al. 2022, Nature Communication, 13, 4325; Muratore et al. 2024, A&A, 692, 135). 1) A. P. Milone, E. P. Lagioia, G. Cordoni, E. Dondoglio, and M. V. Legnardi have extensive experience in photometric and astrometric data analysis. This dataset will be invaluable for F. Muratore's PhD on young and intermediate-age star clusters in the Magellanic Cloud, as well as for E. Bortolan and T. Ziliotto, who study multiple stellar populations. 2) A. F. Marino brings expertise in spectral analysis and chemical abundance determination using multi-band photometry and synthetic spectra. 3) M. Tailo specializes in theoretical stellar evolution and modeling, essential for interpreting the dataset's broader implications. 4) A. Mastrobuono-Battisti focuses on stellar dynamics and will provide insights into the dynamical aspects of multiple stellar populations. Stellar Populations and the Interstellar Medium Stellar Physics and Stellar Types 6 false false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 WFC3/UVIS Imaging No No F275W POS-TARG DITHER 6 LINE false false 2 270 LINE 1 false C:\Users\Fabrizio\AppData\Local\Temp\OPVisit2468485763790346493.xml 1753549143849 The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. 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