2009MNRAS.394.2127B

The very young open cluster (OC) NGC2244 in the Rosette Nebula was studied with field-star-decontaminated Two-Micron All-Sky Survey (2MASS) photometry, which shows the main-sequence (MS) stars and an abundant pre-MS (PMS) population. Fundamental and structural parameters were derived with colour-magnitude diagrams (CMDs), stellar radial density profiles (RDPs) and mass functions (MFs). Most previous studies centred NGC2244 close to the bright K0V star 12Monocerotis, which is not a cluster member. Instead, the near-infrared RDP indicates a pronounced core near the O5 star HD46150. We derive an age within 1-6Myr, an absorption A_V = 1.7±0.2, a distance from the Sun d_☉= 1.6±0.2kpc (~1.5kpc outside the solar circle), an MF slope χ = 0.91±0.13 and a total (MS+PMS) stellar mass of ∼625M_☉. Its RDP is characterized by the core and cluster radii R_c~ 5.6arcmin (~2.6pc) and R_RDP~ 10arcmin (~4.7pc), respectively. Departure from dynamical equilibrium is suggested by the abnormally large core radius and the marked central stellar excess. We also investigate the elusive neighbouring OC NGC2239, which is low mass (m_MS+PMS~ 301M_☉), young (5±4Myr) rather absorbed (A_V = 3.4±0.2), and located in the background of NGC2244 at d_☉= 3.9±0.4kpc. Its RDP follows a King-like function of R_c~ 0.5arcmin ~ 0.5pc and R_RDP~ 5.0arcmin ~ 5.6pc. The MF slope, χ = 1.24±0.06, is essentially Salpeter's initial mass function. NGC2244 is probably doomed to dissolution in a few 10⁷yr. Wide-field extractions and field-star decontamination increase the stellar statistics and enhance both CMDs and RDPs, which is essential for faint and bright star clusters.