Stellar rotation in the intermediate-age massive cluster NGC 1783: Clues about the nature of UV-dim stars

Over the past decade, stellar rotation has emerged as a key factor in shaping the morphology of color-magnitude diagrams of young and intermediate-age star clusters. In this study, we use MUSE integral-field spectroscopy to investigate the stellar rotation of ∼2300 stars in the 1.5 Gyr old cluster NGC 1783 in the Large Magellanic Cloud. The effective temperature, surface gravity, radial velocity, and projected rotational velocity (vsini) of the entire sample were obtained within a Bayesian framework to derive robust estimates of these parameters along with their associated errors. The analysis shows that stars along the extended main sequence turn-off (eMSTO) cover a wide range of rotational velocities, from values consistent with no or slow rotation up to vsini ∼ 250 km s-1. The distribution of stellar rotation velocities appears to play a crucial role in explaining the broadening of the eMSTO in this cluster, and a correlation is observed between vsini and the color of the eMSTO stars, with vsini increasing as the color becomes redder. Among the eMSTO stars, we investigate the peculiar population of stars strongly dimmed in the UV (so-called UV-dim stars), recently discovered in NGC 1783. UV-dim stars show clear photometric evidence of self-extinction and mild spectroscopic signatures typically observed in shell stars, suggesting that they have likely a decretion disk observed nearly equator-on. Interestingly, the study also shows that a significant fraction of UV-dim stars are slow rotators. We discuss potential implications these results may have on our understanding of the formation and evolution of UV-dim stars and we propose that the rotational properties of the UV-dim stars should vary with cluster age.