Massive young star clusters are fundamental building blocks of galaxies, and the most abundant reservoirs of newly born stars in the Milky Way. A robust membership assignment is fundamental to study their populations and physical properties. Traditionally, various methods have been used to separate members from the field stars, including the presence of X-ray emission, infrared excess, spectroscopic youth features, and, to a lesser extent, proper motions. With the advent of the Gaia mission and its precision astrometry, we are now able to study complete stellar populations in massive young clusters at kpc distances, over areas much larger than ever before. In this contribution, I will present an application of the Probabilistic Random Forest algorithm to study the stellar population in the Rosette Nebula, harbouring a young (~2 Myr) cluster NGC 2244 located at a distance of ~1.5 kpc. Our new sample of candidate members doubles the previously identified one, allowing us to derive the most complete Initial Mass Function extending down to 0.1-0.2 MSun, study the spatial structure of the region, detect expansion of NGC 2244, and discuss potential scenarios for its formation.

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