Stellar population of the Rosette Nebula and NGC 2244

Abstract

Context. Measurements of internal dynamics of young clusters and star-forming regions are crucial to fully understand the process of their formation. A basic prerequisite for this is a well-established and robust list of probable members. Aims. In this work, we study the 2.8° ×2.6° region in the emblematic Rosette Nebula, centred in the young cluster NGC 2244, with the aim of constructing the most reliable candidate member list to date. Using the obtained catalogue, we can determine various structural and kinematic parameters, which can help to draw conclusions about the past and the future of the region. Methods. We constructed a catalogue containing optical to mid-infrared photometry, as well as accurate positions and proper motions from Gaia EDR3 for the sources in the field of the Rosette Nebula. We applied the probabilistic random forest algorithm to derive the membership probability for each source within our field of view. Based on the list of almost 3000 probable members, of which about a third are concentrated within the radius of 20′ from the centre of NGC 2244, we identified various clustered sources and stellar concentrations in the region, and estimated the average distance to the entire region at 1489 ± 37 pc, 1440 ± 32 pc to NGC 2244, and 1525 ± 36 pc to NGC 2237. The masses, extinction, and ages were derived by fitting the spectral energy distribution to the atmosphere and evolutionary models, and the internal dynamic was assessed via proper motions relative to the mean proper motion of NGC 2244. Results. NGC 2244 is showing a clear expansion pattern, with an expansion velocity that increases with radius. Its initial mass function (IMF) is well represented by two power laws (dN/dM ∝ M−α), with slopes α = 1.05 ± 0.02 for the mass range 0.2–1.5 M⊙ and α = 2.3 ± 0.3 for the mass range 1.5–20 M⊙, and it is in agreement with slopes detected in other star-forming regions. The mean age of the region, derived from the HR diagram, is ∼2 Myr. We find evidence for the difference in ages between NGC 2244 and the region associated with the molecular cloud, which appears slightly younger. The velocity dispersion of NGC 2244 is well above the virial velocity dispersion derived from the total mass (1000 ± 70 M⊙) and half-mass radius (3.4 ± 0.2 pc). From the comparison to other clusters and to numerical simulations, we conclude that NGC 2244 may be unbound and that it possibly may have even formed in a super-virial state.